Leave the Car at Home: Active Commuting Lowers BMI, Body Fat

Pam Harrison

August 20, 2014

Yet another study is supporting the public-health benefits of actively commuting to work, including lower body mass index (BMI) and body-fat composition, rather than taking the car, UK research suggests.

An earlier study reported by Medscape Medical News showed that walking or cycling to work as well as using public transport lowered the risk of both diabetes and hypertension compared with using private transport, usually the car (Am J Prev Med. 2013;45:282-288).

"We used the same data set as the authors from the earlier study, but they used self-reported diagnosed diabetes and hypertension as their outcomes, and we used nurse-measured BMI and percentage of body fat," Ellen Flint, PhD, a research fellow in social epidemiology at the London School of Hygiene & Tropical Medicine, London, United Kingdom, told Medscape Medical News.

"And because the predominant mode of transportation in Britain is the car, if we can affect a large modal shift away from private transport toward public or active modes of transport, there really is great potential to reap large population health benefits on overweight and obesity," she said.

The study was published online August 19 in the BMJ.

In an accompanying editorial, Anthony Laverty, research associate, and Christopher Millett, PhD, reader in public health, Imperial College London, United Kingdom, say that "the most interesting and perhaps important" finding of this study was the reduction in body fat seen in association with commuting to work using public transport.

"This benefit is likely to accrue because the use of public transport generally involves walking and occasionally cycling to transport access points or interchanges, thus increasing incidental physical activity," they state. The study also highlights the importance of the commute to work as an opportunity to increase population levels of physical activity, they add.

The critical need for active travel to achieve desired health and environmental outcomes is increasingly recognized by national and international policy makers.

"Healthcare professionals are additionally well placed to advise patients to 'leave your car at home,' " the authors note. "This will not only improve their patients' health in the short term but also help reduce the likelihood of hazardous climate change further in the future."

Key Finding: Use of Public Transport as Good as Cycling, Walking

The investigators used data from wave 2 of the Health Assessment subsample of Understanding Society, the UK Household Longitudinal Study (UKHLS). The samples consisted of 7534 subjects who were assessed for BMI and 7424 subjects who were assessed for percentage of body fat, which was measured by electrical impedance.

The mean age for men in the sample was 44, and for women it was 43. Close to three-quarters of the sample commuted by private transport—predominantly by car. Of the subjects, 10% of men and 11% of women reported using public transport as their main way of getting to work. Roughly equal numbers of men, at 14%, and women, at 17%, walked or cycled to work.

Overall, the mean BMI for men was 28 kg/m2, while for women it was 27 kg/m2. Mean body fat percentage was 23% for men and 36% for women.

In fully adjusted models, men who used public transit or who walked or cycled to work had BMI scores 1.10 and 0.97 points lower, respectively, than men who used private transport. Women in turn who used public transport had BMI scores 0.72 points lower than those who used private transport, while women who walked or cycled to work had BMI scores 0.87 points lower than those who took the car.

Results for percentage body fat were similar in terms of magnitude, significance, and direction of effects, the authors point out.

The "key finding" that the effects on BMI and body fat for public transport were actually very similar in size and significance to those for walking or cycling to work is really important, the researchers say.

"Participants were not asked how much walking their public transport involved," Dr. Flint noted. "But the assumption we made is that people who use public transport as their main mode of getting to work also do some walking, so they engage in incidental activity as part of that journey, and that's activity you don't get when you get in your car and drive to the office."

A recent study (Int J Behav Nutr Phys Act. 2014;11:37) also carried out in the United Kingdom found that total weekday physical activity was 45% higher in participants who walked to work compared with those who commuted by car.

Clinically Meaningful Effects Akin to Those From Diet, Exercise

"The magnitude of effects observed in this study were clinically meaningful," the authors note.

For an average man—43 years old, 176 cm tall (~5'9"), weight 86 kg (~189 lb), and a BMI of 27.8 kg/m2—the difference in BMI would equate to a difference in weight of about 3 kg (~7 lbs). For an average women—43 years of age, 163 cm tall (~5'3"), weight 72.8 kg (~160 lb), BMI 27.4 kg/m2—the difference in BMI of around 0.7 kg/m2 would correspond to a difference in weight of about 2.5 kg (5.5 lbs).

"These differences are larger than the effect sizes seen in most individually focused interventions based on diet and physical activity to prevent overweight and obesity," they write.

"The promotion and facilitation of greater use of public transportation, in addition to walking and cycling, should therefore be considered."

Asked what physicians might do to encourage patients to leave their car at home, Dr. Flint suggested that the more clinicians are aware of the benefits active commuting can have, "the more mindful they can be when discussing lifestyle changes in general with patients and bring this aspect into the discussion and ask how patients might make this change in their lifestyle as well."

Dr. Flint is supported by a UK National Institute of Health Research fellowship. Disclosures for the coauthors are listed in the article. The editorialists reported no relevant financial relationships.

BMJ. Published online August 19, 2014. Abstract, Editorial

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