It is true that exercise is beneficial even if you don't lose weight. However, being physically fit while remaining overweight leaves you with health risks that you just don't need. If you can lose weight as well as swim a mile or bike regularly, your chances of avoiding a heart attack or stroke improve.
Many people find that exercise alone doesn't lead to weight loss. In fact, working out leads to increases in muscle mass; this may actually increase the reading on the scale. In addition, exercising can make you ravenously hungry, so you eat enough to replace the calories you have burned, and then some. The disappointing surprise for many people who exercise 30 minutes or more every day is that they don't automatically shed the fat around their waistlines. They are both fit and fat.
Three-F people (fit fat folks) like to believe that, even though they are still carrying around excess fat, they have improved their health outlook — and they definitely have. Exercise makes your blood vessels healthier by making them dilate to carry blood to your muscles. The ability of your arteries to enlarge when needed to ramp up blood flow is what physical fitness is about. When you are “in shape,” your muscles may or may not be stronger, but your arteries are able to deliver blood flow to them more reliably. When your arteries are “in shape,” they are more limber, and you are less prone to high blood pressure, and less likely to get atherosclerosis forming from damage to the blood-vessel walls. The result: a lower risk of heart attacks and strokes.
Physical fitness reduces the risk of dying from cardiovascular disease by about half among men with "metabolic syndrome" — the combination of being overweight, having high blood pressure, high triglycerides, low HDL cholesterol, and a tendency toward diabetes. But that is only part of the story. The rest is that their health outlook would be even better if they would lose body fat, too. No matter how fit you are, being overweight still means having a higher risk of dying and getting heart disease. In other words, if you take two people who lead very sedentary lifestyles (classic couch potatoes), and one is fat and the other thin, the thin one has a greater chance of living longer. Similarly, if you take two people who swim a mile a day at the same speed, one fat and the other thin, the thin person still has a better outlook.
The likely reason is that fat is not just a layer of insulation to keep the body warm. It has a variety of effects that increase your tendency toward diabetes and high blood pressure, which ultimately increase your risk of a whole range of medical problems.
The bottom line is that the sum of the parts is greater than the whole. Working out is one key ingredient of a long and healthy life. Eating right, including cutting the calories and staying away from the bad fats, is another. Put them together and you optimize your chances of having the healthiest and longest life possible.
Thomas H. Lee, M.D. is the chief executive officer for Partners Community HealthCare Inc. He is a professor of medicine at Harvard Medical School. He is an internist and cardiologist at Brigham and Women's Hospital. Dr. Lee is the chairman of the Cardiovascular Measurement Assessment Panel of the National Committee for Quality Assurance.