Before we begin, I just want to re-emphasize that any way you slice it, this study definitively falsifies the version of the carbohydrate-insulin hypothesis that states that carbohydrates must be reduced for fat loss to occur. Here is a quote from Why We Get Fat:
Any diet that succeeds does so because the dieter restricts fattening carbohydrates …Those who lose fat on a diet do so because of what they are not eating—the fattening carbohydrates.This hypothesis is readily testable, and Hall's study directly tested it [note: Hall asked me to clarify that the study was not specifically designed to test Taubes's hypothesis, it just happens to do so]. In this case, "fattening carbohydrates" did not prevent a full pound of body fat from evaporating in six days when dietary fat was specifically reduced (1). This is despite the fact that the low-fat diet was high in sugar (170 g/day; 35% of calories). This hypothesis has previously been falsified by many other studies, but this new study puts a particularly definitive nail in its coffin.
It is true that this study didn't falsify every possible version of the carbohydrate-insulin hypothesis, of which there are many. For example, if your hypothesis is that eating carbohydrate makes you hungrier and makes you eat more, and the resulting increase in calorie intake causes weight gain, this particular study doesn't undermine it because calorie intake was strictly controlled. But again, this study was not intended or designed to test that hypothesis.
OK, on to the critiques.
1. The study was too short. Six days isn't long enough for fat adaptation.
This is the big one that people keep bringing up. The claim is that six days isn't nearly enough time for fat adaptation, so the changes in body fat mass they reported are irrelevant. Let's have a look.
Now, before we dig into this question, let's get clear on what we're talking about. "Fat adaptation" refers to the process of shifting to using fat as the body's main source of energy*. This happens when the diet shifts from carbohydrate-heavy to fat-heavy, or when we're fasting. This process is associated with measurable metabolic changes.
The question is, how long does it take for those metabolic changes to occur? Keep in mind that what we care about here is not how foggy your brain feels, how hungry or cranky you feel, how much energy you feel like you have, or how hard you can exercise. Those things are all irrelevant to the question at hand. For the purposes of evaluating this study, what we care about is how long it takes for the body to maximize its ability to burn fat.
Scientifically speaking, the claim people are making is that six days isn't long enough for fat oxidation to reach its maximal rate. In other words, six days isn't enough time for the body to adjust to burning fat, so Hall's volunteers weren't yet able to tap into their own fat reserves effectively (this concept is shaky to begin with; see discussion below*).
Fortunately, we have sufficient evidence to evaluate this claim. Some of the most relevant data I found are from a 1972 study of prolonged fasting in people with obesity, by William Bortz and colleagues, that Kevin Hall sent me (2). Their study included indirect measurements of the rate of lipolysis, in other words, the rate at which fat exits fat tissue**. These measurements reveal how long it took their volunteers to reach the maximal rate of lipolysis, which corresponds approximately to the maximal rate of fat oxidation.
I've graphed the data out so you can see the results. On the horizontal axis, we have the duration of the fast in days. On the vertical axis, we have the lipolysis rate:
What you can see is that the lipolysis rate ramps up and then plateaus quickly-- in as little as two days-- and then remains stable out to 23 days.
Here is another graph showing the oxidation of fat, carbohydrate, and protein over time during a prolonged fast, from a textbook chapter that Kevin Hall wrote (3):
As you can see, fat oxidation is fully ramped up after 3 days of fasting.
So the consistent picture that emerges is that the body oxidizes fat at the maximum rate within 2-3 days when it is completely deprived of dietary carbohydrate, including in people with obesity. That is less than half the six-day duration of Hall's study.
Furthermore, in Hall's study the volunteers weren't completely deprived of carbohydrate. People in the reduced-carbohydrate arm were still eating 140 grams of carbohydrate per day. Such a modest degree of carbohydrate restriction requires a lot less fat adaptation than a total fast! We might expect them to achieve maximal lipolysis and maximal fat oxidation even sooner.
But let's stop speculating, because Hall's team actually measured fat oxidation over time! In figure 2G, they report the fat oxidation rate on each day of the study for both diets. Have a look for yourself (RC = reduced carbohydrate; RF = reduced fat):
Both according to Hall's model (line) and the observed data (points), fat oxidation in the reduced-carbohydrate group increased rapidly and reached a plateau by day four-- and possibly as soon as day two.
These data allow us to definitively reject the claim that six days isn't enough time to adapt to burning fat. Six days is more than enough time for the body to adapt to withdrawing fat from fat tissue and burning it at the maximal rate, including in people with obesity.
Now, I agree that we have to be careful about extrapolating these findings to longer periods of time. There is still room for longer-term studies to provide direct evidence on what would happen over periods of weeks or months. But the evidence clearly indicates that it is not possible to dismiss the short-term fat loss results of this study on the basis of insufficient time for fat adaptation.
2. The primary reason the low-carbohydrate group lost less body fat is that they were burning through their glycogen stores.
This is a good point, and I think it's basically correct. It is exactly what Kevin Hall's model predicts.
Let me walk through the argument. The average lean human body contains about 1,800 kilocalories (kcals) of carbohydrate, in the form of glycogen stores in liver and muscle tissue (Keith Frayn. Metabolic Regulation. 2010). Obese bodies contain somewhat more than that.
Normally, this stored carbohydrate is used to fuel brain and muscle metabolism. When a person begins a fast, glycogen stores are rapidly depleted in the first few days, and as they go away, the body switches to fat as its primary energy source. A low-carbohydrate diet is basically a milder version of the same process, and when a person goes on such a diet, the body initially taps into its carbohydrate reserves to make up for the carbohydrate shortfall. The less carbohydrate the diet contains, the more glycogen stores are depleted.
So anyway, this glycogen contains calories, and every glycogen calorie the body burns displaces a calorie of fat that would otherwise have been burned. In Hall's study, my calculations indicate that the low-carbohydrate diet caused people to burn 1,920 more kcals of carbohydrate than they ate over the 6-day period. In other words, they burned 1,920 kcals of their glycogen reserves, most of that in the first four days. This is consistent with the fact that they lost water weight, which is a sign of glycogen depletion on low-carbohydrate diets.
Now, here comes the interesting part. If we convert the difference in fat loss between groups into calories, we see that the low-fat group lost 1,962 kcals more body fat than the low-carb group over the 6-day study. That's almost identical to the 1,920 kcal loss of glycogen, suggesting that the glycogen they burned did indeed displace an amount of fat that could roughly explain the difference in fat loss between diets.
Together, this suggests that glycogen depletion in the first few days of the low-carbohydrate diet is the primary reason it caused less fat loss over the 6-day period. Without glycogen depletion, fat loss would have been more similar between diets, although Hall's model predicts that the low-fat diet would still have maintained an edge.
Since glycogen stores are modest, glycogen depletion can't go on for very long, and its effects on body fat mass become negligible in the long run. So it is true that the long-term difference between diets is predicted to be smaller than the 6-day difference Hall's team observed-- a fact they discuss in the paper. Yet the model continues to predict somewhat of a long-term advantage for the very-low-fat diet, primarily due to the fact that carbohydrate has a protein-sparing effect that sustains lean mass and energy expenditure. Longer studies will be necessary to evaluate that prediction.
So yes, glycogen is important, but this in no way undermines the findings or conclusions of the paper. It just means we have to interpret the results a bit to understand their full implications.
3. The study controlled calorie intake, so it missed the effects of carbohydrate intake on appetite.
This, of course, is true, but it misses the point of the study. The purpose of the study wasn't to examine the effects of carbohydrate on hunger or food intake, it was to determine whether dietary carbohydrate suppresses fat loss independently of its calorie content. If calorie intake hadn't been controlled, the study wouldn't have been able to test this hypothesis, and it wouldn't have provided any new evidence.
4. This study is part of a low-fat conspiracy to hide the truth that low-carb is superior in every way.
Give me a break!
There's a lot to chew on with this study-- it just keeps on giving.
I hope it's clear why, despite vociferous objections from certain parts of the diet-health community, this study and its conclusions remain fundamentally sound. Yet at the same time, they do require some interpretation to fully understand.
I also hope it's clear why this study directly falsifies the carbohydrate-insulin hypothesis-- at least the version that proposes that carbohydrate restriction is required for fat loss.
*As an aside, I don't think I even believe the concept that the body has to go through an adaptation period to be able to primarily burn fat. It can primarily burn fat at any time, but whether or not it does so depends on what other fuels are available, because it preferentially burns carbohydrate when it's around (likely because the body's storage capacity for carbohydrate is quite limited, whereas it can store almost unlimited fat). The only reason it doesn't burn primarily fat immediately when dietary carbs run out is that it's burning stored glycogen. As soon as that runs out, it's on to fat without a hitch. There is no period during the transition to primarily fat burning where the metabolic rate drops, suggesting that the body is never struggling to get enough energy out of fat tissue. The body appears to immediately withdraw as much fat as it needs to meet an energy shortfall, whatever the situation. What I can believe is that this process of transitioning to predominantly fat burning causes symptoms like brain fog and reduced physical performance, as tissues adjust to the new fuel source. But this doesn't mean the body isn't burning fat effectively yet-- it definitely is.
** They measured glycerol turnover (= Ra), which is a marker of lipolysis.