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JOURNAL: Journal of Nutrition
AUTHORS: Donald K. Layman
ABSTRACT: A relatively high-protein diet improves body composition, enhances weight loss, and improves glucose and insulin homeostasis.
COMMENTARY: "Amino acids interact with glucose metabolism both as carbon substrates and by recycling glucose carbon via alanine and glutamine; however, the effect of protein intake on glucose homeostasis during weight loss remains unknown," write Donald K. Layman and colleagues from the University of Illinois at Urbana-Champaign.
In this study, 24 adult women who were more than 15% above ideal body weight were assigned to either a predominantly protein diet or a predominantly carbohydrate diet. The protein diet included 1.6 g/kg/day protein, with less than 40% of energy coming from carbohydrate, while the carbohydrate diet included 0.8 g/kg/day protein, with more than 55% of energy coming from carbohydrate. Both diets were equal in calories (7100 kJ/day) and in fat (50 g/day).
After 10 weeks, weight loss was 7.53 ± 1.44 kg in the protein group and 6.96 ± 1.36 kg in the carbohydrate group. Subjects in the carbohydrate group had lower fasting (4.34 ± 0.10 vs. 4.89 ± 0.11 mmol/L) and postprandial blood glucose (3.77 ± 0.14 vs. 4.33 ± 0.15 mmol/L) and an elevated insulin response to meals (207 ± 21 vs. 75 ± 18 pmol/L).
"This study demonstrates that consumption of a diet with increased protein and a reduced carbohydrate/protein ratio stabilizes blood glucose during nonabsorptive periods and reduces the postprandial insulin response," the authors write.
According to a second report from the same study group, "claims about the merits or risks of carbohydrate vs. protein for weight loss diets are extensive, yet the ideal ratio of dietary carbohydrate to protein for adult health and weight management remains unknown."
In this study, 24 women were assigned to either a predominantly carbohydrate diet containing 68 g/day protein with a carbohydrate/protein ratio of 3.5, or to a predominantly protein diet containing 125 g/day protein with a ratio of 1.4. Each diet provided 7100 kJ/day and approximately 50 g/day of fat. Age range was 45 to 56 years and body mass indices were greater than 26 kg/m2.
After 10 weeks, weight loss was 6.96 ± 1.36 kg in the carbohydrate group and 7.53 ± 1.44 kg in the protein group.
Compared with the carbohydrate group, weight loss in the protein group had an increased ratio of fat to muscle loss (6.3 ± 1.2 g/g vs. 3.8 ± 0.9 g/g). Serum cholesterol reduction was approximately 10% in both groups, but only the protein group had significant reductions in triacylglycerols (TAG; 21%) and in the ratio of TAG to high-density lipoprotein cholesterol (23%).
"This study demonstrates that increasing the proportion of protein to carbohydrate in the diet of adult women has positive effects on body composition, blood lipids, glucose homeostasis and satiety during weight loss.
Although it is unlikely that any one diet will be ideal for all individuals, these results indicate that changes in the ratio of protein to carbohydrate toward a higher protein diet can be effective in the control of body weight with parallel improvements in blood lipids.
The National Cattlemen's Beef Association and Kraft Foods helped support this study.