Hot and hungry? How cooling off could slim you down

Ambient temperatures can affect your body weight.

Posted Jul 20, 2010

It is really, really hot in New York at the moment. Too hot to do anything, in fact, except lie here in my apartment in a steaming pool of sweat, blogging about how hot it is.

But what are the effects of ambient heat on body weight? Can you really sweat out calories? Or does summertime inertia and ice-cream lead to fatness in fall?

And do we lose weight in winter because our bodies have to work to stay warm? Or do dark evenings in centrally-heated houses slow down our metabolisms and provoke us to seek solace in buckets of KFC?

In general, research suggests we tend to get heavier and move less in the winter, then lose some weight and move more in the summer. A little bit like bears.

In some cases the seasonal difference is probably related to a touch of SAD-inspired overeating in the colder months (particularly among women).

A fraction of it may be accounted for by summertime loss of water weight through sweating - there is nothing so slimming (or as bad for you) as dangerous levels of dehydration.

And most of it is probably due to seasonal lifestyles - it's more enjoyable being outside in the summer, and being outside generally involves getting off your butt.

But a simple hotness=thinness rule doesn't seem to work when you look at obesity rates across states or countries - people in hotter climates aren't necessarily thinner.

And research emerging last year - on a mysterious substance called brown fat - has made appetite scientists wonder whether being cold, rather than hot, could make us thinner.

Like regular, common-or-garden fat, brown fat contains lipid droplets. But unlike normal fat cells, brown fat cells burn rather than store calories, giving off heat. (All cells give off heat to some degree, but brown fat cells are the best at it.)

Up until 2009 we thought brown fat was only present in babies and animals - but then Swedish scientists discovered deposits of it in PET scans of adult humans, and a US team spotted more of the stuff in lean than overweight people.

The extra-intriguing thing about brown fat in adults is that its magical calorie-burning properties only seem to be activated in cold situations - like sitting in a room at 16 degrees Celsius, or when elegantly dangling one's foot in a basin of ice water.

So does this mean, all other things being equal, we should actually end up skinnier in winter than summer?

Probably not. The extent to which we heat our homes and offices means that we are fairly unlikely to activate brown fat, even if we have some.

And, for that matter, the amount of air-conditioning we use in the US tends to blur the difference between summer and winter anyhow.

(I have never been as cold as when attending an obesity conference in an exhibition center in Florida. At one point my lips actually went blue. And no, I didn't weigh less afterwards.)

Personally, I think the safest bet for weight control through the seasons is to capitalize on the figure-improving outdoor opportunities that summer brings, and to aim to stay active, happy - and warm - throughout the winter too.

Meanwhile I'll keep an eye on the latest news on brown fat and try to work out whether all this ‘global weirding' will ultimately end up creating more obese people or less.

And finally, can somebody please, please get me a cold soda with ice? Actually, on second thoughts, better make that a ‘diet.'

Stay cool everyone!