Age-defying ways to rev the engine. Understanding our metabolism

Rachel Johnson, Ph.D, M.P.H., R.D.
August 19, 2009

Age-defying ways to rev the engine. Understanding our metabolism

Over a cup of tea recently, a 40-something friend confided that she'd had a glimpse of the future and she didn't like it. She had decided to weigh herself that morning, something she hadn't done in a while. When she stepped on the scale, the needle climbed to a point where it hadn't gone since her pregnancy nine years before. "What's happening to me?" she lamented. "My metabolism must be starting its middle-age slowdown."

Metabolism, a greatly misunderstood process of the human body, takes the brunt of many a middle-age whine. People conclude that a slower metabolism is an inevitable part of aging and beyond their control. The truth, however, is more reassuring. Our bodies do change as we age, and metabolism can take a dive as a result, but we hold the key to avert this decline.

Metabolism, the process by which our bodies burn calories (food energy), has three components: resting metabolic rate, the thermic effect of food, and physical activity. Resting metabolic rate (RMR) is the energy we use at rest to perform basic body functions like breathing and sleeping. In most people, this accounts for about 60 to 70 percent of their total daily energy expenditure (about 1,450 calories a day for a 140-pound woman). Because muscle is the body's metabolically active tissue, RMR is almost totally determined by the amount of lean body (muscle) mass a person has. For the most part, we all have the same metabolism per amount of lean body mass. Most women have more body fat in proportion to muscle mass than men, and thus women generally have metabolic rates that are 5 to 10 percent lower than men of the same height and weight. Unfair as it may be, that means most men use up more calories just sitting on the couch than the women sitting next to them do.

The RMR of most people goes down by 2 to 3 percent with each decade once we reach our thirties, a direct result of the loss of muscle mass that often accompanies aging. Luckily, we can prevent this loss with regular strength-training exercises, which are designed to build or preserve muscle.

The thermic effect of food (TEF) is the energy we use to burn calories or, more explicitly, to digest, absorb and metabolize our food. When you eat a 110-calorie snack, for example, 10 of those calories are used for TEF. It is a relatively small portion of our total metabolism: about 10 percent, or 240 daily calories, for a 140-pound woman.

Our greatest control over metabolism lies with physical activity. It's also the most easily thwarted, living as we do in a world of drive-through banks, escalators, leaf blowers and the omnipresent computer. Unless you are one of the rare people whose job requires you to be moving throughout the day, you probably need to work deliberately at increasing your physical-activity level. The less time you have for exercise, the more vigorously you should move. I can jog 21/2 miles in 30 minutes or I can burn the same number of calories on a leisurely hourlong walk. I frequently wear a step-counter to monitor my goal of 10,000 steps a day, the equivalent of 5 miles. After untold hours in front of my computer, if I don't spend at least 45 minutes running or in an exercise class, I don't come anywhere close to my goal. If I exercise enough I can indulge my love of good food and savor a scrumptious dessert or great glass of wine several times a week without adding pounds.

Even fidgeting, which comes naturally to some people, can increase energy expenditure above resting levels by 300 to 600 calories per day. My oldest son is one of those people who seem to be blessed with "thin" genes. But after being around him for more than two decades I think I have a good idea what's going on: he's a fidgeter. He is constantly tapping his foot and shifting in his seat. (When he was little it seemed we were always pleading with him to sit still at the dinner table.) Compared to sitting still, browsing in a store takes twice the energy, while a slow walk (2 to 3 mph) can triple energy expenditure.

I have no doubt my friend will get her weight back to where she'd like it. She may have to invest in some free weights and a little more time running, but that's all under her control. We certainly can't stop the years ticking by, but keeping our metabolism youthful and burning calories at a healthful rate is well within our grasp.


Calories Burned in Action (over 1 hour)

(not counting the 77 calories burned at rest)

Chewing Gum 11

Fidgeting 70

Walking 1 mph 119

Walking 2 mph 158

Walking 3 mph 228