Runners who finished heavier than they started had lower sodium
Of 488 runners tested at the 2002 Boston marathon finish, 13 in 100 were at or below 135 millimoles of sodium per litre. Finishing heavier raised the odds four times.
What the study found
The runners who finished heaviest finished lowest on sodium
Blood sodium falls during a marathon because of water, not lost salt. Of 488 runners tested at the 2002 Boston finish, 13 in 100 were at or below 135 millimoles per litre, and those finishing heavier than they started had four times the odds. What they drank did not matter, only how much.
The caveat · One race on one temperate day
One race, one temperate day in 2002, and a blood sample at the finish line rather than an experiment. Only 488 of the 766 runners who signed up gave a usable sample, so the 13 percent could sit either side of the truth. What people drank was reconstructed from a questionnaire they filled in after four hours of running.