The recent publication of the full text of Kevin Hall’s pilot study on ketogenic vs non-ketogenic weight-reduction diets has sparked a blog battle that doesn’t seem to have any end in sight. It gives reason, apparently, to discuss at least a couple points:
Do ketogenic diets have a metabolic advantage over other diets?
Are ketogenic diets superior to ordinary diets?
It also gives us reason to wonder about few other ideas:
- The significance of honest reporting
- Study design and limitations
- The significance of bias
Before I spend too much time on these three topics, I’ll reveal my own biases and provide some outside sources of great discussions of these. I don’t particularly like Kevin Hall, and it has nothing to do with this study. It has to do with a report he did on the NHANES data some time ago; this gets rather wonky, but the details can’t be spared. There’s a fundamental flaw in the way the NHANES data is used together with food availability data and food loss data. Essentially, the same ratios of losses are assumed for large classes of food that are today used for purposes that they were not used for in the past, at least not to the same extent. Case in point: vegetable oils have not always been the primary fats of deep fryers, but their loss percentages have not been updated. This single data point has the potential to dramatically alter the quantity and ratios of calories in the American diet as provided by food availability and loss data, but that data is used as-is; a point that irks me greatly. You see, if the amount of vegetable oil available in the American food supply were to have its loss ratio updated, then it could be the case that Americans have indeed replaced fats with carbohydrates; but until that data is updated we can’t know. What is known is that the ratio of vegetable oil lost due to cooking has undoubtedly changed since vegetable oils shifted from being the oils of mayonnaise and other dressings to become the primary cooking fats of restaurants and home kitchens (replacing animal fats, which historically had high loss ratios; though also still taking the role of mayo and dressing fats). If these loss ratios are closer to the historical loss ratios of animal fats, then it is absolutely the case that the American diet has shifted away from fats and toward carbohydrates. Kevin Hall’s report on the NHANES data didn’t address this possibility; which I think is somewhat irresponsible.
Moving on: Do ketogenic diets have a metabolic advantage over other diets?
What is a “metabolic advantage” in this context, and what is its significance? A metabolic advantage in the context of a weight reduction diet refers to a diet’s ability to increase metabolic rate. As an example, the exact study in question demonstrated a small but significant increase in metabolic rate, despite Hall’s insistence that it did not. In fact, it used two different measurements to find two different significant increases in metabolic rate.
But I propose that this is a red herring! How important is an increase in metabolic rate?
Why do most diets “fail”? This is actually, and honestly, a contested topic, when it comes to ultimate causes. But just about everyone agrees with what the proximal cause is: Most diets fail because of a lack of adherence. There might be some other causes for non-adherence, but the most significant internal cause is hunger (there may be others, such as a drive for a particular mineral). Significant external causes include social factors.
A simple thought experiment can demonstrate why a metabolic advantage is literally a non-factor in diet success, at least without context.
Suppose we have three hundred identical individuals whose metabolic rate on a standard identical diet is 2000 kcal/day. Let’s say we put 100 of them on a control diet of 2000 kcal/day and 125g of protein. Let’s also put 100 of them on a protein-matched diet A, which consists of 1700 kcal/day and has a metabolic advantage of 200 kcal/day, bringing the caloric expenditure of those on diet A to 2200 kcal/day and creating a caloric deficit of 500 kcal/day. Let’s also put 100 of them on a protein-matched diet B, which consists of only 1200 kcal/day but has a metabolic disadvantage of 300 kcal/day, bringing the caloric expenditure of those on diet B to 1700 kcal/day and creating a caloric deficit of 500 kcal/day. Let’s also suppose that diet A produces excessive hunger and that diet B results in hunger no different from maintenance. Assume no other side-effects or differences in the diet in terms of lean mass lost, effects related to health concerns (or supposed health concerns attributed, falsely or otherwise, to the diet), social pressures, expense, or difficulty of food preparation.
Since each diet produces the same caloric deficit, neither diet has an advantage in terms of weight lost. But if the dieters are free-living, then it’s clear diet B will result in greater success due to adherence.
Now, the thought experiment does leave a few unanswered questions: what caused the metabolic advantage? Because if it was caused, for instance, by an increase in retention of lean mass, then some of our assumptions have to be altered, at least somewhat. Similarly, it could be caused by a potentially deleterious factor, such as an elevated stress response. If we could answer these questions, then we would be able to more fully appreciate the comparisons between the diets, but the thought experiment still reveals a more critical concept: absent contextual understanding, a metabolic advantage cannot be said to confer a real advantage to one diet when compared to another because it alone has no bearing on diet success.
Let that sink in for a minute.
All this hubbub about ketogenic diets not having a metabolic advantage over other diets is a red herring. The most ideal diet might even have a metabolic disadvantage. The crucial question is whether or not you can stick with it.
Whether or not your diet has a metabolic advantage, or how much that advantage is, is probably about as relevant as the names of your cats.
So… What is relevant to diet success? We know the answer: whether or not, and to what extent, you are able to stick to your diet. And when we examine the available evidence, low carb is superior in this regard (here are two references: low carb vs non lo-carb 1 and low carb vs non lo-carb 2, but the story is the same almost whenever these comparisons are made). In these experiments, low-carbers are more likely to complete their protocols, and among those who do complete their protocols, those on the low-carb approaches tend to do somewhat better than those on other protocols.
So… Why is this the case? Because when we start looking at reasons that people might discontinue a diet (other than hunger) low carb doesn’t win on any of the usual fronts: low carb diets are practically equivalent for lean mass retention (perhaps superior?), they have greater social stigmas attached to them, they have medical stigmas attached to them, they are generally more expensive than other diets, preparing interesting meals that fit them is more difficult or at least less familiar, side-effects or symptoms while on a low carb diet are almost always attributed to the diet independent of their causing those symptoms.
In fact, when we consider the mountain of reasons not to go low carb or keto, it can leave us wondering why anyone would do this at all. So, why do people stick to it better than they do other diets?
Not to create a false dilemma, but it appears that if low carb diets have so many things against them, they must have something very significantly in their favor. That something appears to be a modulation of the hunger response to a caloric deficit. Through an unknown mechanism, low carb diets seem to cause people to be able to tolerate caloric deficits better than other diets; and they do so against the weight of all of these stigmas against following them.
Contextually, this modulation of hunger would appear to be the real advantage of a low carb (or ketogenic) diet.
It’s not much of a secret, then, that I think these stigmas against low carb diets are the greatest challenges low carb-ers have to overcome when trying to succeed in their efforts. It’s for that very reason that I find the incessant and energetic denouncement of them to be entirely unfounded, and done in bad faith.