Glycemic Index: Good for Fat Loss or Worthless?

When the Zone Diet and South Beach Diet first hit the market about a decade ago, it caused a greater awareness of the glycemic index. Dieters all over the US were looking up the slow and fast absorbing starches and what the GI of these foods were for fat loss. Glycemic Index or GI is a numerical ranking system used to measure the rate of digestion and absorption of foods and their resultant effect on blood glucose. A food ranking high on the GI produces a large, momentary spike in glucose after it is has been consumed. By contrast, a food with a low GI causes a slower, sustained rise in blood glucose. The supposition of low carbohydrate diets is that throughout the day insulin levels are also lower, allowing (or promoting) greater use of fat as the source of fuel. Those who advocate eating low GI foods believe this eating plan will increase the rate of fat utilization. There have been conflicting studies as to the effectiveness of low glycemic index diets, with some diets showing beneficial effects on fat loss whereas other have found them to be no more effective than low fat diets. A recent published summary from a workshop on glycemic response and health is in agreement, suggesting at this time there is limited evidence to support the role of a low GI diet for weight loss.

Researchers wanted to examine the effects of diets varying in carbohydrate and glycemic index (GI) on changes in body composition, resting metabolic rate (RMR), and metabolic adaptation during and after weight loss. Adults with obesity were randomized to one of four provided-food diets for 17 weeks. Diets differed in percentage energy from carbohydrate (55% or 70%) and GI (low or high) but were matched for protein, fiber, and energy. Body weight, body composition, RMR, and metabolic adaptation (measured RMR-predicted RMR) were measured during weight loss and subsequent weight stability. At the end of the study, no effect of dietary carbohydrate content or GI on body weight loss or percentage of weight lost as fat mass was observed. The researchers found that the subjects lost a mean 6.1kg of total bodyweight in the 12-week weight loss intervention. The amount of weight lost did not differ significantly between groups according to either CHO or GI of the diets.

Metabolic adaptation did not differ by dietary carbohydrate content or GI and was not associated with weight regain 12 months later. The researchers concluded that moderate-carbohydrate and low-GI diets did not preferentially reduce fat mass, preserve lean mass, or attenuate metabolic adaptation during weight loss compared to high-carbohydrate and high-GI diets.

Effects of carbohydrate quantity and glycemic index on resting metabolic rate and body composition during weight loss, by Karl, Roberts, Schaefer, Gleason, Fuss, Rasmussen & Das, in Obesity (2015)

Howlett, J. and Ashwell, M. (2008). Glycemic response and health: summary of a workshop. American Journal of Clinical Nutrition, 87, 212S-216S.