The cold truth about brown fat

Is there really a link between warmth and gestational diabetes? Norman Swan investigates.

Keeping cosy might feel nice, but it may not be as good for you as it feels
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It's lovely to snuggle up by the fire, but maybe we’re better off staying a little cool – especially during pregnancy.

The reason behind this Spartan advice comes from brown fat, the type that gets laid down in pads around the neck when we’re cold. It’s famous for keeping newborns and polar explorers warm by burning glucose. Recent research suggests it also helps body tissues take up glucose more efficiently.

By contrast its evil twin, white fat, makes body tissues more sluggish in their response to the hormone insulin and therefore their ability to take up glucose – a harbinger of type 2 diabetes.

To demonstrate the benefits of brown fat, Paul Lee and colleagues at the Garvan Institute in Sydney kept people in overnight temperatures of 19 degrees Celsius for a month. As published in Cell Metabolism in 2016, it was enough to increase the brown fat deposits in the neck by 40%. It also improved their uptake of glucose.

Findings such as these prompted Gillian Booth and colleagues from St Michael’s Hospital in Toronto to explore the link between brown fat and gestational diabetes, the sort that temporarily develops in mid-pregnancy and strongly predicts type 2 diabetes later in life. Using data from half a million births in the Greater Toronto region over 12 years, they compared the rates of gestational diabetes with the average outdoor temperature in the month prior to the 27th week of pregnancy.

The advantage to carrying out this kind of study in Ontario is that, unlike Sydney, winter and summer temperatures vary immensely. So too, in principle, should the mothers’ brown fat composition.

The researchers took into account factors known to be associated with gestational diabetes, such as obesity, disadvantage and ethnicity.

Even allowing for those, they found that gestational diabetes doubled in summer when the temperature was above 24 degrees Celsius compared to winter when it was below minus-10. For every 10 degrees increase in outdoor temperature, the risk of gestational diabetes went up by 6-9%.

Studies have reported that the prevalence of type 2 diabetes seems to increase in warmer climates. But is this really all about brown fat? Other studies have also reported that the prevalence of type 2 diabetes also seems to increase in warmer climates. But is this really all about brown fat?

Robert Moses, director of diabetes services at Illawarra and Shoalhaven Local Health District in NSW, suspects there could be an alternative explanation.

In a 1997 study published in Diabetes Research and Clinical Practice, he and colleagues at the University of Wollongong tested how temperature affects the results of a glucose tolerance test.

After giving male subjects a sugary drink, he placed them in a temperature controlled chamber for two hours and measured their blood sugar. The result was that the higher the temperature, the higher the blood sugar reading. The biggest increase was seen between men in chambers at 25 degrees Celsius compared to 30. The reason? Most likely the men were trying to cool down by shunting their blood from arteries to peripheral veins.

As a result, the glucose escaped being metabolised by body tissues and the levels were high when sampled from a vein. Perhaps a similar effect might account for the findings with the Canadian mothers? If that explanation is true then a diagnosis of gestational diabetes in summer may be spurious.

So if you’re pregnant you’ll be wondering if you should be hanging out in the cold rather than toasting by the fire. It is possible that both theories are true and co-exist.

Meanwhile the safest strategy is not to miss a diagnosis of gestational diabetes so it can be cared for and the risks to the baby minimised. And there’s little harm in keeping cool in summer and not too hot in winter. For the rest of us who would like a bit more brown fat to help with calorie burn, there are three ways of growing it: darkness, hot chilli, and cool temperatures.

However, weight loss is not guaranteed since cold also increases your food intake and undermines the calorie burn.

Not fair, is it?

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