Researchers from the University of Minnesota tested whether reducing the glycaemic index of a diet already low in calories would have any further benefit for a group of obese adults.
But although the new trial confirmed the benefit of lowering glycaemic index on insulin sensitivity, it did not impact the subjects' weight, they write in this month's issue of the Journal of Nutrition (135:2387-91).
Consumers are increasingly interested in the glycaemic value of foods after some studies suggested that a low-glycaemic diet could help control weight.
But experts have questioned whether there is enough evidence to demonstrate that low GI diets can cause weight loss.
The US researchers used a randomized, controlled feeding trial to compare the effects of three low-calorie diets with different glycaemic loads on 29 obese adults.
All of the diets - high glycaemic index (HGI), low glycaemic index (LGI) or high fat (HF) - provided 3138 kJ less than estimated energy needs.
For the first 12 weeks, all food was provided to subjects and then 22 subjects were instructed to follow the assigned diet for 24 additional weeks (free-living phase).
At 12 weeks, weight changes from baseline were significant in all groups but not different among groups, said the researchers. All groups had improved insulin sensitivity.
During the free-living phase, all groups maintained their initial weight loss and their improved insulin sensitivity but weight loss and improved insulin sensitivity scores were independent of diet composition.
"In summary, lowering the glycaemic load and glycaemic index of weight reduction diets does not provide any added benefit to energy restriction in promoting weight loss in obese subjects," conclude the researchers.