Physical activity and the incidence of multiple sclerosis

Abstract

Objective: To study whether physical activity during adulthood or early life is associated with multiple sclerosis (MS) incidence in 2 prospective cohorts of women.

Methods: Women in the Nurses' Health Study (NHS) (n = 81,723; 1986-2004) and NHS II (n = 111,804; 1989-2009) reported recent physical activity at baseline and in selected follow-up questionnaires. Using this information, we calculated total metabolic equivalent hours of physical activity per week, a measure of energy expenditure. There were 341 confirmed MS cases with first symptoms after baseline. Participants also reported early-life activity. To estimate relative rates (RRs) and 95% confidence intervals (CIs), we used Cox proportional hazards models, adjusting for age, latitude of residence at age 15, ethnicity, smoking, supplemental vitamin D, and body mass index at age 18.

Results: Compared with women in the lowest baseline physical activity quartile, women in the highest quartile had a 27% reduced rate of MS (RR_pooled = 0.73, 95% CI 0.55-0.98; p-trend 0.08); this trend was not present in 6-year lagged analyses. Change in physical activity analyses suggested that women reduced activity before onset of MS symptoms. In NHS and NHS II, higher strenuous activity at ages 18-22 years was weakly associated with a decreased MS rate. However, in NHS II, total early-life activity at ages 12-22 was not associated with MS.

Conclusions: Though higher physical activity at baseline was weakly associated with lower MS risk, this may have been due to women reducing physical activity in response to subclinical MS.

Figure 1. Relative rates and 95% confidence intervals for diagnosis of multiple sclerosis (MS) by early-life activity

(A) Early-life strenuous activity and rate of MS. Months per year engaged in strenuous aerobic physical activity at least twice per week. Follow-up 1986–2004 in Nurses' Health Study (NHS); 1989–2009 in NHS II. Ages 14–17 (squares): 445 NHS II cases; p-trend 0.14. Ages 18–22 (triangles): 575 cases (130 NHS, 445 NHS II); p-trend 0.01, p-het 0.44. (B) NHS II early-life activity and rate of MS. Follow-up 1997–2009; 195 cases. Categories of metabolic equivalent (MET)–h/wk of total activity at the following ages: 12–13 (squares; p-trend 0.81), 14–17 (triangles; p-trend 0.45), 18–22 (diamonds; p-trend 0.73). Analyses stratified by 5-year age groups and time intervals and further adjusted for latitude at age 15 (north, middle, south), ethnicity (Southern European, Scandinavian, other Caucasian, nonwhite), pack-years of smoking (0, <10, 10–24, ≥25), supplemental vitamin D intake (none, <400, ≥400 IU/d), and body mass index at age 18 (<18.5, 18.5–<21, 21–<23, ≥25 kg/m²).

Figure 2. Relative physical activity by time of multiple sclerosis (MS) symptoms or diagnosis

(A) Relative physical activity by time of MS symptoms. p-Trend overall: 0.0002. p-Trend up to and including the year of first symptoms: 0.64. p-Trend after first symptoms: 0.06. (B) Relative physical activity by time of MS diagnosis. p-Trend overall: 0.0003. p-Trend up to and including the year became a case: 0.35. p-Trend after becoming a case: 0.56. Adjusted for age (age, age²), latitude at age 15 (north, middle, south), ethnicity (Southern European, Scandinavian, other Caucasian, nonwhite), pack-years of smoking (0, <10, 10–24, ≥25), cumulative average supplemental vitamin D intake (none, <400, ≥400 IU/d), and body mass index at age 18 (<18.5, 18.5–<21, 21–<23, ≥25 kg/m²).