Although few researchers doubt swimming's ability to improve aerobic fitness, some question how efficient it is at burning fat. A 1989 study of 31 sedentary, overweight adults compared running with swimming and found that swimmers lost less body fat than joggers. Each person exercised five days a week, burning 500 calories per day -- enough to lose one pound per week, said Randall L. Smith, an author of the study and assistant professor of physical therapy at the University of Missouri in Columbia.
The runners lost an average of 2.4 percent body fat at the end of 10 weeks, compared with 2.2 percent for the swimmers. Although each person's skill level was the same, Smith speculated that the participants may not have been exercising at the same level of intensity. "Elite swimmers burn a lot of calories, working harder at a higher intensity," he said. "You have to push yourself more in swimming and realize that just floating around in the pool will not do what you think it will."
Another study of eight swimmers, eight joggers and six competitive triatheletes -- who perform three sports, typically competitive swimming, bicycling and running in rapid succession -- examined how efficiently the body burns calories during exercise.
The results showed that swimmers burned more carbohydrates than fat for energy than the joggers. When examining the triathletes, the researchers found that their bodies responded differently depending on the exercise. When they swam, their responses equaled those of the swimmers -- they burned more carbohydrates than fat. After running, their metabolic responses resembled those of the joggers -- they burned more fat than carbohydrates.
"We speculated that something about swimming causes an adjustment in the choice of how energy is burned," said Michael G. Flynn, an author of the study and assistant professor of exercise science at the University of Toledo.
Despite the limitations of the studies, some fitness experts recommend jogging over swimming for people who want to decrease body fat.