The Salt Manufacturers' Association said a study of salt loss among members of a professional cycling team showed that cyclists could lose up to 3g of salt in a 90-minute training session in mild conditions (15°C).
An earlier study indicated that professional footballers could lose up to 10g of salt in 90 minutes of training.
However, the SMA said levels of salt loss among the cyclists would have been more marked in hotter weather and were a cause for concern in light of the Food Standard Agency's campaign to cut average per capita salt consumption to 6g a day. A spokesman said: "Endurance athletes are more susceptible to a life-threatening condition called hyponatraemia, when vital sodium is flushed from the body through drinking too much water."
Professor Ron Maughan, who led the trial involving the UK's women's cycling team, said one in six people were particularly "salty sweaters", who lost a lot of sodium through sweating.
"Some riders in these trials lost twice as much sodium as others. The government's blanket advice is not appropriate and could indeed be harmful."