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Reduced Fat and Reduced Carbohydrate Diets and Fat Loss – Research Review

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.


Dietary carbohydrate restriction has been purported to cause endocrine adaptations that promote body fat loss more than dietary fat restriction. We selectively restricted dietary carbohydrate versus fat for 6 days following a 5-day baseline diet in 19 adults with obesity confined to a metabolic ward where they exercised daily. Subjects received both isocaloric diets in random order during each of two inpatient stays. Body fat loss was calculated as the difference between daily fat intake and net fat oxidation measured while residing in a metabolic chamber. Whereas carbohydrate restriction led to sustained increases in fat oxidation and loss of 53 ± 6 g/day of body fat, fat oxidation was un- changed by fat restriction, leading to 89 ± 6 g/day of fat loss, and was significantly greater than carbohydrate restriction (p = 0.002). Mathematical model simulations agreed with these data, but predicted that the body acts to minimize body fat differences with prolonged isocaloric diets varying in carbohydrate and fat.

Background on Reduced Fat and Reduced Carbohydrate Diets

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 Zone caught on and then things started to fragment.  Cyclical ketogenic/reduced carbohydrate diets became popular and I even wrote an entire book on the topic.

But now, with the joys of the Internet, the entire dietary world has become very divided.  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.  Nevermind that the entire insulin hypothesis has been completely destroyed.

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.

The Study

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.

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 RC RF
Calories 2740 1918 1918
Protein 101 g 101 g 101 g
Fat 109 g 108 g 17 g
Carbs 350 g 140 g 350 g

A couple of things to note.   First is that refined carbohydrate intake was not changed in the RF diet; neither did 24 hour insulin levels decrease.  They stayed the exact same in the reduced fat group.  The exact same.  They did decrease in the RC group which would be expected.   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.

Carbs didn’t go down and I’m making this point for a reason I’ll explain below.  

Each diet was followed for only 6 days total.

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) 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 RC 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 RF 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.  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 was calculated, the following results were seen.

Diet Net Fat Balance
RC -53.6 g/day
RF -89 g/day

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.  Over the 6 days of the study DEXA was unable to measure any differences in actual fat loss but it would be too small to pick up.

Both diets did result in weight loss and the RC group did better.  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.  And this led to the overall conclusion of the paper: that calorie for calorie, reducing fat intake generates more fat loss than reducing carbohydrate intake.

One final comment, it’s often been claimed by folks like Gary Taubes, who are intent on holding to the insulin hypothesis that low-calorie diets still work by reducing carbohydrate and insulin.  And this paper, done under the most meticulous of conditions shows that that is simply not the case.  Carbohydrates were not reduced in the RF group and neither was insulin. And there was still a measurable negative fat balance.  The researchers even took the time out to mention that explicitly.

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 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.  This study eliminated all of that.  The subjects were locked in a metabolic ward, every calorie they ate was measured and provided to them and their energy expenditure was measured.

The study was, however short and used few subjects but that’s the reality with this kind of work.  Metabolic ward studies are hellishly expensive to do which is why they aren’t done that often.  They are far far more accurate but that comes at a cost.

But there are a couple of other issues I see with the study.  One is the duration.  There can be further adaptations to diet over longer periods.  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 I suspect that glycogen depleting exercise would 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.

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, to match the deficit between the two diets while keeping carbohydrate intake in the RF group from changing; this means that only dietary fat could be reduced and it had to be reduced by a lot.  But it raises a big adherence issue.  Only the most psychotic athlete 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.

And 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.

But regardless of this, 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 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.  Maybe.

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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