Is there re­ally a link be­tween warmth and ges­ta­tional di­a­betes?

Cosmos - - Contents - NOR­MAN SWAN is a doc­tor and multi-award win­ning pro­ducer and broad­caster on health is­sues.

— Cold truth about brown fat

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%.

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 tem­per­a­ture­con­trolled 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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