Amyotrophic lateral sclerosis among cross-country skiers in Sweden.

• A highly increased risk of amyotrophic lateral sclerosis (ALS) has been suggested among professional athletes. We aimed to examine whether long distance cross-country skiers have also a higher risk of ALS and whether the increased risk was modified by skiing performance. We followed 212,246 cross-country skiers in the Swedish Vasaloppet cohort and a random selection of 508,176 general Swedes not participating in the Vasaloppet during 1989-2010. The associations between cross-country skiing as well as skiing performance (i.e., type of race, finishing time and number of races) and the consequent risk of ALS were estimated through hazard ratios (HRs) derived from Cox model. During the study, 39 cases of ALS were ascertained among the skiers. The fastest skiers (100-150 % of winner time) had more than fourfold risk of ALS (HR 4.31, 95 % confidence interval [CI] 1.78-10.4), as compared to skiers that finished at >180 % of winner time. Skiers who participated >4 races during this period had also a higher risk (HR 3.13, 95 % CI 1.37-7.17) than those participated only one race. When compared to the non-skiers, the fastest skiers still had a higher risk (HR 2.08, 95 % CI 1.12-3.84), as skiers who had >4 races (HR 1.88, 95 % CI 1.05-3.35), but those finishing at >180 % of winner time had a lower risk (HR 0.46, 95 % CI 0.24-0.87). In conclusion, long distance cross-country skiing is associated with a higher risk of ALS, but only among the best skiers; recreational skiers appear to have a largely reduced risk.

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