Many people assume that the only way to lose weight is to undertake a period of forced energy restriction and to perform aerobic style exercise. This is done in the belief that a negative energy balance will cause the fat stores to be used as a source of fuel. However, the failure of such types of regimens to cause weight loss is evidenced both anecdotally and in the scientific literature. The reason for this failure is twofold. Firstly, obesity and abdominal obesity (weight gain around the abdomen) is reflective of an underlying metabolic disorder. This metabolic disorder is caused by eating the low quality foods that typify the Western diet. Such foods metabolically poison the liver and this leads to aberrations in the metabolism of energy. The second reason that low calories diets and exercise do not cause weight loss is because under such conditions of metabolic abnormality, the hypothalamus infers a condition of starvation, and any attempt to regulate energy intake downwards is countered by a severe correction in energy efficiency.
The underlying problem with the energy balance theory of weight loss is that it ignores these regulatory mechanisms that can be evoked by the hypothalamus. Of course the law of thermodynamics must be adhered to in a physiological system such as a human. However, there are ways for the energy balance to be maintained other than exercise and eating. It is these other mechanisms that the hypothalamus can evolve to ensure that weight loss is prevented during times of energy ‘starvation’, as is inferred during obesity. For example following exercise, thermogenesis can be regulated up or downwards to increase or decrease energy efficiency (the thermic effect of exercise). Likewise after a meal the hypothalamus controls energy expenditure through the production of heat (the thermic effect of food). The hypothalamus can also control the degree of physical activity performed and its intensity through the control of motivational behaviour. In this regard there are multiple ways that efficiency of energy use can be controlled centrally.
Brown adipose tissue is of interest to this discourse because it is another avenue by which energy can be derailed from performing actual physical work. Brown adipose tissue, sometimes called BAT, is a special type of fat cell that is brown on account of its high concentration of mitochondria. These mitochondria make BAT highly metabolically active, but this activity does not produce work, only heat. Animals and infants are known to use BAT to maintain body temperature, but the role of BAT in adult humans is controversial and not fully understood. However, there have been suggestions that BAT is involved in controlling energy balance in humans. It is suspected that heat produced by BAT is able to decrease energy efficiency during times of overeating and thus allow maintenance of correct body weight despite fluctuations in the amount of food consumed. If true this suggests that overeating is not necessarily a cause of weight gain as proposed by the energy balance theory of weight gain.
The current explosion in cases of obesity in the Western countries has resulted in research to identify the mechanisms of energy balance. To these ends, brown adipose tissue has been researched in order to investigate its role in weight gain and weight loss. For example, in one such study1, researchers assessed the effects of a single high calorie carbohydrate meal (1622 kcal) on the activity of BAT in healthy lean humans. After consumption of the meal (78 % carbohydrates, 12 % protein and 10 % fat), the subjects had their BAT activity measured. A control period of cold exposure, a known stimulator of BAT, was used as a positive control. The activity of the BAT (as measured by a radiolabeled glucose uptake marker) was significantly higher in BAT compared to subcutaneous (skin) and visceral (abdominal) white fat following the meal. It was also observed that the exposure to control caused a significantly higher increase in BAT compared to the high calorie high carbohydrate meal.
However the authors noted that the BAT response to the meal may have been underestimated because of the high uptake of the radiolabeled glucose to skeletal muscle. This is because the demand for glucose from the skeletal muscles deprives the BAT of this source of energy, and as such the marker does not accurately reflect total BAT activity. Disregarding this nuance however, we see that BAT is active in adults and causes a significant release of heat following a meal. This heat is wasted energy, and so those with more efficient BAT systems can more efficiently leak energy from their systems should they overeat. The actual contribution of BAT to energy balance is not known, but even small increases in BAT illustrate the point that energy balance is far more complex than food intake and aerobic exercise, as claimed by some proponents of the energy balance theory of weight gain. The fact that BAT can waste energy following a meal suggests that the control of body weight is automatic, and not something that needs conscious ‘dieting’ to control.
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