The effect that a particular carbohydrate food has on the postprandial plasma blood glucose concentration can be assessed and quantified using the glycaemic index (GI). Foods that elicit a low postprandial rise in blood sugar levels can be said to have a low GI. The GI of foods is measured by comparing the rise in the postprandial plasma blood glucose levels caused by 50g of a particular food, to the rise caused by 50g of a known standard. The use of bread as the standard has superseded glucose because bread is a frequently consumed food and is therefore more acceptable to the tastes of the subject being tested. The GI is calculated from the area under the curve (AUC) for the test food expressed as a percentage of the AUC for the standard control. The glucose scale can be converted to the bread scale by multiplication of 100/70.
The glycaemic load (GL) can also be a useful tool in quantifying the effects of carbohydrate foods on an individual. The GL differs from the GI, because it is an assessment of the total impact of the ingested carbohydrate on postprandial glycaemia. The GL is calculated by taking the GI and multiplying this number by the total grams of carbohydrate, followed by division by 100 to express the value as a percentage. The GL of a meal can be calculated by first taking the mean GI of all carbohydrate foods. The GL therefore has more real world application than the GI, because carbohydrate foods are rarely eaten in isolation. While foods with a GI over 70 are considered to be high GI foods, a number of factors including fibre, protein, fat, starch, pH and particle size can reduce the GL of the food when in a mixed meal.
The evidence that lowering the GI of foods contained within a diet (i.e. lowering the GL) has beneficial effects on weight loss has been reported in the literatures. Meta-analysis of studies have been performed and show that weight loss occurs when the GI of carbohydrate foods are reduced. However, the effects might not be beneficial where energy intake is also restricted, supporting previous findings that show calorie controlled diets are ineffective. Despite the very positive findings in some trials, the amount of research on GL or GI and weight loss is limited, which is surprising given the simplicity by which the changes can be made and the fact that deliberate energy restriction is not required. The benefits to weight loss of increasing the protein content of the diet may also be in part derived from the subsequent reduction in GI caused by the delayed gastric emptying.
Other benefits seen following reductions in the GI or GL of diets include improvements in insulin resistance, which may be reduce subsequent risk of diabetes, cardiovascular disease and metabolic syndrome. Reducing the GI of foods contained within the diet not only reduces the risk of developing type 2 diabetes, but also allows effective control of the disease in those already diagnosed. The reduction in the risk of cardiovascular disease and improvements in dyslipidaemia with diets of a low GI have been reported in many studies. More recently, evidence suggesting that low GI diets may be beneficial in providing protection from some forms of cancer has also been reported. Some studies report significant reductions in breast cancer risk in individuals following a low GI diet. Other meta-analyses have shown significant reductions in endometrial and prostate cancer with lower glycaemic index diet, although the results are controversial.
The protective effects of low GI diets likely relate to the delay to the glucose absorption from the gastrointestinal tract. Reducing the rate of glucose absorption would have favourable repercussions for the subsequent rise in plasma insulin and may reduce circulating levels of the incretin hormones gastric inhibitory peptide (GIP) and glucagon-like peptide 1 (GLP-1). Lowering circulating insulin levels may provide a number of benefits such as decreasing the suppression of fatty acid oxidation and reducing the counter regulatory mechanisms that are triggered upon high levels of circulating insulin. Reducing insulin levels would increase the efficiency of cellular glucose uptake because of improvements in cellular glucose metabolism. As a result fasting blood glucose levels would remain closer to normal baseline levels. The reduced peak glucose levels holds advantages for those with insulin resistance, where control over blood glucose levels may have diminished over time.
RdB
