One of the most frustrating things about getting older is when the pounds pile up along with the years. Keeping weight off can be a challenge, even when you aren't eating more or exercising less.
But don't be discouraged. Aging may be inevitable, but getting fatter need not be.
Obesity affects about 40 percent of American adults, including the young and middle aged, an estimated 93.3 million adults, according to the Centers for Disease Control and Prevention.
It is a serious public health problem, raising the risk of Type 2 diabetes, heart disease, stroke and certain cancers.
One big reason we gain weight as we get older is because we gradually lose muscle mass, about 1 percent every year.
This causes a decrease in our basal metabolic rate, that is, the process of burning calories while we are at rest. The lower the metabolic rate, the fewer calories we burn.
"It may be imperceptible year to year, but compare the amount of muscle mass with the average 80-year-old to the average 20-year-old and it becomes more apparent," says Hensrud, also medical director of the Mayo Clinic's Healthy Living Program.
"The greater the amount of muscle mass we have, the greater our resting metabolic rate."
Also, spontaneous physical activity - separate from exercise - often ebbs with age, he says.
"In general, the average 80-year-old will move less in small and big ways throughout the day compared to the average 20-year-old," Hensrud says.
"And exercise, separate from daily activity, probably declines, although that only affects in a large way the smaller proportion of people who exercise regularly."
"This will also impact metabolic rate."
Changes in hormones - declines in testosterone in men and estrogen and progesterone in women - also can affect weight. But it's a false assumption that postmenopausal women gain more weight than men, Hensrud says.
Rather, both sexes gain, but weight tends to redistribute in women more quickly than in men, often ending up in the abdomen - one reason for this misperception.
"Weight gain seems to affect men and women similarly," he says, typically about a pound or more annually, often between Thanksgiving and New Year's.
"This doesn't seem like much, but on a population-wide basis it adds up to quite a bit," he says.
"It is cumulative. It stays on. So, after 20 or 30 years, it adds up. During menopause, weight gain [in both sexes] is about the same. But [in women] weight shifts more toward the abdominal region, so it appears to be greater weight gain. The same thing happens in men - greater weight gain with age in the abdominal region - but it occurs more gradually."
Researchers studied the fat cells in 54 men and women during a 13-year span, and all of them showed declines in their rate of lipid turnover.
The results indicate that processes in the fat tissue "regulate changes in body weight during aging in a way that is independent of other factors," says Peter Arner.
Experts suggest people keep exercising regularly, monitor calories, lift weights and move throughout the day, avoiding sitting as much as possible.
The good news is that weight gain seems to stabilize after the mid-60s, in part because people often eat less when they get older, Hensrud says.
By Marlene Cimons