We often blame middle-aged weight gain to slowing metabolism. Current belief holds that basal metabolic rate continues to decline through the adulthood. A recent study published in Science [1] involving 6,421 subjects (64% female) aged 8 days to 95 years challenged the view.

What Is Metabolism?

Metabolism: the combination of all chemical processes that occur within a living organism in order to sustain life, including, conversion of energy in food to energy for cellular processes; conversion of food to building blocks of proteins, lipids, nucleic acids, and some carbohydrates; and the elimination of wastes.

Basal metabolic rate (BMR) / Resting metabolic rate (RMR): the minimum energy required to carry out these chemical processes while an organism is fasting and at rest.

Total energy expenditure (TEE): a combination of BMR (50-70%), energy required for food digestion (10-15%), and energy expended in physical activities (20-30%).

BMR only accounts for 50-70% of the TEE, while physical activity (PA) can account for 20-30%, and the thermic effect of food (TEF) – the energy required for digesting food – accounts for 10-15% (Figure 1).

What are the Findings from the Study?

1. Human metabolism peaked at 0.7 year old and declined thereafter (Fig. 2).
2. TEE, BMR, and fat-free mass were stable from ages 20 to 60. BMR started to decline at age 46.5 and TEE started to decline at age 63 (Fig. 3).
3. At about 60 years old, TEE, BMR, fat-free mass and fat mass started to decline. Size-adjusted TEE and BMR declined by 0.7% per year (Fig. 2).
4. TEE and BMR increased with fat-free mass in a power-law manner (Fig. 4)

What are the Take-Home Messages?

Age-related metabolic changes are real and may be different from the continuous model believed previously. Contrary to the common belief that metabolism slows down after 25-30 years old and contributes to weight gain, the study demonstrated that BMR only started to decline after 45 years old, whereas TEE only started to decline after 60 years old. We might not have the metabolism to blame for the middle-age weight gain. It is more likely caused by the reduction of physical activities.

It should be noted though that variations from person to person were considerable even when controlling for fat-free mass, fat mass, sex, and age.

The power-law relationship of TEE and fat-free mass also shed light on how we can increase the metabolism regardless of age and sex. Getting adequate rest, eating a healthy and balanced diet, and maintaining an active lifestyle through both aerobic activities and muscle-building activities help to achieve and maintain a healthy metabolic balance.

References

[1] Pontzer, H., Yamada, Y., Sagayama, H., Ainslie, P.N., Andersen, L.F., Anderson, L.J., Arab, L., Baddou, I., Bedu-Addo, K., Blaak, E.E., et al. (2021). Daily energy expenditure through the human life course. Science 373, 808–812.