The problem with most weight loss diets is that they do not differentiate between weight loss and body fat loss. To most people weight loss is seen as good, while weight gain is seen as bad. However, gaining skeletal muscle mass increases the body weight, and at the same time may decrease the risk of mortality significantly. Generally more muscle improves the aesthetics of the individual because it provides shape and because it decreases the percentage body fat. Likewise, losing muscle mass decreases body weight but can actually be detrimental to the health because muscle mass determines the resting metabolic rate, and as muscles levels fall, resting metabolic rate falls, and this is reflective of a metabolic dysfunction that can be semi-permanent in nature. All calorie restrictive diets tend to cause weight loss but much of this weight can be skeletal muscle. This reduction in skeletal muscle then increases the likelihood of future weight regain when normal eating patterns resume.
Changes in body weight and the subsequent physiological effects they produce have been studied by researchers. For example, in one study1, researchers investigated the effects of weight changes in healthy individuals who ranged from normal weight to obese (50 % of the subjects were obese). The subjects were followed for between 23.5 and 43.5 months and during this time their body compositions were measured twice using magnetic resonance imaging techniques. This study analysed previous data and the weight loss in 30 of the subjects was caused by a low calorie diet. Of the remaining subjects 33 had gained weight spontaneously and 20 were weight stable. The weight changes for the subjects varied during this time from a decrease of 11.2 kg to an increases of 6.5 kg. The results of the study showed that weight loss was associated with a decrease in fat mass in the subjects. Fat mass losses accounted for 72 % of the weight loss, but 28 % of the weight loss was lean mass.
This supports previous studies that have shown significant reductions in lean mass following weight loss. In fact the authors calculated that weight loss was associated with a 52.5 kcal per day decrease in resting metabolic rate. As well as losing skeletal muscle, the size of the heart and kidneys also decrease with weight loss. Losses in skeletal muscle explained 39.4 % of the reduction in resting metabolic rate and decreases in triiodothyronine (T3) explained 5.3 % of the decrease. However, in those subjects that gained weight 87.9 % of the weight gain was due to increases in fat mass, with the rest being explained by lean tissue. This explains the higher resting metabolic rates of obese subjects who have not undertaken dieting. The increase in resting metabolic rate seen with weight gain was estimated to be 26.3 kcal per day. Losing body fat rather than weight is therefore important in order to maintain the resting metabolic rate as the decreases seen with weight loss are greater than the increases seen with weight gain.
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