Effects of the Consumption of Prickly Pear Cacti (Opuntia spp.) and its Products on Blood Glucose Levels and Insulin: A Systematic Review

Background and Objectives: There is confusion as to which component of the Opuntia spp. cacti has demonstrated anti-hyperglycemic effects or anti-diabetic properties. It is important to clarify these health benefits due to the increasing need for prevention and treatment of chronic diseases. The aim of this review is to identify the effects of Opuntia spp. cacti consumption on biomedical measures; glucose and insulin with consideration of its’ components; fruit, leaf and combined or unidentified Opuntia spp. products. Materials and Methods: Prior to commencing the searches, this systematic review was registered with PROSPERO (CRD42018108765). Following the PRISMA 2009 guidelines, six electronic databases (Food Science and Technology Abstracts (EBSCO), Medline, Scopus, CINAHL, Web of Science and Cochrane) were searched for articles investigating the effect of Opuntia spp. consumption on glucose and insulin in humans. Results: Initially, 335 articles were sourced and filtered by exclusion criteria (human interventions, control trials and articles published in English) resulting in 20 relevant articles. The included studies were characterized by such plant components as fruit (n = 4), cladode (n = 12), and other Opuntia spp. products (n = 4), further separated by clinical populations (‘healthy’, hyperlipidemic, hypercholesterolemic, Type 2 Diabetes Mellitus). The findings of this review indicate variations in effects between cacti components and products. Cladode and select Opuntia spp. products predominately demonstrated significant reductions in serum glucose and insulin, indicating potential as a functional food candidate. Prickly Pear fruit was predominately reported to have no significant effects on glucose or insulin. The quality of evidence appeared to vary based on the type of Opuntia spp. product used. Studies that used specifically the fruit or cladode had high risk of bias, whereas studies which used combined Opuntia spp. products had a lower risk of bias. Numerous mechanisms of action were proposed where positive findings were reported, with emphasis on dualistic glucose-dependent and independent actions, however, mechanisms require further elucidation. Conclusion: Currently, there is a lack of evidence to support the recommendation of using Opuntia spp. fruit products as an alternative or complementary therapy in the reduction of risk or management of Type 2 Diabetes Mellitus. The Cladode does however show promise in potential glucose-lowering effects which warrant further investigation.


Introduction
The prevalence of Type 2 Diabetes Mellitus (T2DM) and the corresponding demand for its management is proving to be a global economic burden [1]. Various strategies, including dietary management, exercise-based therapies, novel drug development, personalized medicine, pharmacotherapy, pathogenesis, traditional medicines, and combined anti-diabetic or 'anti-hyperglycemic' therapies are prioritized as various methods for treatment and prevention of T2DM globally [1].
Herbal treatments and traditional plant-based medicines have become increasingly popular, as convenient and low-cost options with potentially fewer side effects [1]. Herbal or plant-based anti-hyperglycemic treatments are reportedly targeted to biochemical pathways, including antioxidant/free radical pathways based on hyperglycemia being associated with mitochondria overproduction of free radicals [1,2]. Additionally, there is increasing interest in management strategies for T2DM and its prevention based on alternative low-cost nutrition strategies, including plant-based functional-foods [2]. However, currently, consensus has only been reached regarding the anti-hyperglycemic effect relating to the high-fiber content of many of the proposed anti-hyperglycemic functional foods. This is primarily based on the fiber content slowing the rate of digestion and the resultant altered rate of glucose absorption [2].
Worldwide, there is a focus on sustainable and responsible preventative health interventions, with the emphasis on socio-political, economic and environmental factors [3]. The social push to include environmental factors has led to the consideration of the effectiveness of low-resource intensive plant-based treatments for the maintenance and prevention of T2DM. This includes the potential therapeutics derived from cacti [3,4]. Specifically, Opuntia spp. cacti, with a strong history of use as a treatment of hyperglycemia in traditional medicine [4], will be the focus of this systematic review. The Opuntia spp. cacti is utilized as a source of food, hedge plant, or as an ornamental plant, particularly in arid and drought-affected regions, becoming more relevant with environmental cases of desertification [5][6][7][8]. The cacti as a crop is utilized, particularly on non-productive agricultural lands, for its resources, that include the fruit, the Prickly Pear (PP) and leaf, the cladode [5][6][7][8]. The Opuntia spp. cacti is reported to have 'anti-hyperglycemic' and 'anti-diabetic' properties in humans and it is prescribed in many traditional and complementary therapies around the world [9][10][11][12]. However, confusion exists within the literature as to which principal cacti component is potentially responsible for such effects. Clarification of this issue could enhance the use of PP for treatment, maintenance, and prevention of chronic diseases and affect progress in the development of new potential anti-hyperglycemic agents.
To be able to further consider Opuntia spp. components for new anti-hyperglycemic agents, there is a need to identify which component(s), if any, are the true source of any potential anti-hyperglycemic effects of Opuntia spp. There are obvious compositional variations between the leaf and fruit which require investigation to define any possible resultant health associations [4]. The fruit of the cacti is also referred to as the 'cactus fruit' or 'Indian fig' and also as the PP [6,7] is predominantly known for relatively high levels of macro-and micronutrients including fiber, mineral composition, numerous amino acids; and significant lipid and phytosterol content [13][14][15][16][17]. In addition, there are a number of different bioactive compounds found in PP such as polyphenols [18,19], flavonols [19][20][21] and betalains [18,22,23], as well as considerably strong antioxidant characteristics [17,18,20,21,[24][25][26][27][28][29][30][31]. The PP fruit is commonly consumed fresh, although it can be formulated into several different including dried fruits, tea's, jams, drinks, tinctures and dietary supplements [6, 32,33].
The Opuntia spp. cacti leaves, often termed the 'cladode' are commonly eaten as a broiled or grilled food product. The cladodes' compositional content varies substantially from the fruit, although it does share common features to include high fiber, mineral, and selection of different bioactives [34]. Similar to PP, the cladode has been extensively investigated for bioactive compounds to include extensive phenolic analysis and antioxidant characterization of various extracts [34]. More recently, the cladode was used in various forms in the food and supplements market such as various pickled products, fiber powders, or capsule-based supplements [35,36]. Therefore, based on the current findings, the primary aim of this systematic literature review is to examine the evidence drawn from well-controlled human clinical trials on the effect of consumption on Opuntia spp. cacti on blood glucose (GLU) and insulin levels (INS) in humans. The outcomes of this systematic review will potentially assist clinicians with decisions regarding the use of Opuntia spp. product (fruit, leaf or combined) consumption, for improvement of T2DM associated clinical measures such as hyperglycemia or insulin regulation.

Materials and Methods
This systematic literature review was conducted following the PRISMA 2009 guidelines [37], and searches were performed in six electronic databases: Food Science and Technology Abstracts (EBSCO), Medline, Scopus, CINAHL, Web of Science and Cochrane databases. The generated results were screened for relevance by title, abstracts and full text (Figure 1), with additional hand searches of included articles reference lists. Results were limited to human quantitative intervention trials based on the consumption of Opuntia spp., investigating the effects on blood GLU or INS resistance parameters, published in English, in peer-reviewed journals since the journal inception. No limitations were made regarding age, gender, race or ethnicity. This systematic literature review is also registered with the PROSPERO registry of the systematic literature reviews held at the University of York (CRD42018108765).
Medicina 2019, 55, x FOR PEER REVIEW  3 of 19 recently, the cladode was used in various forms in the food and supplements market such as various pickled products, fiber powders, or capsule-based supplements [35,36]. Therefore, based on the current findings, the primary aim of this systematic literature review is to examine the evidence drawn from well-controlled human clinical trials on the effect of consumption on Opuntia spp. cacti on blood glucose (GLU) and insulin levels (INS) in humans. The outcomes of this systematic review will potentially assist clinicians with decisions regarding the use of Opuntia spp. product (fruit, leaf or combined) consumption, for improvement of T2DM associated clinical measures such as hyperglycemia or insulin regulation.

Materials and Methods
This systematic literature review was conducted following the PRISMA 2009 guidelines [37], and searches were performed in six electronic databases: Food Science and Technology Abstracts (EBSCO), Medline, Scopus, CINAHL, Web of Science and Cochrane databases. The generated results were screened for relevance by title, abstracts and full text (Figure 1), with additional hand searches of included articles reference lists. Results were limited to human quantitative intervention trials based on the consumption of Opuntia spp., investigating the effects on blood GLU or INS resistance parameters, published in English, in peer-reviewed journals since the journal inception. No limitations were made regarding age, gender, race or ethnicity. This systematic literature review is also registered with the PROSPERO registry of the systematic literature reviews held at the University of York (CRD42018108765).

Search Terminology and Selection Criteria
The following search terms were used, to produce the database outputs: "('Opuntia' OR "Prickly Pear" OR "Cactus fruit" OR "Tuna fruit" OR "Indian fig" OR 'Nopal' OR 'Cladode' OR "Cactus Leaf" OR 'Stems') AND ('Glucose' OR 'Blood glucose' OR 'Glucose tolerance' OR 'Impaired glucose' OR 'Oral glucose' OR 'Glucose metabolism' OR 'glycemia' OR 'Glycaemia' OR 'Hyperglycemia' OR 'Fasting blood glucose' OR 'Glycemic Index' OR 'HOMA' OR 'QUICKI' OR 'GI' OR 'Insulin' OR 'Insulin resistance' OR 'Plasma insulin' OR 'Insulin secretion'). Eligibility for inclusion was determined via satisfaction of criteria; human, in vivo, quantitative intervention trials with control groups or placebo, consumption of Opuntia spp. and published since journal inception until September 2018 (5th). The reference lists of included articles were searched for similar phrases and screened for relevance.

Data Extraction and Outcomes of Interest
Data extraction was conducted independently by two reviewers (C.G., and N.N.) who assessed the eligibility of imported EndNote (v8, Thomas Reuters, Eagan, MN, USA) citations from the database outputs, after removing duplicates. Excluded articles within the screening of titles, abstracts, and full-text articles were excluded on consensus, where disagreement was resolved via discussion. In total, 37 full-text articles were selected for review; and only 20 were included in this systematic review. Prompting reference list searches of included articles, no further articles were identified as suitable for inclusion in this review. Primary outcomes of interest for this systematic review included measures of blood or serum GLU and INS, including biochemical assays, or well-known tools, Homeostatic Model Assessment (HOMA) and 'Quantitative Insulin Sensitivity Check Index' (QUICKI).

Data Analysis
Overall studies were evaluated on; risk of bias; study characteristics to include sample size; and participant characteristics including age and gender, study design and intervention. More specifically tools of analysis and reported outcomes were also analyzed to make meaningful conclusions. Due to the heterogenicity of different food products and non-quantifiable composition of some macroand micronutrients in the provided treatments (food and/or supplement) a meta-analysis was not deemed suitable.

Risk of Bias
The risk of bias for each included study was evaluated using the 'Cochrane Risk of Bias Tool' [38]. The forms of bias considered included; selection, performance, detection, attribution, reporting and 'other' biases, which were scored either 'low', 'high' or 'unclear'.
The included studies were scored for risk of bias (Table 4) by two researchers (C.G. and N.N.) following the Cochrane Risk of Bias Tool [38]. Investigations into single plant products (fruit; cladode) tended to have a high selection (3; 3) and performance bias (3; 9), with the exception of Wiese et al. (2004) [41] and Bacardi-Gascon et al. (2007) [55]. Across all included studies, detection bias was rated as high, or unclear, if not detailed within the methodology of the article. Reporting bias was consistently low for all included studies.         The forms of bias considered included; selection, performance, detection, attribution, reporting and 'other' biases, which were scored either 'low', 'high' or 'unclear'.
Supplementation with the purple PP fruit pulp juice over the period of two weeks [39] resulted in significant differences in fasting GLU levels in 'healthy' males undergoing an exercise intervention (p = 0.01). Wolfram et al. (2002) [40] using a PP fruit pulp (250 g/day; 8 weeks) also reported significant decreases in fasting GLU (p < 0.005) of participants with hypercholesterolemia and hyperlipidemia; and INS (p < 0.005) in participants with hypercholesterolemia but not hyperlipidemia (p > 0.05). Pimienta et al. (2008) [42] reported no significant effects of PP fruit after a single consumption of PP fruit peel, in 'healthy' subjects (n = 14; p > 0.05). Similarly, there were no significant lowering effects inT2DM participants (n = 10) after 5 weeks of treatment for both blood GLU and INS (Both p > 0.05), using Oral GLU Tolerance Tests (OGTT) and 'pre-' and 'post-' intervention serum samples. In addition, Wiese et al. [41] examined the effect of commercially available PP fruit extract capsules on reducing symptom severity in 'healthy' participants with induced alcohol toxicity and reported no significant differences between treatment and placebo on serum GLU (p > 0.05).
Interestingly, majority of the studies reported in this systematic literature review using the cladode as a food delivery product were from Frati and colleagues, where seven articles were included as a part of a separate trial. Firstly, a study by Frati et al. (1983a) demonstrated significant effects in reduction of GLU (p < 0.01-0.05) and INS (p < 0.01-0.02) post-prandial (0-3 h) with ingestion of 100g of broiled cladode in healthy males [45]. When consumed for a longer period (10 days), a study by Frati et al. (1983b) [46] indicated that consumption of the previously established quantity of cladode (100g) had significant reductions in fasted serum GLU (mean: 3.5 mmol/L) for T2DM participants (n = 7; p < 0.01). Additionally, significant reductions were also reported in 'non-diabetic' participants (mean: 0.021 mmol/L p < 0.05). In some cases, the participants of certain comparisons were not defined to the reported study groups but rather generalized as 'healthy' (n = 8) and 'obese' (n = 14; combined study groups) [46]. A study that followed [48] built on the single consumption investigations, where Frati et al. (1987) considered the effects of treatment and OGTT protocol interactions (cladode; cladode before dextrose bolus; cladode mixed with dextrose) to find significantly lower serum GLU (p < 0.025; vs. water) at 60 and 180 min in healthy adults. No significant (p > 0.05) effects were reported for serum INS between the treatments [48]. Comparisons between single consumption of Opuntia spp. cladode and zucchini squash (500g) in T2DM participants (n = 48) were drawn on both GLU and INS parameters [47]. Consumption of the broiled cladode resulted in significant reductions in serum GLU (1 h 8.5 ± 2.4%; 2 h 10.7 ± 1.4%; 3 h 17.6 ± 2.2% µU/mL; p < 0.01-0.025) and INS (p < 0.001; 1 h 22.5 ± 5.2%; 2 h 38.7 ± 6.3%; 3 50.2 ± 8.0%) [47].
A study by Frati et al. (1990) [53] continued to examine the effects of various cladode processing techniques [48] (broiled, raw blended, broiled and blended, blended and broiled; 500 g) on fasted serums GLU in healthy participants. This study it was reported significant reductions in 120 and 190 min (p < 0.001), although no significant (p > 0.05) effects were observed between processing techniques [58]. Investigations into the effects of cladode consumption with established volume and preparation on GLU and INS continued into T2DM participants, whereby the timing of sequential consumption was considered [44]. In this study, there was a significant reduction (p < 0.01) is fasting serum GLU in T2DM participants (2 h, 36.8 mg/dl; 4 h, 92 mg/dl; 6 h, 49 mg/dl) but not in healthy participants (p > 0.05) [44]. A study examining the effect of grilled cladode (500 g) in matched T2DM (n = 14) and healthy (n = 14) participants reported significant reductions in GLU (1 h: 1.19 ± 0.21 mM, p < 0.005; 2 h: 1.57 ± 0.28 mM, p < 0.005; 3 h: 2.26 ± 0.25 mM, p < 0.001) and INS (p <0.01; 1 h: 12.3 ± 15.9; 2 h: 3 h: 36.9 ± 10.8 pM; 56.4 ± 10.8 pM) in T2DM but not in a healthy group [43]. These studies indicate that the consumption of cladode prepared in various ways (cooking procedures) significantly reduced blood GLU and INS levels in T2DM participants. Despite the significant reductions of GLU and INS with consumption of Opuntia spp. Cladode in T2DM participants, there is limited evidence to support the beneficial effects of the vegetable consumption in 'healthy' participants.
The remaining included articles considering cladode products in different forms such as an extract in a capsule [52,53] a meal replacement [49] and in combination with food [49,50,55]. Castaneda-Andrad et al. (1997) [53] reported that single dose cladode consumption, using a capsule (250 mg) significantly decreased serum GLU (319-220 mg/dL) in individuals living with T2DM compared with placebo (p < 0.01). In addition, 6 weeks of supplementation of cladode extracts, provided as a capsule ('NeOpuntia'; 3 × 1.6 g), had no effect on glucose associated risk factors of Metabolic Syndrome in a female cohort [52].
Cladode was also studied in combination with other foods, and it was provided in addition to foods incorporated or provided in a blinded form. A study by Lopez-Romero et al. (2014) [50] conducted two trials within the same paper. In the first experiment, these authors investigated the glycemic responses to consumption of cladode powder (50 g) in healthy participants. The powder significantly lowered serum GLU (p < 0.001) and produced a glycemic index score of 32.5 ± 4.0, and insulinemic index of 36.1 ± 6.1, upon ingestion. In the second experiment, the effect of cladode consumption in combination with various breakfasts (high carbohydrate; high soy protein) on serum GLU and INS in T2DM (n = 14) and healthy participants (n = 7) was investigated [50]. The results showed that T2DM participants nopal consumption in combination with a high carbohydrate breakfast incurred significant decreases in serum GLU (30 min, 45 min and 1 h; p < 0.01), GLU by Area Under the Curve (AUC reduction 287 ± 30 mmol/L; p < 0.001), with no significant differences reported in serum INS [50]. Significant differences were also observed between high carbohydrate and high soy protein breakfasts in GLU levels (p < 0.001).
Lopez-Romero et al. (2014) reported no significant differences (p > 0.05) in cladode powder ingested with a high-soy protein breakfast for parameters of GLU or INS in participants with T2DM, but there was a significant difference for INS AUC in healthy participants (decrease; p < 0.01). A study by Bacardi-Gascon et al. (2007) [55] investigated the cladode consumption by incorporating it into three meals; chilaquiles, burritos, and quesadillas with or without the cladode. Significant decreases in serum GLU (AUC) were apparent in all three meals (chilaquiles p = 0.018; burritos p = 0.025; quesadillas p = 0.019). A separate study by Guevare-Cruz et al. (2012) investigated the effects of a dietary pattern containing cladode among other ingredients to include chia seeds, oats, and soy protein against a less nutrient and bioactive dense placebo dietary pattern over a 2-month period [49]. The mentioned dietary pattern was found to have a significant difference on AUC for INS (p < 0.05) and diet-time interactions for INS as well (p < 0.001). The GLU lowering effects of Opuntia spp. of the cladode are supported by literature; however, more evidence is recruited to make conclusions regarding INS.

Results of Combined or Unspecified Opuntia spp. Included Studies
Combined Opuntia spp. plant products ( Table 3) were studied in both males and females, using the specifically-formulated tortillas and bars [51] or commercially available 'OpundDia © ' capsule dietary supplement, reported to contain 75% Cladode and 25% PP fruit extract. The Opuntia spp. products were investigated with control [36,51] and cross-over [56,57] designed trials in healthy [51,56,57] and obese [36] participants. Investigations into combined fruit and cladode products were consistent, where controls were established as placebo capsules [36,56,57] or formulated products for investigation into single consumption models.
Investigations considering combined Opuntia spp. products considered the efficacy of the established products, with the exception of Guevera-Arauza et al. (2011) [51], who developed several products. Acute and chronic effects of 'OpunDia © ' in obese participants were investigated by Godard et al. (2010) [36]. Acute trial (n = 29; 400 mg) indicated significant differences in serum GLU for time points 1, 2, and 3 h (p < 0.05) between treatment and control (placebo capsules). Guevera-Arauza et al. (2011) [51] concluded that the chronic supplementation of 'OpunDia © ' over 16 weeks induced significant reductions in GLU both pre-and post-treatment (p < 0.05) and placebo (p < 0.05); and had no significant effects on INS [36].
A study by Guevera-Arauza et al. (2011) [51] differed from other included combined Opuntia spp. in that the Opuntia spp. products were developed by investigators for the purpose of the study. The aim of this study was to determine the effect of the Opuntia spp. PP fruit and cladode in placebo and fortified food products including various bars and tortillas were tested on serum GLU. The results demonstrated a significant lowering effect (p < 0.05; 4.43 mmol/L) on serum GLU in healthy participants upon product consumption.

Discussion
The Opuntia spp. cacti and its components have been reported to have 'anti-diabetic' or 'anti-hyperglycemic' properties [56]. However, confusion exists within the literature between which Opuntia spp. components are responsible for the reported health effects [34,56]. The Opuntia spp. cladode demonstrated significant positive health effects, indicating its potential as a functional food for potential short-term treatment of hyperglycemia in multiple sample participants.
Proposed mechanisms underlying how plant-based products may induce anti-hyperglycemic effects include individual or combined effects via the modulation of gastrointestinal absorption or regulation of INS secretion and sensitivity [56]. The anti-hyperglycemic activities provided by plant-based products are diverse and are a result of a multiple contributing factors, including the presence of INS-like substances or 'factors' within the plant material [59], upregulation of INS via β-cells [60], increased insulin binding activity of receptors [60], increase GLU metabolism [61], high fiber content reducing the rate of absorption [62], or potential regeneration of pancreatic cells [63]. Various aspects of potential mechanisms are proposed following positive effects of specific Opuntia spp. products reported within this review. The findings of this review provide rationale for the use of Opuntia spp. products in applications such as incorporation into functional foods or nutraceuticals, given the additional substantial bioactive contents [18][19][20][21][22][23][24][25][26][27][28][29][30][31].

Opuntia Ficus Indica Fruit
Overall, the results of this systematic review suggest no significant effects on blood GLU [39,41,42] and INS [42], upon investigations into Opuntia ssp. PP fruit pulp [40], pulp juice [39], fruit peel [42], and fruit extracts [41]. The conclusions made regarding the results of this systematic review do consider the disagreements of significant differences in the reduction of GLU in hypercholesterolemic (n = 12) and hyperlipidemic (n = 12) sample participants; and INS in a hypercholesterolemic sample population (n = 12) [40]. Although serum GLU was investigated in every included study regarding the PP fruit, it is important to recognize that INS was only studied in two of the four included fruit-based investigations [40,42]. The conclusions drawn with respect to Opuntia spp. fruit are limited by high risk of bias with relation to selection, performance, and detection biases, but did show strengths in attribution and reporting bias.
Interestingly, the widely accepted associations of fiber as an anti-hyperglycemic effect [62] were observed within the fruit-based studies included in this review. Anti-hyperglycemic effects are often reported in trial foods with a higher fiber content [62], such as that expected of fruit flesh, unlike a majority of the investigations within this review. The included fruit-based studies investigating fruit products with substantially less fiber to include; juice [39], peel [42] and extracts [41]. Future research should investigate the effects of PP fruit consumption regarding the INS-GLU relationship, with consideration of fiber. The results of this review do not encourage the prescription of PP fruit as an alternative medicine in treatment of hyperglycemia or associated conditions, until further evidence is developed.
Consistent with previous hypotheses regarding GLU lowering mechanisms, the cladode leaf is reported to contain substantial amounts of fiber [34,62]. Currently, due to the cladode's fiber content, it is utilized for the development of many food products and supplements, such as flours and fiber supplements [34]. Future research should consider investigation into other potential mechanisms of action, and inter-relationships should be considered regarding the anti-hyperglycemic effects observed via consumption. In conclusion, the consumption of the cladode has noticeable effects of hyperglycemia; however, it should be recommended with caution and consideration of commonly-associated effects of food with high fiber content.

Opuntia Ficus Indica as Combined Fruit and Cladode Leaf
Of the included articles within this review, four were identified as combined Opuntia spp. products; 'OpunDia © ' capsule-based investigations [36,56,57] and cladode-based food products [49]. Both Opuntia spp. product investigations reported significant reduction in serum GLU. To our knowledge, the literature surrounding the effects of Opuntia spp. products on INS, were only studied using the 'OpunDia © ' capsule; however, they reported conflicting results [36,56,57].
The beneficial effects of the combined supplements (cladode and fruit) may be due to the greater cladode to fruit proportions within the 'OpunDia © ' capsules. Noticeably, the studies examining the effects of the combined Opuntia spp. products do not control for the anti-hyperglycemic effects of fiber [62]. A study by Deldicque et al. (2013) [56] based investigation into 'OpunDia © ' on the proposed mechanism of direct upregulation of INS by the Opuntia ficus indica spp, rather than indirect production of INS, in response to increased serum GLU. Multiple studies propose the direct insulinogenic actions of the Opuntia spp. products, described as acute and rapidly acting [56], a result of upregulation of pancreatic β-cell INS secretion [56,57]. Although, a study by Van Proeyen et al. (2012) [57] proposed a dualist mechanism stemming from both GLU-induced and GLU-independent INS production of the combined Opuntia spp. product consumption. However, this finding was in a population undergoing an exercise intervention, in which they were characteristically more susceptible, or had improved INS sensitivity [57]. The use and effectiveness of combined Opuntia spp. products are largely specific to the product, where further evidence is required on mechanisms of action before making conclusions regarding the effectiveness or advised supplementation.

Conclusions
The findings of this systematic literature review indicate that the cladode leaf and selection of combined Opuntia spp. products (typically containing a mix of cladode leaf and fruit) may have significant effects of serum GLU reduction. However, effects on INS were less consistent. The PP fruit was found to not have significant 'anti-hyperglycemic' effects, though the definitive evidence was conflicting. This suggests that there might be variability in the potential anti-hyperglycemic effects of different plant parts of Opuntia spp., which warrants further investigation. Study characteristics including design, exposure, quantity, and participants, as well as the treatment product and compositions, varied extensively, limiting the conclusions within this review. Future research should investigate the serum glucose-insulin relationships within consumption-based investigation and the inter-relationships within the proposed mechanisms of action, ideally following standardized and reproducible methodologies.