[willsun]

In case anyone is interested, this thread includes a more specific set of methodological critiques made by Dr. Kevin Hall at this week's ObesityWeek conference (unfortunately does not include Dr. Ludwig's rebuttal): https://twitter.com/YoniFreedhoff/status/1062760576869371910

Hall's research focus is on mathematical metabolism models, so his critique comes from that angle: it is possible doubly-labeled water measurements of energy expenditure may behave differently in a lower-carb environment, so the observed EE difference may not be meaningful. Ludwig actually briefly addresses this in their published paper, citing a few other studies suggesting that DLW is accurate and carb intake does not mess with isotopic measurements.

Hard to tell if this is a meaningful advance for the low-carb crowd, or just an artifact of insufficiently validated methodology.

[koliber]

I am interested in what the article did not say, as it may shed some light on this critique. Specifically, the weight differences between the groups at the end of the second phase of the study.

The second phase of the study involved breaking the participants into three groups. They were fed the same number of calories. Each group had several biomarkers tracked. This included daily calories burned and ghrelin (a hormone) levels. I understand the critique targets the measurement of calories burned, via the doubly labeled water measurement of the metabolic date.

What I wonder is what the average weight change was for each of the three groups after 5 months. If the claim is that the high-fat diet group burned an average of 250 calories more, then that should be reflected in an additional weight loss of about 10 pounds (250 calories per day * 5 months * 30 days in a month / 3700 calories in 1 pound of fat). This would be because all the people across all groups were fed the same number of calories. If one group used 250 calories more per day, these extra calories had to come from somewhere. They would have to come from fat stores and muscle tissue inside of the body, resulting in a weight loss.

If the weight loss for that group correlates with the doubly labeled water measurement then it gives more credence to the study's results. If it does not correlate with weight loss, it is suspicious to claim that one group of people burned 250 more calories than the other over 5 months while eating the same number of calories but did not lose any weight.

[willsun]

Yup - this line of reasoning is also expressed by Dr. Hall, in the slide here [1]. To answer your question, the study specifically states: "On average, body weight changed by less than 1 kg during the test phase, with no significant difference by diet group in either the intention-to-treat (P=0.43) or per protocol (P=0.19) analysis."

It is interesting that the study authors highlight the higher energy expenditure as an advantage for weight loss, but do not seem to directly address that they did not observe this advantage during the 20 week study period.

[1] https://twitter.com/YoniFreedhoff/status/1062762047677571079

[gamblor956]

Already commented elsewhere, but the researchers noted that the fed the participants different amounts of food each day as necessary to maintain weight. They were testing metabolic differences associated with the ratio of carbohydrates in the diet.

Studies have already shown that weight loss affects metabolic rates.

[willsun]

Thanks for clarifying - this is an important point and seems to explain why there was no noticeable weight loss.

From the paper: "During the test phase, participants’ energy intake was adjusted periodically to maintain weight loss within 2 kg of the level achieved before randomization."

[gamblor956]

The purpose of the study was to test metabolism, not weight loss. The tested variable was the amount of calories/day attributable to carbohydrates.

A key part of the study was that they fed the participants more or less each day to maintain weight.

In other words, there should not have been any weight change at all, as weight change would have confounded the results since weight loss is known to effect metabolism.

[koliber]

If that is the case, then it makes sense that they did not lose weight.

However, from a layman perspective, it seems like it would be nice to be able to collaborate the measure of increased metabolism not only through one method (doubly labeled water) but also through weight loss over time.

It's like having an inaccurate measuring stick and basing the results of a study solely on measurements made by that one stick, while at the same time making an effort to make sure that no other means of measuring exist.