Planning your New Year’s resolution—which diet is best?

January 1, 2008 By Benjamin A. Olenchock, M.D. Ph.D. [mailto:bolenchock@partners.org]

Adelaide, Australia Two types of diets, one with very low carbohydrates and high fat (VLCHF) and one with high-carbohydrates and low fat (HCLF), were compared in a randomized study of obese adults. Unlike previous diet comparisons, total calorie intake was matched between the study arms. The findings, published in the Journal of the American College of Cardiology, demonstrate that the two diets achieved equal weight loss. The HCLF diet, however, had a better effect on lipid profile.

Investigators in Australia randomized 118 obese adults to one of two diets. Participants were matched by age, gender and body mass index prior to randomization. The VLCHF diet was designed to provide 4% carbohydrate, 35% protein, and 61% fat (20% saturated fat). The HCLF diet, in contrast, was 46% carbohydrates, 24% protein and 30% as fat (less than 8% saturated fat). Patients had to remain on the diet for 24 weeks. Food was provided for the first 8 weeks, and thereafter vouchers were provided. Around 80% of patients in both arms completed the course of diet, leaving 89 patients to analyze. Baseline characteristics were very similar between groups. The average age was 50 years old and the average body mass index was 33. Baseline lipid profiles and medication use were similar. The diets had a planned 30% energy restriction.

The mean weight loss was around 10 kg, and there was no difference in weight loss between the two study arms. Plasma ketone levels were higher in those on the VLCHF diet—levels peaked at 8 weeks and were much lower by week 24. Weight loss correlated with plasma ketone level in patients on the VLCHF diet. Total cholesterol and LDL cholesterol decreased significantly in patients randomized to the HCLF diet (decrease of 0.54 and 0.46 mmol/l, respectively) but not those eating VLCHF. Triglycerides, however, were better lowered by the VLCHF diet (decrease of 0.64 mmol/l) than on the HCLF diet (decrease of 0.35 mmol/l). Weight loss reduced blood pressure, fasting glucose and insulin levels, with similar effect in both diets.

There are a number of purported benefits to a VLCHF diet. The better weight loss achieved by this diet in previous studies has been suggested to be due to reduced efficiency in using fats as energy source. The authors note that changes in energy intake on the VLCHF diet likely explains much of the benefit to weight loss observed in previous studies. When calorie intake was equalized, the current study saw no difference in weight loss. The beneficial effect of the VLCHF diet on triglycerides suggests that perhaps this diet would be more suitable for an obese individual with impaired glucose tolerance, visceral adiposity, and hypertriglyceridemia.

Again, we are left with the unfortunate law of conservation of energy.

1. Jeannie Tay, Grant D. Brinkworth, Manny Noakes, Jennifer Keogh, and Peter M. Clifton. Metabolic Effects of Weight Loss on a Very-Low-Carbohydrate Diet Compared With an Isocaloric High-Carbohydrate Diet in Abdominally Obese Subjects ''J Am Coll Cardiol 2008 51: 59-67. ''