The number of calories you burn in a triathlon depend on a number of factors including the length of the triathlon swim, cycle and run sections. Additionally, your calories burned will vary according to your weight and environmental factors such as the temperature of the water. In general, the heavier you are, the more calories you will burn completing a triathlon.
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In a sprint-distance triathlon, the swim is 750 m, the cycling portion covers 20 km and the run is 5 km. In pool-based triathlons, the swim may be a reduced distance such as 400 or 500 meters. For a 150-lb person, swimming burns 682 calories per hour. Results from the 2010 Burlington VT sprint triathlon indicate that the swim typically takes between 7.5 and 20 minutes. Therefore, the swim section of a sprint triathlon would burn between 85 and 227 calories.
At a fast cycling speed of 20 miles per hour, completing the 20-km cycle section of a sprint triathlon would take 37.5 minutes. For a 150-lb triathlete, cycling this distance at this speed would burn approximately 682 calories. At a body weight of 120 lb, a triathlete completing the same distance in the same time would burn only 545 calories. Slowing down the cycling pace will generally increase the number of calories burned over the same distance, as the overall time taken on the cycle section will be longer.
Running at a speed of six miles per hour -- equivalent to a 10-minute mile pace -- a 150-lb triathlete would burn 352 calories over the 5 km distance of the sprint triathlon run. At the same body weight, running significantly faster slightly decreases the overall calories burned -- at a speed of 10 miles per hour -- a 6-minute mile -- the same athlete would burn only 345 calories over the 5 km.
Your body weight is the biggest variable determining the number of calories you burn in a triathlon. For example, a 150-lb swimmer will burn 682 calories per hour, but a 90-lb swimmer will burn only 409 calories for every hour of swimming. For a 150-lb athlete completing the swim in 20 minutes, the cycling section in roughly 38 minutes and the 5k run in 31 minutes, the total calories burned in a sprint-distance triathlon would be 1,261. Additional calories would be burned during transitions between the disciplines.