A Low Metabolic Rate Will Predispose You To Obesity

Continuing with citations from my article in Obesity Reviews on an aeteological framework for assessing obesity, we now turn to the importance of metabolic rate:

Any assessment of obesity should begin with an estimate of energy requirement – specifically recognizing that any decrease in metabolic rate, without a corresponding decrease in energy intake and/or increase in activity will result in weight gain. Thus, in anyone presenting with weight gain, without any notable change in energy intake or activity levels, it is safe to assume that the only explanation can be a reduction in energy metabolism.

As a rule of thumb: the lower the total energy requirements, the greater the risk of obesity (simply stated: over‐eating is less likely for someone who needs 4000 kcal d−1 than for someone who needs 1500 kcal d−1). In sedentary individuals, resting metabolic rate is responsible for dissipating the vast majority of daily ingested calories (60–75%) and is therefore a key determinant of energy expenditure. Thus, even a small, sustained percentage reduction in resting metabolic rate, without a compensatory adjustment of energy intake or activity, can account for a large cumulative caloric excess over time (e.g. an unbalanced 3% reduction in resting metabolic rate in an individual with a total energy expenditure of 1800 calories can lead to a caloric excess of 32.4 kcal d−1, which can translate into 972 kcal excess per month).

Numerous factors can determine and/or affect metabolic rate. These include genetic and epigenetic factors, gender, aging, neuroendocrine function, sarcopenia, metabolically active fat, certain medications and prior weight loss.

Commentary: of course the numeric relationship between caloric intake and weight gain is not as straightforward as many people may think. This is because changes in caloric balance will in turn change caloric expenditure – remember, we are dealing here with physiology, not physics! Thus, a 20 kcal daily excess will only lead to weight gain until the higher body weight uses up the extra 20 kcal to maintain itself, at which point the 20 kcal are no longer in excess of demands and a new caloric balance is found (weight-gain plateau – the reverse happens with caloric restriction). Thus, to continue gaining weight, one has to continue increasing caloric intake to ensure that they stay above actual requirements. This self-limiting nature on the effect of a change in caloric intake (increase or decrease) on weight gain is often forgotten when people make simplistic assumptions that small increases in caloric intake have large effects on body weight over time – they don’t! Nevertheless, the lower your caloric requirements, the greater your risk of eating too many calories.

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