Physical Activity: From Genes to Policy

In preparation for the start of the Summer Olympics, The Lancet has dedicated virtually all of its current issue to articles on physical activity.

One article by Adrian Bauman, on behalf of the “Lancet Physical Activity Series Working Group”, particularly caught my attention as it encourages us to look beyond the classical ‘ecological model’ of physical activity, which essentially looks at environmental rather than ‘biological’ determinants of physical activity.

Pointing out the limitations of the model in fully explaining inter-individual variability in physical activity and the rather modest impact that interventions based on this model have so far had in terms of noticeably increasing physical activity in the general population, the authors briefly also summarize the potential ‘biological’ factors that may be important in determining who does what and how much.

With regard to genetic determinants of physical activity, the authors write:

“Genetics is a possible determinant of physical activity—ie, a heritable component affects activity behaviours, not just measures of fitness. Similar to other behaviours, such as eating (appetite), evidence from human and animal studies indicates that physical activity is regulated by intrinsic biological processes. Animal studies suggest that CNS mechanisms might regulate daily physical activity. Twin and family studies have shown that genetic factors contribute to variation in reported daily physical activity levels, with heritability estimates ranging from small (h2 <30%) moderate (h2=30—65%),53—58 and even high (h2=78%).”

“Substantial individual differences have been noted in the acute averse and rewarding effects of physical activity, implicating genetic factors. Specifically, reward systems will be activated in individuals with above-average abilities, those who crave activity, and those who feel rewarded by accomplishing an activity; adverse effects will be reported in those who feel pain, fatigue, or even exertion. As such, candidate genes might be part of the reward systems and pain sensation.”

However, they also point out that so far genome-wide studies have not yet identified a ‘robust association’ between specific genes and inter-individual differences in daily physical activity levels.

The authors also discuss how evolutionary biology may relate to differences in physical activity levels.

As they point out,

“…many components of our physiology are adapted to a range of expected behaviour.”

and ask

“Is there evidence that people became physically active out of necessity and biological adaptation, and then had to reduce activity because of mechanisation and culturally and technologically induced decreases in the need for energy expenditure?”

When defining levels of physical activity as the ratio of total energy expenditure to basal metabolic rate,

“Ancestral foragers—of larger body size on average than are contemporary foragers—had estimated mean physical activity levels of roughly 1·7 (range 1·5—2·1), which is little different from those in industrialised populations with moderate activity levels. Non-human primates do less activity than do human beings (1·2—1·5), suggesting that our species adapted to increased physical activity for foraging.

Subsistence farmers have variable levels of activity, with a mean of about 1·9 in men and 1·8 in women, but ranging up to roughly 2·5.76 However, in urban populations, the most sedentary individuals do little activity (about 1·5). Overall, people could be encouraged to achieve levels of about 1·75, as was recommended by WHO and the Food and Agriculture Organisation for health in 2004, but this value is much higher than is that of sedentary populations.”

(Unfortunately, neither the authors (nor the WHO) tell us exactly how such ‘encouragement’ would actually work in practice.)

Finally, the authors also discuss whether or not being less physically active is indeed causally related to obesity.

“The notion that physical activity is a key determinant of body fat in individuals and populations is common, seemingly supported by the logic of the energy-balance equation and empirical reports of cross-sectional associations between adiposity and activity. On this basis, clinicians assume that physical activity will induce weight loss in overweight individuals, but secular declines are also judged a key driving factor in the worldwide obesity epidemic.

However, in the past decade, studies have begun to challenge both these assumptions, suggesting instead that adiposity could be a determinant of physical activity. In several longitudinal studies, baseline activity did not predict follow-up adiposity, whereas baseline adiposity predicted follow-up activity. Promotion of physical activity has little effect on prevention of obesity in children, adolescents, or adults.

Although long-term trends in mechanisation and transport, and equivalent behaviours such as rural—urban migration, could reduce activity and hence cause weight gain, whether substantial reductions in physical activity in industrialised populations have occurred since the 1980s is a matter of debate. Conversely, some believe that real decreases in total physical activity have occurred, which indicates a possible role of physical activity in obesity prevention. Clearly, further work is needed, but evidence does suggest that increasing obesity could be a contributor to high levels of inactivity in human populations.”

Whichever the direction of this relationship (remember, the Canadian Measurement Survey found little association between activity levels and BMI in both kids and adults), this paper certainly suggests that the link between being physically active and body weight may be far less straightforward than the sport-fitness industrial complex would like us to believe.

None of this should distract from the important impact of the ‘ecological’ determinants of physical activity including the importance of recreation facilities and locations and transportation environments but also aesthetics and cultural norms.

However, ignoring the ‘biological’ determinants in such discussions may explain why some of these ecological model-based measures have been less successful than expected.

I would certainly like to hear from readers, who have experienced inter-individual (perhaps even within families) differences in their ‘liking’ and ‘wanting’ of physical activity.

Station touristique Duchesnay, QC