A pound of fat is 3500 calories. If a chess player eats nothing at all they are unlikely to burn that many calories in a day. Chess players do eat during a tournament. Therefore weight loss must be water.

That's not how weight loss works- you don't lose a pound by "burning" fat.

How else would it work? You burn fat (as in: you turn it into energy, powering your own living cells), turning it into CO2 and H2O, which are then breathed out or expelled with urine.

What? Yes it is. Your fat cells are turned into ATP via Beta-oxidation and used for cellular energy. The excess carbon from the processes is then exhaled as CO2 during normal respiration and the water is sweated or urinated.

When you lose about 3500 calories of energy via this process, you have burned a pound of fat.

The fat in the cells are turned into energy, but generally the cells remain.

Yes that is true, the water and lipids in the cell are extracted, but the cell stays there until its natural cell death (which can take years).

That doesn't really have anything to do with the conversation though.

Do people who burn 3500 calories weigh one pound less? Are you sure? From what I can tell, weight changes happen only partly due to CO2 exchange and partly due to rates of consumption and excretion.

The biophysical origin (I'm a biophysicist) of weight change as a function of energy use, eating and excretion is a bit more complicated than that.

So wait if I fasted for two days, could I responsibly expect to come out the other side one pound lighter? That’d be a roughly 3600 calorie deficit.

Yes.

Assuming that they burned purely fat for energy, what else could it be? Are you saying that it's not just fat being burned, or that 'burning' the fat is not going from '1' to '0' in terms of weight?

Give or take, yes.

Like you said, it’s a bit more complicated than that and every body is different. But burning roughly 3500 calories will result in the body weighing roughly 1lb less (accounting for food and water intake and loss).

If it didn’t work this way, every bodybuilder and fitness enthusiast on the planet would have a very different approach to their diet.

The chemical process that consumes fat is a combustion reaction.

Yes, that part is true, I agree. That's not what I was saying, though.

I totally agree. My personal opinion here is:

1. They're stressed and therefor eating less, especially if they're spending the daytimes of the tournament playing games - hardly conducive to appetite, and in fact I expect many will avoid food purposefully to try to not get that sleepy-full feeling.

2. To a much lesser degree, increased stress == increased heartrate == increased BMR.

3. Maybe, and I hate to speculate because of the stigma (personally I don't care if chess players are doing drugs), some amphetamine use, or at least caffeine, all of which can suppress appetate further.

I personally very much doubt that it's the thinking hard that's actually burning calories.

Unfortunately, I can't even read the linked post! It's paywall'd.

Absolutely water weight.

I can drop 10 pounds this week trivially by triggering a water flush (change in diet), essentially at will. Wife absolutely hates it, but that’s water weight.