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Do Ketogenic Diets Have a Metabolic Advantage?

Without fail, every decade seems to see the resurgence of the idea that extremely low-carbohydrate/ketogenic diets diets have a “metabolic advantage” over carb-based diets.  By that I mean that advocates claim that more weight/fat will be lost at the same (or even a higher) calorie level.  But is it true?

In examining the issue, I will be looking at the following paper:

Johnston CS et. al. Ketogenic low-carbohydrate diets have no metabolic advantage over nonketogenic low-carbohydrate diets. American Journal of Clinical Nutrition. (2006) 83: 1055-1061

A Disclaimer about Ketogenic Diets

Before I continue let me say that I have nothing against low-carbohydrate/ketogenic diets.  My first book The Ketogenic Diet was about nothing that topic although nowhere in that book did I specifically recommend them.   At the same time, at least some of my books have included low- or lowered carbohydrate phases in them.  Either those low-carbohydrate phases generate specifically biological effects (i.e. helping to mobilize stubborn fat) or are simply required to allow sufficient dietary protein and fat while keeping calories low.

Certainly increasing amounts of research shows that, in many cases, lowering carbohydrates can have profound health benefits.  In cases of insulin resistance/metabolic syndrome or conditions such as PCOS, removing all carbohydrates except for fruits and vegetables may be profoundly beneficial.   And while that is certainly a reason to consider them, that is outside of the claim of a true metabolic advantage.

Do Ketogenic Diets Provide a Metabolic Advantage?

As well, research clearly shows that, for some people, lowering carbohydrates can have profound health benefits and in some cases a near removal of dietary carbohydrates (except for things like fruits and vegetables) may be profoundly beneficial.

However, the weight and fat loss claims are a bit trickier.  There is certainly an element of truth to the idea that low-carb diets generate more total weight loss but this issue is confounded by the issue of water loss.  Between a drop in insulin (insulin causes the kidney to resorb water) and a dehydrating effect of ketones themselves, very-low carbohydrate diets can cause significant water loss.

As I discussed at length in The Ketogenic Diet, water loss can range from 1-15 pounds depending on size (even small individual may lose a rapid 3-4 pounds, of primarily water, in the first days of carbohydrate restriction).  This tends to make comparisons of weight loss pretty meaningless.  This is even more true when you consider that the difference in total weight loss between low- and high-carb diets is usually only a few pounds anyhow.

However, other recent studies have used more accurate methods of measuring body composition and several do seem to find a greater fat loss for the low-carbohydrate diet compared to the higher carbohydrate diet.   Often with the low-carbohydrate diets reporting that they are eating the same (or occasionally) more calories.  Aha, a metabolic advantage.


I have two primary issues with most of the studies that have been done, one of which I referred to above.  That is the issue of caloric self-reporting.  The grand majority of studies done to date have allowed people to self-report their food intake and this introduces a staggering number of issues.  Because, simply, people really suck at it.  Relying on them to tell you how much they are actually eating means not having any real idea as to what’s going on.

Protein Provides a Metabolic Advantage

The other, potentially bigger issue, revolves around protein intake.  In general, and this is especially true if you are comparing a typical very-low carbohydrate diet to a high-carbohydrate diet, the lower carbohydrate diet will contain significantly more protein.

This is simply a function of what foods can be eaten on the diet.  Logically, when most of your diet revolves around meat, you will tend to eat more protein than when it doesn’t.  But this tends to make comparing the diets problematic for reasons I have discussed elsewhere on this website.

Now, at this point there is little to no debate that higher protein diets have a number of inherent benefits to lower protein diets.  Benefits to dieters include increased satiety, better maintenance of metabolic rate while dieting, better blood glucose maintenance, less lean body mass loss and others I’m sure I’m forgetting.

But at this point we’re not talking about “low-carbohydrate/ketogenic” diets per se.  Rather, we’re talking about “high-protein” diets.  That is, the comparison is no longer about the carbohydrate content of the diet but rather the protein intake.    Some researchers argue that the primary “benefit” of ketogenic diets comes from the increased dietary protein intake rather than the carbohydrate content or development of ketosis per se.

Shockingly, many low-carb advocates are incapable of making this distinction.  It is eminently possible to increase dietary protein intake in the context of a carbohydrate based diets. Athletes have done it for decades.  Yes, in the general public, adopting a ketogenic diet tends to spontaneously increase protein intake.  But this is ultimately beside the point.

To make a fair comparison of ketogenic and non-ketogenic diets requires setting protein at identical and sufficient levels for both diets and then varying carbohydrate and fat content.  Then and only then can the carbohydrate content of the diet itself be examined.  And that brings me to today’s study.

Comparing Ketogenic and Non-Ketogenic Diets

Researchers recruited 20 overweight subjects, both women and men. Admittedly this is a small sample size.  The subjects were then placed on either a ketogenic/low-carbohydrate diet (essentially the Atkins diet) or a moderate-carbohydrate non-ketogenic diet.  The non-ketogenic diet was similar to The Zone and Barry Sears is actually one of the paper’s authors.  Exercise was not performed.

The researchers examined a number of different variables including fat loss, insulin sensitivity, blood lipid levels, inflammation and energy levels.  Resting metabolic rate was also measured.

One nice thing about this study is that the researchers went out of their way to make sure that both diets were equal in both calories and protein (almost anyhow, the very low carb diet was a touch higher in protein).  The composition of both diets appears in the table below:

Ketogenic vs. Non-Ketogenic Diet

Note: The ketogenic diet was also slightly higher in saturated fat than the non-ketogenic diet (21% vs. 9% of the total calories).

You might be wondering how the researchers ensured that the diets were actually followed as described.  Another strength of this study is that all foods were provided to the study subjects.  Lunches were actually prepared and served to the subjects at the testing facility Monday through Friday.  All other meals were prepared for the subjects to take home with them for consumption.

The Results?

In terms of weight and fat loss, at the end of 6 weeks both groups had lost roughly the same amount of weight: 6.3 kg (13.9 lbs) for the ketogenic diet versus 7.2 kg (15.8 lbs) for the non-ketogenic diet.  This was not statistically significant.    The ketogenic diet group lost 3.4 kg (7.5 lbs) of fat versus 5.5 kg (12.1 lbs) for the non-ketogenic diet group.  Despite being nearly 5 lbs higher, this was not a statistically significant difference.  There was no change in fat free mass/lean body mass for either diet.

I should mention that body composition was measured with a Tanita BIA device which is far from ideal.  BIA has its share of problems and can be thrown off by changed in hydration status.  And hydration status can be impacted by carbohydrate intake.   The study would have been stronger had it used a  better method for determining changes in body composition.

Hunger ratings improved for both diets with no difference between diets.  An oft-heard claim is that ketogenic diets cause hunger blunting due to the presence of ketones or what have you; but this study does not support that.  Given that protein is the most filling nutrient, the effect seems to be mediated by the increased protein content, not decreasing carbohydrates (or ketosis) per se.

Somewhat oddly, the researchers found that weight adjusted resting energy expenditure went up but as it did so equally in both groups.  This was more likely related to the protein content of the diets (remember, identical) than the carbohydrate intake per se.

Looking at blood lipids, the results were actually quite mixed.  Low-density lipoprotein (LDL) went up in 5 of the ketogenic dieters and down in the other 4.  LDL went up in 2 of the non-ketogenic dieters and down in the remaining 8.  HDL levels fell in both groups.

The researchers also found that one marker of inflammation was significantly higher in the ketogenic diet compared to the non-ketogenic diet but no explanation was given in the discussion for why this might be the case.  Some studies link saturated fat to inflammation and the slightly higher saturated fat intake in the ketogenic diet might have been part of this.

Insulin sensitivity was improved in both groups with no differences between the diets and this is typical: when weight and fat is lost, insulin sensitivity invariably improves almost regardless of dietary composition.

In terms of energy levels, there were no differences except that the non-ketogenic dieters scored higher on a rating of vigor-activity than the ketogenic dieters.  I’d note that, in my experience, the response to full blown ketogenic diets is highly variable: some people feel awful and others feel completely energized.

Summing up, the researchers conclude:

In the current study, the KLC [ketogenic low-carb] diet did not offer any significant metabolic advantage over the NLC [non-ketogenic low-carb] diet.  Both diets were effective at reducing total body mass and insulin resistance, but, because blood ketones were directly related to LDL-cholesterol concentrations and because inflammatory risk was elevated with adherence to the KLC diet, severe restrictions in dietary carbohydrate are not warranted.


My Comments

This study had several strengths.  One is the controlled calorie and matched protein levels along with the food being provided to the subjects.  Weaknesses include the relatively small sample size and the use of BIA to track body composition.    A larger study with a better method of body composition would have been better but would have cost a lot more money.

With that said, this study adds to the data set suggesting that, if calories and protein intake are identical, there is little to no metabolic advantage (in terms of fat or weight loss) to ketogenic diets over non-ketogenic diets.  They work at least as well, mind you, but not better.  At least not in terms of fat loss.

Which isn’t to say that I think all diets are equal or best for all situations.   And I don’t really agree with their strongly worded conclusion that carbohydrate restriction is not warranted.  As I discussed when I compared the diets  there still may be circumstances where very-low carbohydrate/ketogenic diets may still be preferred, even given the researchers comments above.

For many individuals, the biggest “metabolic advantage” of very-low carbohydrate diets may be one of food/calorie control.   For a great many dieters, especially those who are insulin resistant, eating carbohydrates tends to make them want to eat more carbohydrates.  In some cases, removal of nearly all dietary carbohydrates save fruits and vegetables is necessary to allow them to adhere to the diet.  And adherence trumps everything.

Over time, removing all of the tasty carbohydrate foods also tends to change taste preferences.  Taste buds renew themselves every 4-6 weeks and many report that a long enough period on a low-carbohydrate diet allows them better dietary control after they have reintroduced carbs.

I mentioned in the introduction that severely insulin resistant individual seem to get health benefits from extreme carbohydrate restriction.  For leaner dieters, as I discuss in both The Ultimate Diet 2.0 and The Stubborn Fat Solution, extreme carbohydrate restriction tends to help with stubborn lower body fat mobilization.

In other situations, moderate carbohydrates diets (still containing sufficient protein mind you) may be superior.  For people who can control their food intake, many simply feel better with more moderate carbohydrate intakes, their training (especially high intensity training such as weights) doesn’t suffer as much, and they don’t feel quite as lethargic.  Again, these issues are discussed in detail in elsewhere on the website.

Context is Key

I guess my point is that there is too much variance between individuals and their needs to claim that any single diet is inherently superior for all people and all situations.  As this study suggests, given identical calories and protein intake, there doesn’t seem to be any inherent metabolic advantage in terms of total fat loss to a full blown ketogenic diet, at least not when compared to a moderate carbohydrate diet with an identical amount of protein.

But that doesn’t mean that there aren’t other potential advantages to be had.  As a great deal of recent research has found that the best diet is the one that a person can adhere to.  If a low-carbohydrate/ketogenic diet improves adherence for some reason it may be superior for a given individual.  If it doesn’t, it won’t be.

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