We've all heard the mantra: we put on weight as we age because our metabolism slows down. It is the rationale for our universal loss of the battle of the bulge. Yet researchers have been unable to find any significant decrease in resting metabolism in fat-free mass when adjusted for weight. I've written about this previously in Getting Older...Weight Gain. I conclude that what we presume is metabolic slowdown is actually declining total energy expenditure arising from the loss of muscle mass resulting from declining physical activity. Our metabolic rate remains steady mainly in life. We confuse decreasing total energy expenditure with declining metabolic rate.
A new study and position paper, reported in The New York Times and many other sources, corroborate this directly. Using the most accurate techniques available, researchers worldwide studied the metabolic rates of 6,500 people, ranging in age from 8 days to 95 years. They found that the metabolic rates of neonates were roughly 50% higher than that of adults. Beyond the first year, it slowly declines to adult levels by age 20, unaffected by teen growth spurts. Between 20 and 60, the per-pound metabolic rate of fat-free tissue stays approximately constant. Unfortunately, after 60, it declines by roughly 0.7 percent a year. Of course, the studied group was picked to represent the population at large. Nothing is known about how the rate of decline after 60 is affected by fitness or training.
This brings me back to my comments in Getting Older. Declining total energy expenditure is due to loss of muscle mass throughout most of life. Loss of muscle mass is both preventable and reversible. The "dad bod" is not written into genetics or the passing years. What we think is aging is more due to the slow accretion of the self-indulgences we allow ourselves over time. It is a choice.