High-protein, energy-restricted diet shows beneficial metabolic effects - Diet Composition

Nutrition Research Newsletter, August, 2003

It is well established that insulin resistance increases the risks of cardiovascular disease and type 2 diabetes. Weight loss, in the range of 4.5 kg to 13 kg, especially involving a reduction in visceral adipose tissue, has been shown to significantly improve upon insulin sensitivity, glycemic control, and dyslipidemia in overweight subjects inflicted with insulin resistance. Replacing some dietary carbohydrate with protein, combined with a low total (<30%) and saturated (<10%) fat content, has been demonstrated to enhance weight loss in free-living subjects. There is also a small amount of evidence supporting the reduction or elimination of insulin resistance, independent of weight loss, with consumption of a high-protein diet. However, the beneficial effects of high-protein diets on serum lipids during either energy restriction or weight maintenance require confirmation. Additionally, there has been some concern expressed regarding the safety of high-protein diets, including the possibility of enhanced calcium excretion and bone resorption.

Recently, investigators set out to compare the effects of two isoenergetic, energy-restricted diets with either a high or standard content of dietary protein (27% or 16% of energy, respectively, as protein) on body composition, glucose and insulin homeostasis, lipid concentrations, bone turnover, and blood pressure in obese subjects with hyperinsulinemia.

Subjects included 57 obese and overweight individuals, aged 20 to 65 years who had a fasting serum insulin concentration of> 12 mU/L and a body mass index (BMI) of 27 to 43. The majority of subjects was sedentary at baseline and was asked to maintain their usual levels of physical activity and to refrain from drinking alcohol throughout the study. The prescribed diets were either high in protein [HP diet: 30% of energy as protein (-110 y/d), 40% as carbohydrate, and 30% as fat] or contained a standard amount of protein [SP diet: 15% of energy as protein (-60 g/d), 55% as carbohydrate, and 30% as fat]. The diets were prescriptive fixed-menu plans, and subjects were supplied with key foods that made up 60% of their energy intake. Each subject completed weighed daily checklists of all foods consumed and was assessed by the same dietitian at 2-week intervals.

The study was conducted on an out-patient basis for 16 weeks. Both dietary groups underwent 12 weeks of energy restriction, followed by 4 weeks of energy balance with the same macronutrient composition. On two consecutive days at each of weeks 0, 4, 8, 12, and 16, body weight and blood pressure were measured and venous blood samples were taken and analyzed for plasma glucose, serum insulin, and lipid concentrations. At weeks 0 and 16, a 3-hour meal tolerance test was performed with meals that were representative of the diet to which the subjects were assigned.

Weight loss (7.9 [ or -] 0.5 kg) and total fat loss (6.9 [ or -] 0.4 kg) did not differ between the two diet groups. In women, total lean mass was significantly (P = 0.02) better preserved with the HP diet (-0.1 0.3 kg) than with the SP diet (-1.5 [ or -] 0.3 kg). Those in the HP diet group had significantly (P < 0.03) less glycemic response at weeks 0 and 16 than did those fed the SP diet. Following weight loss, the glycemic response decreased significantly (P < 0.05) more in the HP diet group. The reduction of serum triacylglycerols concentrations was significantly (P < 0.05) greater in the HP diet group (23%) than in the SP diet group (10%). Markers of bone turnover, calcium excretion, and systolic blood pressure were unchanged.

In this study, replacing carbohydrate with protein from meat, poultry, and dairy foods had beneficial metabolic effects and no adverse effects on markers of bone turnover or calcium excretion.

E. Farnsworth, Luscombe, M. Noakes, et al. Effect of a high-protein, energy-restricted diet on body composition, glycemic control, and lipid concentrations in overweight and obese hyperinsulinemic men and women. Am J Clin Nutr; 78:31-39 (July, 2003). [Correspondence: PM Clifton. CSIRO Health Sciences and Nutrition, PO Box 10041 BC, Adelaide SA 5000, Australia. E-mail: peter.Clifton@csiro.au].