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