Australasian Science: Australia's authority on science since 1938

The Back Roads to Obesity

By Tim Olds

Obesity isn’t just a matter of eating too much of the wrong foods. Several other factors in modern life have been playing a role.

Most of us come to obesity by the highway, zipping past the gyms and stopping off at the fast-food restaurants, but recent research has suggested that some less direct by-ways can unexpectedly lead us to the same place. Environmental pollution, changes in indoor temperature, the age at which women first give birth, prescription drugs, giving up smoking, and too little sleep may also take us there. So let’s buckle up our seat belts and take a drive down some of the new roads to obesity.

Let’s wind down the windows as we go. On second thoughts, let’s not. The polycyclic aromatic carbons found in cigarette smoke and car exhaust increase inflammation, and chronic low-grade inflammation is associated with obesity – although the causal connections are not clear. A longitudinal study found that infants exposed to second-hand smoke and near-roadway pollution had adult BMI scores 2.15, or about 8%, higher than those without exposure. However, obese people breathe in more air because they are bigger, so it may be that obesity leads to pollution exposure rather than the other way around.

With the windows up, let’s switch on the air conditioning. Or maybe that’s not such a good idea. A zone of thermal comfort called the thermo-neutral zone (TNZ) is the temperature at which the body’s heat production exactly matches heat loss. For a naked man standing upright in still air it’s around 27°C. Any temperature above or below the TNZ requires us to use more energy.

Both human and animal experiments show that spending more time in the TNZ results in greater levels of fatness – and we’re in the TNZ much more nowadays. The proportion of Australian households with an air conditioner has almost doubled from 1994 to 2008, while between 1970 and 2000 the average home temperature in the UK increased from 13°C to 18°C. The first mobile air conditioner was an axle-driven fan that blew air over ice in a horse-drawn carriage in 1884. In 1961, only 8% of US cars had air-conditioning, compared with 100% today.

Mum’s in the back seat enjoying the view, but she may be partly responsible for your obesity. Your risk of obesity is greater if your mother was older when you were born. Every year of increase in the mother’s age at birth is associated with a 3% increase in the obesity risk of her child. Fifty years ago, the median age at which Australian mothers had their first child was 22. Today it is close to 29, so over the past 50 years this change alone would account for an 18% increase in the prevalence of obesity.

Mum has been putting on a few kilos recently, too. Maybe it’s when she gave up smoking. Smoking increases your metabolic rate and suppresses your appetite. An unfortunate side-effect, however, is death. One meta-analysis found that people put on 5–6 kg when they give up cigarettes.

Or it may be the drugs Mum’s been taking. When people start on beta-blockers they put on 1.2 kg. Antipsychotics are worse: they add 4 kg, and worst of all are oral contraceptives – you can expect to put on 5 kg.

Recent work has even pointed the finger at antibiotics. Kids with early life infections are more likely to end up obese, and early exposure to broad-spectrum antibiotics increases the obesity risk by16%.

Mum’s now dozed off, but nowadays even sleeping can be dangerous. Too little or too much sleep is associated with adult obesity. It may be that late bedtimes increase the amount of time we spend munching fast food in front of the telly, or that short sleep increases our appetite by releasing the hunger hormone ghrelin, or that we’re just too tired to exercise. There’s a lot of debate about whether adults are sleeping less these days. Kids are, but a recent meta-analysis suggests that adults are sleeping about the same amount.

At least we can say this: we know where we’re going, and it seems that almost any road will take us there. Anyway, it’s time to pull over – there’s a KFC.

Professor Tim Olds leads the Health and Use of Time Group at the Sansom Institute for Health Research, University of South Australia.