Dietary protein and amino acids

High-protein diets for weight management have being revisited in recent years (reviewed in references 46 and 129). Proteins are more thermogenic (see Section 4.2.3) and satiating (see Chapter 2) than fats and carbohydrates. There is convincing evidence from human intervention studies that a higher protein intake (25% or more of the total energy as protein) increases ther-mogenesis and satiety, and reduces subsequent energy intake in the short-term compared with diets having the usually recommended protein content (15% or less of total energy as protein).46 1 29 There is also evidence that higher-protein diets can result in an increased weight loss and fat loss as compared with diets lower in protein, probably due to reduced perceived hunger and energy intake.46'129'130 Higher fat loss with high-protein diets is evident, however, even under isocaloric conditions, where total weight loss is not affected, pointing to a metabolic effect of protein favoring energy repartitioning towards lean body mass. Increasing the protein intake (from 15 to 18% total energy) has also been shown to limit weight regain and favor the regaining of fat-free mass at the cost of fat mass during weight maintenance after weight loss, under ad libitum energy intake conditions.131

How can a higher protein intake affect body composition? Layman et al.132 reported that substituting dietary protein for carbohydrate in energy-restricted diets brought about endocrine changes (maintenance of thyroid hormones T3 and T4 and reduced insulin response to a test meal) consistent with higher rates of lipolysis. In addition, an increased amount of dietary protein has been shown to reduce nitrogen losses associated with very low energy diets and to sustain muscle protein anabolism during catabolic conditions (reviewed in reference 78). Hence, the changes in body composition associated with the higher protein diets may be associated with either targeting of body fat or sparing of muscle protein or both.132

A high intake of branched-chain amino acids (BCAAs: leucine, valine and isoleucine), and specifically of leucine, may be of special interest in the context of body weight-loss strategies, because of the effects of BCAAs on glycemic control and the specific effects of leucine in promoting muscle protein synthesis and/or inhibiting muscle protein breakdown (see Section 4.4.3) (reviewed in references 78 and 133). These metabolic roles for BCAAs can only be sustained by diets that provide them at levels exceeding the requirements for BCAAs as substrates for protein synthesis, which is their primordial metabolic destiny.78 Of note, the BCAAs are the only amino acids not degraded in the liver, so that dietary intake directly impacts plasma levels and concentrations in peripheral tissues. BCAAs account for 15-25% of the total protein intake, with whey protein and dairy products being particularly rich sources.

Major concerns about using higher-protein diets, particularly those rich in animal products, are an increased risk of renal failure and the association of cholesterol and, especially, saturated fatty acids with cardiovascular disease. There is little evidence for adverse effects of high-protein diets on renal function in individuals without established renal disease,134 although it is obvious that caution should be exerted in the case of susceptible groups. Likewise, it appears that moderately high protein diets are not harmful to cardiovascular health and may indeed be beneficial.135-137 In any case, although recent evidence supports potential benefits, rigorous longer-term studies are needed to investigate the safety and effects of high-protein diets on weight loss and weight maintenance.

It is important to emphasize that a high-protein diet does not necessarily mean a very low carbohydrate, high-fat Atkins diet (the latter diets having ~10% of the energy as carbohydrate and ~60% as fat). Various studies have found greater fat losses with diets consisting, for instance, of 25-30% protein, 40-45% carbohydrate, 30% fat compared with diets close to the usually recommended 15% protein, 60% carbohydrate, 25% fat macronutrient balance (see reference 46).

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