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More Thoughts On The Recent Low-Fat Vs. Low-Carb Metabolic Ward Study

The recent low-carb vs. low-fat study has provoked criticism from parts of the diet-health community.  Let's examine these objections together with reckon how they handgrip upwards to scientific scrutiny.

Before nosotros begin, I simply desire to re-emphasize that whatsoever way yous while it, this study definitively falsifies the version of the carbohydrate-insulin hypothesis that states that carbohydrates must live reduced for fatty loss to occur.  Here is a quote from Why We Get Fat:
Any diet that succeeds does thus because the dieter restricts fattening carbohydrates …Those who lose fatty on a diet do thus because of what they are non eating—the fattening carbohydrates.
This hypothesis is readily testable, together with Hall's study direct tested it [note: Hall asked me to clarify that the study was non specifically designed to seek Taubes's hypothesis, it simply happens to do so].  In this case, "fattening carbohydrates" did non foreclose a total pound of trunk fatty from evaporating inwards 6 days when dietary fatty was specifically reduced (1).  This is despite the fact that the low-fat diet was high inwards refined saccharify (170 g/day; 35% of calories).  This hypothesis has previously been falsified past times many other studies, but this novel study puts a specially definitive blast inwards its coffin.

It is truthful that this study didn't falsify every possible version of the carbohydrate-insulin hypothesis, of which at that topographic point are many.  For example, if your hypothesis is that eating refined saccharify makes yous hungrier together with makes yous consume more, together with the resulting increment inwards calorie intake causes weight gain, this item study doesn't undermine it because calorie intake was strictly controlled.  But again, this study was non intended or designed to seek that hypothesis.

OK, on to the critiques.

1.  The study was likewise short.  Six days isn't long plenty for fatty adaptation.

This is the big i that people maintain bringing up.  The claim is that 6 days isn't nearly plenty fourth dimension for fatty adaptation, thus the changes inwards trunk fatty majority they reported are irrelevant.  Let's own got a look.

Now, earlier nosotros dig into this question, let's larn clear on what we're talking about.  "Fat adaptation" refers to the procedure of shifting to using fatty every bit the body's primary root of energy*.  This happens when the diet shifts from carbohydrate-heavy to fat-heavy, or when we're fasting.  This procedure is associated amongst measurable metabolic changes.

The inquiry is, how long does it take for those metabolic changes to occur?  Keep inwards hear that what nosotros attention most hither is non how foggy your encephalon feels, how hungry or cranky yous feel, how much unloosen energy yous experience similar yous have, or how difficult yous tin exercise.  Those things are all irrelevant to the inquiry at hand.  For the purposes of evaluating this study, what nosotros attention most is how long it takes for the trunk to maximize its powerfulness to flame fat.

Scientifically speaking, the claim people are making is that 6 days isn't long plenty for fatty oxidation to compass its maximal rate.  In other words, 6 days isn't plenty fourth dimension for the trunk to adjust to burning fat, thus Hall's volunteers weren't yet able to tap into their ain fatty reserves effectively (this concept is shaky to start out with; reckon give-and-take below*).

Fortunately, nosotros own got sufficient prove to evaluate this claim.  Some of the most relevant information I constitute are from a 1972 study of prolonged fasting inwards people amongst obesity, past times William Bortz together with colleagues, that Kevin Hall sent me (2).  Their study included indirect measurements of the charge per unit of measurement of lipolysis, inwards other words, the charge per unit of measurement at which fatty exits fatty tissue**.  These measurements unwrap how long it took their volunteers to compass the maximal charge per unit of measurement of lipolysis, which corresponds around to the maximal charge per unit of measurement of fatty oxidation.

I've graphed the information out thus yous tin reckon the results.  On the horizontal axis, nosotros own got the duration of the fast inwards days.  On the vertical axis, nosotros own got the lipolysis rate:

What yous tin reckon is that the lipolysis charge per unit of measurement ramps upwards together with thus plateaus quickly-- inwards every bit piffling every bit 2 days-- together with thus remains stable out to 23 days.

Here is some other graph showing the oxidation of fat, carbohydrate, together with poly peptide over fourth dimension during a prolonged fast, from a textbook chapter that Kevin Hall wrote (3):

As yous tin see, fatty oxidation is fully ramped upwards afterwards three days of fasting.

So the consistent flick that emerges is that the trunk oxidizes fatty at the maximum charge per unit of measurement inside 2-3 days when it is completely deprived of dietary carbohydrate, including inwards people amongst obesity.  That is less than one-half the six-day duration of Hall's study.

Furthermore, inwards Hall's study the volunteers weren't completely deprived of carbohydrate.  People inwards the reduced-carbohydrate arm were nevertheless eating 140 grams of refined saccharify per day.  Such a little grade of refined saccharify restriction requires a lot less fatty adaptation than a total fast!  We mightiness expression them to attain maximal lipolysis together with maximal fatty oxidation fifty-fifty sooner.

But let's halt speculating, because Hall's squad truly measured fatty oxidation over time!  In figure 2G, they study the fatty oxidation charge per unit of measurement on each twenty-four hours of the study for both diets.  Have a expression for yourself (RC = reduced carbohydrate; RF = reduced fat):

Both according to Hall's model (line) together with the observed information (points), fatty oxidation inwards the reduced-carbohydrate grouping increased rapidly together with reached a plateau past times twenty-four hours four-- together with perchance every bit presently every bit twenty-four hours two.

These information allow us to definitively turn down the claim that 6 days isn't plenty fourth dimension to accommodate to burning fat.  Six days is to a greater extent than than plenty fourth dimension for the trunk to accommodate to withdrawing fatty from fatty tissue together with burning it at the maximal rate, including inwards people amongst obesity.

Now, I handgrip that nosotros own got to live careful most extrapolating these findings to longer periods of time.  There is nevertheless room for longer-term studies to supply direct prove on what would conduct house over periods of weeks or months.  But the prove clearly indicates that it is non possible to dismiss the short-term fatty loss results of this study on the footing of insufficient fourth dimension for fatty adaptation.

2.  The primary argue the low-carbohydrate grouping lost less trunk fatty is that they were burning through their glycogen stores.

This is a adept point, together with I remember it's basically correct.  It is just what Kevin Hall's model predicts.

Let me walk through the argument.  The average lean human trunk contains most 1,800 kilocalories (kcals) of carbohydrate, inwards the cast of glycogen stores inwards liver together with musculus tissue (Keith Frayn. Metabolic Regulation. 2010).  Obese bodies comprise somewhat to a greater extent than than that.

Normally, this stored refined saccharify is used to fuel encephalon together with musculus metabolism.  When a individual begins a fast, glycogen stores are rapidly depleted inwards the starting fourth dimension few days, together with every bit they operate away, the trunk switches to fatty every bit its primary unloosen energy source.  A low-carbohydrate diet is basically a milder version of the same process, together with when a individual goes on such a diet, the trunk initially taps into its refined saccharify reserves to brand upwards for the refined saccharify shortfall.  The less refined saccharify the diet contains, the to a greater extent than glycogen stores are depleted.

So anyway, this glycogen contains calories, together with every glycogen calorie the trunk burns displaces a calorie of fatty that would otherwise own got been burned.  In Hall's study, my calculations dot that the low-carbohydrate diet caused people to flame 1,920 to a greater extent than kcals of refined saccharify than they ate over the 6-day period.  In other words, they burned 1,920 kcals of their glycogen reserves, most of that inwards the starting fourth dimension iv days.  This is consistent amongst the fact that they lost H2O weight, which is a sign of glycogen depletion on low-carbohydrate diets.

Now, hither comes the interesting part.  If nosotros convert the departure inwards fatty loss betwixt groups into calories, nosotros reckon that the low-fat grouping lost 1,962 kcals to a greater extent than trunk fatty than the low-carb grouping over the 6-day study.  That's almost identical to the 1,920 kcal loss of glycogen, suggesting that the glycogen they burned did indeed give the axe an sum of fatty that could roughly explicate the departure inwards fatty loss betwixt diets.

Together, this suggests that glycogen depletion inwards the starting fourth dimension few days of the low-carbohydrate diet is the primary argue it caused less fatty loss over the 6-day period.  Without glycogen depletion, fatty loss would own got been to a greater extent than similar betwixt diets, although Hall's model predicts that the low-fat diet would nevertheless own got maintained an edge.

Since glycogen stores are modest, glycogen depletion can't operate on for really long, together with its effects on trunk fatty majority operate negligible inwards the long run.  So it is truthful that the long-term departure betwixt diets is predicted to live smaller than the 6-day departure Hall's squad observed-- a fact they beak over inwards the paper. Yet the model continues to predict somewhat of a long-term payoff for the very-low-fat diet, primarily due to the fact that refined saccharify has a protein-sparing outcome that sustains lean majority together with unloosen energy expenditure.  Longer studies volition live necessary to evaluate that prediction.

So yes, glycogen is important, but this inwards no way undermines the findings or conclusions of the paper.  It simply agency nosotros own got to translate the results a fight to empathise their total implications.

3. The study controlled calorie intake, thus it missed the effects of refined saccharify intake on appetite.

This, of course, is true, but it misses the dot of the study.  The purpose of the study wasn't to examine the effects of refined saccharify on hunger or nutrient intake, it was to create upwards one's hear whether dietary refined saccharify suppresses fatty loss independently of its calorie content.  If calorie intake hadn't been controlled, the study wouldn't own got been able to seek this hypothesis, together with it wouldn't own got provided whatsoever novel evidence.

4. This study is purpose of a low-fat conspiracy to cover the truth that low-carb is superior inwards every way.

Give me a break!


There's a lot to chew on amongst this study-- it simply keeps on giving.

I promise it's clear why, despite vociferous objections from sure as shooting parts of the diet-health community, this study together with its conclusions rest fundamentally sound.  Yet at the same time, they do require some interpretation to fully understand.

I also promise it's clear why this study direct falsifies the carbohydrate-insulin hypothesis-- at to the lowest degree the version that proposes that refined saccharify restriction is required for fatty loss.

*As an aside, I don't remember I fifty-fifty believe the concept that the trunk has to operate through an adaptation menstruum to live able to primarily flame fat.  It tin primarily flame fatty at whatsoever time, but whether or non it does thus depends on what other fuels are available, because it preferentially burns refined saccharify when it's around (likely because the body's storage capacity for refined saccharify is quite limited, whereas it tin shop almost unlimited fat).  The solely argue it doesn't flame primarily fatty straight off when dietary carbs run out is that it's burning stored glycogen.  As presently every bit that runs out, it's on to fatty without a hitch.  There is no menstruum during the transition to primarily fatty burning where the metabolic charge per unit of measurement drops, suggesting that the trunk is never struggling to larn plenty unloosen energy out of fatty tissue.  The trunk appears to straight off withdraw every bit much fatty every bit it needs to reckon an unloosen energy shortfall, whatever the situation.  What I tin believe is that this procedure of transitioning to predominantly fatty burning causes symptoms similar encephalon fog together with reduced physical performance, every bit tissues adjust to the novel fuel source.  But this doesn't hateful the trunk isn't burning fatty effectively yet-- it definitely is.

** They measured glycerol turnover (= Ra), which is a marking of lipolysis.


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