GI diet gets health boost
low-glycemic-index diet, and could influence future debate on
whether food makers can make claims for low 'GI' products or those
containing fibres that release sugar slowly.
The glycemic index (GI) is a numerical system of measuring how fast a food or ingredient triggers a rise in circulating blood glucose; the higher the GI, the greater the blood sugar response. There is already some evidence from human trials that a low-glycemic index diet can benefit heart health and weight loss.
For example, a study published in the European Journal of Clinical Nutrition online (doi:10.1038/sj.ejcn.1601992) found that dietary GI was positively associated with cardiovascular disease risk factors among Japanese women who consumed white rice as a staple food.
But the authors of the new trial say most studies so far have not ruled out the possible benefits from other aspects of the subjects' diets, such as fibre or overall caloric intake.
The US scientists reported in Friday's issue of The Lancet that a low-GI diet led to weight loss, reduced body fat, and reduction in risk factors for diabetes and cardiovascular disease among rats.
"The study findings should give impetus to large-scale trials of low-GI diets in humans," said senior author David Ludwig, director of the Optimal Weight for Life obesity programme at Children's Hospital Boston in the US.
The rats were fed tightly controlled diets with identical nutrients, except for the type of starch. Both diets were 69 percent carbohydrates, but 11 rats were randomly assigned to a high-GI starch and 10 to a low-GI starch. Food portions were controlled to maintain the same average body weight in the two groups.
At follow-up, the high-GI group had 71 percent more body fat and 8 percent less lean body mass than the low-GI group, despite very similar body weights. The fat in the high-GI group was concentrated in the trunk area, a risk factor for cardiovascular disease.
The high-GI group also had significantly greater increases in blood glucose and insulin levels on an oral glucose tolerance test, and far more abnormalities in the pancreatic islet cells that make insulin (all changes that occur with diabetes). And the high-GI group had blood triglyceride levels nearly three times that of the low-GI group, another heart disease risk factor.
Further experiments on mice and another on rats that were switched from a low to high-GI diet (they showed greater increases in blood glucose and insulin than rats that were switched from high to low GI), confirmed the initial findings.
"What the study shows is that glycemic index is an independent factor that can have dramatic effects on the major chronic diseases plaguing developed nations - obesity, diabetes, and heart disease," said Ludwig."This is the first study with hard endpoints that can definitively identify glycemic index as the active dietary factor."
Children's Hospital Boston is now recruiting adult subjects for a large-scale, 18-month human study of the low-GI diet.
The low-GI diet is seen as a more moderate approach than the Atkins and other no- or low-carb diets, yet one which also capitalizes on increasing awareness that carbohydrates, and not only calories, can influence weight and heart health. Unlike the popular Atkins diet, which seeks to minimize carbohydrate intake, the low-GI diet makes distinctions among carbs.
Dieters are advised to avoid high glycemic-index foods, such as white bread, refined breakfast cereals, and concentrated sugars, which are rapidly digested and raise blood glucose and insulin to high levels. Instead, it emphasizes carbohydrates that release sugar more slowly, including whole grains, most fruits, vegetables, nuts, and legumes.
The food industry in the US has already started to take note and has begun producing GI foods. One of the most recent offerings was the fiber-rich Natureal GI (glycemic index) oat bran concentrate from GTC Nutrition.
The ingredient is rich in beta glucan, the soluble fiber found in oats that studies indicate is effective in helping maintain healthy blood sugar levels.
More than 100 million Americans have elevated cholesterol levels. For the 65 million who are 'borderline high', (200-239 mg/dL), lifestyle modifications such as diet can play a role in maintaining normal cholesterol levels, according to the National Institutes of Health (NIH) National Cholesterol Education Program (NCEP).
