Nuts and Dried Fruits: An Update of Their Beneficial Effects on Type 2 Diabetes

Nuts and dried fruit are essential foods in the Mediterranean diet. Their frequent consumption has been associated with the prevention and/or the management of such metabolic conditions as type 2 diabetes (T2D), metabolic syndrome and cardiovascular diseases. Several previous reviews of epidemiological studies and clinical trials have evaluated the associations of nuts and/or dried fruit with various metabolic disorders. However, no reviews have focused on the mechanisms underlying the role of nuts and/or dried fruit in insulin resistance and T2D. This review aims to report nut and dried-fruit nutritional interventions in animals and humans, and to focus on mechanisms that could play a significant role in the prevention and treatment of insulin resistance and T2D.


Introduction
Nuts and traditional dried fruit (i.e., with no added sugar) are key food categories in the Mediterranean diet and other regional diets [1]. Several prospective studies, clinical trials and research in animals have reported beneficial effects after nut consumption [2]. However, the benefits of dried fruits (DF), mainly raisins, have been less explored [3].
Over time, food consumption has varied. More than 30 years ago, the consumption of nuts and DF was discouraged because of their high fat and sugar content, respectively. However, at the beginning of the 1990s, several randomized clinical trials (RCT) and animal experiments demonstrated their potential beneficial effect on cardiovascular diseases (CVD). Nuts and DF contain various macro and micronutrients together with other important bioactive compounds that may synergically contribute to modulate specific metabolic diseases such as hypercholesterolemia, hypertension and type 2 diabetes (T2D) (reviewed in [3,4]). Even so, the specific role of nuts and DF in the development and progression of insulin resistance (IR) and T2D are still controversial.
In this review, we focus on the role of nuts and DF in the prevention and treatment of T2D. We summarize published in vivo, in vitro, epidemiological and clinical studies, and we review the potential mechanisms that could explain the beneficial role of nut consumption on glucose and insulin metabolism, both of which are altered in T2D and in other glucose-impaired states. Given that the present article is not a systematic review, we may not have identified some studies and publication bias should be acknowledged. However, all authors independently conducted the literature search.

Nuts
Nuts have been part of the human diet since prehistoric times [5,6]. They are an independent food group and are one of the cornerstones of the Mediterranean diet (MedDiet) [7]. According to the botanical definition, a nut is simply a dried fruit with one seed (rarely two) in which the ovary walls are very hard (stony or woody) at maturity, and the seed is unattached or free within the ovary wall. However, the word "nut" is commonly used to refer to any large, oily kernel in a shell that can be eaten as food. In this review, we use the term "nuts" to refer to almonds, Brazil nuts, cashews, hazelnuts, macadamias, peanuts, pecans, pine nuts, pistachios and walnuts. Although peanuts are actually classified as legumes because of their similar nutrient composition and their proven cardiovascular health benefits, they are commonly regarded as being a nut.

Dried Fruits
To extend their shelf life, fresh fruits are processed by various techniques to become DFs [3]. Dried fruits are a concentrated form of fresh fruits with a lower moisture content. Fruits can be dried whole (e.g., apricots, berries and grapes), in halves, or in slices (e.g., kiwis, mangoes and papayas). In this form, they are easy to store and distribute, they can be available throughout the year, and they are a healthier alternative to salty or sugary snacks. Apples, apricots, currants, dates, figs, peaches, pears, prunes, and raisins are referred to as "conventional" or "traditional" DFs. Meanwhile, such fruits as blueberries, cranberries, cherries, strawberries and mangoes are commonly infused with different types of sugar solution (or fruit juice) concentrate before drying [8] so are not included in the aforementioned category. Moreover, we have also excluded dried tomato because although it is botanically a berry-type fruit, it is culinary considered a vegetable and it shares nutrient composition with this food category.

Nutritional Composition of Nuts and Dried Fruits
Nuts and DFs are a matrix of important bioactive compounds such as Vitamins (Vitamin E, niacin, choline and/or folic acid), minerals (magnesium, potassium, calcium and/or phosphorus), phenolic compounds, carotenoids and/or phytosterols [9]. Importantly, some nuts and DFs are among the 50 foods with the highest antioxidant capacity [10] and are also a known source of bioactive compounds, including plant sterols [11]. Pistachios are particularly rich in β-carotenes which have been widely associated with a protective T2D role [12,13]. In addition, pistachios are the only nuts that contain significant amounts of lutein and zeaxanthin [9]. Sun-dried raisins retain the minerals and most of the phytochemicals and antioxidants of the grape, including its resveratrol [14,15]. In fact, sun-drying enhances the antioxidant content of raisins. Because of the dehydration process, phytonutrients are more concentrated in raisins than in grapes. However, the concentration of some compounds is decreased by the sun-drying process in DFs and by dry roasting techniques in nuts [9]. Polyphenols and tocopherols from nuts and DF have proved to be rapidly accessible in the stomach, thus maximizing the possibility of absorption in the upper small intestine, and contributing to the beneficial relation between nut and DF consumption and health-related outcomes [8,16].

Diet Quality in the Context of Nut and Dried Fruit Consumption
Epidemiological studies conducted in children and adults have demonstrated a significant positive association between nut consumption and diet quality [17,18]. Furthermore, the results of a clinical trial conducted in obese (Ob) subjects (n = 124) showed that the nutritional dietary quality of nut consumers (reporting to eat 42 g hazelnuts/day for 12 weeks) was remarkably higher than among other groups consuming chocolate, potato crisps or no additional foods [19]. Moreover, including nuts in energy-restricted diets reduced attrition and increased weight loss, indicating that nuts enhance palatability and compliance with diets without compromising health [20].
Several studies have examined the associations of whole fruit or 100% fruit juice [21] with nutritional or health outcomes such as T2D but there is a lack of studies examining potential links between DF and diet quality. A prospective study conducted in adult participants (n = 13,292) in the 1999-2004 National Health and Nutrition Examination Survey (NHANES) demonstrated an association between DF consumption and diet quality [22]. DF consumption was associated with improved nutrient intakes, a higher overall diet quality score, and lower body weight (BW)/adiposity measures [22]. Moreover, in a cross-sectional study in healthy adults (n = 797) from Hong Kong, an inverse association has been found between the intake of vegetables, legumes, fruits, dried fruits and Vitamin C and the prevalence of metabolic disorders such as non-alcoholic fatty liver disease (NAFLD) [23].
The 2015 Dietary Guidelines for Americans included the following three healthy dietary patterns: a Healthy US-style Pattern, a Healthy Vegetarian Pattern and a Healthy Mediterranean-style pattern. Fruits, nuts, and seeds play a prominent role in all three of these food-based dietary patterns, which recommend 350-440 g/day of fruit, and 16-28 g/day of nuts and seeds [24].

In Vivo and In Vitro Studies
Even though much of the research on nuts, dried fruits and T2D is based on observational studies and human trials, some in vitro and in vivo studies also evaluate their modulatory effect on glucose and insulin metabolism. In this regard, the effect of nuts on glucose and insulin metabolism has been investigated by evaluating nut extracts [25,26] or nuts as a whole [27][28][29] mainly in mice or rats. However, dried fruit has been investigated-like their non-dried counterparts-mainly in in vitro studies [30][31][32][33]. Almost all the research has focused on the extracts from non-edible parts, such as the shell [34], leaves [35,36], stems [37] and roots [38], and very little on the nut or fruit kernel [27][28][29]39]. However, this review focuses on those studies evaluating specific nutrients (e.g., polyphenols) or edible parts in both traditional nuts and dried fruits with outcomes related to glucose metabolism, IR, and the T2D oxidation/inflammation axis [32,40].

Nuts
The in vivo studies on nuts and T2D-related parameters are summarized in Table 3. These studies were mainly performed using extracts from peanuts or walnuts. Peanut oil supplementation for 42 days in diabetes-induced rats significantly reduced glucose and glycated hemoglobin (HbA 1c ) concentrations and improved lipid metabolism compared to normal rats [27]. Other researchers have found similar improvements in glycaemia in genotypes of diabetes in rats fed with peanut oil extract [41,42] or peanut aqueous extract [25]. In the case of walnuts, a polyphenol-rich walnut extract (PWE) for 4 weeks significantly decreased urinary 8-hydroxy-2 -deoxyguanosin levels (an in vivo marker of oxidative stress) and improved serum TG in db/db mice [26]. Moreover, an HFD with a 21.5% of energy from walnuts tested in mice for 20 weeks significantly reduced TG compared to nut-free HFD and tended to improve glucose and IR [29]. In vitro assessment of the T2D-related antioxidant and inflammatory capacity of nuts has largely been conducted by examining the ability of extracts to increase the resistance of human plasma or low density lipoprotein (LDL) to oxidation. Extracts of walnut [43], almond and almond skins [44,45], pistachio [46], and hazelnut [47] have been found to increase the lag time oxidation of LDL. However, little research has focused on their in vitro effects on glucose and insulin metabolism. Specifically, a hydro-ethanolic extract of cashew nut and its principal compound, anacardic acid, significantly stimulated glucose uptake in C2C12 muscle cells in a concentration-dependent manner, suggesting that it may be a potential anti-diabetic nutraceutical [48]. Moreover, cytoprotective activity of pistachio extracts (methanolic, water or ethyl acetate) against oxidative (reactive oxygen species formation) and carbonyl stress has also been reported in a T2D model in hepatocytes from rats [49].

Dried Fruits
Both in vitro and in vivo research has mostly focused on grape. Overman and collaborators showed that a grape powder extract (GPE) significantly attenuated lipopolysaccharide (LPS)-mediated inflammation in macrophages and decreased the capacity of LPS-stimulated human macrophages to inflame adipocytes and cause IR [30]. Moreover, GPE further attenuated tumor necrosis factor-α (TNF-α) mediated inflammation and IR in primary cultures of human adipocytes [31]. Grape polyphenol extract modulated in vitro membrane phospholipid fatty acid (FA) composition but also decreased muscle TG content and increased muscle glucose transporter type 4 (GLUT4) expression in high-fat-high-sucrose diet-fed rats. Overall, it improved insulin resistance status (i.e., HOMA-IR parameter) [50]. This is of considerable importance because the accumulation of muscle TG content and the modification of the muscle phospholipid fatty acid pattern may have an impact on lipid metabolism and increase the risk of developing T2D [51]. Mice fed with grape skin extract showed hypoglycaemic and anti-hyperglycaemic effects (independent of an increase in insulin release) but are probably dependent on an increase in insulin sensitivity resulting from the activation of the insulin-signaling cascade in skeletal muscle [52]. Furthermore, grape seed aqueous extract protected the pancreas against oxidative stress, inflammation and apoptosis-induced damage while preserving pancreatic function at near normal levels in diabetic rats [53].
Other DF extracts have also been evaluated. Treatment with a date fruit extract was effective at decreasing behavioral, neurophysiological, and pathological alterations induced by diabetes in the peripheral nerves of streptozotocin (STZ)-induced diabetic rats (well-characterized animal model of type 1 diabetes) [33]. Moreover, daily consumption of a low-or high-fat diet supplemented with 1% black currant powder extract (with 32% of anthocyanins) for 8 weeks reduced body weight gain and improved glucose metabolism [54].
The beneficial effect of fruit juices and fermented grape juice (i.e., wine) have also been an important focus of research. Schmatz and collaborators investigated the ex vivo effects of a moderate consumption of red wine (RW) and grape juice (GJ), and the in vitro effects of various substances (resveratrol, caffeic acid, gallic acid, quercetin and rutin) on STZ-induced diabetic rats. They demonstrated decreased platelet aggregation in diabetic-induced rats after moderate RW and GJ consumption for 45 days [55]. Similarly, resveratrol increased the hydrolysis of adenosine triphosphate (ATP), while quercetin decreased it in platelets [55]. These results were extended using a wine grape powder supplementation, which prevented hyperglycemia and IR, and reduced oxidative stress in a rat model of metabolic syndrome (MetS) [56].
Specific compounds found in both nuts and DFs were also further investigated. Quercetin and trans-resveratrol are plant polyphenols which have showed a significant reduction of IR and inflammation associated with obesity. Eid et al. showed that quercetin (isolated from lingonberry) exerted an anti-diabetic activity by stimulating adenosine monophosphate-activated protein kinase (AMPK) [40]. Moreover, quercetin also enhanced basal glucose uptake in mouse myoblast C2C12 muscle cells in the absence of insulin [40] via a mechanism which is highly analogous to metformin. Importantly, quercetin seems to be as effective as or more effective than resveratrol in attenuating TNF-α-mediated inflammation and IR in primary human adipocytes and macrophages [32,57].

Epidemiological Studies on Nuts
The relationship between the consumption of different food categories and the incidence or prevalence of metabolic disorders has been explored all over the world. As is shown in Table 4, numerous epidemiological studies have assessed the associations between nut consumption and T2D. However, no studies have been published on the link between DF consumption and risk of T2D.
As far as nuts are concerned, several studies have found an inverse association between the frequency of consumption and the development of this metabolic pathology [58][59][60][61][62][63][64]. In the Nurses' Health Study (NHS) and NHS II cohort, the intake of different types of nut was explored [61,64]. Total nut, walnut and peanut butter intake were associated with a lower risk of T2D in women. In the NHS (n = 83,818 female subjects), those with the highest nut consumption (28 g/day; ≥5 days a week) had a lower relative risk (RR) of developing T2D (0.73 [95% confidence interval (CI), 0.60-0.89]) than those who never/almost never consume (0.92 [95% CI, 0.85-1.00]) [61]. Results were similar for peanut butter: the RR was 0.79 (95% CI, 0.68-0.91) in those women who had a higher intake (5 times or more a week) than those who never/almost never ate peanut butter [61]. After combining NHS and NHS II cohorts, in a total of 137,953 female subjects, Pan and collaborators found that walnut consumption was inversely associated with risk of T2D [64].
These results are in line with those of a cross-sectional study of 7,210 subjects at high CV risk within the context of the PREvención con DIeta MEDiterránea (PREDIMED) study, where the upper category of nut consumption had a lower prevalence of T2D than the lower category [63]. A recent prospective study also associated the consumption of nuts-higher than 4 times a week-with a lower risk of T2D [60]. A cross-sectional study performed in the context of the National Health and Nutrition Examination Survey (NHANES) established a relation between the homeostatic model assessment of insulin resistance (HOMA-IR) and tree nut consumption. Decreased insulin resistance and lower levels of β-cell function markers were found in the nut consumers than the non-consumers [59]. In a large cohort of the Netherlands Cohort Study (NLCS), the total nut intake was associated to lower T2D cause-specific mortality in men and women [65].
Even though current evidence shows that nuts have a strong protective effect against the progress of T2D, especially in women, some epidemiological studies have not identified this relation [66,67]. The latest systematic review and meta-analysis designed to assess the relation between nut consumption and risk of cancer and T2D was published in 2015. It included five studies that were linked to T2D. After pooling data from studies conducted in both genders, it was found that there was no statistically significant association between nut consumption and risk of developing T2D (RR = 0.98 [95% CI, 0.84-1.14]), even though the heterogeneity was significant [68].

Human Clinical Trials
The effect of nut consumption on glucose and insulin metabolism has also been researched in acute and chronic clinical trials. Acute studies mostly demonstrate a decrease in postprandial glycaemia and hyperinsulinemia after nut consumption. In contrast, chronic randomized clinical trials designed to analyze the effects on glucose metabolism provided controversial results. Table 5 summarizes various acute clinical studies of nut consumption, most of which focus on almonds [69][70][71][72][73]. In a randomized crossover trial in healthy subjects, almond intake decreased postprandial glycaemia and insulinaemia [69]. In a dose-response study conducted in healthy individuals, almond intake attenuated the postprandial glycaemic response of white bread [70]. In impaired glucose-tolerant subjects, it was observed that the consumption of almonds with a meal also decreased blood glucose in plasma [71]. In a recent crossover study performed in pre-diabetic subjects, a preload of almonds decreased postprandial glycaemia [73]. In healthy and diabetic individuals, Cohen et al. also showed a 30% reduction in postprandial glycaemia after almond consumption compared with a starchy meal [72].

Acute Clinical Trials on Nuts
The effect of pistachio intake on postprandial glycaemia was investigated by Kendall et al. in two randomized studies. In overweight (Ow) healthy subjects, pistachio consumption was reported to have a minimal effect on postprandial glycaemia. When pistachios were included in a carbohydrate meal, the relative glycaemic response (RGR) was attenuated [74]. In a crossover trial with 20 subjects with MetS, postprandial glycaemia decreased after the consumption of pistachios (85 g) compared to white bread. In the same study, a peripheral increase in the glucagon-like peptide-1 (GLP-1) concentrations was also observed after pistachio consumption compared with the consumption of white bread [75].
For peanuts, a randomized crossover trial conducted in 13 healthy subjects reported a decreased postprandial glycaemic response after the consumption of a breakfast containing 63 g of one of the following types of peanuts: raw with skin, roasted without skin and ground-roasted without skin [76]. Similarly, in a parallel study conducted in 65 overweight and obese men, peanut consumption reduced postprandial insulinaemia levels compared to high-oleic peanut consumption [77].
Nut consumption was observed to have similar beneficial effects on glucose and insulin metabolism in the only study to analyze the effect of a mix of nuts (almonds, macadamias, walnuts, pistachios, hazelnuts and pecans). In this study conducted in 10 diabetic and 14 non-diabetic subjects, nut consumption decreased the RGR compared to white bread. Importantly, this study also reported that nut consumption improved short-term glycaemic control in patients with T2D [74].
In summary, nuts seem to have beneficial postprandial glycaemic effects when consumed alone or in combination with high carbohydrate foods, and so may potentially help to prevent and manage impaired glucose states.

Chronic Clinical Trials on Nuts
Most of the RCTs have compared nut-enriched diets with control diets in order to analyze their effects on lipid profile, and blood glucose and insulin concentrations as a secondary outcome. However, some, mainly conducted in subjects with T2D [72,[79][80][81][82][83][84], but also in pre-diabetic [85], hyperlipemic [86], overweight/obese [87] and healthy individuals [88], were specifically designed to assess changes in glucose or insulin metabolism after nut consumption (Table 6).
In 2002, Lovejoy and coworkers evaluated the effect of an almond-enriched diet in 20 healthy and 30 T2D subjects in two different studies [79]. Healthy subjects, supplemented with 100 g almonds/day for 4 weeks and advised to reduce their energy intake by an equivalent amount, did not change their insulin sensitivity, whereas their body weight increased and their lipid profile improved. In a second study, subjects with T2D were randomized following a crossover design to one of 4 diets with different fat contents (25% or 37%) with 10% of fat from almonds or from olive or canola oil, with a minimum washout of 2 weeks between periods. Fat source (almond vs. oil) or fat level (high fat vs. low fat) were not observed to have any significant effect on either the glucose or insulin index. In contrast, in a crossover study conducted in 20 Chinese patients with T2D and mild-hyperlipidemia assigned to either a control diet (National Cholesterol Education Program (NCEP) step II diet) or an almond-enriched diet (with almonds replacing the 20% total daily calorie intake) for 4 weeks, a significant decrease in fasting insulin and glucose concentrations together with an improvement of HOMA-IR were reported during the almond phase [80]. In a similar crossover study conducted in 48 diabetic patients who received 50 g/day of pistachios and a pistachio-free diet (12 weeks each period), with an 8-week washout, a significant improvement in fasting blood glucose (FBG) and HbA 1c was observed during the pistachio consumption, while no changes in HOMA-IR were reported [83]. Recently, Gulati and coworkers conducted a pre-post intervention study in a group of 50 Asian Indians who consumed 20% of total energy in the form of whole raw almonds for 24 weeks preceded by a control diet free of nuts. Although no changes in FBG were observed during the almond consumption, the authors found a significant reduction in glycosylated hemoglobin, together with an improvement in other T2D risk factors such as waist circumference or inflammation status [84]. To determine whether the beneficial effect of nut consumption on glucose and insulin metabolism could also be extended to pre-diabetic subjects our group conducted a randomized crossover study in 54 pre-diabetic subjects who consumed a pistachio-supplemented diet (55 g pistachio/day) and a control diet (nut-free diet), each for 4 months with a 2-week washout period. We found a beneficial effect of pistachio intake on fasting glucose, insulin, and HOMA-IR. Other cardiometabolic risk markers such as fibrinogen, oxidized LDL, platelet factor 4 and GLP-1 were also modified appropriately during the consumption of pistachios [85].
Changes in fasting glucose or insulin levels, HOMA-IR and glycosylated hemoglobin have also been assessed as secondary outcomes in several clinical feeding trials with different designs (parallel, crossover) and subject characteristics (i.e., healthy, overweight/obese, TD2, MetS), mainly using walnuts and almonds, and with different intervention lengths (from 2 weeks to 2 years). The results obtained are controversial. Although most studies found no improvement in glucose/insulin metabolism [89][90][91][92][93][94][95][96][97][98][99][100][101][102][103], others reported a significant reduction in FBG levels [89,92,104,105], fasting insulin or insulin resistance [90,92,106,107] and HbA 1c [92]. However, a meta-analysis of RCTs including 25 trials with a total of 1,650 particpants who were otherwise healthy or had dyslipidaemia, metabolic syndrome or type 2 diabetes mellitus showed that the consumption of tree nuts led to modest decreases in fasting blood glucose compared with control diet interventions [108]. Addition of 20 kcal/kg-BW of roasted peanuts to the regular caloric intake.
Plasma-insulin and C-peptide increased in the candy group, but not in the peanut group. FBG was not modified.
Energy intake increased similarly in both groups. BW and WC increased significantly only in the candy group.
At the end of the study LDL-C and ApoB/ApoA-1-ratio were higher in the candy group than in the peanut group. Almonds were added to the control diet to replace 20% of total daily calorie intake.
Compared with subjects in the control diet, those in the almond diet reduced the levels of fasting insulin, FBG, and HOMA-IR.
Almond intake decreased TC, LDL-C, and LDL-C/HDL-C. The almond diet enhanced plasma α-tocopherol level compared with control diet. No changes in BMI were reported.
Significant HDL-C reduction in control group was observed. Although the hazelnut group achieved a greater reduction in TG concentrations than the control group, these changes were non-significant.

Acute Clinical Trials on Dried Fruits
Less research has been carried out into DFs than into nuts. However, the findings to date point to a beneficial effect of DFs on postprandial glucose regulation and glycaemic control in T2D subjects.
The putative effect of DFs on postprandial glycaemia and insulinaemia has been studied mainly using raisins (Table 7). However, some research into dried plums has also been published.
First, in 1989 Rasmussen and coworkers evaluated in healthy and T2D subjects the postprandial effects of three meals: raw rolled oats, oatmeal porridge, or a mixture of raw rolled oats with raisins, compared to a control glucose ingestion [109]. The substitution of 25% of the starch meal with raisins (i.e., simple sugars) did not affect blood glucose or insulin responses. In addition, a similar glucose and insulin response in both normal and T2D subjects were reported [109]. Other researchers investigated the postprandial effect of raisins consumed alone. When the GI was investigated in three different groups (sedentary, aerobically trained or pre-diabetic subjects), no significant differences were found among groups, even though the GI (55-69) it seemed moderate for aerobically trained adults, and low (GI, ≤55) for the other groups [110]. Kanellos and collaborators found a moderate GI of raisins in healthy and T2D subjects [111], whereas Esfahani et al. found that raisins were low-GI and glycaemic load (GL) foods in healthy subjects [112]. Recently, researchers have found that even though the same available CHO content from raisins and glucose generated a similar postprandial response, raisins significantly modulated the levels of GIP, ghrelin and ghrelin/obestatin ratio, with important implications in terms of appetite regulation and overall insulin secretion [113]. In overweight women, researchers determined that dried plums had a lower plasma glucose and insulin incremental area under the curve (IAUC) than an isoenergetic low-fat cookie meal [114].
Overall, results suggest that raisins have a beneficial postprandial glucose and insulin effect, which may cautiously be extrapolated to other DFs considering their overall macronutrient composition. GIP was lower after raising intake compared to glucose intake at 60 and 120 min postprandially. Ghrelin was lower after raisin compared to glucose intake at 120 and at 180 min post-ingestion. No differences were reported for GLP-1, apelin or obestatin in either trial.

Chronic Clinical Trials on Dried Fruits
The study of the beneficial effects of chronic DF consumption also focuses on raisins (Table 8). Randomized clinical trials have been conducted in healthy [115], overweight or obese [116,117], T2D subjects [118,119], or a combination of the three [120]. In a 6-week parallel trial in healthy subjects, Puglisi et al. included 150 g/day of raisins into subjects' habitual diet or increased their physical activity (or combined them both). Neither FPG nor insulin levels were different among groups or compared to baseline. However, the inflammation status reported by plasma TNF-α significantly decreased in the raisin intervention [115].
The results for overweight or obese subjects do not show a significant improvement in glucose or insulin levels after the consumption of raisins [116] or dried plums [117]. However, these studies are only short (2 weeks) which makes it impossible to analyze the chronic effect. In fact, in a parallel study conducted in Ow/Ob subjects, comparing raisin consumption (270 Kcal/day) with a snack (300 Kcal/day) for 12 weeks, researchers [120] found that even though FGP or insulin were not affected by either intervention, HbA 1c levels and postprandial glucose levels had been reduced by raisin consumption by the end of the trial. This suggests a beneficial effect of raisin consumption on glycaemic control in Ow/Ob subjects with pre-D. Importantly, raisin intake also improved systolic blood pressure (SBP) and diastolic blood pressure (DBP) and had a null effect on body weight [120].
Two studies have been conducted in individuals with T2D. A parallel study comparing the consumption of 36 g of raisins versus the habitual diet free of raisins or grapes for 24 weeks did not find any change in either body weight or in glycaemic control and lipid profile, but the total antioxidant capacity increased and the DBP decreased after raisin consumption [118]. Likewise, the consumption of raisins as a snack (84 g/day, 270 Kcal/day) for 12 weeks significantly reduced the postprandial glucose response in T2D subjects compared to an alternative snack (300 Kcal/day) for 12 weeks. A non-significant trend to a reduction in fasting glucose and HbA 1c was also observed in the same group of raisin consumers [119].
The results of DF consumption on glycaemia/insulinaemia point to a beneficial effect. However, novel acute and long-term RCTs assessing other types of DF should be carried out in order to corroborate and expand what is known about raisins. No changes were found in plasma glucose or insulin levels in any intervention.
Plasma TG concentration was unchanged by dried plum consumption and was higher after the consumption of low-fat cookies. Incorporation of dried plums or low-fat cookies into the diet did not alter energy intake or BW. Fasting HbA 1c levels were significantly reduced after raisin intake, whereas FBG and insulin levels were not significantly affected by the intake of raisins or snacks. Postprandial glucose levels were significantly reduced by raisin intake vs. snacks.
Raisin intake was associated with reductions in SBP and DBP. BW did not significantly change within or between groups. No change in CRP was observed. A significant difference in plasma circulating p-hydroxybenzoic acid was observed between groups at the end of the trial. Compared to the snack group, those who consumed raisins reduced their postprandial glucose levels, and an NS trend to a reduction in fasting glucose and HbA 1c . NS changes in BW, fasting insulin, HOMA-IR or lipid profile between intervention groups.
Compared to alternative processed snacks, those who consumed raisins had a significant reduction in SBP but not a significant reduction in DBP.

Nut-and Dried Fruit-Related Nutrients and Their Role in Glucose and Insulin Metabolism
Fiber Content in Nuts and Dried Fruits Both nuts and DFs are high in dietary fiber [9]. Diets rich in complex CHO and fiber are associated with increased insulin sensitivity and reduced plasma insulin levels, promoting better glycaemic control in diabetic patients [121]. Soluble fiber increases gastric distension, viscosity in the gastrointestinal tract, and slower absorption of macronutrients [122]. As a consequence, the speed of CHO absorption and the concentration of postprandial glucose tend to be lower after the ingestion of fiber-rich foods than foods or meals poor in fibers [123].
Fiber is resistant to enzymatic digestion in the small intestine and thus susceptible to fermentation by bacteria in the colon. It produces short chain fatty acids (SCFA, e.g., acetate, propionate and butyrate) which reduce the production of hepatic glucose and stimulate the secretion of GLP-1 [124,125]. Incretins such as GLP-1 and gastric inhibitory polypeptide (GIP) stimulate the secretion of insulin by β-cells and promote the proliferation of these cells, favoring the maintenance of normal blood glucose levels [125]. The secretion of GLP-1-which is mainly performed by enteroendocrine L-cells of the gastrointestinal tract -is partly mediated by monosaccharides, peptides and amino-acids, MUFA and PUFA as well as by SCFA. Therefore, the positive influence of GLP-1 on blood glucose homeostasis, appetite sensations, and food intake provides a strong rationale for its therapeutic potential in the nutritional management of T2D and obesity [126].
Overall fiber contained in nuts and DFs is also able to decrease postprandial glycaemic levels and this could be a strategy for increasing insulin sensitivity which improves T2D and several other CV risk factors for chronic diseases [74,127].

Carbohydrate Content-Glycaemic Index of Nuts and Dried Fruits
It should be noted that nuts are relatively low in CHO (approximately 15% of the total energy) whereas DFs have a high amount of CHO (60-80%). Nuts have a low glycaemic index and therefore increase the blood glucose level less and require less insulin secretion, thus favoring the control of T2D. However, because DFs are high in carbohydrates and fiber, their specific GI has been the object of considerable study. The GI of raisins was first evaluated in three heterogeneous groups of subjects (aerobically trained, sedentary or pre-diabetic) and was described between 49 and 69, therefore corresponding to the low-to-moderate GI foods [110]. However, later studies have reported that raisins are in the low GI category in healthy subjects (a GI of 49.4 and an insulinemic index of 47.1) [15]. This suggests a favorable postprandial glucose and insulin response [112], that could be explained by the high proportion of fructose that DFs contain.
Overall, the inclusion of both nuts and DFs in a balanced diet may reduce the overall glycaemic index of the diet, with benefits to glycaemic and insulinemic control in both healthy and T2D subjects [128].

Fat Content in Nuts
The high content of MUFA and PUFA in nuts seems to enhance the reduction of IR, thus consequently reducing the risk of developing T2D [129][130][131]. However, the mechanisms by which these fatty acids (FA) affect insulin sensitivity are not yet fully understood [132]. It is believed, however, that the FAs present in the phospholipids of different cell membranes are affected by the type of fatty acid intake, thus affecting insulin sensitivity. The different unsaturated FAs that are part of the cell membrane influence the action of insulin via affecting the binding or affinity of insulin to its cellular receptor [133]. It is hypothesized that a higher unsaturation of FA in the cell membrane facilitates the movement of the glucose receptor to the cell surface, thus increasing insulin sensitivity [134].
In addition, as we have seen, unsaturated fatty acids act through the stimulation of GLP-1 secretion, thus improving the efficiency of β-cells. SFA and MUFA act on the lipogenic gene expression, while PUFA inhibit the expression of these genes, partly by binding and activating nuclear receptors, such as those activated by peroxisome proliferators (PPAR) [131]. It has been suggested that the activation of PPAR has a therapeutic role in the treatment of T2D, because it induces fatty acid clearance by the adipose tissue, decreasing its plasma levels and thus increasing insulin sensitivity in the muscle [135].

Mineral Content in Nuts and Dried Fruits
Nuts contain relatively high quantities of different minerals such as potassium, magnesium and calcium, whereas DF contains more moderate quantities (mainly potassium, magnesium and boron). Magnesium has been associated with beneficial effects on glycaemic control [136]. Low magnesium levels have been implicated in IR development [124]. In fact, kidneys lose the ability to retain magnesium in periods of acute hyperglycemia. It is excreted in the urine, leading to low mineral blood levels. It has been shown that correcting this depletion improves response and insulin action. Magnesium deficiency also interferes with the reactions that use or produce ATP, thus altering the enzyme cascade involved in the CHO metabolism and favoring the development of T2D [137].
Therefore, the minerals present in nuts and DF could explain the relationship observed between magnesium intake and the lower risk of developing T2D and other chronic diseases [138,139].

Other Bioactive Compounds in Nuts and Dried Fruits
The other bioactive compounds contained in nuts and DF could partially explain their protective anti-oxidant and anti-inflammatory properties [2] and their implication in glucose and insulin metabolism (and therefore in T2D). Significant evidence suggests that polyphenol-rich diets have the ability to protect against diabetes [140]. It appears that anthocyanins (or anthocyanin-rich foods) are inversely associated to the risk of T2D, but there is no association for other polyphenol subclasses [141,142]. Dietary polyphenols have some benefits for T2D: they protect pancreatic β-cells against glucose toxicity and they have anti-inflammatory and anti-oxidant effects, among other things. Quercetin is a member of the flavonoid class of polyphenols which is abundant in such foods as fruits, vegetables, nuts and seeds [143]. Several in vitro studies have sought to elucidate the mechanisms behind the antidiabetic properties of quercetin: for example the inhibition of the α-amylase and α-glucosidase activities, and the prevention of the lipid peroxidation of pancreatic tissue homogenates [144]. Moreover, ellagic acid (EA) is another polyphenol which naturally occurs in some fruits, such as berries (strawberries and red and black raspberries), nuts, and pomegranates [145]. There is emerging in vitro evidence that EA may ameliorate symptoms of chronic metabolic diseases-by decreasing chronic inflammation at the expression level-which include dyslipidemia, IR and T2D [146][147][148]. Despite the growing amount of information on EA, a definitive mechanism of action has not been established. This may be attributed to the complexity of EA metabolism, which is governed by various factors [145].
A growing body of evidence suggests that dietary agents and the non-nutrient components of fruits, vegetables and nuts can affect epigenetic processes related to T2D [149,150]. Epigenetics generally refers to heritable and reversible changes affecting gene expression and chromatin organization that are not due to alterations in the DNA sequence [151,152]. No specific data is available on the role of nuts and DFs in T2D epigenetics. However, dietary polyphenol resveratrol-most abundant in the skin of grapes and raisins, but also found in peanuts and cranberries [153]-and other phytochemicals (e.g., curcumin) have proved to be effective agents against cancer and to act through epigenetic mechanisms that affect the global epigenome [154,155]. However, future studies are needed to determine the biological importance of the altered tissue-specific DNA methylation in T2D resulting from nutritional modifications [152].

Gene Expression
Some studies have analyzed the effect of nut and DF consumption on changes in the gene expression of particular cells and/or tissues and its relationship with glucose and insulin metabolism or T2D. In particular, a crossover study conducted in pre-diabetic subjects consuming pistachio for 4 months reported a downregulation of interleukin-6 (IL-6) and resistin (RETN) genes in leukocytes, whereas the pistachio diet led to a lower significant increase of solute carrier family 2 member 4 (SLC2A4, codifying GLUT4) [85]. Consistent with this attenuation in the expression of glucose transporters, leukocytes significantly reduced the uptake of glucose following pistachio consumption, suggesting a decrease in the cell hyperactivity described in T2D.
To our knowledge no published studies have evaluated gene expression after the consumption of DF. However, a translational study conducted in 120 healthy young subjects has shown that individuals in the highest tertile of fruit and vegetable consumption had statistically lower values of the expression of some pro-inflammatory marker genes such as intercellular adhesion molecule-1 (ICAM-1), IL-6 and TNF-α, in peripheral blood mononuclear cells (PBMC) [156].
Therefore, these results shed new light on the nutrient-gene interactions in nuts and DFs. However, further research is needed.

MicroRNAs
The modulatory role of nuts and DF on the expression of genes related to inflammation, oxidative stress and glucose metabolism can also be mediated by the effect of nutrients on microRNAs. MicroRNAs (miRNAs) are RNA molecules that belong to a family of small non-coding RNAs with 20 to 25 nucleotides [157,158] that post-transcriptionally and negatively regulate gene expression.
Research into the nutritional modulation of miRNA is still in its infancy and few studies have assessed whether a specific dietary pattern, food, supplements or a particular nutrient can influence miRNA levels [159]. In humans, consumption of grape extract rich in resveratrol, and Vitamin supplementation modulated specific miRNAs towards a healthier status [160]. Similarly, a PUFA-enriched diet including almonds and walnuts was effective at modifying several miRNAs [161]. More recently, pistachio consumption in pre-diabetic subjects significantly diminished the levels of miR-375 and miR-192, both related to the glucose and insulin metabolism [162]. However, there is still much to discover about the exact mechanisms linking miRNAs with the glucose and insulin metabolism and the dietary modulation of miRNA expression.

Microbiota and Metabolomic Modulation
In recent years, many studies have pointed out that the gut microbiome might make an important contribution to the development of insulin resistance and T2D. Several mechanisms related to the composition of gut microbes-including changes in bowel permeability, endotoxemia and interaction with bile acids-may contribute to the onset of insulin resistance. On the other hand, it is well established that long-term dietary patterns and shorter-term dietary variation influence gut microbiota composition [163]. Many potential prebiotic components can be present in a particular food. For example, the fermentation of fiber from nuts and DF to beneficial end-products (e.g., butyric acid) and the biotransformation of phytochemicals (i.e., tocopherols, phytosterols, and phenolic compounds) are associated with the transition to a healthier microbiota composition [2,164].
There is little information on the putative effect of nuts and DF on microbiota modulation. In fact, only two publications have evaluated the effect of nut consumption (almonds and pistachios) on changes in microbiota. Mandalari et al. conducted an in vitro study with almond skins and they found that almond fiber significantly altered the composition of gut bacteria, specifically Bifidobacteria, thus showing that almond skins had a potential prebiotic use [165]. Some years later, Ukhanova and collaborators carried out two separate crossover feeding trials with nuts and pistachios, and showed that they both significantly affected microbiota modulation [166]. However, a greater prebiotic effect was found after pistachio intake. In fact, pistachio increased the number of butyrate-producing bacteria, which are potentially beneficial, whereas the numbers of Bifidobacteria were not affected by the consumption of either nut [166].
Recently, dietary polyphenols have been found to be involved in gut microbiota dysbiosis processes which may lead to a reduction of fat storage, inflammation and IR (reviewed in [167]). In fact, a cranberry extract also altered the gut microbiome of mice by increasing mucin-degrading bacteria, a potential link to reverse the dysbiosis and metabolic inflammation underlying T2D [168].
Very few studies have been made on metabolomic modulation after nut and DF intake. Of the studies that have been made, some have evaluated changes in metabolites after a chronic consumption of nuts. They showed a modulation in the levels of metabolites such as raffinose, (12Z)-9,10-dihydroxyoctadec-12-enoate, sucrose, together with some modulations of plasma amino acids and fatty acids [169], and a modulation of gut-related metabolites and cis-aconitate, an intermediate of tricarboxylic acid [170], after pistachio consumption.
Therefore, several specific nutrients-together with their synergic effects-in both nuts and DFs may explain their beneficial role in glucose and insulin metabolism, which helps to prevent or maintain T2D ( Figure 1). collaborators carried out two separate crossover feeding trials with nuts and pistachios, and showed that they both significantly affected microbiota modulation [166]. However, a greater prebiotic effect was found after pistachio intake. In fact, pistachio increased the number of butyrate-producing bacteria, which are potentially beneficial, whereas the numbers of Bifidobacteria were not affected by the consumption of either nut [166].
Recently, dietary polyphenols have been found to be involved in gut microbiota dysbiosis processes which may lead to a reduction of fat storage, inflammation and IR (reviewed in [167]). In fact, a cranberry extract also altered the gut microbiome of mice by increasing mucin-degrading bacteria, a potential link to reverse the dysbiosis and metabolic inflammation underlying T2D [168].
Very few studies have been made on metabolomic modulation after nut and DF intake. Of the studies that have been made, some have evaluated changes in metabolites after a chronic consumption of nuts. They showed a modulation in the levels of metabolites such as raffinose, (12Z)-9,10-dihydroxyoctadec-12-enoate, sucrose, together with some modulations of plasma amino acids and fatty acids [169], and a modulation of gut-related metabolites and cis-aconitate, an intermediate of tricarboxylic acid [170], after pistachio consumption.
Therefore, several specific nutrients-together with their synergic effects-in both nuts and DFs may explain their beneficial role in glucose and insulin metabolism, which helps to prevent or maintain T2D (Figure 1). Figure 1. Role of nutrients from nuts and dried fruits in glucose and insulin metabolism, and cellular and molecular mechanisms related to T2D/IR. CHO, carbohydrate; CMF, cellular membrane fluidity; GI, glycemic index; IR, insulin resistance; MUFA, monounsaturated fatty acid; PUFA, polyunsaturated fatty acid; T2D, type 2 diabetes.

Concluding Remarks
Undoubtedly, the specific composition of nuts and dried fruits means that they can be used to efficiently counteract metabolic diseases such as type 2 diabetes. Their unique profile of macronutrients, micronutrients and other bioactive compounds may explain the beneficial effects observed in clinical and epidemiological studies. However, the exact mechanisms by which they modulate glucose and insulin metabolism and influence T2D have yet to be fully discovered. They contain fiber, fat, minerals and other bioactive molecules that modulate several gene mechanisms at the cellular and molecular level. This may explain some of their beneficial effects. However, further

Concluding Remarks
Undoubtedly, the specific composition of nuts and dried fruits means that they can be used to efficiently counteract metabolic diseases such as type 2 diabetes. Their unique profile of macronutrients, micronutrients and other bioactive compounds may explain the beneficial effects observed in clinical and epidemiological studies. However, the exact mechanisms by which they modulate glucose and insulin metabolism and influence T2D have yet to be fully discovered. They contain fiber, fat, minerals and other bioactive molecules that modulate several gene mechanisms at the cellular and molecular level. This may explain some of their beneficial effects. However, further basic and translational research is needed in order to extend their positive health benefits and to find novel mechanisms and targets to explain their contribution to the management of type 2 diabetes.