Longevity Briefs: Suffering From ‘Middle Age Spread’? Your Metabolism Might Not Be To Blame

Longevity briefs provides a short summary of novel research in biology, medicine, or biotechnology that caught the attention of our researchers in Oxford, due to its potential to improve our health, wellbeing, and longevity.

Why is this research important: We know that with age, total daily energy expenditure declines, meaning that fewer calories need to be consumed in order to avoid a calorie surplus. Most of the energy we expend each day (60-70%) is the result of our resting metabolic rate – the baseline level of energy expenditure required to maintain bodily functions at rest. There is a common conception that the tendency to gain weight during middle age is caused in part by a decline in resting metabolic rate, which is not matched by a sufficient reduction in calorie intake.

What did the researchers do: In this study, researchers studied the energy expenditure of 6,421 people, aged 8 days to 95 years, across 29 countries. Total energy expenditure was measured using a doubly labelled water technique, which involves subjects drinking water containing isotopes of oxygen and hydrogen, and measuring the presence of these isotopes in the urine. The disparity between the presence of these two isotopes is related to the rate of carbon dioxide production (and hence calories used). Resting metabolic rate was measured separately in 2008 people using indirect calorimetry, which calculates energy expenditure by measuring carbon dioxide production or oxygen consumption.

Key takeaway(s) from this research: Energy expenditure was unsurprisingly higher in people with greater lean mass. After adjusting for this, researchers found that while both total and resting average energy expenditure vary greatly throughout childhood and adolescence, they appear to remain quite stable during midlife, and only begin to decline around the age of 60.

This seems to suggest that a slowing down of the intrinsic metabolic rate might not be responsible for the tendency to gain weight in one’s 30s and 40s – dietary changes may instead play more of a role. However, it should be noted that while the basal and total metabolic rates may remain stable on average throughout midlife, there is still opportunity for variation amongst individuals, and so it is still possible for someone to experience weight gain in middle age due to a reduction in physical activity and hence total energy expenditure. Furthermore, a loss of lean mass will certainly result in a reduction in absolute resting energy expenditure, which may then lead to a surplus of calories if eating habits are not adjusted to compensate.

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