A comprehensive study funded by the Agricultural Research Service might allay any lingering debate about why metabolism slows as people age. The new findings show that the gradual loss of body cells, especially those high-energy-consuming muscle cells, can help explain why older people burn fewer calories while at rest–which so often leads to creeping weight gain.
Some scientists have suggested that changes in hormones, immune function or other factors may depress resting metabolism with aging. But this study–a statistical analysis conducted by researchers at the Human Nutrition Research Center on Aging at Tufts University in Boston–showed a direct association between metabolic rate and cell mass, also known as lean- or fat-free mass.
And that’s good news. It means that older people may regain some of their youthful resting metabolic rate by regular muscle-building exercises. Increasing muscle mass would help seniors get off the slow boat to obesity, according to the researchers.
For their analysis, they used measurements of body composition and resting metabolic rate for 131 healthy men and women taken over the last five years at the Boston center. The subjects ranged in age from 18 to 87, giving the researchers a broad sample for detecting small changes in cell mass across the years.
But they found that only one of the six methods used to measure the subjects’ body composition exposed the decline in cell mass occurring with age. That was a high-tech method for measuring the body’s total potassium–a mineral found almost entirely inside of cells.
Related studies at the center further support an age-related loss of cell mass. Researchers found a definite decline in muscle mass of middle-aged and senior men and women over 10- and 12-year periods when they measured the subjects’ leg muscles by computerized tomography–or CAT scan. The shrinkage of muscle tissue explained at least half of the subjects’ loss of strength in those muscles.
ARS is the U.S. Department of Agriculture’s chief research arm.