Couch potatoes and hunter-gatherers

Further to Aseem Malhotra's absurd claims about obesity not being linked to physical activity, here he is at a recent conference last month making another of his frequent assertions—that there has been "little change in exercise levels in the past 30 years".

Ignore the silly stuff about the Big Sugar 'playbook' for a moment. (It is, incidentally, not true that the tobacco CEOs swore under oath that they didn't believe smoking caused lung cancer. They famously said they didn't believe nicotine was addictive. The bit about cancer has been added by Malhotra.)

I want to focus on the second point on the slide.

Little change in exercise levels in the past 30 years whilst obesity has rocketed

Perhaps because this is a counter-intuitive claim in an era of office drones and couch potatoes, Malhotra goes to the trouble of providing a reference. It is a reference I have come across before because Action on Sugar cited it in a briefing paper sent to politicians.

The title of the study - 'Hunter-gatherer energetics and human obesity' - provides a clue that it is not going to offer much evidence about changes in Western levels of physical obesity in the last thirty years. And it doesn't. It actually looks at a hunter-gatherer tribe in Tanzania, whose members apparently have similar levels of energy expenditure as that seen amongst a sample of Americans.

What is the relevance of this study? Presumably Malhotra's thinking goes like this.

1. The lifestyle of hunter-gatherers in Africa is similar to that of Westerners in days of yore.

2. Hunter-gatherers in Africa are no more physically active than Westerners today.

3. Levels of physical activity have not changed for thousands of years and therefore cannot have changed in the last few decades.


Hmm. This theory is problematic for a number of reasons. For a start, the study of Tanzanian bushmen actually did find higher levels of physical activity ("As expected, physical activity level, PAL, was greater among Hadza foragers than among Westerners.") It also found higher levels of physical activity amongst peasant farmers, in line with previous studies. It was only total energy expenditure after adjusting for height, weight etc. that was similar in both societies—a finding that appears to be due to physiological differences between the two groups.

(Incidentally, the authors note that "many hunter-gatherers seasonally consume a large portion of their daily calories as honey, which has high concentrations of glucose and fructose". This poses a bit of a problem for people like Malhotra who see fructose as the main cause of obesity and diabetes, both of which are vanishingly rare amongst tribesmen.)

But even if taken at face value, this is still only one study. If you look at the the evidence in the round, a rather different picture emerges... 

Most analyses of hunter-gatherer diets assume caloric intakes of approximately 3000kcal day, a surprisingly large figure that exceeds typical contemporary intakes. The level of energy expenditure necessitated by pre-agricultural lifestyles, however, was much greater than that for average modern individuals. For instance, total energy expenditure in the !Kung and Ache peoples averaged 206kJ/kg/d, compared to roughly 134kJ/kg/d for contemporary humans.

Hunter-gatherers who consume 3000 calories a day rarely, if ever, become obese. Westerners who eat 3000 calories a day frequently become obese. The difference is that hunter-gatherers burn off all those calories whereas Westerners generally don't.

What does this tell us about physical activity levels over the last 30 years? Not very much, to be honest, but it is worth noting that in the mid-twentieth century the UK government recommended people eat 2,900 calories a day, based on good evidence showing that if people consumed less than that, they lost weight. Today, the government recommends people consume 2,250 calories a day (more for men, less for women). Why, if there has been no decline in energy expenditure, has the advice changed so much?

The fact is that the average Briton who consumed 2,700 calories in the 1940s would lose weight. Today, the average Briton who consumes 2,700 calories gains weight. Conversely, if the average Brit in the mid-twentieth century followed today's guidelines they would lose weight to a rapid and dangerous extent. The only rational explanation for this—supported by masses of empirical evidence—is that physical activity has declined over the years.

As I mentioned in a previous post, the decline in physical activity in the West is acknowledged by the World Health Organisation, Public Health England, the NHS, the Harvard School of Public Health and countless other organisations. If you want to challenge that view, you'd better have some pretty compelling evidence. Malhotra's killer piece of evidence—the one study that he repeatedly cites and puts up on slides at conferences—is not only of little relevance to the claim he is supposed to be debunking, but is an outlier even within its own narrow field.

All of which is a good indication that he hasn't got much evidence at all.