Common wisdom has always held that our metabolism slows as we age. People often gain weight around middle age; scientists themselves have speculated this is influenced by a decline in our bodies' energy needs, which results in unintentional overeating. However, a new transnational study published in the esteemed journal Science suggests that our metabolism is actually incredibly stable between ages 20 and 60. After 60, the researchers discovered, it begins to consistently decline to a slower metabolism. This reorganization of our metabolic timeline will have huge impacts across medicine, dietary, and exercise science.
Researchers collaborating across 29 countries tested the metabolism of 6,421 people with an age range of 8 days old to 95 years old. To measure metabolism, the researchers used a mixture of heavy hydrogen and oxygen isotopes in water known as doubly labeled water. The isotopes were tracked through ingestion and elimination, giving the researchers the unique opportunity to measure the individual subject's daily energy expenditure. The researchers then normalized their findings by weight to create a pound-for-pound comparison of metabolism at every age.
The results surprised even the experienced scientists working on the experiment. At birth, a baby's metabolism is the same as the mother upon which it recently biologically depended. Their metabolic rate swiftly increases to a lifetime peak around age one. At this time, a baby's metabolism is 50% higher than it will be as an adult. This means the child's cells are burning energy at an exceptional rate.
From age one, the rate declines gently until age 20. Surprisingly, the researchers found no spikes of metabolism during the growth spurts and changes of puberty. Another shocking piece of data concerns the middle life. During adulthood from age 20 to 60, metabolism was incredibly stable. The researchers noted that this implies the bodily thickening associated with middle age is likely not due to metabolism slowing, but probably because of other psychological changes of aging.
After age 60, metabolism does begin to decline. The rate of energy burning decreases by about 0.7% a year. By their 90s, a person needs 26% fewer calories every day versus their needs at middle age. The cells slow their activities and muscle mass is reduced. In short, we begin life with racing metabolisms, spend much of our years stably consuming energy, and finish life by slowing down.
Why is this research important? Well, firstly it helps debunk commonly held ideas about metabolism's role in weight gain during adulthood. Secondly, data on metabolism can be critical to insuring medication is prescribed in the right dosages based on phase of life. Lastly, the data highlights the incredible importance of adequate nutrition in the first year of life, when a child's metabolism is racing to help with all the critical growth of that time period. This data should be used to influence policy on reducing child hunger and malnutrition around the world, as the effects of the first year of life reverberate through the body and mind for years to come.