The Ability to Burn Body Fat Decreases With Age

Posted by: Kevin Flatt

POST UNDER Aging & Health AND Weight Loss

In a study comparing the fat-burning ability of eight women in their 20s with another eight in their 60s and 70s, the seniors kept pace with their juniors after eating 250- and 500-calorie meals. But they couldn’t match the fat-burning rate of the younger group after a 1,000-calorie meal. Fat oxidation was about 30 percent lower in the older women after the big meal.

As people age, they may be able to reduce their risk of gaining weight by eating smaller, more frequent meals, according to U.S. Department of Agriculture findings reported in the October issue of the American Journal of Clinical Nutrition.

“Dietary fat that doesn’t get burned gets stored as body fat,” said I. Miley Gonzalez, Under Secretary for Research, Education, and Economics. “That’s what happens when older people overindulge.

So the scientists recommend that seniors eat fewer calories at a sitting, but eat more often to ensure getting enough nutrients.”

Study leader Susan Roberts heads energy metabolism research at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston.

Roberts said the study is the first to measure fat oxidation after eating, noting that further studies are needed to confirm the findings.

She and colleagues are conducting the research, funded by USDA’s Agricultural Research Service, to get at underlying causes behind the age-related increase in body fat.

“Body fat typically doubles between the ages of 20 and 50 to 60 years,” she explained. That increase is linked to several diseases, including cardiovascular disease and non-insulin dependent diabetes. Several studies suggest a drop in fat oxidation may play a role, said Roberts.

The women in the study ate two peanut butter and jelly sandwiches plus a large glass of milk to get 1,000 calories, which represents a moderately large meal, Roberts said.

On average, U.S. women in their 60s and 70s consume about 1,400 to 1,500 calories a day, according to the latest USDA survey data. So 1,000 calories is about two-thirds of a day’s calorie intake for this age group. “It simulates going out to a restaurant and having a big meal,” Roberts noted.

She said the 250-calorie meal, representing a snack, and the 500-calorie meal, representing a small meal, were also peanut and jelly sandwiches and milk, but in smaller portions. All the test meals contained 35 percent calories from fat.

Roberts believes the drop in fat-burning ability is due to hormonal changes. The older women had higher levels of glucagon. This hormone triggers the release of sugar into the blood, the opposite effect of insulin.

With more sugar available to fuel body processes, the women burned less fat. “That’s not good,” said Roberts, “because you want to get rid of the fat.”

She cited several factors that could contribute to the drop in fat reduction. Older people have less skeletal muscle, the primary site of fat oxidation. They also are less physically active and thus don’t burn as much fat. In addition, they tend to eat higher-fat diets.

The scientists suggest that older people exercise to increase skeletal muscle and fitness. This may offset the fat-burning deficiency by increasing their capacity to burn fat while resting.

In earlier studies comparing energy metabolism in young and older men, Roberts found that basal, or resting, metabolism doesn’t increase in older men when they overeat as it does in young men. This also helps explain why body fat creeps up with age.


Judy McBride, U.S. Department of Agriculture.

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