Very-low-calorie diets with high and low protein content: impact on triiodothyronine, energy expenditure, and nitrogen balance

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Abstract
Optimal composition of reducing diets remains controversial. Seventeen obese inpatients received 440 kcal/d, either 41% protein plus 55% carbohydrate (CD) or 95% protein (PP), for 3 wk. There were no significant diet effects (all data CD vs PP) in weight loss (8.88 +/- 1.01 vs 8.74 +/- 0.79 kg), loss of lean mass (2.10 +/- 0.35 vs 1.61 +/- 0.39 kg), metabolic rate reduction (15.3 +/- 2.8 vs 13.0 +/- 5.2%), or meal-stimulated thermogenesis (26.6–37.9 vs 29.0–26.1 net kcal/3 h [time NS also]). Triiodothyronine (T3) responses differed (2.35 +/- 0.11 to 1.57 +/- 0.14 vs 2.43 +/- 0.11 to 1.47 +/- 0.12 nmol/L, p less than 0.01) as did free T3 (3.4 +/- 0.2 to 2.6 +/- 0.2 vs 3.2 +/- 0.2 to 2.0 +/- 0.2 pmol/L, (p less than 0.01]; thyroxine declined similarly in both groups. Subjects fed CD gained no advantage over subjects fed PP. Regression analyses revealed no relationship between thyroid hormones, energy deficit, or lean mass with nitrogen losses, suggesting that other or more complex processes govern endogenous protein metabolism during weight loss.