When it comes to di­ets, low fat beats low carb: study

But the real chal­lenge is to keep calo­ries down, lead re­searcher says

Times Colonist - - Life - MELISSA HEALY

It’s a cen­tral dogma of the low-carb lifestyle: That while avoid­ing car­bo­hy­drates will force the hu­man body into fat-burn­ing mode, any diet that fails to sup­press in­sulin will trap body fat in place and thwart a di­eter’s hope of shift­ing to a leaner, health­ier body type.

But re­searchers from the U.S. Na­tional In­sti­tutes of Health have found that the hal­lowed creed of Atkins acolytes doesn’t hold up in the meta­bolic lab, where di­eters can’t cheat and res­pi­ra­tory quo­tients don’t lie.

The au­thors of a study pub­lished Thurs­day in the jour­nal Cell Me­tab­o­lism con- ducted a high-tech throw­down pit­ting a car­bo­hy­drate-re­stricted diet against a weight-loss reg­i­men that re­duces di­etary fat. Con­fined for a to­tal of four weeks in an NIH me­tab­o­lism lab, re­search sub­jects got equal calo­ries in each con­di­tion (low carb, low fat, each for two weeks).

The sub­jects in each con­di­tion also had equally scant op­por­tu­ni­ties to cheat, shave or mis­re­mem­ber what they ate. Night and day, ma­chines mea­sured not only how much fuel their bod­ies were burn­ing, but what kind of fuel.

In the end, the obese sub­jects lost weight re­gard­less of which diet they were on (and low-carb di­eters lost a lit­tle over a pound more than did low-fat di­eters over two weeks). But obese sub­jects on a low-fat diet lost more body fat than did those on a diet low in car­bo­hy­drates.

The dif­fer­ences were barely per­cep­ti­ble over a sub­ject’s two-week stay in each of the two diet con­di­tions. But the study’s au­thors de­vised a com­puter model and pro­jected that over six months, sub­jects who stuck with a low-fat weight-loss diet would lose 6.5 pounds of body fat more than those who ad­hered to a diet that re­stricted carbs.

“In con­trast to pre­vi­ous claims about a meta­bolic ad­van­tage of car­bo­hy­drate re­stric­tion for im­prov­ing body-fat loss, our data and model sim­u­la­tions sup­port the op­po­site con­clu­sion when com­par­ing the re­duced-fat and re­duced-car­bo­hy­drate di­ets,” the au­thors wrote. “Fur­ther­more, we can defini­tively re­ject the claim that car­bo­hy­drate re­stric­tion is re­quired for body-fat loss.”

Cu­ri­ously, low-carb di­eters did show sus­tained in­creases in fat ox­i­da­tion — us­ing stored fats to pro­duce energy — while low-fat di­eters showed no such in­crease.

One dif­fer­ence be­tween the two groups may have played a key role in help­ing the low-fat di­eters lose more body fat: com­pared with carb-re­stricted di­eters, those on re­duced-fat di­ets tended to burn more calo­ries while they slept.

In the war of words be­tween low-fat zealots and the car­bo­hy­drate-averse, these find­ings are a small but sig­nif­i­cant vic­tory for pro­po­nents of re­duced-fat di­ets. But while this bat­tle has gone to the low-fat camp, lead study au­thor Kevin D. Hall said, the war is far from over.

“Our study sug­gests it’s prob­a­bly the calo­ries in a diet that mat­ter much more than the car­bo­hy­drates or the fat,” Hall said. Get­ting those calo­ries down — and keep­ing them down for the long haul — is the key chal­lenge for di­eters, he added.

There may be some sat­is­fac­tion in punc­tur­ing low­carb cham­pi­ons’ claims of meta­bolic su­pe­ri­or­ity, Hall said. And the find­ings should re­as­sure those who pre­fer to whit­tle their di­etary fat and en­joy the oc­ca­sional carb that their ef­forts to shape up are not in vain, he added.

But for di­eters out­side the me­tab­o­lism lab, “it’s whether you can stick to a diet that’s the most im­por­tant thing,” said Hall. For some di­eters, par­ing back on carbs is a small price to pay for the sat­is­fac­tion of con­sum­ing a bit more fat. For oth­ers, mak­ing toast and pasta off-lim­its is a for­mula for diet sabotage.

Those real-world pref­er­ences, and the neu­ral bases for them, will be the sub­ject of on­go­ing re­search in Hall’s bi­o­log­i­cal mod­el­ing lab at the NIH’s Na­tional In­sti­tute of Di­a­betes and Di­ges­tive and Kid­ney Dis­eases.

As the 19 sub­jects re­cruited for the cur­rent study dieted their way through four weeks of low­carb and low-fat reg­i­mens, Hall and his col­leagues con­ducted brain scans and other tests to glean how di­ets with dif­fer­ing nu­tri­ent com­po­si­tions af­fected their mood, mo­ti­va­tion and sense of sat­is­fac­tion.

That, said Hall, should be­gin to shed light on the fac­tors that shape in­di­vid­u­als’ abil­ity to stick to di­ets.

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