The scenario plays out time and again: last year’s clothes fit more snugly and the number on the scale reads higher. The mindless munching, countless nights out drinking and partying, long hours at a job that takes away from time to work out, winter weight gain (did you know the average person gains a little over one pound between September and February?), and forgotten New Year’s resolutions (for most, the winter weight gain is never lost) have taken their toll.
Whether due to a drop in metabolism or inattention to nutrition and physical activity, with each passing year, most people gain weight.
Weight Gain Over the Years in Numbers
The worldwide epidemic of obesity has spared few cultures, though U.S adults have been particularly hard hit. While the current rates of childhood obesity and overweight are alarming — 17 percent obese and 30 percent overweight or obese — the adult rates are startling. 35 percent of U.S. adults obese and over 67 percent overweight or obese. Do the math, and you can see that there are a whole lot of normal weight children and teens becoming overweight adults. The obesity rates don't seem to be decreasing, either.
The data plays out the unfortunate reality that weight gain throughout adulthood is the norm.
The National Health and Nutrition Examination Survey follow-up study found that among adults 25 to 44 years old, men gained about 3.4 percent of their body weight each 10-year interval; women gained 5.2 percent per 10 years.
That means that the typical normal weight six-foot and 170-pound, 25-year-old man would likely end up an overweight 182-pound, 45-year-old. The average 120-pound, 25-year-old woman can expect to weigh about 133 pounds by the time she is 45 years old.
Some research suggests that susceptibility to permanent weight gain seems to be highest during adolescence, pregnancy, and midlife for women and the period after marriage for men. For most, the weight gain doesn’t end in middle age.
A new study released by the CDC shows that adults over 60 years old were more likely to be obese than younger adults. That may be due to a decades-long slow and steady "energy imbalance."
The concept of energy imbalance is easy: Eat more calories than you burn and you will gain weight. Eat fewer and you will lose weight. With 3500 calories in a pound of fat, it doesn’t take much to put on a pound or two per year. It could be as little as a two extra sodas every month or a few too many neglected 20-minute evening walks.
Metabolism’s Role in Age-Related Weight Gain
It turns out that for most people, age-related weight gain is due in large part to a dramatic decrease in calories burned. While lower levels of physical activity play a large role in the decreased energy expenditure, an age-related decline in metabolic rate is also to blame.
A study evaluating total energy expenditure (TEE) – the sum of calories burned from the basal metabolic rate (metabolism), the energy required to digest and absorb food, and physical activity – confirmed what most people already know: energy expenditure decreases with age.
Basal metabolic rate, which accounts for about 50 to 70 percent of TEE, is thought to decrease about one to two percent per decade. That is, after a person reaches 20 years old, daily energy expenditure decreases about 150 calories per decade. The decline is probably due to decreased muscle mass (which is highly metabolically-active) and increased fat mass (which is relatively metabolically-inactive).
Some studies have also found that, even when controlling for fat-free mass, basal metabolic rate is five percent lower in older compared with younger adults. It is not clear why, but some researchers speculate that it may be due to an unavoidable loss of very metabolically-active organ tissue, or a decreased metabolic rate within muscle tissues. Decline seems to be most rapid after 40 years old in men and 50 years old in women.
In sum, the number of calories burned per day decreases with age. This reality is widely accepted and is even built in to formulas that estimate resting energy expenditure. The age-related decline in energy expenditure is largely due to decreased metabolic rate — which results from decreased lean mass and increased fat mass — and decreased levels of physical activity.
While a small decrease in daily energy expenditure is probably inevitable, with a committed fitness program, "aging" adults (anyone over 20 years old) can avoid sizeable decreases in metabolic rate.
The Key to Fighting Age-Related Weight Gain and a Declining Metabolic Rate
Incorporate these elements a committed fitness program:
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- Flegal KM, Carroll MD, Kit BK, Ogden CL. Prevalence of Obesity and Trends in the Distribution of Body Mass Index Among US Adults, 1999-2010. Jama. Jan 20 2012.
- Williamson DF. Descriptive epidemiology of body weight and weight change in U.S. adults. Ann Intern Med. Oct 1 1993;119(7 Pt 2):646-649.
- Roberts SB, Dallal GE. Energy requirements and aging. Public Health Nutr. Oct 2005;8(7A):1028-1036.
By Natalie Digate Muth
Natalie Digate Muth, MD, MPH, RDNatalie Digate Muth, MD, MPH, RD is the Senior Nutrition Consultant for the American Council on Exercise, a community pediatrician, registered dietitian, mom, and author of “’Eat Your Vegetables!’ and Other Mistakes Parents Make: Redefining How to Raise Healthy Eaters.”