Metabolism Declines in Late Life
The research noted here is one of many different views into the decline in cell and tissue activity that takes place in old age. One can look at the way in which cancer rates decline with age after peaking in the 60s and 70s, for example. Or the phenomenon of diminished protein synthesis in old tissues. Or reduced calorie intake in older people. Many of the manifestations of age are reactions to underlying causes. A general slowdown in cell activity has the look of something that depends upon environment, given the various studies showing that many types of cell taken from old individuals can still perform to youthful levels if given a youthful environment.
To come up with a number for total daily energy expenditure, researchers relied on the "doubly labeled water" method. It's a urine test that involves having a person drink water in which the hydrogen and oxygen in the water molecules have been replaced with naturally occurring "heavy" forms, and then measuring how quickly they're flushed out. Scientists have used the technique to measure energy expenditure in humans since the 1980s, but studies have been limited in size and scope due to cost. So multiple labs decided to share their data and gather their measurements in a single database, to see if they could tease out truths that weren't revealed or were only hinted at in previous work.
Energy needs shoot up during the first 12 months of life, such that by their first birthday, a one-year-old burns calories 50% faster for their body size than an adult. "Something is happening inside a baby's cells to make them more active, and we don't know what those processes are yet." After this initial surge in infancy, the data show that metabolism slows by about 3% each year until we reach our 20s, when it levels off into a new normal.
Midlife was another surprise. Perhaps you've been told that it's all downhill after 30 when it comes to your weight. But while several factors could explain the thickening waistlines that often emerge during our prime working years, the findings suggest that a changing metabolism isn't one of them. In fact, the researchers discovered that energy expenditures during these middle decades - our 20s, 30s, 40s and 50s - were the most stable.
The data suggest that our metabolisms don't really start to decline again until after age 60. The slowdown is gradual, only 0.7% a year. But a person in their 90s needs 26% fewer calories each day than someone in midlife. Lost muscle mass as we get older may be partly to blame, since muscle burns more calories than fat. But it's not the whole picture. "We controlled for muscle mass. It's because their cells are slowing down."