It could also influence future debate on whether food makers can make claims for low 'GI' products or those containing lower sugar carbohydrates or fibres that release energy more slowly than refined carbs.
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 per cent 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 per cent more body fat and 8 per cent 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 in 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 diet is expected to have a bigger influence on Europe's food industry than the currently popular Atkins diet, or similar low-carb diets, radically impacting the US. While Europe, like the US, has a rising obesity rate - it is now thought that one-third of western European consumers are currently overweight and by 2006 this will increase to almost half - the Atkins diet has had sustained negative media coverage as in its biggest European market, the UK, because of reactions from nutritionists.
The low-GI diet is seen as a more moderate approach 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.
Ingredients firms have picked up on the trend, with the Netherlands-based Acatris promoting its fenugreek extract FenuLife as ideal for the 'slow-carb' trend. Meanwhile National Starch has found that more than 80 per cent of 'nutrition-influencing professionals' in the UK recommend the glycemic index. It promotes its Hi-maize resistant starch as a significant opportunity for food makers.
Communicating the benefits of a low GI diet to consumers remains a challenge however and unless included in the proposed European legislation on health claims, legislation preventing claims for weight loss could hold back the development of this emerging category.