Calorie counting diets are detrimental to the health and this explains their long term failure rates. Generally calorie restrictive diets are detrimental because while they cause weight loss initially, this weight includes a high amount of skeletal muscle. In fact, during the initial stages of a calorie restrictive diet up to half of the weight lost is skeletal muscle and other lean tissue. The result of this muscle loss is a reduction in the resting metabolic rate, which is detrimental because the resting metabolic rate determines the amount of energy consumed at rest. In an average 70 kg man, the resting metabolic rate accounts for 60 to 70 % of the total energy expenditure (roughly 1500 kcal of a total 2500 kcal energy expenditure). When the resting metabolic rate falls during calorie restriction weight regain is more likely as the amount of food required to maintain the body weight is reduced. Resuming a low quality diet at the previous maintenance calorie intake therefore causes the accumulation of body fat because the underlying cause of the weight gain (over consumption of poor quality foods) has not been addressed.
Calorie counting diets therefore result in cycles of muscle loss and fat gain and a gradual deterioration in body composition. This explains the low resting metabolic rates seen in serial dieters. Research suggest that the reduction in the resting metabolic rate seen with calorie restrictive diets is semi-permanent and can last for decades. The reduction in skeletal muscle is actually caused by the catabolism of skeletal muscle tissue for cellular energy needs. As skeletal muscle is catabolised, resting metabolic rate is reduced through hypothalamic downregulation of thyroid hormones. A drop in the amount of skeletal muscle is therefore mirrored by a fall in the circulating levels of the active form of thyroid hormone (T3) and an increase in a biologically inactive form of thyroid hormone (reverse T3 or rT3). Researchers have measured the effects of calorie restriction on circulating levels of thyroid hormones in healthy humans in order to assess the relationship between energy intake and metabolic rate.
For example in one study1 healthy subjects eating normal mixed diets had their thyroid hormone levels measured before and after a period of calorie restriction. Following the calorie restrictive phase T3 levels fell and rT3 levels increased suggesting that the hypothalamus was downregulating the metabolic rate in order to increase the energy efficiency of the body. Calorie restriction therefore causes a decrease in resting metabolic rate that is concomitant to a reduction in active thyroid hormone production. When the subjects were allowed to eat a normal mixed diet again in a period of refeeding, the T3 levels did not recover to their baseline levels, but only returned to the halfway point between the nadir of the dieting phase and the apogee of the baseline level. The fall in the levels of thyroid hormones were not dependent on the macronutrient composition of the diet, as both a low calorie carbohydrate and protein diet caused the same effect as a low calorie fat and protein diet.
Dr Robert Barrington’s Nutritional Recommendation: These results support other studies that show that calorie restrictive diets have detrimental effects on thyroid hormone levels. The cause of weight gain is not the consumption of too much food, but the consumption of too much low quality food that causes metabolic dysfunction such as insulin and leptin resistance. The way to cause successful long term weight loss is through improvements in diet quality while consuming adequate calories to maintain metabolic rate. A positive nitrogen and energy balance spares muscle tissue and maintains levels of circulating biologically active thyroid hormones, while the higher quality foods reverse the metabolic dysfunction and allow the correct regulation of energy needs.
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