Cycling to Work Could Cut Obesity-Related Mortality

Liam Davenport

April 29, 2019

GLASGOW — Obese individuals who actively commute to and from work, particularly those who cycle, may be able to reduce their risk of all-cause mortality and cardiovascular events versus those who go to work by car, say UK researchers in findings presented at the 26th European Conference on Obesity.

Edward Toke-Bjolgerud, a fifth-year medical student at Glasgow University, and colleagues examined data on more than 160,000 participants in the UK Biobank who commuted to work by car, on foot, or on a bike.

Compared with normal weight people who either cycled or walked to work, obese car commuters had a 32% increased risk of death from any cause and a 59% increased risk of cardiovascular events.

The increased risk of all-cause mortality was no longer present when they looked at obese commuters who either cycled or walked to work, although they still had an 82% increased risk of cardiovascular events.

When the team examined further, they found that the risk of cardiovascular events was far lower among obese individuals who cycled to work than those who commuted by car but was increased in those who walked, suggesting that intensity of exercise may play a role in the degree of benefit.

Protective Effect?

Toke-Bjolgerud underlined that, due to the observational nature of the study, it is not possible to say whether or not the impact of mode of commuting on outcomes in obese individuals is causal.

However, he said that their results suggest that "active commuting could play a role in attenuating the relationship between obesity and all-cause mortality, but achieving improvements in cardiovascular endpoints may require a more vigorous active commuting mode, such as cycling".

He went on to say that the question the study raises is: "What is causing this protective effect?

"Is it consistent, daily, moderate to vigorous exercise or indeed is it the displacement of inactive, at times sedentary, behaviour when commuting by car with exercise," he asked, or a combination of the two?

Toke-Bjolgerud added that, if the relationship is shown to be causal, "is there anything holding back further benefit?

"Specifically, we’re talking about air quality and pollution but also road safety, and these are things which will be particular concerns to people who are maybe taking up active commuting."

Toke-Bjolgerud said: "Maybe if those were addressed, we could see even further benefits."

'Intriguing Possibilities'

Session co-chair, Simon Williams, PhD, who is chair of the Association for the Study of Obesity, told Medscape News UK that it is "an extremely interesting" study that "raises some intriguing possibilities".

He added: "As the presenter suggested, it’s a long way from being conclusive because of the methodological issues within the study, but I think it’s very, very interesting."

More individuals taking up active commuting would require societal changes, however, to create the right circumstances, and Williams said that more work is needed to make sure that there is "good evidence" on which to base those changes.

He noted: "We can see examples in other countries, where the environment has changed to support more physical activity, and some pockets of good examples in this country as well."

Exercise Guidelines

Toke-Bjolgerud began by noting that the UK physical activity guidelines from the Chief Medical Officer recommend 75 minutes of vigorous intensity or 150 minutes of moderate intensity exercise per week, or a combination of the two.

However, figures from the British Heart Foundation published in 2017 suggest 39% of adults in the UK, representing around 20 million people, do not meet those recommendations.

Among the reasons cited by people for not doing more exercise is a lack of time due to other commitments.

Toke-Bjolgerud suggested that one cause of that lack of time could be the time spent on commuting to and from work.

He said that the average daily commute in Britain is estimated at 1 hour and 38 minutes per day, or 8.1 hours per week, which could be spent in either a sedentary or active fashion.

UK Biobank Data

Previous research using UK Biobank data has suggested that active commuting, primarily cycling, halved the risk of all-cause mortality and heart disease versus car commuting.

To investigate further, Toke-Bjolgerud and colleagues set out to determine whether the mode of commuting moderates the relationship between obesity and all-cause mortality and cardiovascular outcomes.

Again looking to the UK Biobank, they gathered information on 163,149 individuals aged 37–73 years at baseline who were in paid employment or self-employed but did not work at home.

They excluded individuals with a condition that would restrict physical activity and those with conditions that would affect either their mortality risk or risk of cardiovascular outcomes.

They divided the participants, of whom 50.8% were women, into normal weight, overweight, and obese categories, based on World Health Organisation definitions.

The researchers then defined the mode of commuting as going to work either by car only, cycling only, walking only, or a mixture of cycling and walking.

Linking the records to healthcare databases, they examined rates of death from any causes and combined fatal and non-fatal cardiovascular events.

Over a mean follow-up of 5.0 years, 2425 individuals died and 7973 developed cardiovascular disease.

Compared with normal weight individuals who reported cycling or walking or both, those who were normal weight but commuted by car had a non-significantly increased risk of all-cause mortality, at a hazard ratio of 1.21, and of cardiovascular events, at a hazard ratio of 1.09.

As expected, obese participants who commuted by car had a significantly increased risk of both all-cause mortality and cardiovascular events versus normal weight active commuters, at hazard ratios of 1.32 (p=0.041) and 1.59 (p<0.0001), respectively.

In contrast, obese individuals who were active commuters had a significantly increased risk of cardiovascular disease versus normal weight active commuters, at a hazard ratio of 1.82 (p<0.0001), but not of all-cause mortality, at a hazard ratio of 1.04.

Cardiovascular mortality was also significantly increased in obese car commuters versus active normal weight commuters, at a hazard ratio of 2.06 (p=0.036), but was there was only a trend for increased risk among obese active commuters (hazard ratio 2.10; p=0.089).

Obese Active Commuters

To explore the apparent disconnect between all-cause mortality and the incidence of cardiovascular events in obese active commuters, the researchers looked at intensity of physical exercise.

They found that obese cyclists had a 44% lower risk of cardiovascular events versus obese car commuters, but obese walkers had a 19% higher risk than obese car commuters.

Intensity of exercise also had an impact when the team defined obesity based on body fat percentage, rather than the traditional body mass index.

This showed that, compared with normal weight cyclists, obese cyclists had a reduced risk of cardiovascular events, at a hazard ratio of 0.54, while obese car commuters had an increased risk, at a hazard ratio of 2.07.

However, versus normal weight walkers, obese walkers had an increased risk of cardiovascular events, at a hazard ratio of 1.52, which was only marginally less than that see in obese car commuters, at a hazard ratio of 1.55.

Toke-Bjolgerud pointed out a number of limitations to their study, the very nature of which means that they cannot prove causality.

For example, there is possible confounding from active commuters potentially engaging in other forms of physical exercise alongside cycling and walking to and from work.

In addition, the commuting mode was self-reported, and could have changed over the course of the study.

He said that the study was nevertheless based on a large cohort he and his colleagues were able to adjust for a wide range of confounders, including, to a certain extent, air pollution.

No conflicts of interest or funding declared.

26th European Conference on Obesity: Abstract IS2.04. Presented 28 April.

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