We identified 156 incident Parkinson’s disease cases during 6 years of follow-up (2000–2006) in the HPFS and 402 cases over 24 years of follow-up (1982–2006) in the NHS. Participants with fewer bowel movements were more likely to use laxatives regularly (≥1 time/week), had a lower body mass index, and consumed larger amounts of caffeine and less alcohol and lactose than those with a bowel movement daily or more often (), but no differences were observed with respect to the other covariates.
Baseline Characteristics According to Bowel Movement Frequency in the Health Professionals Follow-up Study (2000) and the Nurses’ Health Study (1982)a
In the HPFS, we found a significant association between fewer bowel movements and higher risk of developing Parkinson’s disease in the next 6 years of follow-up (). The multivariate relative risks were 0.75 for those with >1 bowel movement/day, 3.08 for those with bowel movements every 2 days, and 4.98 (95% confidence interval (CI): 2.59, 9.57) (Ptrend < 0.0001) for those with bowel movements every 3 days or less in a comparison with men who had daily bowel movements. In the NHS, the results appeared to depend on the years of follow-up under investigation. When the analysis was limited to the first 6 years of follow-up, we identified an association that was similar to what we found in men. The multivariate-adjusted relative risks as compared with daily bowel movements were 0.74 for >1 per day, 1.42 for every 2 days, and 2.15 for every 3 days or less (95% CI: 0.76, 6.10) (Ptrend = 0.10). When we pooled the 2 cohorts together, the combined relative risks for Parkinson’s disease during the first 6 years of follow-up were 0.75 for >1 per day, 1 (referent) for daily, 2.62 for every 2 days, and 3.93 for every 3 days or less (95% CI: 2.26, 6.84) (Ptrend < 0.0001) (). The association remained in both men and women after excluding participants who reported use of laxatives ≥1 time/week: The multivariate relative risk comparing a bowel movement every 3 days or less with daily was 4.35 (95% CI: 1.80, 10.5) (Ptrend < 0.0001) in men and 2.98 (95% CI: 1.09, 8.14) (Ptrend = 0.03) in women. Similar results were observed when we included only Parkinson’s disease cases confirmed by a neurologist: The multivariate-adjusted relative risk for a bowel movement every 3 days or less versus daily was 5.10 (95% CI: 2.63, 9.88) (Ptrend < 0.0001) in men and 3.07 (95% CI: 1.10, 8.5) (Ptrend = 0.08) in women. Further adjustment for consumption of fruits and vegetables, supplements of iron or calcium, physical activity, or use of drugs that may result in constipation (i.e., aspirin, diuretics, H-2 blockers, methyldopa, and valium or other minor tranquilizers) did not materially change the results observed (Ptrend = 0.10 in women and < 0.0001 in men).
Relative Risk of Developing Parkinson’s Disease According to Frequency of Bowel Movement in the Health Professionals Follow-up Study (2000–2006) and the Nurses’ Health Study (1982–2006)
Figure 1. Pooled relative risk (RR) of Parkinson’s disease onset during the first 6 years of follow-up in the Health Professionals Follow-up Study (2000–2006) and the Nurses’ Health Study (1982–1988). Adjusted for age (in months), (more ...)
In the NHS, however, we did not find a significant association between baseline bowel movement frequency and risk of Parkinson’s disease onset beyond the 6 years of follow-up (). The multivariate-adjusted relative risks comparing bowel movements every 3 days or less with daily were 1.25 (95% CI: 0.61, 2.59) for Parkinson’s disease onset during years 7–12, 0.54 (95% CI: 0.25, 1.19) for years 13–18, and 0.88 (95% CI: 0.43, 1.79) for years 19–24. Excluding regular users of laxatives did not materially change the results.
We did not find a significant interaction between age (years), smoking status (never vs. ever), use of hormone replacement therapy (never vs. ever, in women only), and caffeine intake (high vs. low, based on median value) and bowel movement frequency in relation to Parkinson’s disease risk (Pinteraction > 0.2 for all).