Short article that includes a bunch of information from this new paper: https://advances.sciencemag.org/content/5/6/eaaw0341

I read that paper shortly after it came back and sent this info to a friend:

I've read the entire paper and I don't buy their conclusion. Basically, I think they're saying that at > 2.5x BMR, you simply are burning more calories than you can process. I think that a more likely culprit is body damage, which although vague, is definitely something different. Here's a telling portion from their own article:

> An alimentary limit of ~2.5× BMR is consistent with the SusMS versus activity duration curve, which flattens out below ~3× BMR (Fig. 1B), but raises directions for further research. First, in at least two studies (Tour de France cyclists and elite Nordic skiers; table S1), subjects maintained metabolic scopes of 3× to 5× BMR without substantial weight loss.

The weasel word "substantial" is present and I guess one could argue that perhaps they had small amounts of weight loss that would add up, but I don't think that's what they're really saying. Between Marshall Ulrich's experience (where he set the age record and then was kind of fucked up for about a year, IIRC) to my own experience where the harder I run the more it beats me up, even in short distances where I can and do eat (and drink!) more post-race and hence gain weight, my guess is that they're being fooled by the correlation between the increase in damage that's done the harder one runs and the corresponding increase in calories needed. As such, they come up with a useful rule of thumb, but their explanation of "alimentary limit" (which is in the title "Extreme events reveal an alimentary limit on sustained maximal human energy expenditure") is just not the case.

Although I don't think it would be easy to find subjects to do such a test, my guess is if you could find a low-impact protocol for raising BMR higher than 2.5x and you were to do it long enough, with "good" nutrition that it would be fairly easy to surpass their magic number. However, that's trickier than it sounds. Riding a stationary bicycle, for example, probably is lower impact than the "running" that was done, however, there are reasons to believe that while shorter distances are lower impact, longer distances may not be, because a bicyclist has less freedom to alter which joints/tendons/muscles he/she stresses and hence you hit the weak-link-in-the-chain syndrome, even though it may take a while to get there. Also, riding a stationary bicycle is boring as hell.

Really, if you take out the "substantial" weasel word from the paragraph I quoted, I think it's pretty damning. However, I think they had a hypothesis and wanted to justify it. On one hand the authors have much greater scientific chops than I do. On the other hand, I'm a fairly analytical person and I have a lot of experience with endurance events, although not much multi-day stuff and my longest multi-day event, the Tahoe Two Hundred, was marred by a (minor, but cumulatively very annoying) toe/foot injury that I sustained on the second day.

Oh, and back to that quoted paragraph: I think the participants in Tour de France and elite Nordic skiers) are much more likely to be significantly more fit (both from a cardiovascular perspective as well as tougher skins, ligaments, and muscles) than the RAUSA runners and that would be in keeping with my hypothesis.