[XFrequentist]

Anecdatum, but I play competitive basketball on a very low carbohydrate diet (<20g/day), and feel just fine, so necessary seems necessarily false.

The missing concept (in the brief quote in your comment) might be "fat adaptation", which takes (in my experience) 2-4 weeks (details: http://www.artandscienceoflowcarb.com/the-art-and-science-of... )

One certainly can fuel activity with carbohydrates, but a fat-adapted person should have no trouble with physical activity on a (very) low carbohydrate diet. Interestingly, a fat-adapted athlete can still use carbohydrates for supplemental fuel before, during, or immediately following extreme exertion (no adaptation is required for short-term use of carbohydrates).

[bitexploder]

The book The Lore of Running covers this as well. For normal day to day things this is fine. However, it is extremely understudied with minimal evidence it works for everyone.

https://www.google.com/amp/s/www.runnersworld.com/newswire/d...

It is still unclear, and dubious to claim, that you can operate at 80+ percent VO2 max on hard work outs. Maybe for short workouts or workouts with bursts of activity e.g. basketball.

You can also undo fat adaptation pretty easily. The answer is it depends, but there is little evidence it has any large negative effects. Questions on peak performance vs "feeling fine" will be interesting to see in studies.

[lazerpants]

I wonder if many people have tested VO2 max like this. Anecdotally, I know that I have a very difficult time getting my heart rate over 150 if I don't consume carbs, while I can easily get it 160+ if I've consumed carbs. I don't notice a qualitative difference, except during HIIT exercise, but I believe there is a subtle one based on my heart rate.

[bitexploder]

I have. I found a cool place that did lactate threshold (via constant blood sampling) Dexa fat scans and VO2 max).

The thing is you can use RPE. We basically mean sustained 4/5 effort (rating of perceived on a scale of 1-5). It's a good enough proxy. VO2 max is also tricky because some people have higher or lower VO2 max but go faster or slower. Higher max != Faster times.

I think the sweet spot for very low carb diets is ultra marathon where you keep your HR around 130-140 anyway, wherein you would use fat anyway. Simple fact is you can't max your power output in an aerobic capacity without carbs. This is not disputed. What we want to know is where the cut off point is in terms of percentage of VO2 max.

Edit: just adding, Noakes talks about ultra performance in his book. It covers everything I saw in that book. He dives deep into the body systems and energy production of the body to suss out the science involved.

[XFrequentist]

Great comment, I'm curious about all this too. I'd be interested in more precise data than my own vague n = 1 impressions!

I actually spent a fair bit of time delivering VO2 max tests to athletes (in a previous life as an exercise physiology lab volunteer).

This comment has me wondering if I still know any active researchers in this area, looking for ideas...

[bitexploder]

I have tried it too. I did not PR in the 5k I tried it for. I think on long runs I never noticed, but 50 minute sessions with 5k pace intervals never felt the same zip as I did on carbs. I was consistent with it for 3-4 months so I am sure I was fat adapted. I noticed my water weight was less unstable (my weight didn't fluctuate as much day to day.) I still like periods of low carb, but I found it onnerous to stay I'm that 20g or less range always. Especially with kids and a wife not on the same diet. So I am optimistically on board the low carb diet scheme, it was fine for me. I cautiously think fine for almost everyone. But... :)

[fasteo]

OP here. Fully agree. I do 2-3 keto cycles (5 weeks) per year. Not against keto diet.

But note that the context of this study is healthspan and mortality, not athletic performance, weight loss, etc.

It was just warning that this study does not warrant that keto diet is good for your health in the long term. The title is a bit misleading.

[XFrequentist]

Fair to say the study results themselves (as opposed to the authors' assertion quoted in the gp) don't really pertain to the health effect of low carb/keto, at all (ie null rather than evidence for or against)?