by Lily O’Hara, BSc, Postgrad Dip Hlth Prom, MPH, PhD (c)
In Part 1 of this article I addressed some of the less well known contributors to increased body weight, including the strong role played by genetics, proteins and bacteria in the gut, and infections with bacteria and adenovirus. I also discussed factors such as inadequate sleep duration and quality, chronic work or life stress, and exposure to endocrine disrupting chemicals. Finally I discussed the studies that demonstrate the contribution of dieting and weight control behaviors to weight gain. In part 2 of this article, I examine a range of environmental factors and their contribution to increased body weight. The article draws on the PhD thesis I am currently completing, and is therefore written in an academic tone, and includes a number of references.
In recent years, antiobesity researchers, policy makers and health professionals have increasingly pointed to the contribution of environmental change to the obesity epidemic [1-8]. So called ‘obesogenic’ environmental factors are most commonly described as those environmental factors that contribute to changes in nutrition and physical activity – the ‘Big Two’  – by making unhealthy behaviors the easy or default choice for people . On the nutrition side, ‘obesogenic’ environmental factors are purported to include the heavy promotion of fast food, energy dense snacks and sweetened beverages, the ready availability of these foods in schools, the low cost and large serving sizes of these foods, the density of fast food outlets in poor neighborhoods, the high cost of fresh foods, the lack of time to prepare fresh meals and the reduction in family meal time. On the physical activity side, ‘obesogenic’ environmental factors are purported to include changes to the urban environment and perceptions of safety which have led to reduced use of active transport, increased car use, reduced outdoor play and increased indoor (sedentary) play and recreation, technological advancements which have resulted in reduced need for physical activity and increased opportunity for sedentary behaviors at home and at work, and reduced time dedicated to physical activity in schools.
The arguments for the ‘Big Two’  ‘obesogenic’ environments have relied on a combination of ‘common sense’ about presumed mechanisms of action together with results from ecological studies that show associations between the specific ‘obesogenic’ environmental factors and aggregate population rates of ‘obesity’ prevalence or incidence. However critics point out that correlation is not the same as causation  and the ‘ecological fallacy’ refers to the inability of ecological studies to attribute any causal relationship between exposure to any putative ‘obesogenic’ factor and the development of disease in individuals . As such these studies are considered to be useful for generating hypotheses about the causation of changes in body weight and health outcomes, but not for testing them .
Only a small number of scientific studies have investigated the relationship between ‘obesogenic’ factors and their purported ‘obesogenic’ behavioral correlates. A systematic review of 28 studies examined the relationship between physical, social, cultural and economic environmental factors, ‘obesogenic’ dietary behaviors and body weight in adults . BMI was consistently associated with the food environment, whereas ‘obesogenic’ dietary behaviors were not. Living in a socio-economically deprived area was the only environmental factor consistently associated with ‘obesogenic’ dietary behaviors. There were no other consistent relationships between ‘obesogenic’ environments and ‘obesogenic’ dietary behaviors.
On the physical activity side, ‘obesity’ researchers and public health policy makers have focused a lot of attention on the physical environment and its relationship with active transport (walking or riding a bicycle as a means of transport rather than for recreation or leisure). The assumption, based on ‘common knowledge’ about the relationship between physical activity and body weight, is that people who use active transport are more physically active than those who don’t, and will therefore have lower body weight. This ‘common knowledge’ has been tested in numerous studies, and the findings are at best equivocal. A recently published systematic review of studies focused on adults concluded that there is “limited evidence” that active transport is associated with more physical activity or with body weight . A systematic review of the evidence with respect to active school transport by children and adolescents showed that children who walked or rode their bikes to school tended to be more physically active overall than passive commuters, however only one study of the 10 that examined the effect showed any impact of active school transport on body weight. The authors concluded that “evidence for the impact of active school transport in promoting healthy body weight for children and youth is not compelling .
These studies on just a few select components of the Big Two ‘obesogenic’ environments show that the ‘common knowledge’ about the environmental influences on eating and physical activity, and therefore on body weight, may not be quite as straightforward or simple as portrayed. Although the research literature and public health policy continues to be dominated by the Big Two, some studies have examined other factors for their potential effect on body weight. The remainder of this article focuses on these factors.
A narrative review of ‘obesogenic’ environmental factors beyond the Big Two proposed 10 factors for which there is strong evidence of a causative role in increased average weight in the population . In addition to sleep debt, exposure to endocrine disruptors, and in-utero effects of under and over-nutrition discussed in part 1 of this article, the review found strong evidence of a range of other environmental and social factors that contribute to increased body weight.
Reduction in variability in ambient temperature has resulted from increases in the temperature control of living, working and leisure environments. The ‘thermoneutral zone’ is the range of ambient air temperatures in which the body does not need to expend any energy to remain at a comfortable temperature. Ambient temperatures outside of this zone – too hot or too cold – require energy expenditure, and spending time in temperatures above the thermoneutral zone also reduces food intake. As countries become more industrialised, the proportion of homes with central air conditioning increases, and people spend more time in the thermoneutral zone. The energy expenditure required to maintain physical comfort has therefore been significantly reduced and the review authors propose that this reduction has contributed to increased body weight .
Another widespread social change, in western countries in particular, is decreased smoking rates. Nicotine has a thermogenic (heat generating) effect and is also well known as an appetite suppressant, both of which contribute to body weight regulation. Smoking rates have decreased steadily since their peak rates in the 1940s and the Centers for Disease Control and Prevention in the US estimate that the reduction in the prevalence of smoking has made a significant contribution to increases in average body weight .
Increases in the prescription rates of medications that lead to weight gain is proposed as a likely cause of population weight gain . Medications known to contribute to weight gain include antipsychotics, antidepressants, mood stabilizers, anticonvulsants, antidiabetics, antihypertensives, steroid hormones, contraceptives, antihistamines, protease inhibitors and HIV antiretroviral drugs. Most of these drugs have either been introduced to the market in the same period that average body weight increased, or their use increased dramatically. The authors therefore regard the case for this putative cause as very strong.
Demographic changes have also been proposed as likely candidates for increased average body weight at the population level . These include changes in the distribution of ethnicity and age, and increased average age of childbirth, all of which are associated with increased average body weight.
Finally the authors of the review point to biological factors related to mating that may have contributed to increased average population weight . Firstly they present evidence that there is a reproductive selection bias for higher BMI, which means that fatter people are more likely to have more babies, and that this genotype is therefore more likely to be passed on to their offspring. Secondly, they propose that assortative mating means that there is a higher probability that phenotypically similar individuals will mate – in other words that fat people are more likely to mate with other fat people .
A recent study proposed that increases in acidic load from rising atmospheric carbon dioxide have contributed to increases in average weight for humans . A large study published in 2011 examined changes in average mid-life body weight over the past few decades of over 20,000 animals from 24 populations living in close proximity to humans including primates and rodents in research colonies, domestic dogs and cats, and feral rodents . Across all of the animal populations studied there were significant increases in average mid-life body weight, providing further evidence that the aetiology of increasing body weight is not yet well understood.
Despite the significant body of evidence on the contribution of genetics and a multitude of other factors to weight gain, antiobesity researchers and policy makers continue to posit the ‘common knowledge’ that eating too much and moving too little results in fatness, and that people have the capacity to consciously change these behaviours and thereby change their body weight. This two part article sheds light on some ‘uncommon knowledge’ about factors that contribute to body weight at the individual and population levels. Already it is clear that ‘common knowledge’ about body weight is about as accurate as the 16th Century ‘common knowledge’ that the sun revolves around earth. No doubt, with the enormous sums of money currently being invested in ‘obesity’ research, we will witness even more discoveries that shed new light on the incredible complexity of body weight regulation.