More Thoughts on the Recent Low-fat vs. Low-carb Metabolic Ward Study

The recent low-carb vs. low-fat study has provoked criticism from parts of the diet-health community. Let's examine these objections and see how they hold up to scientific scrutiny.
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A New Human Trial Undermines the Carbohydrate-insulin Hypothesis of Obesity, Again

The carbohydrate-insulin hypothesis of obesity states that carbohydrates (particularly refined carbohydrates and sugar) are the primary cause of obesity due to their ability to increase circulating insulin, and that the solution to obesity is to restrict carbohydrate intake. Numerous studies have tested this hypothesis, more or less directly, in animals and humans. Despite the fact that many of these studies undermine the hypothesis, it remains extremely popular, both in the popular media and to a lesser extent among researchers. A new human trial by Kevin Hall's research team at the US National Institutes of Health offers very strong evidence that the carbohydrate-insulin hypothesis of obesity is incorrect. At the same time, it offers surprising and provocative results that challenge prevailing ideas about diet and weight loss.

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Does high protein explain the low-carb "metabolic advantage"?

In 2012, David Ludwig's group published a paper that caused quite a stir in the diet-nutrition world (1). They reported that under strict metabolic ward conditions, weight-reduced people have a higher calorie expenditure when eating a very low carbohydrate diet (10% CHO) than when eating a high-carbohydrate diet (60% CHO)*.

In other words, the group eating the low-carb diet burned more calories just sitting around, and the effect was substantial-- about 250 Calories per day. This is basically the equivalent of an hour of moderate-intensity exercise per day, as Dr. Ludwig noted in interviews (2). The observation is consistent with the claims of certain low-carbohydrate diet advocates that this dietary pattern confers a "metabolic advantage", allowing people to lose weight without cutting calorie intake-- although the study didn't actually show differences in body fatness.

In Dr. Ludwig's study, calorie intake was the same for all groups. However, the study had an important catch that many people missed: the low-carbohydrate group ate 50 percent more protein than the other two groups (30% of calories vs. 20% of calories). We know that protein can influence calorie expenditure, but can it account for such a large difference between groups?

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Recent Interviews

For those who don't follow my Twitter account (@whsource), here are links to my two most recent interviews.

Smash the Fat with Sam Feltham.  We discuss the eternally controversial question, "is a calorie a calorie"?  Like many other advocates of the low-carbohydrate diet, Feltham believes that the metabolic effects of food (particularly on insulin), rather than calorie intake per se, are the primary determinants of body fatness.  I explain the perspective that my field of research has provided on this question.  We also discussed why some lean people become diabetic.  Feltham was a gracious host.

Nourish, Balance, Thrive with Christopher Kelly.  Kelly is also an advocate of the low-carbohydrate diet for fat loss.  This interview covered a lot of ground, including the insulin-obesity hypothesis, regulation of body fatness by the leptin-brain axis, how food reward works to increase calorie intake, and the impact of the food environment on food intake.  I explain why I think proponents of the insulin-obesity hypothesis have mistaken association for causation, and what I believe the true relationship is between insulin biology and obesity.  Kelly was also a gracious host.  He provides a transcript if you'd rather read the interview in text form.

Thoughts on the McDougall Advanced Study Weekend

For those of you who aren't familiar with him, Dr. John McDougall is a doctor and diet/health advocate who recommends a very low fat, high starch, whole food vegan diet to control weight and avoid chronic disease. He's been at it for a long time, and he's a major figure in the "plant-based diet" community (i.e., a diet including little or no animal foods).

Dr. McDougall invited me to participate in his 3-day Advanced Study Weekend retreat in Santa Rosa, CA. My job was to give my talk on insulin and obesity, and participate in a panel discussion/debate with Dr. McDougall in which we sorted through issues related to low-carb, Paleo, and the health implications of eating animal foods. I was glad to receive the invitation, because I don't see myself as a diet partisan, and I believe that my evidence-based information is applicable to a variety of diet styles. I saw the Weekend as an opportunity to extend my thoughts to a new community, challenge myself, and maybe even learn a thing or two. It was particularly interesting to compare and contrast the Advanced Study Weekend with the Ancestral Health Symposium, which is more Paleo- and low-carb-friendly.

General Observations

The attendees were a lot older than AHS attendees. I estimate that most of them were in their 60s, although there were some young people in attendance.

I don't place too much emphasis on peoples' personal appearance at conferences like this. You don't know what a person's background, genetics, or personal struggles may be, you don't know how closely they adhere to the program, and you don't know to what degree a group of people might be self-selected for particular traits*. But I will note that Dr. McDougall, his family, and many of the other starch-based/plant-based diet advocates tended to be extremely lean with low fat and muscle mass. They also tended to have a healthy and energetic appearance and demeanor. As I would expect, decades of exceptionally high starch intake hasn't made them obese or obviously ill.

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What about the Other Weight Loss Diet Study??

The same day the low-fat vs low-carb study by Bazzano and colleagues was published, the Journal of the American Medical Association published a meta-analysis that compared the effectiveness of "named diet programs". Many people have interpreted this study as demonstrating that low-carbohydrate and low-fat diets are both effective for weight loss, and that we simply need to pick a diet and stick with it, but that's not really what the study showed. Let's take a closer look.

Johnston and colleagues sifted through PubMed for studies that evaluated "named diet programs", such as Ornish, Atkins, LEARN, Weight Watchers, etc (1). In addition, the methods state that they included any study as low-carbohydrate that recommended less than 40% of calories from carbohydrate, was funded by the Atkins foundation, or was "Atkins-like". These criteria weren't extended to the low-fat diet: only studies of name-brand low-fat diets like the Ornish diet were included, while the meta-analysis excluded low-fat diet studies whose guidelines were based on recommendations from government and academic sources, even though the latter group represents the majority of the evidence we have for low-fat diets. The inclusion criteria were therefore extremely asymmetrical in how they represented low-carb and low-fat diets. This fact explains the unusual findings of the paper.

The abstract immediately activated my skeptic alarm, because it states that at the one-year mark, low-carbohydrate diets and low-fat diets both led to a sustained weight loss of about 16 pounds (7.3 kg). Based on my understanding of the weight loss literature, that number seems far too high for the low-fat diet, and also too high for the low-carbohydrate diet.

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Low-carbohydrate vs. Low-fat diets for Weight Loss: New Evidence

A new high-profile study compared the weight loss and cardiovascular effects of a low-carbohydrate diet vs. a low-fat diet. Although many studies have done this before, this one is novel enough to add to our current understanding of diet and health. Unlike most other studies of this nature, diet adherence was fairly good, and carbohydrate restriction produced greater weight loss and cardiovascular risk factor improvements than fat restriction at the one-year mark. Yet like previous studies, neither diet produced very impressive results.

The Study

Lydia A. Bazzano and colleagues at Tulane University randomly assigned 148 obese men and women without cardiovascular disease into two groups (1):

1. Received instructions to eat less than 40 grams of carbohydrate per day, plus one low-carbohydrate meal replacement per day. No specific advice to alter calorie intake. Met regularly with dietitians to explain the dietary changes and maintain motivation.
2. Received instructions to eat less than 30 percent of calories from fat, less than 7 percent of calories as saturated fat, and 55 percent of calories from carbohydrate, plus one low-fat meal replacement per day. No specific advice to alter calorie intake. This is based on NCEP guidelines, which are actually designed for cardiovascular risk reduction and not weight loss. Met regularly with dietitians to explain the dietary changes and maintain motivation.

Participants were followed up for one year, with data reported for 3 month, 6 month, and 12 month timepoints. This study actually measured body fat percentage, but unfortunately did so using bioelectrical impedance (like on some bathroom scales), which is essentially meaningless in this context.

Results

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Fat and Carbohydrate: Clarifications and Details

The last two posts on fat and carbohydrate were written to answer a few important, but relatively narrow, questions that I feel are particularly pertinent at the moment:

• Was the US obesity epidemic caused by an increase in calorie intake?
• Could it have been caused by an increase in carbohydrate intake, independent of the increase in calorie intake?
• Does an unrestricted high-carbohydrate diet lead to a higher calorie intake and body fatness than an unrestricted high-fat diet, or vice versa?
• Could the US government's advice to eat a low-fat diet have caused the obesity epidemic by causing a dietary shift toward carbohydrate?

However, those posts left a few loose ends that I'd like to tie up in this post. Here, I'll lay out my opinions on the relationship between macronutrient intake and obesity in more detail. I'll give my opinions on the following questions:

• What dietary macronutrient composition is the least likely to cause obesity over a lifetime?
• What dietary macronutrient composition is best for a person who is already overweight or obese?
• Is fat inherently fattening and/or unhealthy?

From the beginning

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Fat vs. Carbohydrate Overeating: Which Causes More Fat Gain?

Two human studies, published in 1995 and 2000, tested the effect of carbohydrate vs. fat overfeeding on body fat gain in humans. What did they find, and why is it important?

We know that daily calorie intake has increased the US, in parallel with the dramatic increase in body fatness. These excess calories appear to have come from fat, carbohydrate, and protein all at the same time (although carbohydrate increased the most). Since the increase in calories, carbohydrate, fat, and protein all happened at the same time, how do we know that the obesity epidemic was due to increased calorie intake and not just increased carbohydrate or fat intake? If our calorie intake had increased solely by the addition of carbohydrate or fat, would we be in the midst of an obesity epidemic?

The best way to answer this question is to examine the controlled studies that have compared carbohydrate and fat overfeeding in humans.

Horton et al.

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Glucagon, Dietary Protein, and Low-Carbohydrate Diets

Glucagon is a hormone that plays an important role in blood glucose control. Like insulin, it's secreted by the pancreas, though it's secreted by a different cell population than insulin (alpha vs. beta cells). In some ways, glucagon opposes insulin. However, the role of glucagon in metabolism is frequently misunderstood in diet-health circles.

The liver normally stores glucose in the form of glycogen and releases it into the bloodstream as needed. It can also manufacture glucose from glycerol, lactate, and certain amino acids. Glucagon's main job is to keep blood glucose from dipping too low by making sure the liver releases enough glucose. There are a few situations where this is particularly important:

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