But if your plan is to adopt a low glycemic diet to achieve your goal, you may want to think twice. Enticed by the promise of being able to enjoy carbohydrates at every meal, devotees have created a lot of buzz around low glycemic diets.
Unlike their no-carb counterparts, low glycemic eating plans do not restrict carbohydrate intake altogether.
Instead, followers abide by the glycemic index GI , a ranking system that assigns a number from 0 to to carbohydrates based on their effect on blood sugar levels. Foods with a number of 55 or lower, such as broccoli and apples, are considered low GI foods, and adherents of the diet can eat these at every meal. Weight loss is best done with a combination of reducing calories in your diet and increasing your physical activity and exercise. Selecting foods based on a glycemic index or glycemic load value may help you manage your weight because many foods that should be included in a well-balanced, low-fat, healthy diet with minimally processed foods — whole-grain products, fruits, vegetables and low-fat dairy products — have low- GI values.
For some people, a commercial low- GI diet may provide needed direction to help them make better choices for a healthy diet plan. The researchers who maintain the GI database caution, however, that the "glycemic index should not be used in isolation" and that other nutritional factors — calories, fat, fiber, vitamins and other nutrients — should be considered.
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Matthan NR, et al. Estimating the reliability of glycemic index values and potential sources of methodological and biological variability. American Journal of Clinical Nutrition. Bosy-Westphal A, et al. Impact of carbohydrates on weight regain. Liu S, et al. To determine the glycemic index GI of a food, healthy volunteers are typically given a test food that provides 50 grams g of carbohydrate and a control food white, wheat bread or pure glucose that provides the same amount of carbohydrate, on different days 4.
Blood samples for the determination of glucose concentrations are taken prior to eating, and at regular intervals for a few hours after eating. The changes in blood glucose concentration over time are plotted as a curve. The GI is calculated as the incremental area under the glucose curve iAUC after the test food is eaten, divided by the corresponding iAUC after the control food pure glucose is eaten. The value is multiplied by to represent a percentage of the control food 5 :. In contrast, cooked brown rice has an average GI of 50 relative to glucose and 69 relative to white bread.
In the traditional system of classifying carbohydrates, both brown rice and potato would be classified as complex carbohydrates despite the difference in their effects on blood glucose concentrations. While the GI should preferably be expressed relative to glucose, other reference foods e. Additional recommendations have been suggested to improve the reliability of GI values for research, public health, and commercial application purposes 2 , 6.
By definition, the consumption of high-GI foods results in higher and more rapid increases in blood glucose concentrations than the consumption of low-GI foods. Over the next few hours, the increase in blood insulin concentration hyperinsulinemia induced by the consumption of high-GI foods may cause a sharp decrease in the concentration of glucose in blood resulting in hypoglycemia. Many observational studies have examined the association between GI and risk of chronic disease , relying on published GI values of individual foods and using the following formula to calculate meal or diet GI 9 :.
Yet, the use of published GI values of individual foods to estimate the average GI value of a meal or diet may be inappropriate because factors such as food variety, ripeness, processing, and cooking are known to modify GI values. In a study by Dodd et al. Besides the GI of individual foods, various food factors are known to influence the postprandial glucose and insulin responses to a carbohydrate-containing mixed diet.
A recent cross-over , randomized trial in 14 subjects with type 2 diabetes mellitus examined the acute effects of four types of breakfasts with high- or low-GI and high- or low- fiber content on postprandial glucose concentrations. Plasma glucose was found to be significantly higher following consumption of a high-GI and low-fiber breakfast than following a low-GI and high-fiber breakfast.
However, there was no significant difference in postprandial glycemic responses between high-GI and low-GI breakfasts of similar fiber content In this study, meal GI values derived from published data failed to correctly predict postprandial glucose response, which appeared to be essentially influenced by the fiber content of meals. Since the amounts and types of carbohydrate, fat, protein , and other dietary factors in a mixed meal modify the glycemic impact of carbohydrate GI values, the GI of a mixed meal calculated using the above-mentioned formula is unlikely to accurately predict the postprandial glucose response to this meal 3.
Using direct measures of meal GIs in future trials — rather than estimates derived from GI tables — would increase the accuracy and predictive value of the GI method 2 , 6. In addition, in a recent meta-analysis of 28 studies examining the effect of low- versus high-GI diets on serum lipids , Goff et al. Therefore, a stricter use of GI cutoff values may also be warranted to provide more reliable information about carbohydrate-containing foods.
The glycemic index GI compares the potential of foods containing the same amount of carbohydrate to raise blood glucose. However, the amount of carbohydrate contained in a food serving also affects blood glucose concentrations and insulin responses. Yet, one serving of watermelon provides 11 g of available carbohydrate, while a medium doughnut provides 23 g of available carbohydrate.
The concept of glycemic load GL was developed by scientists to simultaneously describe the quality GI and quantity of carbohydrate in a food serving, meal, or diet. The GL of a single food is calculated by multiplying the GI by the amount of carbohydrate in grams g provided by a food serving and then dividing the total by 4 :. Using the above-mentioned example, despite similar GIs, one serving of watermelon has a GL of 8, while a medium-sized doughnut has a GL of Dietary GL is the sum of the GLs for all foods consumed in the diet.
It should be noted that while healthy food choices generally include low-GI foods, this is not always the case. For example, intermediate-to-high-GI foods like parsnip, watermelon, banana, and pineapple, have low-to-intermediate GLs see Table 1. The consumption of high-GI and -GL diets for several years might result in higher postprandial blood glucose concentration and excessive insulin secretion.
A US ecologic study of national data from to found that the increased consumption of refined carbohydrates in the form of corn syrup, coupled with the declining intake of dietary fiber , has paralleled the increased prevalence of type 2 diabetes In addition, high-GI and -GL diets have been associated with an increased risk of type 2 diabetes in several large prospective cohort studies. Moreover, obese participants who consumed foods with high-GI or -GL values had a risk of developing type 2 diabetes that was more than fold greater than lean subjects consuming low-GI or -GL diets However, a number of prospective cohort studies have reported a lack of association between GI or GL and type 2 diabetes The use of GI food classification tables based predominantly on Australian and American food products might be a source of GI value misassignment and partly explain null associations reported in many prospective studies of European and Asian cohorts.
Nevertheless, conclusions from several recent meta-analyses of prospective studies including the above-mentioned studies suggest that low-GI and -GL diets might have a modest but significant effect in the prevention of type 2 diabetes 18 , 25, A meta-analysis of 14 prospective cohort studies , participants; mean follow-up of Three independent meta-analyses of prospective studies also reported that higher GI or GL was associated with increased risk of CHD in women but not in men A similar finding was reported in a cohort of middle-aged Dutch women followed for nine years Overall, observational studies have found that higher glycemic load diets are associated with increased risk of cardiovascular disease, especially in women and in those with higher BMIs.
A meta-analysis of 27 randomized controlled trials published between and examining the effect of low-GI diets on serum lipid profile reported a significant reduction in total and LDL - cholesterol independent of weight loss Yet, further analysis suggested significant reductions in serum lipids only with the consumption of low-GI diets with high fiber content. In a three-month, randomized controlled study, an increase in the values of flow-mediated dilation FMD of the brachial artery, a surrogate marker of vascular health, was observed following the consumption of a low- versus high-GI hypocaloric diet in obese subjects A reduction in the expression of the gene coding for 3-hydroxymethylglutaryl HMG -CoA reductase, the rate-limiting enzyme in cholesterol synthesis , in blood cells further confirmed an effect for the low-GI diet on cholesterol homeostasis Evidence that high-GI or -GL diets are related to cancer is inconsistent.
A recent meta-analysis of 32 case-control studies and 20 prospective cohort studies found modest and nonsignificant increased risks of hormone -related cancers breast, prostate , ovarian, and endometrial cancers and digestive tract cancers esophageal , gastric , pancreas , and liver cancers with high versus low dietary GI and GL A significant positive association was found only between a high dietary GI and colorectal cancer Yet, earlier meta-analyses of prospective cohort studies failed to find a link between high-GI or -GL diets and colorectal cancer Another recent meta-analysis of prospective studies suggested a borderline increase in breast cancer risk with high dietary GI and GL.
Adjustment for confounding factors across studies found no modification of menopausal status or BMI on the association This highlights that using GI in isolation may not always be the best predictor of blood sugar levels.
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