ESSAY- Keep pedaling: How exercise helps your weight, life

As a nation, Americans getting fatter and fatter. Yet we seem ever more confused about how to lose weight. We're particularly fuzzy on the question of how big a role exercise plays, or whether we just have to count calories.

So here's the deal. Yes, you can count calories or weigh yourself every day. If your weight is up today compared with yesterday, you probably ate more calories than you burned. If it's less, you burned more than you ate– provided you didn't drink gallons of liquid the day before, throwing the scale off.

It comes down to simple arithmetic, and you've heard it before: Calories in, calories out. You will absolutely, inevitably gain weight if you eat more calories than you expend in basic metabolism– breathing, digesting, sleeping, etc.– plus whatever else you do, such as chasing the kids around, walking, vacuuming, or going to the gym.

Except that most of us can't– or won't– do the math, probably because it's so depressing.

We routinely overestimate the number of calories we spend in physical activity, and underestimate the calories from food. For instance, when I swim hard for an hour, which I do regularly, I probably use up 400 to 600 calories. But when I eat a blueberry muffin, which I'm afraid I also do regularly, I take in nearly 400 calories. So, I have to swim pretty fast for 40 minutes just to offset one lousy muffin. It's not fair! I swim for health (and fun). But if my only goal were weight loss, it would be easier to just not eat that muffin.

"The problem is people's inability to know how many calories they burn and eat,'' says Dr. George Blackburn, associate director of the Division of Nutrition at Harvard Medical School. "If you put a person in a metabolic chamber, where you know exactly what they eat and what they burn, the calories in, calories out idea is always reconfirmed.''

So, if it takes an awful lot of exercise to make a dent in the calories in-out equation, is exercise pointless? No way. It's essential for good health. Regular physical exercise reduces the risk of early death, coronary heart disease, stroke, high blood pressure, type 2 diabetes, colon and breast cancer, and depression, according to the 2008 Physical Activity Guidelines for Americans.

Moreover, even if exercise doesn't help much in the battle to lose weight, it is essential to maintain weight loss, says Dr. Timothy Church, director of Preventive Medicine Research at the Pennington Biomedical Research Center in Baton Rouge.

"This whole thing is not rocket science,'' he says. "You can take weight off through a whole variety of strategies. But people don't lose weight and keep it off unless they are physically active. There are tons and tons of studies on this.''

Among them is a series of studies by researchers at the University of Pittsburgh who showed last year that it takes a considerable amount of exercise– expending 2,000 calories, which requires four or more hours of exercise– per week to maintain a 10 percent weight loss, even on a low-calorie diet.

If your goal is weight loss, as opposed to overall health, does it matter what you eat? No. And yes. And it goes without saying that any diet should involve lots of fruits and veggies, whole grains, and reasonable– not gigantic– portions.

But it still comes down to calories. In February of 2009, a two-year study of more than 800 overweight adults showed that people can lose weight if they reduce calories, regardless of the percentages of fat, protein, and carbohydrates in their diets. The study, by researchers from the Harvard School of Public Health, the Pennington Biomedical Research Center, and the National Institutes of Health, was published in the New England Journal of Medicine.

Some foods are more "addictive'' than others because they have a bigger effect on the brain chemicals that control the "reward'' circuits in our brains. From a neurobiological point of view, sweets, fats, and salty foods make us want to eat more of the same, as Dr. David S. Kessler, the former head of the US Food and Drug Administration, makes clear in his book The End of Overeating, which was released in paperback last month. Obviously, eating too much leads to weight gain.

Overall, says Blackburn, the body can't store calories from protein as amino acids, so it either makes protein from them or converts them to carbohydrates. Excess dietary carbohydrate is first stored as glycogen, but if the body already has enough of that, excess carbs are stored as body fat. Excess dietary fat also gets stored as body fat.

And what about the question of whether exercise increases or decreases appetite?

Exercise can suppress appetite, says Blackburn, because it triggers not only the chemical dopamine, which governs the brain's reward system, but also endorphins, those feel-good brain chemicals. These substances act on the hunger and satiety areas of the brains for as long as four hours afterward. "You don't need cigarettes, or drugs, or food, all those things in the pleasure areas of the brain, because exercise has already activated them,'' says Blackburn.

A review article in 2007 from researchers at Tufts University also concluded that there is a "spontaneous reduction in hunger associated with participation in exercise.''

Psychologically, as opposed to biochemically, some experts theorize that exercise might lead people to believe they can reward themselves with treats afterward or that they may be tempted to be less active for the rest of the day. And some studies, says exercise physiologist William J. Evans of the University of Arkansas for Medical Sciences, suggest that if you exercise, say, for 40 minutes a day, you will "then compensate by decreasing how active you are at other times of the day, leaving total energy expenditure unchanged'' or that you might reward yourself with extra food. Then again, other studies say both of those theories are wrong.

One factor that matters without question, in terms of controlling food intake, is how fast you eat. "It takes about 20 minutes for food to get digested and formulated into hormones for your brain to know what you did, to get that signal to the brain,'' says Blackburn. In other words: if you wolf down your food, you'll finish your second helping before your brain has registered your first.

As for the perennial question of how much exercise you need, the federal guidelines say adults "gain substantial health benefits'' from getting two and a half hours a week of moderate intensity aerobic activity, or 1 hour and 15 minutes of vigorous physical activity.

Moderate activity means walking briskly, water aerobics, ballroom dancing, and even gardening. Vigorous activity means racewalking, jogging or running, swimming laps, jumping rope, or hiking uphill. The guidelines also recommend weight training at least two days a week.

Boiled down, my personal mantra is this: You have to do both– diet to keep caloric intake under control, and exercise for fitness and fun.


Judy Foreman, a former staffer at the Boston Globe, writes a biweekly column about health for the Globe, which is where this essay first appeared.