Physical activity and health
Authors: Lars Bo Andersen
Journal: BMJ (2007)

Evidence that physical activity improves health is convincing,1 but we lack knowledge about how to increase physical activity in individuals and populations. Taking part in sport may improve health, but sport is only taken up by a small proportion of the adult population, and mainly by the better educated. 

In this week's BMJ, a systematic review by Ogilvie and colleagues assesses the effect of interventions to improve walking on how much people walk, physical activity, fitness, disease risk factors, and wellbeing.2 It found that interventions tailored to people's needs, which targeted the most sedentary or those motivated to change, can increase walking by up to 30-60 minutes each week. Few studies included in the review assessed clinical benefits from the increased walking, and this remains to be shown in randomised controlled trials.

So what is the evidence so far on the effects of interventions on other types of physical activity? A recent Cochrane review of randomised controlled trials found that trials promoting physical activity in general significantly increased self reported physical activity (standardised mean increase of 0.31, 95% confidence interval 0.12 to 0.50), and fitness (0.40, 0.0.9 to 0.70).3 The review by Ogilvie and colleagues also included non-randomised studies, which, although considered weaker forms of evidence, are necessary to assess the effect of population level interventions such as bike lanes, walking paths, and recreational areas.

One non-randomised community intervention in Odense, Denmark, promoted bicycling through many initiatives and increased the number of bicycle trips by more than 20% over five years.4 At the same time, the number of accidents involving cyclists was 20% lower than in the rest of the country.

Another study found that children who cycled to school were 8% more fit than children who used other modes of transport including walking.5 It concluded that a 10-15 minute session of cycling twice a day would be enough to increase aerobic fitness in children.5

Observational studies have consistently shown that children who walk or cycle to school engage in more physical activity (other than the travel activity) than those who travel by other means.6 7 This extra activity may reflect selection (children who are generally more active choose active transport) or it may be that children who are encouraged to take up active transport go on to engage in other activities. However, because of the lack of cycle lanes in many countries it may be difficult to promote increased cycling for safety reasons.

A weakness in many of the trials of walking interventions is the lack of assessment of health gains; however, epidemiological studies suggest that health benefits of active transport are substantial. The nurses health study found that women who increased both walking distance and speed had a lower risk of cardiovascular disease, type 2 diabetes , and all cause mortality.8 9 The risk in the upper quintile of walking was around half that seen in the sedentary group. Similarly, another study found a 30% lower mortality rate in participants who cycled to work than in non-cyclists after adjusting for general physical activity level, socioeconomic background, and smoking.10

Ogilvie and colleagues' study shows that interventions can increase the amount of walking. It has not yet been proved that the lower rates of disease and mortality seen in people who walk is caused by walking itself, but even this low intensity type of exercise probably improves metabolic control and other health parameters. The challenge now is to make politicians work for an environment that promotes walking, and to call on doctors to encourage patients to walk, especially those with disorders such as hypertension, metabolic syndrome, or raised fasting insulin.11