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Systematic review and meta-analysis of reduction in all-cause mortality from walking and cycling and shape of dose response relationship

Background and objective: Walking and cycling have shown beneficial effects on population risk of all-cause mortality (ACM). This paper aims to review the evidence and quantify these effects, adjusted for other physical activity (PA).

Data sources: We conducted a systematic review to identify relevant studies. Searches were conducted in November 2013 using the following health databases of publications: Embase (OvidSP); Medline (OvidSP); Web of Knowledge; CINAHL; SCOPUS; SPORTDiscus. We also searched reference lists of relevant texts and reviews.

Study eligibility criteria and participants: Eligible studies were prospective cohort design and reporting walking or cycling exposure and mortality as an outcome. Only cohorts of individuals healthy at baseline were considered eligible.

Study appraisal and synthesis methods: Extracted data included study population and location, sample size, population characteristics (age and sex), follow-up in years, walking or cycling exposure, mortality outcome, and adjustment for other co-variables. We used random-effects meta-analyses to investigate the beneficial effects of regular walking and cycling.

Results: Walking (18 results from 14 studies) and cycling (8 results from 7 studies) were shown to reduce the risk of all-cause mortality, adjusted for other PA. For a standardised dose of 11.25 MET.hours per week (or 675 MET.minutes per week), the reduction in risk for ACM was 11% (95% CI =4 to 17%) for walking and 10% (95% CI =6 to 13%) for cycling. The estimates for walking are based on 280,000 participants and 2.6 million person-years and for cycling they are based on 187,000 individuals and 2.1 million person-years. The shape of the dose-response relationship was modelled through meta-analysis of pooled relative risks within three exposure intervals. The dose-response analysis showed that walking or cycling had the greatest effect on risk for ACM in the first (lowest) exposure interval.

Conclusions and implications: The analysis shows that walking and cycling have population-level health benefits even after adjustment for other PA. Public health approaches would have the biggest impact if they are able to increase walking and cycling levels in the groups that have the lowest levels of these activities.

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Alex’s Notes: Ever since I received my Fitbit I have been aiming to walk five miles per day, which for me is about 10,000 steps. I also try and ride my bicycle for short commutes when weather permits. No doubt this incredibly simple movement is part of an evolutionary ritual, as Carl rightly points out,

“One of our evolutionary gifts is our ability to stand on two legs and ambulate! It was something that we as a species did not do easily… We were slower and less stable…  But we stuck with it and as a result developed an arch in our feet and the geometry of our pelvis changed. Our spine elongated and our rib-cage shortened. The hip muscles rearranged and our gluteus minimums muscles took on a predominant role to hold us erect. Over the course of millions of years we became efficient bi-peds and used this to our advantage.”

But is walking and cycling worth it in the end? This is a question the current meta-analysis sought to answer. But the researchers took things a step farther to ensure robustness to their analysis. They standardized the exposures of walking and cycling to international physical activity guidelines and only considered studies that adjusted for other physical activity. Moreover, only cohorts of healthy populations were included.

The reported levels of walking and cycling were converted into metabolic equivalents (MET) per week as this allowed for consideration of both intensity and duration. If not otherwise stated in the included studies, walking was defined as 4.0 METs (“walking to work or class”) and cycling was defined as 6.8 METs (“to/from work, self-selected pace”). Moreover, if walking was reported as distance, it was assumed 4.0 METs corresponded to a 3.3 mph (5.3km/h) pace, and if walking/cycling were reported as sessions per week, it was assumed a session lasted 30 minutes. Current global recommendations for adults’ physical activity are 150 minutes of moderate-intensity aerobic activity per week, and these recommendations define moderate intensity as 3-6 METs. The researchers thus took the mid-point of 4.5 METs to convert the guidelines to their standard measure of 11.25 METs.

Finally, the Newcastle Ottawa Scale (NOS) was used for quality assessment. It looked at variables such as control for the main variable (age), control for any additional factors (physical activity), sufficient follow-up time (5 years) and percentage of follow-up persons, and also the representativeness of the cohorts as healthy and generalizable to the greater population.

Overall, 14 studies presented a total of 18 results on walking and all-cause mortality, and 7 studies presented a total of 8 results on cycling and all-cause mortality.

Walking

The vast majority of the walking studies came from Western Europe and the USA, with a few from China and Japan. Most the studies also showed a reduced risk of all-cause mortality, although only 5 of the 18 were statistically significant. All the studies were high quality with none scoring less than a 7/9 on the NOS, and the studies totaled 280,000 persons aged 20-93 years (average 56.6 years) and 2.6 million person years.

Cycling

All but one study came from Western Europe (the other was China) with all being of high quality. Additionally, all but one showed a statistically significant or insignificant benefit towards cycling. They contained 187,000 persons with the same age range and average as the walking studies and 2.1 million person years.

Meta-analysis

Meeting the international recommendation of 11.25 METs per week, in the form of walking and cycling as transportation, reduced the risk of all-cause mortality by 11% and 10%, respectively. Sensitivity analyses through changing the average intensities of the activities (e.g. using 3.0 METs instead of 4.0 METs for walking) only made a one percentage point difference in the results, and testing various non-linear dose-response relationships changed the relative risk reduction for walking by 1% and for cycling by 6%, suggesting that the results were very robust. When looking at the dose-response relationship specifically, the greatest rate of risk reduction was found between 1-120 minutes of walking and 1-100 minutes of cycling, with a sharp decline thereafter.

The Takeaway

This is literally the largest walking and first cycling meta-analysis to estimate the effects of these exposures on all-cause mortality after adjustment for other forms of physical activity. Walking or cycling for transportation for 150 minutes per week (just over 20 minutes per day) can reduce the risk of all-cause mortality by 11% and 10%, respectively. However, the benefits are most pronounced for every extra minute up to 120 minutes of walking and 100 minutes of cycling. This isn’t to say more isn’t beneficial, but it provides a strong incentive to at least meet these minimum amounts.

This study is massive, with the estimates for walking based on 280,000 participants and 2.6 million person-years and based on 187,000 individuals and 2.1 million person-years for cycling. All the studies were high quality, and all the studies controlled for other forms of physical activity. That said, the different ways and extents to which the studies assessed other physical activity is a limitation, as well as the fact that for walking there was considerable heterogeneity, representing differences in walking categorization or sample bias. Moreover, the majority of studies came from industrialized nations, and results for other nations where environmental factors such as air pollution might be different. Finally, the researchers made several assumptions that no doubt may have introduced some errors, which would be compounded by the self-report nature of the included studies.

Regardless, this study strongly supports the notion that we were born to walk (or cycle). With all the limitations in mind, it appears that walking and cycling for the sake of getting from A to B will do wonders for your lifespan. So don’t be an active couch potato, move!

By the way, dogs are great for this.

 
 

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