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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.

AMS
Station touristique Duchesnay, QC

5 Comments

  1. I have no doubt that most of us have a baseline of physical activity that our bodies prefer. However, a lot of us have outside forces – and inside forces – that prevent us from being as active as our bodies want us to be. Have you ever suppressed the urge to go do something physically active because you needed to finish some sit-down work that you were in the middle of? I have, and I’m working on listening to my body when it tells me to move, rather than pushing the urge aside. That’s something I can do.

    However, not everything that discourages people from being active is under their control.

    I’m a fairly normal looking middle aged woman. Nothing unusual or shocking in my appearance other than the fact that I’m a bit heavier than average. I’m on the low end of plus-sized. When I go out for a walk, I’ll sometimes get young men making a point of belching or spitting as they pass me on the sidewalk. It’s rude, but I can ignore it. “Yeah, I get it. You don’t think I’m hot. Whatever. You’re probably young enough to be my kid, and I’m sure glad you aren’t.”

    I rarely get verbal abuse, but I’m well aware that people who are heavier than I am often do have to put up with it. I’m even heard of people having fast food containers thrown at them from cars. Who the hell is going to stay physically active when they’re treated like that every time they go out in public? Most people would rather take a nice walk rather than going all hamster-wheel with their exercise, but not when they have to deal with abuse from random strangers.

    If I was going to give other fat people advice on how to be more physically active on a day-to-day basis, I’d say “find reasons to walk places and make movement a natural part of your day,” but I’d also acknowledge that being fat in public can be stressful because of how we’re treated when we’re out in public.

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  2. I think we will discover that the reward mechanism that responds to exercise is individualized according to activity. In my early weight-loss maintenance I found that I had a reward mechanism that responded specifically to running — once I broke through the anaerobic threshhold. It wasn’t a “high” per se, but I entered a netherworld where my thoughts entered into free associations, kind of like dreaming awake. It helped me work through problems and it evened out my mood. At the end of a run I couldn’t remember precisely where I’d been, but I knew I’d followed my routine route because my watch told me I’d run for the particular period of time it took to run it.

    Now that injury and aging have taken me out of running, I haven’t found an activity that replicates the feeling. I do aerobics, weighted aerobics, etc., and work up the same sweat I did running, but I don’t like it much. I just do it. I also tap dance, and it gives me a different reward. It’s limbic, I think. The tap ladies share a connectedness that also happens to include some physical activity.

    My husband, I think, has a reward response to weight lifting. I think we will someday discover that this is a common “guy thing.” I look at him and just shake my head. I find weight lifting tedious. He drags himself to my annual tap recitals (which, indeed, are a bit ridiculous — the old ladies doing their routines betwixt all these youngsters doing theirs). He shakes his head and does not get the point.

    I also get no “high” from competition (or the sporting type activities that promote it — golf, tennis, etc.). I bet we’ll discover that some people may have a genetic desire to compete and will put up with some physical discomfort to get that reward.

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  3. I have had Sjogren’s Syndrome all my life. I now realize that I have challenges keeping my body temperature where it should be, which makes exercise a huge challenge. The ambient temperature where I can exercise in comfort and safety is quite narrow, as Sjogren’s includes Reynaud’s. I didn’t get diagnosed until a few years ago, but the subconscious recognition of the need for temperature control was a huge control on my exercise.

    I also experienced increasing fatigue beginning in my late 20s, which greatly reduced my exercise tolerance. When you have to choose between exercising and getting done what you need to get done, guess what wins?

    There are many people in early stages of autoimmune disease, or with other sub-clinical problems (which won’t be diagnosed until the person is ill enough that even a doctor can tell) who probably shouldn’t be exercising because it will increase their illness.

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  4. Is it physical activity levels that we should be concerned about or too much sitting? By looking only at the amount of moderate-to-vigorous physical activity a person or population does, are we are missing an important factor – time spent sitting or sedentary behaviour.

    Our world increasingly exposes us to environments that encourage, or force, us to sit for prolonged periods of time. The way we communicate, commute, live, work, and entertain ourselves have all changed in a way that has resulted in reduced demands on us to be active.

    Sedentary behaviour is different to a lack of physical activity and not simply an absence of moderate-to-vigorous physical activity, that is, too much sitting is different from too little exercise. A person can be sufficiently physically active and meet the physical activity guidelines, but still spend extended periods of time sitting at work, sitting while commuting, or watching television. And, there is increasing evidence that this time spent sitting, independent of physical activity, may have deleterious health consequences (including increased waist circumference and BMI – and particularly in children).

    If we simply look at levels of physical activity, are we missing an important piece in the puzzle?

    (Note – I’m not saying that moderate/vigorous physical activity is not important. Sedentary behaviour should be seen as an addition to, and not a replacement of, moderate-to-vigorous physical activity.)

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  5. Maybe this explains why I’ve never managed to experience the elusive exercise high that devout exercisers describe as addictive. I participated in our local Fit ‘n Firm contest a few years back. I knew I had no chance of winning as I wasn’t willing to go to the extremes some other contestants did; I just wanted a framework to get me started on a regular exercise routine (aside from my twice daily dog walks). Anyway, I found the extra exercise (1 hr/day) tired me out too much and as much as I was proud of myself for sticking with it for the three month contest period, it was very easy to drop the habit once the contest ended. So for now I am happy with my dog walks and getting in my 10,000+ steps over the course of the day and enjoying whatever exercise opportunities present themselves as they come (hiking with friends, swimming when it’s hot out, dancing class, etc.).

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