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Are Low Fat or Low Carb Diets Superior?

A long-standing debate in the field of dieting for fat loss is over the relative superiority of low fat and low carb diets in terms of which is superior.  Today I want to address the issue by examining the following research paper by Kevin Hall.

Hall, KD et. al. Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity Cell Metabolism  Cell Metabolism (2015) 22: 1–10.

For years the debate over reduced fat or reduced carbohydrates has gone on and it shows no sign of stopping.  The pendulum has actually swung over the years.  In the 70’s, the Atkins diet drove interest in very low/reduced carbohydrate diets.

In the 80’s, reduced fat diets came into vogue as it looked like dietary fat was more easily stored as body fat and it looked like, so long as fat intake was kept low enough, weight and fat loss would happen (this was true until people went nuts and started overeating low fat foods in excess).

In the 90’s, somehow the (overly complex) Zone caught on and then things started to fragment.  In the mid-1990s, cyclical ketogenic/reduced carbohydrate diets such as Dan Duchaine’s Bodyopus and the Anabolic Diet by Mauro DiPasquale became popular.   Interest in them led me to write my first book, The Ketogenic Diet.

But now, with the joys of the Internet, the entire dietary world has become very divided and all dietary approaches have their own sets of hardcore (and often hardheaded) zealots.  With the publication of a book that I shall not name the idea that insulin was the cause of obesity, that reducing insulin was either required or would magically cause fat loss came back into vogue.

Regardless, the debate goes on with various studies coming in on both sides of the debate.  They are often problematic, relying on self-reporting of diet or poor measurements of actual changes (i.e. weight versus body fat).  Over the long-term, there is invariably little to no difference in the overall results anyhow and, as I’ve said before and will reiterate below, the best diet is really the one someone can stick to.

Regardless of that, a new study has been blowing up the Internet and since I keep getting or seeing questions about it, I want to weigh in (ha ha) on it.

Low-Carb vs. Low-Fat Diets

The study recruited 19 obese subject, 9 females and 10 males.  Their body fat percentage was 32% for the men and 48% for the women.  All subjects were placed in a metabolic ward for two two week periods with a 2-4 week washout period (this means that they weren’t in the metabolic ward).

By metabolic ward, I mean they were locked in a room/rooms so that every aspect of their metabolism, activity, etc. could be measured.  Every morsel of food was given to them and measured.  This is important since uncontrolled studies invariably have enormous confounds with people eating different amounts of calories and protein.

They exercised for one hour per day on the treadmill.  During the entire study, their 24 hour total daily energy expenditure was measured.  For the first 5 days, all subjects were given a baseline diet of 50% carbs, 35% fat and 15% protein at maintenance levels.  This was to make sure that nothing about their previous eating habits would impact on the study.

After this 5-day washout period, they were put on a 30% reduced calorie diet and either a reduced carbohydrate or reduced fat diet.  Protein intake was unchanged and the same for both groups (a key aspect in making any sort of useful comparison).  I’ve summarized the basic diet below.


Baseline, Low-carb and Low-fat Diet Composition

There are a couple of things to note.   First is that refined carbohydrate intake was not changed in the reduced fat diet and 24 hour insulin levels were not decreased.     Also note that there was no reduction in the carbohydrate intake from baseline.  The entire deficit was created simply by cutting carbs.

The second is that the RC carb group was not a ketogenic diet.  That is generally defined as any diet with less than 100-120 grams of carbohydrate per day.  It was simply reduced relative to baseline.  24 hour insulin secretion was also reduced here.  Also note that carbohydrate intake did not change in the RF diet.  They  created the entire deficit by reducing fat intake.

Each diet was followed for only 6 days total.  This is admittedly a short time period but this is the reality of metabolic ward studies due to the insane cost involved.

Changes in Nutrient Oxidation

In the metabolic ward, respiratory quotient (RQ), a measure of fuel utilization was also measured.  An RQ of 1 indicates 100% carbohydrate oxidation and an RQ of 0.7 is 100% fat oxidation. Protein is generally ignored and you will also see non-protein respiratory quotient (NPRQ) used for this reason. But measuring RQ and energy expenditure allows the amount of carbohydrate and fat that is being burned by each subject to be measured/estimated.

And when that estimation was made, the reduced carbohydrate group increased fat oxidation from baseline by 51 grams per day and carbohydrate oxidation dropped by 148 grams per day.  This is a known effect of carbohydrate reduction and, as I’ve discussed, it’s the reduction in carbs and not an increase in fat that drives this (note that fat intake didn’t change). In the reduced fat group, carb oxidation went up by 36 grams per day and fat oxidation fell slightly by about 10 grams per day from where it started.

But don’t start drawing conclusions yet. Because while fat oxidation went up in the RC group, they were also eating more total fat.  And even though fat oxidation went down a little bit in the RF group, they were eating less total fat.

And this is important because fat loss is determined by the difference between fat oxidation and fat intake.    That is, it is the fat balance that is important here rather than the absolute values for either fat oxidation or fat intake.

Fat oxidation can be very high but if fat intake is also very high, the difference can be small.  Even if fat oxidation is lower, if total fat intake is lower, the difference can be larger.  Again, it’s the difference between the two that matters.

And this is what the study showed and what everybody is going on about online.  Because when that difference between fat oxidation and fat intake was calculated, the following results were seen.

Net Fat Balance for Low-Fat and Low-Carb Diets

So even thought the RC diet caused more fat to be burned, since they were eating more total fat, the net fat balance was lower than in the RF group where less fat was burned but much less was eaten.  And this is where the conclusion that the low-fat diet is superior for fat loss comes from in this study.

I would note that the short duration of the study made it impossible for DEXA to measure any actual fat loss.  But physiologically, in terms of losing fat from the body, the reduced carbohydrate diet was superior.

Both diets did result in weight loss and the RC group did better in this regard.  This is no surprise given that reducing carbohydrates causes water to be lost.  And ultimately fat loss is far more important than just weight loss. 

All of this led to the overall conclusion of the paper: calorie for calorie, reducing fat intake generates more fat loss (technically a more negative fat balance) than reducing carbohydrate intake.

The Insulin Hypothesis

Up above, I made a point that the reduce fat diet did not lower carbohydrates from baseline.  Nor did it see a difference in 24 hour insulin levels.  This is important because one of the recent goal post shifts of the Insulin Hypothesis zealots is that low-calorie diets still only work by reducing carbohydrates and insulin levels.

And this paper, done under absolutely meticulous conditions showed that this was not the case.

My Comments

Ok, since everybody is drawing wild and often differing conclusions from this paper, here are my comments.  First and foremost this is one of the most meticulously designed studies you could do.  As per the introduction, most studies give dietary advice (some even give the subjects a copy of the diet book being tested) and then let people go.

They rely on notoriously inaccurate self-reporting of food and even if it probably is more realistic to the real world, you can get flawed results.   In many of the low-carbohydrate vs. low-fat diet studies subjects end up eating different amounts of total calories and invariably the low-carb group increases their protein.      And this doesn’t say anything about what happens if you match calories and protein.

This study eliminated all of that.  The subjects were locked in a metabolic ward, every calorie they ate was measured and provided to them, the diets contained identical amounts of protein and they were measured under meticulously controlled conditions.     The only thing that would have made the paper stronger would have been a longer study duration.  But that’s simply an issue of cost.

That isn’t to say that the study was perfect. The short duration does raise some questions as there may be further adaptations to diet over longer time periods (i.e. it takes about 3 weeks for the body to full adapt to using ketones for fuel). It’s known for example that the obese often lose metabolic flexibility; this just means that their bodies are often poor at switching from using carbohydrate for fuel to fat for fuel.  As I discussed last week, their muscles are often full of glycogen and intramuscular triglycerides (IMTG) and the insulin resistance that develops is part of this.

This is reversible with exercise and glycogen depletion causes this to occur very quickly (it works in the lean too).  The exercise performed by the study subjects was low intensity and  glycogen depleting exercise might have affected the results and increased fat oxidation in the RC group.  Of course, that might be argued to fail the reality check: how many folks who switch to low-carb diets who are obese do glycogen depleting exercise? Probably not many.

Perhaps the biggest problem I see with this study is that the fat intake was insanely low in the RF group.  It was an 8% fat diet with only 17 grams of fat (the researchers even made this point).  It’s clear why the researchers did this.  They wanted to match the deficit between the two diets without changing carbohydrate intake. That meant that dietary fat had to be reduced by a lot.

And outside of the pure physiological results, this raises a big adherence issue.  Only the most psychotic individual is able to maintain this type of diet in the long-term and there are some arguably long-term problems with fat-soluble vitamin absorption and even EFA intake.

In the long-term of real-world fat loss, adherence trumps everything else and even if one diet or another has a slight advantage, it doesn’t matter if someone won’t stick to it.  The best diet for someone is the one that they can maintain in the long-term and I doubt most people could stick to an 8% fat diet for very long.

When you consider that the difference in net fat oxidation was about 30 grams (80 vs. 50 g/day), I have to wonder what would have happened if the fat intake had been raised to a more realistic level.  Triple it to 25% (still a low fat diet) and you add 35 grams of fat which just offsets the difference.

Are Low-Fat or Low-Carb Diets Superior?

Accepting the limitations of the research (a necessary evil of metabolic ward studies), a couple of things become clear.  The first is that the assertion that low-calorie diets work because they reduce insulin or carbohydrates is false.  Neither happened in the RF group here and there was still fat burning going on.  So Gary Taubes and Tim Noakes and all of the other zealots can suck it.

The second is that, again within the limitations of this study, there doesn’t seem to be an advantage in terms of FAT LOSS (which is what actually matters) to a reduced carbohydrate diet.  Yes, weight loss is always greater but water loss can range from 1-15 lbs.  It happens in the first few days, ultimately means nothing since water contains no energy, and pinning a metabolic advantage on water loss is pretty silly.

Now one criticism that I suspect will come from the low-carbohydrate camps is that the diet wasn’t truly low-carbohydrate.  140 grams per day of carbs is above the 100-120 g/day cutoff for ketosis and they will argue that there would have been a difference in the results if it had been fully ketogenic.  Perhaps.  Maybe if Hall can get it funded, that can be the follow-up study.

And if you want another good non-popular science interpretation of the paper read this. Which basically says the same thing as I wrote above.

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7 thoughts on “Are Low Fat or Low Carb Diets Superior?

  1. Lyle, are you sure that in the RF group insulin levels weren’t reduced?

    Or is fasting insulin something different from 24h insulin?

  2. “…the RF diet (it’s a crossover study so most subjects did both diets) saw a reduction in fasting insulin in men (-26%) whereas the RC diet dropped the fasting insulin in women by ~1/3rd. The men were insulin sensitive, the women somewhat insulin resistant. 24h insulin secretion via C-peptide was reduced in RC fig 2B but same in RF.

    The fasting insulin levels in these subjects were fairly low and not at the level that would strongly inhibit release of fat in the first place. There’s no data of 24h insulin profiles.

    The large increase in fat oxidation ties in with the reduction in insulin secretion in RC. I suspect we need to see individual data to get a true picture.

    Yes fat loss occurred by reducing fat intake at constant carbohydrate in this case, so it would be fair to say that fat loss is not wholly dependent on carb restriction, just as sugar intake doesn’t prevent fat loss or other absolutes.”

    is this a correct interpretation?

  3. V, in that your interpretation is exactly what I said happened.

  4. ” The RF diet did not reduce refined carbohydrates from baseline and resulted in no significant changes in 24-hr insulin secretion.”

  5. why is 24-hour insulin secretion more important to look at than fasting insulin?

  6. Thanks for the great article Lyle – did the RF diet lose more fat due to the thermic effect of food? Since the TEF is 6-8 percent for carbohydrates and 2-3 percent for fat therefore the effective caloric load was smaller for the RF diet – so is this just simple math?

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