Dietary Glycemic Index and Load and the Risk of Type 2 Diabetes: Assessment of Causal Relations.
Plain language summary
It is generally accepted that certain diet and lifestyle choices contribute to a person’s risk of developing type 2 diabetes (T2D). In this meta-analysis, researchers set out to review previous studies and assess whether there is any evidence that the amount and type of carbohydrate (measured by Glycaemic Index (GI) and Glycaemic Load (GL)) in a person’s diet has a direct influence on their risk of developing T2D. The authors concluded with a high level of confidence that eating high GI and GL foods can lead to a higher risk of developing T2D. They suggest that nutrition advice that favours low GI and GL foods could produce significant cost savings for public healthcare.
While dietary factors are important modifiable risk factors for type 2 diabetes (T2D), the causal role of carbohydrate quality in nutrition remains controversial. Dietary glycemic index (GI) and glycemic load (GL) have been examined in relation to the risk of T2D in multiple prospective cohort studies. Previous meta-analyses indicate significant relations but consideration of causality has been minimal. Here, the results of our recent meta-analyses of prospective cohort studies of 4 to 26-y follow-up are interpreted in the context of the nine Bradford-Hill criteria for causality, that is: (1) Strength of Association, (2) Consistency, (3) Specificity, (4) Temporality, (5) Biological Gradient, (6) Plausibility, (7) Experimental evidence, (8) Analogy, and (9) Coherence. These criteria necessitated referral to a body of literature wider than prospective cohort studies alone, especially in criteria 6 to 9. In this analysis, all nine of the Hill's criteria were met for GI and GL indicating that we can be confident of a role for GI and GL as causal factors contributing to incident T2D. In addition, neither dietary fiber nor cereal fiber nor wholegrain were found to be reliable or effective surrogate measures of GI or GL. Finally, our cost-benefit analysis suggests food and nutrition advice favors lower GI or GL and would produce significant potential cost savings in national healthcare budgets. The high confidence in causal associations for incident T2D is sufficient to consider inclusion of GI and GL in food and nutrient-based recommendations.
A multifunctional diet improves cardiometabolic-related biomarkers independently of weight changes: an 8-week randomized controlled intervention in healthy overweight and obese subjects.
European journal of nutrition. 2016;(7):2295-306
PURPOSE A multifunctional diet (MFD) was previously shown to reduce blood lipids, CRP and blood pressure in a 4-week intervention under weight-maintenance conditions. Here, MFD effects were evaluated in an 8-week intervention with no restriction for weight changes. METHODS Healthy subjects consumed MFD (23 subjects) or a control diet (CD) devoid of the functional components (24 subjects) in a "free-living" randomized controlled experiment. MFD included several functional concepts: low-glycemic-impact meals, antioxidant-rich foods, oily fish, viscous dietary fibers, soybean and whole barley kernel products, almonds and plant stanols. Measured outcomes were fasting blood values of lipids, glucose, insulin, GGT, CRP, HbA1c, PAI-1, GLP-1, GLP-2, body weight, blood pressure and breath hydrogen. RESULTS At baseline, participants were 51-72 years old, with BMI between 25 and 34 and fasting glycemia ≤ 6.1 mmol/L. Consumption of both diets resulted in similar weight loss after 8 weeks (-4 %; P < 0.001). Compared to baseline, consumption of MFD reduced total serum cholesterol (-26 %; P < 0.0001), LDL cholesterol (-35 %; P < 0.0001), triglycerides (-16 %; P < 0.05), LDL/HDL (-27 %; P < 0.0001) and ApoB/ApoA1 (-15 %; P < 0.0001). There were important net differences between diets, which remained significant after adjustment for body weight. Reduced systolic blood pressure, circulating GGT, HbA1c and insulin concentrations were observed with both MFD and CD with no difference between diets. The Reynolds cardiovascular risk score was decreased by 36 % (P < 0.0001) with MFD. MFD increased breath hydrogen levels (120 %; P < 0.05). CONCLUSIONS Consumption of MFD decreased blood lipids and improved several other aspects of the cardiometabolic risk profile. This effect was not dependent on weight loss.
Effects of GI vs content of cereal fibre of the evening meal on glucose tolerance at a subsequent standardized breakfast.
European journal of clinical nutrition. 2008;(6):712-20
OBJECTIVE To investigate if the improved glucose tolerance previously observed at breakfast following an evening meal with boiled barley kernels derives from colonic events related to the fermentation of the elevated amounts of indigestible carbohydrates present and/or from the low-GI features. SUBJECTS/METHODS Twenty healthy volunteers aged 19-30 years. DESIGN High-GI white wheat bread (WWB), WWB+barley dietary fibre (DF) corresponding to the DF content of barley kernels, low-GI spaghetti+ barley DF, spaghetti+double amounts of barley DF (2(*)DF), spaghetti+oat DF, or whole grain barley flour porridge, were provided as late evening meals. At a subsequent standardised WWB breakfast, B-glucose, s-insulin, p-SCFA, p-FFA, and breath hydrogen (H(2)) were measured. RESULTS The B-glucose response (incremental areas under the curves (IAUC) 0-120 min and total areas under the curves 0-180 min) to the standardized breakfast was significantly lower after consuming spaghetti+2*DF in the evening compared with barley porridge (P=0.012). The spaghetti+2*DF meal also resulted in the highest breath H(2) excretion (P<0.02). The glucose IAUC (0-120 min) after the standardized breakfast was positively correlated to fasting p-FFA (r=0.29, P<0.02), and the total glucose area (0-180 min) was negatively correlated to the p-propionate level (0-30 min) (r=-0.24, P<0.02). CONCLUSIONS The prolonged digestive and absorptive phase per se, like with a low-glycaemic index (GI) spaghetti evening meal, did not induce overnight benefits on glucose tolerance. Addition of barley DF in high amounts (2*DF) was required to improve overnight glucose tolerance. The correlations observed between glycaemia and p-propionate implicate colonic fermentation as a modulator of glucose tolerance through a mechanism leading to suppressed free fatty acids levels. It is proposed that the overnight benefits on glucose tolerance previously reported for boiled barley kernels is mediated through colonic fermentation of the prebiotic carbohydrates present in this product.
Effects of GI and content of indigestible carbohydrates of cereal-based evening meals on glucose tolerance at a subsequent standardised breakfast.
European journal of clinical nutrition. 2006;(9):1092-9
OBJECTIVE To evaluate the impact of four low-glycaemic index (GI) and one high-GI cereal-based evening meals on glucose tolerance at a subsequent standardised breakfast. DESIGN Wheat kernels, barley kernels, spaghetti, spaghetti with added wheat bran and white wheat bread (WWB) were consumed in the evening in a random order at five different occasions. At the subsequent breakfast, blood glucose, serum insulin, plasma short chain fatty acid, plasma free fatty acid (FFA) and breath hydrogen were measured. SETTING The study was performed at Applied Nutrition and Food Chemistry, Lund University, Sweden. SUBJECTS Fifteen healthy volunteers were recruited. One subject was later excluded owing to abnormal blood glucose values. RESULTS The blood glucose response (0-120 min) to the standardised breakfast was significantly lower after consuming barley kernels in the evening compared with evening meals with WWB (P=0.019) or spaghetti+wheat bran (P=0.046). There were no significant differences in insulin concentrations at breakfast. Breath hydrogen excretion at breakfast was significantly higher after an evening meal with barley kernels compared with WWB, wheat kernels or spaghetti (P=0.026, 0.026 and 0.015, respectively), and the concentration of plasma propionate at breakfast was significantly higher following an evening meal with barley kernels compared with an evening meal with WWB (P=0.041). In parallel, FFA concentrations were significantly lower after barley kernels compared with WWB (P=0.042) or spaghetti evening meals (P=0.019). CONCLUSIONS The improved glucose tolerance at breakfast, following an evening meal with barley kernels appeared to emanate from suppression of FFA levels, mediated by colonic fermentation of the specific indigestible carbohydrates present in this product, or, to the combination of the low-GI features and colonic fermentation.
An examination of the possibility of lowering the glycemic index of oat and barley flakes by minimal processing.
The Journal of nutrition. 2000;(9):2207-14
Differences in glycemic responses to various starchy foods are related to differences in the rate of starch digestion and absorption. In this study, the importance of the degree of gelatinization and the product thickness for postprandial glycemic and insulinemic responses to rolled oats and barley were studied in healthy subjects (5 men and 5 women). Thick (1.0 mm) rolled oats were made from raw or preheated (roasted or steamed) kernels. In addition, thin (0.5 mm) rolled oats were made from roasted or roasted and steamed (processed under conditions simulating commercial production) oat kernels. Finally, steamed rolled barley kernels (0.5 or 1.0 mm) were prepared. All thin flakes elicited high glucose and insulin responses [glycemic index (GI), 88-118; insulinemic index (II), 84-102], not significantly different from white wheat bread (P: > 0.05). In contrast, all varieties of thick oat flakes gave significantly lower metabolic responses (GI, 70-78; II, 58-77) than the reference bread (P: < 0.05). Thick barley flakes, however, gave high glucose and insulin responses (GI, 94; II, 84), probably because the botanical structure underwent more destruction than the corresponding oat flakes. We conclude that minimal processing of oat and barley flakes had a relatively minor effect on GI features compared with the more extensive commercial processing. One exception was thick oat flakes, which in contrast to the corresponding barley flakes, had a low GI.