We found a decreased risk of developing MS with higher walking speed, jogging and high levels of LTPA in both men and women whereas walking volume and light physical activity were not associated with MS development. Our data indicated that the risk of developing MS may be reduced as much as 35–50% in subjects who are more physically active.
MS is highly prevalent worldwide25
and MS incidence is considered epidemic in nature.13
In the present study population of 21–98-year-old Danish men and women living in Copenhagen in 1994, the average MS prevalence was 21% in women and 27% in men. In a similar Swedish study MS prevalence was 19% in women and 24% in men.6
Even higher prevalences have been reported,7
but direct comparisons between studies are obviously difficult due to differences in study population characteristics and choice of MS definitions and diagnostic criteria. Cumulated MS incidence during the 10-year follow-up was 15.4%, ranging from 19.4% in the sedentary group to 11.8% in the moderately to highly active group. In a meta-analysis by Ford and Li,13
incidences of 10.4–17.5% were reported with follow-up time ranging from 4 to 23 years. Thus MS constitutes an increasing problem with important economic and public health implications.
Our results confirm the association between physical activity and MS risk found in earlier cross-sectional studies.3
However, several longitudinal studies have failed to demonstrate an association between physical activity and MS incidence, suggesting that a low LTPA level could be the result of MS, rather than its cause.9
In a population of 862, 50-year-old men free of diabetes and MS it was found that BMI and self-reported LTPA were independent predictors of insulin resistance at 20-year follow-up, indicating that obesity and physical inactivity could increase insulin resistance by independent pathways.9
However, only BMI—and not LTPA—was a significant predictor of MS at follow-up. Correspondingly, a study of 714 White, Black and Hispanic men and women without MS at baseline followed for an average of 5.2 years found that self-reported physical activity at baseline did not predict MS status at follow-up.17
In the CCHS, Berentzen et al27
demonstrated that in 4808 men and women aged 21–81 years, LTPA did not have a significant effect on changes in waist circumference—a key component in the MS—during a 10-year follow-up period. In contrast to this, a 15-year follow-up study of 4192 young (18–30 years of age) adult men (45%) and women (55%) showed that physical activity was inversely associated with MS risk.28
However, after multivariable adjustment the association was only significant when comparing the highest and lowest physical activity groups (OR 0.65). This finding suggests that there is a threshold below which physical activity does not confer protection against MS. In concordance with this, our longitudinal analyses showed that subjects who were physically active 2–4 h/week but who did not engage in physical activity with more intensity (fast walking, jogging) were not protected against the development of the MS. Interestingly, this finding also seems to apply to high-risk populations. A recent study including 486 middle-aged men and women at high risk of developing type 2 diabetes found that increases in moderate-to-vigorous LTPA reduced MS risk and could even lead to its resolution in individuals already diagnosed with the MS.18
Changes in low-intensity LTPA did not affect MS development or resolution.
Our analyses did not indicate any evidence of a beneficial effect of increasing walking volume whereas walking speed and jogging both reduced MS risk after adjusting for volume of physical activity (LTPA), thus supporting the perception that intensity plays a key role in MS prevention. In a cohort of 612 middle-aged men with assessment of both LTPA and VO2max
, MS risk was lower at 4-year follow-up in men engaging in >3 h/week of moderate or vigorous LTPA and in men in the upper VO2max
again pointing to an effect of the intensity of physical activity. In a cross-sectional study of 1069 middle-aged men, cardiorespiratory fitness was inversely associated with MS whereas there was no effect of low-intensity activity.29
Other studies have confirmed cardiorespiratory fitness as a predictor of less progression towards the MS7
even in high-risk individuals.10
Similarly, Hassinen et al19
found that high levels of cardiorespiratory fitness decreased the risk of developing MS and could even increase the probability of MS resolution in a population of 1226 men and women aged 57–78 years. In the CCHS, Schnohr et al20
have previously shown that both jogging and walking speed rather than duration were protective of all-cause and CVD mortality. A recent meta-analysis of walking volume and pace on risk of coronary heart disease suggested a more pronounced dose–response protection effect for walking pace than walking volume.21
A similar meta-analysis by Hamer and Chida22
regarding the effect of walking on cardiovascular disease and all-cause mortality found that walking pace was a stronger predictor of overall risk than walking volume. Although the present study did not point towards any association between duration of walking and MS risk, potential health benefits could still ensue from increasing walking volume. Regardless, it seems that increasing walking intensity rather than walking volume would be the sensible strategy in attempt to reduce MS risk.
The primary strength of the present study is the considerable study population size that represents a random sample of the Copenhagen population making our results applicable to the general population. The longitudinal study design enables us to investigate the causal inference between physical activity and the MS. Combining a longitudinal study design with a large study population provides useful information about the relationship between physical activity and MS risk. Weaknesses of our study include the use of self-administered questionnaires to assess LTPA level, which may result in poorer precision compared to more objective measures of physical activity. However, earlier studies have shown that self-administered questionnaires can produce reliable data when estimating habitual physical activity.31
Our data do not include direct fitness level measurements or exact information regarding volume and intensity of physical activity, but it seems plausible that the combined information on jogging, walking speed and physical activity habits yield a fair estimate of volume as well as intensity. The study comprised relatively few joggers and the questionnaires used did not take jogging intensity into consideration. As a result of this we could not draw any conclusions regarding the effect of jogging intensity versus jogging volume on MS risk.
Furthermore, the questionnaires did not provide any information regarding diet or nutrition. Obviously, potential differences in dietary habits between the LTPA groups could have significant impact on the results of the present study. The use of a modified cut-off level for plasma-glucose in consequence of the lack of fasting-plasma glucose measurements is unlikely to have introduced a significant bias towards an overestimation of patients with MS as the cut-off level deliberately was above levels normally seen in individuals with normal glucose metabolism.