The metabolism myth: why we gain fat as we age
A slowing metabolism is seen as an inevitable part of aging—as natural as day following night. In fact, there is no biological reason why our metabolism should decline on its own over the years. What we lose is muscle mass, and that distinction changes everything.
Why metabolism gets the blame unfairly
In Western society, men start gaining an average of 500 g of fat a year from the age of 35—and continue doing so until roughly their sixties. Women’s annual gain is even greater. Almost everyone blames an aging metabolism.
But the numbers tell a different story. High-school boys have an average body fat percentage of 15%; among men aged 20–30, it is already 25%. For women, the figure rises from 25% during their school years to as much as 35% in adulthood. Doctors say even these averages should not be considered normal—the threshold for being overweight should be 20% for men and 30% for women.
What actually happens as we age
A team of researchers at the University of Minnesota, led by Dr. Ancel Keys, measured the energy requirements of people of different ages. The result was strikingly clear: in both men and women aged 20–60, energy expenditure per unit of fat-free mass was remarkably constant. At rest, everyone burned an average of 1.28 kcal per hour per 1 kg of fat-free mass—regardless of age or sex.
William Evans, Ph.D., and Irwin H. Rosenberg, M.D., confirm the same finding in their book “Biomarkers.” The muscle that remains is just as metabolically active as it has always been. What changes is the amount of muscle, not its “fuel requirement.”
These findings also appear in the exercise physiology textbook “Exercise Physiology” (William D. McArdle, Frank I. Katch and Victor L. Katch, Lea & Febiger, 3rd Edition, 1991) and the book “The Weighting Game” (Lyle Stuart, 1988).
A vicious cycle: less muscle, slower metabolism
Muscle tissue is active—it requires energy even at rest. Fat tissue is a passive store of energy; it simply sits there. As muscle mass declines, daily energy needs fall too. According to Evans and Rosenberg, you would need to reduce your daily calorie intake by about 100 kcal every ten years to maintain your weight.
Most of us don’t do that. We keep eating just as much—some of us even eat more as we age, simply to pass the time. Low activity levels, declining muscle mass and excess calories combine to produce one result: fat accumulation. The cycle feeds itself, getting worse each year unless you take action.
Restoring your metabolism with strength training
The good news is that a slowing metabolism can largely be reversed. Any physical activity burns calories and keeps your metabolism elevated for a while after your workout. A light training load produces an afterburn lasting about an hour. After intense strength training, energy expenditure is still above normal 12 hours later.
The more muscle mass you have, the more calories you burn—both during training and on the couch. That is why strength training is a key part of the equation. Aerobic training burns calories while you do it; strength training builds the machine that burns them all the time.
And age is no barrier. Evans and Rosenberg documented a study in which people aged 87–96 increased their strength and muscle mass by 10% through an eight-week strength training program. As Dr. Lawrence Lamb put it: “Training old muscle is just like training young muscle.” You need to be more careful with the training load, but progress is assured.
Food helps too. Avoid high-calorie foods and choose foods that provide more volume, fewer calories and plenty of dietary fiber—fruit, root vegetables and whole grains. But physical activity remains your main weapon against an aging metabolism.
Frequently asked questions about metabolism
Does metabolism really slow down as you age?
Per unit of body weight, yes—but only because muscle mass declines. Energy expenditure per kilogram of muscle stays almost the same throughout adulthood, regardless of age.
Is strength training suitable after the age of 60?
Yes. Studies show clear gains in strength and muscle mass even in people in their nineties. You need to be more careful with the training load, but you can still make progress.
What matters most if I want to keep my body weight under control?
Physical activity and strength training come first, followed by calorie intake. Without muscle mass, a diet remains a short-term solution.
Translated by Janar Rückenberg
Come work out! ArtGym
