Low-Carb vs. Low-Fat Diet for Young Adults who are Obese
Obesity is among the most important medical problems in the United States, with prevalence in one third of the population, and identification of effective dietary treatment has become a major public health priority. Three popular diets- low fat, low carbohydrate, and low glycemic load—have recently received much attention. Many inconsistencies have presented themselves in former studies, including the physiological differences among study participants in insulin secretion.
Determining whether insulin secretion affects weight loss is a subject of primary interest. Diets with a high glycemic load (the mathematical product of the glycemic index and the carbohydrate amount) result in higher postprandial insulin concentration, calorie for calorie, than those with a low glycemic load. High insulin concentration after meals has been postulated to decrease availability of metabolic fuels several hours after a meal, causing hunger and overeating.
In a study published in JAMA in May 2007, researchers purposed to determine whether insulin secretion affects body fat loss among obese individuals consuming self-prepared diets. They conducted an 18-month randomized controlled trial to compare the efficacy of a low-glycemic load/higher-fat diet with ad low-fat/higher-glycemic load diet. Treatment intensity, treatment fidelity, nutrition education and dietary counseling strategies, and physical activity prescription were consistent between the diet groups.
The study followed 73 participants aged 18-35 years with BMIs of 30 and above, not exceeding 140kg (308 lbs) in total body weight, non-smokers, no recent adherence to a weight loss diet, and no other major illness (i.e. diabetes mellitus) as assessed by a medical history and laboratory screening tests.
In the Low-Glycemic Load Diet, participants were counseled to consume low-glycemic load foods (particularly nonstarchy vegetables, legumes and temperate fruits) and to limit intake of high-glycemic load foods (such as refined grains, starchy vegetables, fruit juices, and sweets). Attention was also directed toward consuming sources of healthful fat including nuts, seed and oils. Participants were instructed in appropriate serving sizes of each type of food. The target macronutrient composition was 40% of energy from carbohydrate from low glycemic sources, 35% from fat, and 25% from protein.
In the Low-Fat Diet, Participants were counseled to consume low-fat grains, vegetables, fruits and legumes and to limit intake of added fats, sweets, and high-fat snacks. The Target macronutrient composition was 55% of energy from carbohydrate, 20% from fat and 25% from protein. The aim was to prescribe a diet consistent with low-fat guidelines. Participants in this group were also instructed on appropriate serving sizes of high-fat foods (e.g. butter) and sweets.
The diets were prescribed in increase satiety and therefore promote a negative energy balance. Proposed mechanisms for this presumption involve improved access to metabolic fuels on the low-glycemic load diet, and decreased energy density on the low-fat diet.
Researchers found that changes in body weight did not differ between the diet groups overall. However, Insulin concentration at 30 minutes after a dose of oral glucose was used as an effect modifier, and for subjects with an insulin concentration at 30 min above the median, the low-glycemic diet produced a greater decrease in weight (-5.8 vs. -1.2kg) and body fat percentage (-2.6% vs. -0.9%) than the low-fat diet at 18 months. There were not any significant differences in these end points between diet groups for those with insulin concentration at 30 minutes below the median level.
In conclusion, reducing glycemic load may be especially important to achieve weight loss among individuals with high insulin secretion. Of note, in the full cohort, plasma HDL cholesterol and triglyceride concentrations improved more on the low-glycemic load diet, whereas LDL cholesterol concentration improved more on the low-fat diet.
JAMA, May 16, 2007—Vol 297, No. 19 pp 2092-2101