Swimming competitively is a highly demanding activity. Olympic record-holding legend Michael Phelps ate 12,000 calories per day when training for competition. How much an individual swimmer needs will vary according to personal metabolism and training regimen.
Baseline Calorie Needs
The USDA recommends that typical men eat between 2,000 and 2,500 calories per day, with a recommendation for women at 1,800 to 2,300. Since swimmers are active and have better muscle tone than most others, the baseline calorie need will be on the high end of or above this scale.
Swimming Calories Burned
A 150-pound swimmer burns about 210 calories in a half-hour of moderately vigorous swimming, or 330 in the same period during a vigorous workout. Lighter swimmers will burn fewer calories, and heavier swimmers will burn more. Competition counts as vigorous swimming but is rarely continued for more than a few minutes at a time.
How Many Calories
To get a ballpark estimate of the calories you need daily, add the total burned in your practices to your daily baseline. For example, if your daily practice consisted of two hours evenly divided between vigorous and moderate training, your daily need would be 2,870 and 3,370 calories for a man, 200 or so less for a woman.
Common Sense Caution
The differences between individual needs varies widely according to many factors. Competitive swimmers should have access to coaches and medical trainers. Defer to them, and to the messages from your own body, when considering your daily diet.