Swimming is also a whole body workout, making use of all four limbs moving at the same time, through dense liquid. Pro swimmers can achieve power physicists estimate at a maximum of 1,200 watts—three times that of a cyclist.

Moreover, the body loses a lot of heat to the surrounding pool and has to expend a greater amount of energy on temperature regulation. There’s also a theory that submerging the body in temperatures below 20ºC (68ºF) triggers hunger, releasing hormones that signal for the body to store more fat for insulation. No wonder, then, that swimmers need and want a lot of fuel.

This can be a problem when swimmers retire. Phelps, now 31, reportedly put on 25 pounds after the 2008 Beijing Games, and says his calorie intake has dropped considerably, partly because of lifestyle and training changes after rehab for alcohol abuse, but also because, sadly, he no longer has the body and metabolism of a 23-year-old.

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