The traditional strategy for losing weight is to increase physical activity and to decrease energy intake. There is undeniable evidence in the scientific literature that such a strategy causes weight loss. However, confusion between weight loss and fat loss is common amongst both scientists and laypersons. Weight loss is easy to measure through use of a balance, but body mass reductions do not give an accurate representation of the beneficial progress of a weight loss strategy unless the fat loss is also calculated. Muscle loss can be extreme during energy restriction diets, and can account for up to 50 % of the weight lost. This is one reason that energy restriction should not be used as a strategy to improve body composition. Such muscle loss is damaging to the metabolic rate and can significantly increase the risk of future increases in body fat. However, bone loss is also substantial during energy restriction, and this can cause long term problems including increasing the future risk of bone disorders such as osteoporosis.
Obesity increases the risk of a number of diseases including cardiovascular disease, cancer and type 2 diabetes. However, the traditional strategy of reversing obesity causes general weight loss, and it has been estimated that a 10 % reduction in weight results in a 1 to 2 % bone loss at the hips, and total body bone loss can be as high as 3 to 4 % if upper body losses are included. This rate of bone loss is double the rate observed in weight stable individuals, and may increase the risk of degenerative bone diseases such as osteoporosis. In one study, individuals who have previously lost weight had a prevalence of osteoporosis of 31 %, whereas those who had remained weight stable had a prevalence of only 4 %. It is unclear if the rate of weight loss effects the rate of bone loss, but evidence suggest that increasing rates of muscle loss occur with increasingly restrictive energy intakes, and therefore lean tissue loss accelerates as energy intakes fall further away from the normal energy requirement of the individual.
A weight loss of 5 % has been reported to increase the risk of bone loss and increase the risk of bone fractures in both men and women. Cycles of dieting increase the risk of fracture and osteoporosis further. While the bone loss has been reported mainly in older individuals, there is no physiological reason to assume this does not occur in younger individuals. The main difference may be that physical activity is protective of bone loss and as such younger more active persons may be at a lower risk if they perform some degree of weight bearing physical activity. The mechanisms by which bone loss occurs during weight loss may include hormonal changes. For example weight loss may decrease circulating levels of sex hormones and at the same time increase the sex-hormone binding globulin in the blood, and this may reduce osteoblastic activity. Energy restriction also brings reductions in mineral intake, particularly calcium, which can hinder the ability to remodel bone.
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