A High Carb Diet Burns More Fat?
A High Carb Diet Burns More Fat?

The debate between low-carb diets and low-fat diets is one that never seems to end. Proponents on each side of the weight loss diet spectrum champion their side as the one true "king" of fat loss diets.

Well, researchers finally decided to pit the two diets against each other in a no-holds barred fight for fat loss diet supremacy, and the results may surprise you.

Related - Is the Keto Diet Right for You?

The Study on High Carb Diets

19 obese adults (9 females and 10 males) were admitted to the metabolic unit at the National Institute of Health (NIH) Clinical Center and placed in a metabolic ward for two 2-week periods with a 2-4 week "washout period."

If you're not familiar with metabolic wards, essentially the test subjects were locked in a room in order for the team of researchers to accurately measure every aspect of their food intake, metabolism, activity, sleep, etc. This is very different from most weight loss studies which simply go off of diet recall, which is notoriously inaccurate.

Researchers collected baseline metrics on all subjects, including body fat percentage, which on average was 32% for men and 48% for women. For the first 5 days of the trial, all subjects consumed a baseline diet consisting of 50% carbs, 35% fat and 15% protein at maintenance calorie levels.

Researchers administered the baseline to ensure none of the subjects' previous dietary protocols were affecting the results of the study.

Following the 5 day baseline diet, subjects were placed on a 30% reduced calorie diet that was classified as either low fat or low carb. Protein intake was the same for both groups. This table summarizes the diets used:

Low Carb Vs. Low Fat
Baseline Low Carb Low Fat
 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

Each diet was followed for only six days, and at the conclusion of the trial, researchers documented that on an equal calorie basis, restricting dietary fat led to greater body fat loss than limiting dietary carbohydrate intake in adults with obesity. Both diets showed similar reductions in leptin, ghrelin, and cholesterol levels.

One thing that's important to note is that researchers accounted for changes in body fat by calculating daily fat balance as the difference between fat intake and net fat oxidation. This is important, and something we'll address next.

Limitations of This Study

Scale

The study at hand can be commended for placing subjects in a metabolic ward. It is not cheap or convenient, but provides substantially more accurate data than self-reporting, which most other dietary studies use. Now, for the bad news...

There are several glaring errors with this study, led off by the fact that the data for the men and women were combined for the analysis. From the outset, it was evident there were significant differences between the sexes across all the variables that were measured.

Secondly, the study fails to account for the subjects' hydration status; it's difficult to interpret the body composition data with no documentation of lean mass or hydration status. Next, the length of the study is almost absurdly short. 6 days is not long enough to see what kind of long term metabolic adaptations can occur as a result of a long-term change in diet.

As for the diets, the reduced-fat diet restricted subjects fat intake to 8% of their total daily calories. This is horrendously low, and if carried out longer than the six days of the trial could lead to all sorts of complications including joint aches and problems with hormone output, not to mention the fact it's completely unsustainable for the long-term.

Additionally, the reduced carb diet was by no means a low carb diet, as subjects were consuming 140g/day of carbohydrates. To truly see the effects of a reduced/low carb diet on fat loss, researchers should have restricted carb intake to keto diet levels of between 35-50g / day.

Now, going back to the conclusion of the study - that reduced fat was superior to reduced carb for fat loss. We need to look a little closer at how the results were calculated.

In the study, fat loss was determined by the difference between fat oxidation and fat intake. If fat intake is incredibly low (as it was here) and fat oxidation is high, you have a greater difference and it can make it appear that one diet is superior to another. This is exactly what happened in this study.

The reduced carbohydrate group also had a high rate of fat oxidation, but they also had a high fat intake, which made the difference between the two numbers very small. This gave the implication that the reduced fat diet was superior, since there was a greater difference in fat loss, and this is why so many people are championing low-fat diets as the superior weight loss diet.

In reality, both diets resulted in weight loss, but the reduced carbohydrate group actually did better in both body composition changes and important other metrics included: lowered 24 hour energy expenditure, respiratory quotient, and carbohydrate oxidation, as well as increased 24 hour fat oxidation.

Takeaway

So, what's the takeaway from this study?

Well, both diets do work for weight loss, and in the long run, the best weight loss diet is the one you can stick to. For some that could be Paleo, low fat, low carb, Zone, or IIFYM. However, subjects in the low fat group in this study consumed only 8% of their calories from fat, which is neither healthy nor sustainable long term.

Science continues the search for the one true holy grail of fat loss diets, and until it's revealed (if that ever happens), we'll continue to promote the practice of moderately reducing calorie intake and increasing physical activity as the optimal way to go about fat loss.

References
Hall KD, Bemis T, Brychta R, et al. Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity. Cell Metab. 2017;22(3):427-436. doi:10.1016/j.cmet.2015.07.021. http://www.cell.com/cell-metabolism/fulltext/S1550-4131(15)00350-2