Running generally burns more fat and calories than swimming, because running is more apt to build fat- and calorie-burning muscle, your heart beats faster when you run and exercising in air increases your metabolism for a much longer period than exercising in water. Swimming, however, causes far fewer injuries than running, so it can be a better long-term weight-loss exercise.
Your body stores most of your energy in carbohydrates and fats. When you run, swim or do any other kind of exercise, you burn calories from your stored carbohydrates and fats. You lose 1 pound when you burn 3,500 calories. Less active people burn a higher percentage of fat than carbohydrates, and more active people burn a higher percentage of carbohydrates than fat, according to "The Complete Guide to Walking." However, more active people burn more fat and more carbohydrate calories. "Burning one or the other does not appear to make for better long-term weight loss," wrote Mark Fenton, author of "The Complete Guide to Walking."
Building your muscles in water is more difficult than building your muscles out of water because water "restricts the type of movements the muscles make," according to "Choosing the Right Exercise," a report in The Merck Manuals Online Medical Library. Muscles burn more calories, including calories from fat, than body fat. Men burn fat and calories more easily than women because the average man has about twice as much muscle mass as that of the average woman who weighs the same as he does, according to the textbook "An Invitation to Health."
You burn more calories, including calories from fat, when you are exercising more intensely. Your heart rate measures your intensity. When you are under water, your heart rate is slower than it is on land, according to "An Invitation to Health" author Dianne Hales. Calories-burned studies reflect this disparity. The most intense running exercise burns 1,553 calories per hour in a 190-pound person, but the most intense swimming exercise burns 949 calories per hour in the same person, according to a study by the state of Wisconsin.
Running helps you burn calories during exercise and for up to 18 hours afterward. Swimming only helps you burn calories during exercise. This is true because air increases your body temperature and water doesn't, according to the "Choosing the Right Exercise" report. A higher body temperature causes you to burn more fat and calories after you're done exercising. Your body temperature remains higher than normal for 18 hours. Consequently, you burn more calories standing in line, for example, after you have finished running than after you have finished swimming.
Walking, swimming and pedaling a stationary bicycle are the exercises that cause the fewest injuries, according to The Merck Manual of Medical Information. Running, on the other hand, causes several times more injuries than swimming, and more than 60 percent of people who begin exercising drop out within six weeks because of an injury.