Type 2 diabetes is a common chronic disease with great global health and economic burden. The prevalence is still increasing due to lifestyle changes, especially in developing countries [1
]. Diabetic education, nutrition therapy, physical activity, pharmacotherapy and glucose monitoring are key components of diabetes management. Lifestyle intervention including diet control is recommended as the fundamental approach for all patients with type 2 diabetes. Diabetic patients are suggested to consume at least the amount of fibers and whole grains recommended for the general public, which is 14 g fiber/1000 kcals daily or about 25 g/day for adult women and 38 g/day for adult men [3
]. Dietary fibers promote one or more of the beneficial effects such as laxation, reduction in blood lipids, modulation of blood glucose due to their non-digestibility in the small intestine and fermentation in the colon. Oats are a good source of soluble dietary fiber rich in β-glucan, which is considered as a bioactive component in reducing postprandial glucose and insulin responses, improving insulin sensitivity, maintaining glycemic control and regulating blood lipids [4
]. The United States Food and Drug Administration (FDA) suggested that the consumption of 3 g or more per day of β-glucan from oats or barley may reduce the risk of coronary heart disease [8
A number of studies have reported the beneficial metabolic effects of oats or β-glucan on people with and without type 2 diabetes [9
]. A modified diet with β-glucan from oats was reported to be superior to the American Diabetic Association’s diet in improving metabolic and anthropometric profiles in well controlled type 2 diabetic patients: larger decreases in glycosylated hemoglobin A1c (HbA1c), weight and body mass index (BMI); greater increase in high-density lipoprotein cholesterol (HDL-C) [9
]. A high dose of barley β-glucan supplement (6.31 g β-glucan) improved the glucose and insulin responses when added to a high-carbohydrate food in lean, healthy men without type 2 diabetes [10
]. For overweight or obese patients and patients with metabolic syndrome, oats fiber also improved glucose intolerance and insulin sensitivity [11
]. However, the European Food Safety Authority (EFSA) reported that the evidence remained insufficient to prove the relationship between β-glucan consumption and the long-term maintenance of normal blood glucose level [13
]. Accordingly, the aim of this systematic review was to comprehensively evaluate if oats intake is beneficial for both the short-term glucose response and the long-term glucose control as well as other metabolic parameters such as lipid and anthropometric profiles in type 2 diabetic patients.
The present systematic review of 16 studies has demonstrated a moderately beneficial effect of oats intake on glycemic control and lipid profiles in patients with type 2 diabetes. To our knowledge, this is the first systematic review of oats consumption in patients with type 2 diabetes. On the whole, this review has revealed an improvement of glucose, insulin sensitivity and lipid profiles after oats consumption. Compared with a control meal, a single meal of oatmeal also showed superiority of acute glucose and insulin responses.
Among the eight studies investigating HbA1c, three randomized, parallel controlled studies [9
] showed a significant reduction in HbA1c from baseline in the oats diet group (absolute change: −0.28%, −0.40% and −2.22%, respectively). Ma et al.
] revealed the greatest beneficial effect of oats intake on diabetic patients with the following features: First, compared with common oats products, naked oats maintain the most ingredients and beneficial nutrients of the whole-oat grains, which indicates naked oats might be better for patients with diabetes. Second, a relatively large sample size (260 participants) in this study seemed to be more likely to get a positive result. Third, the baseline glucose level was relatively high (mean HbA1c 9.87%, mean FBG 9.99 mmol/L, mean PBG 18.77 mmol/L). Forth, a diet with low energy, low fat and high fiber was provided to all the participants in both the intervention and the control groups, indicating oats consumption might show its benefits especially when the general energy intake was low. However, Kabir et al.
] showed that adding 3 g of β-glucan from oats to a low-glycemic index breakfast with cereal, milk, bread and butter could not lead to a significant chronic changes (four week-baseline) in FBG, FINS and HbA1c. It may be due to the fact that the original study mainly aimed to evaluate the effects of a low-glycemic index breakfast on the glucose and lipid metabolism in type 2 diabetic patients. Thus, the test meal was focused on the glycemic index of food rather than the ingredients of food such as oats. Therefore, the results of this study are less meaningful for evaluating the beneficial effects of oats intake on type 2 diabetes. On the other hand, it suggests that a background diet with added oats is important for the total effect. The above evidence suggests that adding naked oats to a calorie-restricted diet might help type 2 diabetic patients to get a more obvious hypoglycemic effect especially in those with a high level of blood glucose. The amounts of β-glucan were greater than or equal to 3 g in most oats dietaries of the included studies. Tappy et al.
] revealed a dosage-dependent association between the amount of β-glucan in breakfast cereal and the response of postprandial glucose. Additionally, this inverse liner relationship was more obvious at low doses of β-glucan (below 6 g). The results of this study were confirmed by previous reports, which also showed a significant dose-dependent relationship between the hypoglycemic effect and the amount or the log viscosity of oats [31
]. These findings will help in deciding the appropriate dose of oats or β-glucan included in the whole food system. As the UK Prospective Diabetes Study (UKPDS) Group revealed, a 1% reduction in HbA1c was associated with a 21% and 14% reduction in the risk of death related to diabetes and all-cause mortality, respectively [33
]. That is to say, the magnitudes of the statistically significant reduction in HbA1c in the present review would translate to a clinically significant reduction in the risk of death related to diabetes (−8.82%) and overall mortality (−5.88%).
Compared with the controls, oats intake significantly reduced the concentrations of TC and LDL-C. The findings in the present review are consistent with previous systematic reviews or meta-analyses which also showed a significant reduction in TC and LDL-C after oats or oats β-glucan consumption at the general population level [34
]. This review also revealed a decreasing tendency in TG, which was omitted previously [34
]. This decreasing tendency may partly be explained by the relatively high baseline level of TG in type 2 diabetic patients in our review. Interestingly, two oats intervention groups in one study [16
] showed a slight reduction from baseline in HDL-C (−0.06 and −0.08 mmol/L, respectively; both p
< 0.05), while two studies [9
] showed a slight increase in HDL-C from baseline (+0.15 and +0.05 mmol/L, respectively; both p
< 0.05). The slight reduction in HDL-C in this study may partly be due to the side effect of a low-cholesterol and saturated-fat diet as the author of the original study discovered [37
]. Whether this slight reduction would produce clinical significance remains to be determined. Some inconsistent results about the effect of oats intake on HDL-C at the general population level were also reported, Tiwari et al.
] revealed an increase in HDL-C after oats intake, while Thies et al.
] found a non-significant effect of oats intake on HDL-C. A characteristic pattern of diabetic dyslipidemia, which consists of a mild to marked elevation of TG and low level of HDL-C [38
], may partly account for the discrepancy between the general population and the diabetic patients. Therefore, further analysis is necessary to confirm the lipids (especially HDL-C and TG) changes after oats consumption in the diabetic and non-diabetic people separately. Previous evidence showed that each 1% reduction in TC or LDL-C was associated with a 2% or 1% reduction in the risk of coronary heart disease, respectively [39
]. This means the effect of oats-containing diets in this review would translate to an additional 4.00 to 25.60% reduction in coronary heart disease risk due to the lipid benefits from oats intake.
Overall, oats intake was associated with a slight decrease in body weight and BMI, but the difference was not significant. To be noted, body weight increased slightly following the oat-enriched diet compared with standard dietary advice in only one study [26
], with an excess total energy and the glycemic load in the oat-enriched dietary plan. It indicated that total energy as well as other dietary components should be very carefully considered during the assessment of oats consumption in patients with diabetes.
Oats are classified as a kind of whole grain which is different from other grains. They are particularly high in soluble fiber, β-glucan and some micronutrients such as magnesium. The unique components and special physic-chemical properties largely decide the beneficial effects of oats. The beneficial effects of oats on glycemia and blood lipids are mainly related to oats β-glucan, a soluble and fermentable fiber, which cannot be decomposed and absorbed in the small intestine but can be fermented in the colon. The β-glucan is reported to increase the viscosity of food bolus, delay gastric emptying and lengthen intestinal transit time, slow the absorption of nutrients especially the carbohydrates, and enhance the satiety [6
]. It was also reported that β-glucan could slow the appearance of glucose in plasma, resulting in longer-lasting insulin secretion which exert a prolonged inhibition of endogenous glucose production and lipolysis [44
]. Apart from β-glucan, oats are also a rich source of magnesium, which is an important co-factor for many enzymes including enzymes involved in the metabolism of glucose and insulin. Additionally, an inverse association between magnesium in relation to type 2 diabetes was reported [45
]. A group of phenolic compounds named avenanthramides have been found in oats. Avenanthramides are traditionally considered a kind of antioxidant. Some other important effects of avenanthramides, such as enhanced endothelial function and anti-inflammatory properties, were reported recently. Thus, avenanthramides as well as some other antioxidants including vitamin E from oats could synergistically contribute to the beneficial effects on diabetes and the subsequent complications such as dyslipidemia, atherosclerosis and cardio-cerebrovascular diseases [46
]. The dosage, chemical structure, molecular weight (MW), solubility and viscosity are key influential factors for the health effects of oats. Additionally, the above factors are affected by the variety and growing conditions, the processing and food preparations, and even the physiological disposition of oats in vivo
]. The mechanisms of lowing cholesterol are not very clear, but it is suggested that β-glucan can bind with bile acids and increase the intestinal viscosity, thereby decreasing cholesterol absorption and increasing fecal bile acid excretion [48
]. The variety of oats may also be an important source of the heterogeneity among studies included in the present systematic review.
The argument of oats might be raised due to its potential association with asthma, coeliac disease, dermatitis and some other allergic conditions. However, another different viewpoint has indicated that the possible association may result from a wheat contamination which contains gluten. Gluten is a group of seed storage proteins of cereals. It is also widely used in food manufacturing, usually as an ingredient and processing aid, due to its viscoelastic properties [49
]. Pure oats contain avenins, which are less likely to cause allergies. However, gluten is still added to most oat breads to produce the needed elasticity and structure of bread [48
]. In the current review, we did not find evidence about the relationship between oats consumption and allergic reactions or diseases. Caution is still needed to add oats to the diet of wheat hypersensitive patients. It is better to use pure oats without wheat contamination. The relationship between infant exposure to oats and the development of type 1 diabetes has been thoroughly discussed recently. Introducing oats early (<4 months of age) or late (≥6 months of age) in the infancy was reported to be related to the development of type 1 diabetes [52
]. The American Academy of Pediatrics also recommended to introduce solid foods including oats between 4 and 6 months of age [54
]. For children with susceptibility to type 1 diabetes, the introduction of oats would be with great caution. Further investigation about the safety of oats consumption in diabetic patients is required.
There are several limitations in the present review. Firstly, the limited number of studies included and the small number of participants involved in each study might not have sufficient power to detect a definite effect. Secondly, we failed to find evidence of oats consumption in patients with type 1 diabetes, which has a different pathogenesis and clinical feature from type 2 diabetes. Thirdly, the safety of oats consumption was not assessed due to insufficient data.