I don't think this is accurate, or at least not universally so. The pandemic has given me the opportunity to experiment a bit with losing weight. These days I mostly do eat one meal a day (sometime between noon and 6pm). I'll occasionally have a light snack aside from that one meal. I lost a good 15 lbs in just a few months this way. It was mostly fat. I kept up a running regimen (8-12 mi per week). I felt amazing. Much more energy, flatter stomach, better-defined chest, arms, thighs, and calves.

Unfortunately I've gone back to some of my old ways over the past 9 months or so with restaurants and bars reopening, and I've gained back some of the weight. This tells me that it's not really how many meals I have, but what's in them, and how many calories. So I'd probably be fine with two or three meals a day, just healthier choices and fewer overall calories. But I don't believe one meal a day is a problem; just it isn't necessary, and you can still lose weight with a more normal meal pattern.

> Eating only once a day is very bad advice.

This meta-review seems to conclude fasting + resistance training might not only be harmless but actually beneficial https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7468742/

Correct. I would be doing some form of weight training or resistance type training as well to make sure my muscles don't evaporate. I'd also eat a high protein meal for my one meal to make sure my body had enough protein to repair/keep muscle on.

I think for most people who are looking for weight loss - this is not as much of a concern and going to the gym would likely be an extra hurdle for them that would make it too hard. So, 1 meal a day is relatively straight forward.

Anyone who has actually fasted knows this is all nonsense. Sure, you can lose muscle mass — that's always true of going into a caloric deficit, which is the only way to lose weight.

Nonsense? I take you've never actually measured body fat percentage during diet periods.

It is true that caloric deficit always causes muscle; the issue is how much one loses.

I don't see anyone here questioning that you lose muscle, but questioning the idea that confining your eating to a narrow window (or a single meal) is bad.

I'm not aware of any evidence to support that. Fasting patterns have been studied, and I'm not aware that any studies on fasting have shown the effect you describe, but I've not followed the research for some years.

In terms of anecdotal evidence, I have regularly measured body fat percentage during diet periods, and I do see steep drops in body fat percentage when I've done intermittent fasting with the vast bulk of my intake coming in a single meal (2k calories plus during one period). As mentioned elsewhere I now prefer 4-5 days of drastic cuts and eating normally a few days a weak, and without confining it to a narrow period, but I also did IF for many months while at my peak of lifting both in a deficit and not, after having previously done slight deficit dieting in the past, and could compare the difference, and I didn't see the slightest hint of the effect you're claiming.

The middle east has significantly lower rates of cancer attributed partially to fasting in Ramadan and throughout the year.

Eating once a day is a type of intermittent fasting, which has been well studied and shown to have some of the health benefits of fasting.

Unless you're a competitive weightlifter or similar, you're unlikely to see significant enough muscle loss with very significant calorie restriction over the kind of time frames people would stick to very strict diets like that for it to matter much.

I wouldn't do it every day, but mostly because I find it too hard, but I do currently aim for ~600 kcal 4-5 days a week to drop down. I do it in multiple small meals, but I've also done intermittent fasting with narrow feeding windows. On the remaining days, I eat normally - I don't binge.

It'd likely have more of a negative effect on my muscle mass if I did it every day, but if I did, I'd also reach my target weight far faster, so even if I'd had to spend time bulking up again later I'd have done that if I was able to stick to it, but I know from experience I'm not.

I also power lift (not competitively), and I've had steady progress while doing that, with the caveat I'm not at my peak - I wouldn't expect to progress with this diet if I was. But most people are so far off hitting limitations caused by food intake in their exercise it matters for quite few people.

People should keep up resistance exercise while on diets to reduce muscle loss.

It's not sustainable over long periods of time in that sooner or later you get to an inflection point where it impedes your progress, but it works when your body fat percentage is high, and for most people looking to lose weight it's simply not what they should be worrying much about.

That you accumulated fat with what you think was a net daily deficit sounds to me like it wasn't a net daily deficit. Intermittent fasting works, and limiting yourself to a single daily meal is not nearly long enough fasting periods to mess much with body composition. But it may have messed with your activity levels. It takes little change in e.g. dozing off on the couch rather than walking around to have an effect.

People are mostly more sedentary than they think and burn fewer calories than they think. When I was at my peak, exercising every day and lifting heavily 3 times a week, to maintain my weight I needed to keep my intake around 1800kcal because my life outside of that was not very active. I'm 185cm, and at the time I was about 93kg, with body fat around 15%.

Someone with a more active life outside of the gym might burn more. Many burn less. Kcal guidelines for adults have remained static for way too long with declining levels of physical activity, and people really should count calories for a few weeks and adjust their intake and figure out how little they actually burn...

> Unless you're a competitive weightlifter or similar, you're unlikely to see significant enough muscle loss with very significant calorie restriction over the kind of time frames people would stick to very strict diets like that for it to matter much.

I've always done weightlifting, although not competitively, as general support of the fitness regime (which may include other sports or not).

What I've observed is that when a dieting cycle goes bad, a short time after the end of it (to allow glycogen and water levels to come back to normal), I end it with approximately the same weight before the start. If/when I observe an increase in measured fat (which happens most of the times, on bad diet cycles), then muscle mass went down.

If a diet ends up with little enough weight loss that you'll just bounce back to the same weight, the changes are small enough that I really doubt you can reliably say much about change in muscle mass. For me the bounce back at the end of a diet is pretty much always ~2kg, so a "loss" below 2kg is really not an actual loss, but easily accounted for by less fluid retention and less food in my system. Yes, you're likely to lose some, but again I'l stress: not much for it to matter much for most people. Especially when the point of comparison is not doing nothing but just dieting slower. You'll lose muscle mass while dieting slower too. I've seen little to indicate there's a particularly meaningful difference in proportion other than for pretty advanced lifters for whom a "meaningful" difference in loss is much smaller than for most people.

When I was pushing my body to the max every week, then the difference mattered to me too. But most people are not there, or anywhere near those kinds of limits.

Tell that to the 1.8 billion muslims that don't eat for a month out of the year.

Err, that don't eat between dawn and sunset for a month out of the year...