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Short-term studies in humans report loss of muscle mass and strength and reduced bone mineral density.[20] This is to be expected as part of the weight loss that accompanies CR. Beyond using lean tissue as an energy source, the presence of catabolic hormones, such as cortisol, and the lack of anabolic ones, such as insulin, disrupts protein synthesis, amino acid uptake, and immune response. People who lose weight as a result of CR but who are sedentary have a reduced capacity to perform exercise compared with those who lost similar amounts of weight from exercise alone,[21] emphasizing the need for strength training in CR practitioners. A study of long-term CR practitioners "who had been eating a CR diet (approximately 35% less calories than controls) for an average of 6.8 ± 5.2 years (mean age 52.7 ± 10.3 years)" found that they had reduced bone mineral density at the level of hip and spine, in accordance with a previous one-year weight-loss trial,[22] but that after initial weight loss they had achieved a stable, normal level of bone turnover and that the microarchitectural structure of their bones was healthy; the researchers concluded that "These findings suggest that markedly reduced BMD is not associated with significantly reduced bone quality in middle-aged men and women practicing long-term calorie restriction with adequate nutrition."[23] Some specialists say that minor mineral losses can be minimized with regular physical activity and vitamin D and calcium supplements.[24] Similarly, despite acute reductions in muscle mass at onset, CR retards the age-related loss of muscle structure and function (sarcopenia) in nonhuman primates[25][26] and rodents;[27][28] however, no longitudinal data are available on this subject in humans. The authors of a 2007 review of the CR literature warned that "[i]t is possible that even moderate calorie restriction may be harmful in specific patient populations, such as lean persons who have minimal amounts of body fat."[29] |
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