Associations between active commuting, body fat, and body mass index: population based, cross sectional study in the United Kingdom
BMJ 2014; 349 doi: https://doi.org/10.1136/bmj.g4887 (Published 19 August 2014) Cite this as: BMJ 2014;349:g4887
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Dear Sir/Madam,
Flint and colleagues' large cross-sectional, observational study provides persuasive evidence of a positive association between self-reported active commuting modes and objectively assessed markers of obesity. As the authors stress, however, the presence or direction of a causal relationship cannot necessarily be assumed, and further longitudinal or quasi-experimental studies are suggested. No tests of the effects of resource allocation or health promotion on commuting behaviours are reported in the current study.
The authors' first stated objective, therefore, which is "to determine if promotion of active modes of travel is an effective strategy for obesity prevention (using the above results)..." does not therefore appear to be directly addressed by the study design. Future studies will undoubtedly add further to our understanding of this crucial question.
Competing interests: No competing interests
The authors' conclusions that men and women who commuted to work by active or public modes of transport had lower BMI and %body fat compared to people who commuted to work using private transport raises the tantalising idea that encouraging people to find active ways to get to work may improve body composition. The authors acknowledge a major limitation of cross-sectional epidemiology studies that we cannot determine causality i.e. whether active commuting decreases BMI or whether people with a healthy BMI are more likely to be active commuters. This is highlighted with the paradoxical association between skimmed milk use and a higher BMI mentioned by the authors, which may be due to overweight or obese individuals buying skimmed milk because it's lower fat but still consuming excess daily calories from other sources.
The authors argue that the lower %body fat between active (1.35% CI 0.29 to 2.41%) or public transport (1.48% CI 0.32 to 2.65%) compared to the private transport commuters in the adjusted analysis was clinically significant. However, bioelectrical impedance devices typically have day-to-day reliability of 2-4%. Therefore, arguably the differences in %body fat observed in the active or public transport commuters are well within the bounds of technical/biological variability of BIA measurements and hence of little clinical significance. Nevertheless, if a group of private transport commuters were recruited and randomised into a group with subsidies to promote active commuting versus a group with subsidies continue commuting using private transport for six months perhaps the changes in body composition observed would exceed the 2-4% test-retest reliability of BIA.
Competing interests: No competing interests
Dear Sir/Ma'am
Thank you for publishing such an intriguing piece (Flint E, et al. BMJ 2014;349:g4887). It is incredible in this post-modern age of an ever expanding increase in disease burden, diagnostic challenges, prognostic indicators and peculiar rule out safety nets ruling in the possibility of the ridiculously rare, instead of factoring in the extremely common, in every day practice that a plethora of studies exist stating, re-stating and stating again common sense observations, i.e. that if your commute means you expend more energy, than you surely must possess a lower BMI.
I quote from the conclusion
"This study suggests that the incorporation of greater levels of physical activity into the daily commute independently predicts lower bodyweight and healthier body composition for both men and women. Effect sizes and significance levels were similar for both active modes (walking and cycling) and public transport. The promotion and facilitation of greater use of public transportation, in addition to walking and cycling, should therefore be considered. Given that most commuters in the UK use private transport as their main mode, there are potentially large population-level health gains to be made by shifting to more active modes of travel"
It is tragic, in post-modern Britain that an academic paper needs to be composed to qualify an opinion expounded for millennia that the more active you are during your day, e.g. getting to and from work, that you are going to be anatomically, biochemically and physiologically healthier. This is not just obvious, been thoroughly validated elsewhere, umpteen number of times, but I find it incredible that a journal possessing the stature of the mighty, life changing and influential British Medical Journal (BMJ) publishes this.
We do not exist in the early 1900s during earlier days of medicine, when such observations would have been applauded as peculiarities worthy of publication but in 2014, and considering the plethora of significant diseases that warrant tackling, and subsequent publishing to stimulate rigorous intellectual and clinical debate to tackle some of the most difficult subjects in medicine, such a piece relegates the BMJ to a jocular scrap of paper that most doctors use to get a bonfire going.
This, is no fault of the authors as the piece is interesting but it is surprising to read it in the BMJ. Or, perhaps, it reflects and endemic change in quality of the BMJ as one has noticed, increasingly, that the majority of the print BMJ is dominated by advertisements for jobs, instead of research as it used to be. It is tragic that doctors are increasingly becoming dependent on numbers to validate obvious observations. One wonders who, and indeed what, is driving this.
Competing interests: No competing interests
The NICE guidelines recognise that a multifocal approach is required whereby a combination of exercise promotion and dietary advice is crucial in the prevention of obesity. Within its literature, the National Institute for Health and Care Excellence (NICE) cite ‘energy balance’ as key to maintaining a healthy weight with energy intake via food not exceeding an individual’s required energy expenditure (REE) through daily activities and exercise (1). The study here highlights the importance of exercise and that the incorporation of even mild physical activity into a person’s daily routine promotes a healthier lifestyle with improved BMI and body fat content. This in turn confers long-term health benefits to the individual. A number of methods exist to calculate the REE of an individual. The revised Harris-Benedict Equation is one such method and can provide an estimate of a person’s Basal Metabolic Rate utilising their height, weight and age. It can then be adjusted according to a person’s activity levels during the week to provide an approximated daily calorie intake (2). The current UK Government guidelines state that ‘within a healthy balanced diet, a man needs approximately 2,500kcal per day to maintain his weight while a woman requires around 2,000kcal per day’ (3). However, when applied to two individuals (male and female) of average size and age, the Harris-Benedict Equation indicates that this guidance may in fact recommend excessive calorie intake in the population and consequently could actually promote gradual weight gain rather than weight stabilisation.
For example, for a 30 year old male measuring 177.8cm (5’10’’) and weighing 75kg (BMI 23.7), the Harris-Benedict Equation calculates that with little or no exercise every week, an intake of 2131 calories per day would be sufficient to maintain his weight. If the individual were to undertake light exercise (1-3 days per week), calorie intake should be increased to 2442 calories and when undertaking moderate exercise (3-5 days per week), calorie intake should be further increased to 2752 calories. Similarly, a 30 year old female measuring 162.56cm (5’4’’) and weighting 65kg (BMI 24.5) would require 1707 calories per day to maintain her weight. This would increase to 1956 calories if undertaking light exercise and 2205 calories with moderate exercise.
These figures consequently illustrate that there may be a need to reconsider the current UK guidelines on daily calorie intake with specific relation to weekly physical activity levels. As the study here illustrates, strategies to encourage the incorporation of physical activity into an adult’s daily lifestyle are crucial in improving physical health and preventing obesity. However, it is the belief of the author that such a strategy needs to be promoted in tandem with a reconsideration of the current public health guidelines relating to recommended daily calorie intake. For the current UK guidelines on calorie intake to be applicable, an average person would need to undertake 3-5 days of moderate exercise per week to maintain a stable weight. As a large proportion of the UK population are relatively sedentary, these guidelines do not apply and it can therefore be taken that if adhering to the advice provided, the general public will continue to gain weight.
(1) http://www.nice.org.uk/guidance/CG43/chapter/Working-with-people-to-prev...
(2) http://ajcn.nutrition.org/content/40/1/168.full.pdf+html
Competing interests: No competing interests
If more commuters were to walk, cycle or use public transport, then there would be more room on the roads for the rest of us to drive to work. As always, the unintended consequences are difficult to predict.
We have known for 60 years - thanks to research by Morris et al - that bus conductors, who are active, are healthier than bus drivers, who are sedentary, yet that has not stopped most of the rest of us wanting to use our own cars rather than public transport.
And that has not stopped schoolchildren being taken to school by car in preference to walking, cycling or using public transport, which from personal observation over 10 years that trend has continued. The underlying perception is possibly that cars - especially Chelsea tractors - are somehow safer, and that belief needs to be disproven.
Competing interests: No competing interests
Re: Associations between active commuting, body fat, and body mass index: population based, cross sectional study in the United Kingdom
McGregor and Foley draw attention to the design of our study on commuting mode and body composition.[1-3] We concur with their comments that cross-sectional data do not allow causal inference. But previous studies using quasi-experimental study designs have provided indirect evidence that active and public transport uptake is associated with improvements in BMI.[4] However, further research is required in order to unpack causal processes and mechanisms. Longitudinal datasets that offer a combination of relevant exposure variables (mode, frequency, duration of commute); objectively measured health outcomes; and the necessary range of socioeconomic and behavioural covariates, are unfortunately rare. At present, the Understanding Society study does not have repeated objective health outcome measures. In the future, when the same individuals have been revisited for the second wave of Health Assessment data, it will be possible to investigate whether a change in commuting mode (for example a switch from private transport to public or active transport) between the two time-points may independently predict a change in BMI or percentage body fat. In addition to utilising panel studies, opportunities to exploit natural experiments (for example, the introduction of new public transport schemes) should be taken, as these also provide exciting avenues for disentangling cause and effect.
Regarding the point about differences in percentage body fat being within the bounds of normal variability of BIA measurements; we take the view that it is important to distinguish between individual level and population level effects. At the individual level, a body fat change of 2-4% may be within the margin of technical/biological variability. However, across a large sample of individuals, we would expect some body fat measurements to be higher than the ‘true’ value and some to be lower. This variability is expected to be cancelled out when looking at mean effects across groups containing large numbers of individuals.
Dr Ellen Flint
Department of Social and Environmental Health Research, London School of Hygiene and Tropical Medicine, London WC1H 9SH, UK
Professor Steven Cummins
Department of Social and Environmental Health Research, London School of Hygiene and Tropical Medicine, London WC1H 9SH, UK
Professor Amanda Sacker
ESRC International Centre for Lifecourse Studies in Society and Health, Research Department of Epidemiology and Public Health, University College London, London WC1E 6BT, UK
REFERENCES
1 McGregor, RA. BMJ 2014;349:g4887
2 Foley, P. BMJ 2014;349:g4887
3 Flint E, Cummins S, Sacker A. Associations between active commuting, body fat, and body mass index: population based, cross sectional study in the United Kingdom. BMJ2014;349:g4887.
4 Webb, Netuveli, Millet. Free bus passes, use of public transport and obesity among older people in England. JECH, 2011;66:176-180.
Competing interests: No competing interests