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July 25, 2006 — Diets high in carbohydrates (CHOs) with low glycemic index (GI) are best for cardiovascular risk reduction, according to the results of a randomized controlled study reported in the July 24 issue of the Archives of Internal Medicine. The editorialist suggests that it is time to incorporate the concepts of glycemic index and glycemic load into clinical practice to reduce cardiovascular risk.
"Despite the popularity of low-glycemic index (GI) and high-protein diets, to our knowledge no randomized, controlled trials have systematically compared their relative effects on weight loss and cardiovascular risk," write Joanna McMillan-Price, MNutrDiet, from the University of Sydney in Australia, and colleagues. "A unifying hypothesis is that a high dietary glycemic load (GL; the contribution to postprandial glycemia of all foods in a diet) increases the difficulty of weight control because rapidly digestible CHOs can cause marked fluctuations in blood glucose and insulin levels, in turn stimulating hunger and inhibiting fat oxidation. Both low-GI and high-protein diets have caught the public's attention, but clinicians and health professionals remain skeptical, calling for greater scientific evidence on which to base advice to patients."
In this study, 129 overweight or obese young adults (body mass index [BMI], ≥ 25 kg/m2) were randomized to 1 of 4 reduced-fat, high-fiber diets for 12 weeks, and changes in weight, body composition, and blood chemistry profile were studied. Diets 1 and 2 were high carbohydrate (55% of total energy intake), with high and low GIs, respectively, whereas diets 3 and 4 were high protein (25% of total energy intake), with high and low GIs, respectively. The glycemic load was highest in diet 1 and lowest in diet 4.
Mean weight loss was similar in all groups (diet 1, -4.2% ± 0.6%; diet 2, -5.5% ± 0.5%; diet 3, -6.2% ± 0.4%; and diet 4, -4.8% ± 0.7%; P = .09). However, the proportion of subjects in each group who lost 5% or more of body weight varied significantly by diet (diet 1, 31%; diet 2, 56%; diet 3, 66%; and diet 4, 33%; P = .01). Women on diets 2 and 3 lost approximately 80% more fat mass (-4.5 ± 0.5 [mean ± SE] and -4.6 ± 0.5 kg) than did those on diet 1 (-2.5 ± 0.5 kg; P = .007).
Mean low-density lipoprotein cholesterol levels decreased in the diet 2 group (-6.6 ± 3.9 mg/dL [-0.17 ± 0.10 mmol/L]) but increased in the diet 3 group (+10.0 ± 3.9 mg/dL [+0.26 ± 0.10 mmol/L]; P = .02). Goals for energy distribution were not achieved exactly, in that both CHO groups ate less fat, and the diet 2 group ate more fiber.
"Both high-protein and low-GI regimens increase body fat loss, but cardiovascular risk reduction is optimized by a high-carbohydrate, low-GI diet," the authors write. "At least in the short term, our findings suggest that dietary GL, and not just overall energy intake, influences weight loss and postprandial glycemia. Moderate reductions in GL appear to increase the rate of body fat loss, particularly in women."
Study limitations include inexact achievement of dietary goals for energy distribution and study duration limited to 12 weeks.
"Diets based on low-GI whole grain products (in lieu of whole grains with a high GI) maximize cardiovascular risk reduction, particularly if protein intake is high. Reassuringly, this advice can optimize clinical outcomes within current nutrition guidelines, without the concerns that apply to low-CHO diets. Multicenter studies to evaluate weight reduction, weight maintenance, and long-term outcomes, particularly in individuals with established risk factors, are clearly warranted."
The National Heart Foundation of Australia and Meat and Livestock Australia supported this study. Two of the authors have disclosed writing books about low GI diets.
In an accompanying editorial, Simin Liu, MD, ScD, from the University of California, Los Angeles, notes that until recently, knowledge of responses to GL has not been comprehensive and applied to the development of food-composition tables in guiding dietary practice.
"Accumulating data now indicate that postprandial glycemia is an important risk factor in the development of CVD [cardiovascular disease], which can be controlled through both pharmacologic and dietary means that delay gastrointenstinal absorption of carbohydrates," Dr. Liu writes. "Given the fact that a large segment of the population is following many popular diets that emphasize low-GI and/or low-carbohydrate intake for weight loss, future feeding trials and mechanistic studies such as the one reported by McMillan-Price and colleagues will continue to provide welcome insights in a field that is often shrouded with confusion."
Dr Liu has disclosed no relevant financial relationships.
Arch Intern Med. 2006;166:1438-1439, 1466-1475
