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.
Glycemic response and health--a systematic review and meta-analysis: relations between dietary glycemic properties and health outcomes.
The American journal of clinical nutrition. 2008;87(1):258S-268S
Plain language summary
All carbohydrates are ranked based on their glycaemic impact, or the response they provoke in blood glucose after consumption. It has been proposed that diets with a low impact on blood glucose are associated with a reduction in disease risk, however the specific dietary factors and degree of influence of this association remains unclear. The aim of this meta-analysis was to assess which glycaemic impact factors are important for affecting the maintenance of health and management of disease. Data from 45 relevant publications were included to investigate various health risk markers of overweight and obesity, diabetes mellitus and coronary heart disease. This review found that diets with a reduced glycaemic impact and higher unavailable carbohydrate are associated with favourable changes for all examined health risk markers. The authors conclude that these interventions led to improved glycaemic control, therefore provide a compelling diet for managing chronic disease.
BACKGROUND Reduction of dietary glycemic response has been proposed as a means of reducing the risk of diabetes and coronary heart disease. The impact of glycemic response on markers of health remains to be elucidated. OBJECTIVE We assessed the evidence relating the glycemic impact of foods to measures relevant for health maintenance and management of disease. DESIGN This was a systematic review and synthesis of interventional evidence from literature reported on glycemic index and markers of health through the use of meta-analyses and meta-regression models. RESULTS Data from 45 relevant publications were found to January 2005. Lower glycemic index (GI) diets reduced both fasting blood glucose and glycated proteins independently of variance in available and unavailable carbohydrate intakes. Elevated unavailable carbohydrate added to improvements in both blood glucose and glycated protein control. These effects were greater in persons with poor fasting blood glucose control. No effects were seen on fasting insulin<100 pmol/L; above this, study numbers were few but consistent with prevention of hyperinsulinemia in some but not all overweight persons. Insulin sensitivity according to a variety of measurement methods was improved by lower GI, higher unavailable carbohydrate interventions in persons with type 2 diabetes, in overweight and obese persons, and in all studies combined. Fasting triacylglycerol in addition to body weight reduction related more to glycemic load than to GI. Glycemic load reduction by >17 g glucose equivalents/d was associated with reduced body weight. CONCLUSIONS Consumption of reduced glycemic response diets are followed by favorable changes in the health markers examined. The case for the use of such diets looks compelling. Unavailable carbohydrate intake is equally important.
Energy expenditure and net substrate utilization in men ingesting usual and high amounts of nonstarch polysaccharide.
The American journal of clinical nutrition. 1998;(4):820-6
BACKGROUND Diets intrinsically high in nonstarch polysaccharides (NSPs) are frequently advised for body weight regulation and health, but the consequences for energy expenditure and fuel selection are undetermined. OBJECTIVE We determined whether energy expenditure and fuel selection differ when men consume a diet intrinsically higher in NSP than a usual mixed diet. DESIGN A randomized crossover design was used in which 12 healthy men were fed a maintenance diet for approximately 3 wk in a metabolic suite. By judicial choice of food exchanges, the usual- and high-NSP diets were similar in protein, fat, and carbohydrate contents. Twenty-four-hour, indirect, open-circuit calorimetry was performed, including measurements of total hydrogen gas and methane. Participants were weight stable (within 2 kg for 3 wk), entered an 11-m3 calorimetry chamber for 36 h with measurements taken in the last 24 h, and underwent a strictly controlled program of moderate physical activity (1.3 x basal metabolic rate). RESULTS The mean total 24-h energy expenditure and percentages from protein, fat, and carbohydrate metabolism were 10 MJ/d and 16%, 35%, and 48%, respectively. Differences (mean+/-SEM) between the 2 diets were only -0.005+/-0.130 MJ/d, -0.3+/-1.3%, -0.2+/-2.0%, and 0.6+/-2.2%, respectively, and were nonsignificant (P> 0.2). CONCLUSIONS There was no thermogenic response to the high-NSP diet, which would be advantageous for body weight control, and no short-term influence on body composition, as may be judged from a lack of change in protein, fat, or carbohydrate metabolism.
Is the rat a suitable model for humans on studies of cereal digestion?
European journal of clinical nutrition. 1996;(11):710-2
OBJECTIVES To compare the extent of starch digestion from barley flake and flour in humans and rat. Is the rat a useful model? DESIGN Four healthy male ileostomy volunteers consumed, in random order, flapjacks containing flaked barley on one occasion and barley flour on another. Ileostomy fluid was collected for analysis of starch and nonstrach polysaccharide (NSP) hourly for 12 h. The ratio of starch to NSP in ileostomy fluid was compared with that in the terminal ileum of eight randomly selected male rats consuming the flaked barley flapjack and another eight rats consuming the barley flour flapjack. RESULTS After consuming flaked barley the terminal ileal starch-NSP ratio was 20 times lower in rats than in ileostomists and was 0.04 (s.e.m. 0.006) g/g compared with 0.89 (s.e.m. 0.05) g/g respectively. By contrast the starch-NSP ratio was very low after consuming the barley flour in both species; however the ratio was still lower in rats than in ileostomists and was 0.02 (s.e.m. 0.01) g/g compared with 0.05 (s.e.m. 0.01) g/g respectively. Using the NSP as an indigestible marker, starch from barley flake that resisted small intestinal digestion was calculated to be 0.7 (s.e.m. 0.02)% in rats compared with a higher value of 17 (s.e.m. 1)% in ileostomists. CONCLUSION The cell walls of barley flake limit the extent of starch digestion in humans but in rats this appears not to be a limiting factor. In the present instance it was not possible to extrapolate results from rats to humans.