Glycemic load has no long term impact on weight loss, study
weight loss in the long term, according to new research.
Conducted at the Jean Mayer US Department of Agriculture Human Nutrition Research Center on Aging (USDA HNRCA) at Tufts University, the study was the first phase of an examination into the link between calorie-restricted diets, aging, and age-related disease. The first phase of the study, which was conducted over a period of 12 months, revealed that calorie-restricted diets differing substantially in glycemic load can result in comparable long-term weight loss. "Participants in our pilot study achieved and maintained comparable weight loss after one year, regardless of whether they were on a low-glycemic-load or a high-glycemic-load diet," said Dr Susan Roberts, director of the USDA HNRCA's Energy Metabolism Laboratory. "The goal was for both groups to restrict calories by 30 percent and, after one year, both groups had lost an average of 8 percent of their original body weight. We found that the two groups did not differ significantly in their average body fat loss, energy intake, metabolic rate, or reports of hunger and satiety." The findings could provide significant clues for developers of low glycemic index foods, which have gained popularity in recent years as a new diet plan. Although the glycemic index (GI) has been around for 20 years, initially intended as a tool for diabetics, it has been embraced by many food manufacturers since 2003 as a way to give a marketing edge to health or diet products - particularly in the aftermath of the low-carb, Atkins phenomenon. Packaged Facts, a division of MarketResearch.com, recently published a report entitled Low Glycemic Index Products in the US, in which it predicts that sales will experience compound annual growth rate of 45.7 percent through 2011, when they will be worth $1.8bn. It estimates that 2006 sales have been in the region of $350m. The CAGR from 2003 to 2005 was 55.4 percent, and the continuing growth indicates that it has a long way to go before the trend starts to run out of steam. GI is perhaps the most consumer-friendly measure of carbohydrates' effect on blood sugar levels. It ranks individual foods according to their impact on blood sugar levels. But some believe that the index is too basic a measure and advocate that glycemic load may be more helpful, since it also considers the amount eaten and the foods' context as part of the overall diet, thereby quantifying the potential glycemic impact of foods. The glycemic load is calculated by multiplying the glycemic index by the grams of carbohydrates in a serving. In the new study, 34 overweight but otherwise healthy men and women were assigned randomly to a low-glycemic-load (LG) or high-glycemic-load (HG) diet. At six months, the LG group had lost an average of 10.4 percent body weight, while the HG group had lost an average of 9 percent body weight. By 12 months, participants in both the LG and HG groups had lost an average of 8 percent of their starting body weight. "Unlike several other long-term studies, which have reported greater weight loss with low GL diets at six months but no differences by 12 months, our data show no significant short-term or long-term differences," said Dr Sai Das, scientist at the USDA HNRCA and first author of the study. "However, we did detect a greater tendency for weight and body-fat regain among LG participants. This finding suggests that reduced calorie intake may be harder to sustain on LG diets over time." The researchers said the two study diets were carefully matched for factors known to influence food intake during weight-loss efforts, such as palatability, dietary variety, and fiber. The LG diet contained 40 percent carbohydrate, 30 percent fat, and 30 percent protein, while the HG diet contained 60 percent carbohydrate, 20 percent fat, and 20 percent protein. Examples of foods provided as part of the LG diet include bean and barley stew, low-fat cottage cheese, and pumpernickel bread. The HG diet included foods like bagels, candied sweet potatoes and shepherd's pie with mashed potatoes. Both diets were designed to restrict calories by 30 percent, relative to a person's baseline energy requirements, while providing the recommended amounts of vitamins, minerals, and essential fatty acids, said the researchers. All participants attended weekly behavioral support groups and met individually with a dietitian. "An important difference between our study and other weight-loss trials is that we did not rely on self-reported intakes. Underreporting of caloric intake can vary between 5 and 50 percent. By providing the study food for the first six months, we did not have to worry as much about lifestyle factors like shopping and cooking habits interfering with dietary change," said Das. Recruitment is currently underway for participation in the second phase of the Comprehensive Assessment of Long-term Effects of Restricting Intake of Energy (CALERIE) study at Tufts, which will examine the relationship between calorie-restricted diets, aging, and age-related disease.
