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Comment by adekok | Hacker News Reader

2 pounds a day. That's how much CO2 you breath out, and the hard limit for weight loss.

You can lose more than that, (see boxers before a weigh-in), but that's all water loss, and is temporary. And, with a strong likelihood of death.

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Fat can break down into ketones which can be passed out via urine (i.e. leave the body without being oxidized to H2O and CO2).

This is noted in the article, in the caption under the second figure.

There is a hard limit of ketogenesis: about 185g of ketone bodies per day [0].

(That's about 2 lbs every 3 days).

[0] https://www.diapedia.org/metabolism-insulin-and-other-hormon...

That's a matter of liver health, right? So you could theoretically go higher (by combining antidiabetic drugs with beta-adrenergic agonists, and then sitting in a hospital in a cold water bath hooked up to a liver dialysis machine.)

Not a viable alternative to dieting and exercise, of course, but maybe an alternative to liposuction or gastric-bypass surgery.

I'd suspect the more exercise you do the more that number moves up, you'd be breathing harder. I'd hesitate to try and put a 'hard limit' on anything like this.

Also take daylight savings into account, you could get an extra hour one day a year. Thats a lot more breaths.

Not sure about you but I keep breathing 24 hours a day.

you're clearly not trying hard enough.

Supercharge your weight loss by running west.

But optimize this by not running too close to c (or is it the contrary ? My restrained relativity courses are rusty)

In your reference frame nothing will be different, so it really depends on whether you want to loose weight for yourself or for others.

Except he had to accelerate to speed and back, so only our reference frame matters.

Supercharge the whole planet's weight loss by spinning counterclockwise... Robbing the planet of it's angular momentum and extending night and day.

It's pretty difficult to double that number. Can you really double your metabolism without ill effect? Or without massively changing your lifestyle?

The normal expectation is maybe 10-25% change. Any more than than would require things likes 8 hours of exercise per day.

Losing eight pounds during 3-4 hours run (marathon event) is not unheard of: http://www.letsrun.com/forum/flat_read.php?thread=3319287

Like the previous commenter mentioned, that's mostly water. I've seen various numbers between 100 and 150 calories burned per mile for runners, which at the high end is just a bit over one pound's worth of calories for a marathon.

I’ve read about people who do things like swim the channel, and bulk up (10’s of lbs) before doing so. Given that they lose this weight in the process, how does that happen?

I think that's mostly independent. A swimmer bulking up for a channel swim is probably putting on a mix of water, fat, and muscle in the slow lead up to the swim (and maybe extra water in the days before) and then losing water during the swim. Here's [0] a discussion of swimmers strategizing about weight gain, and you can see that it's largely about issues like insulation from cold, buoyancy, and strength, and not because they're worried about losing the weight during the swim itself.

[0] http://marathonswimmers.org/forum/discussion/90/when-to-gain...

5-6000 Calories is normal for an 8 work hour shift at -10c.

https://academic.oup.com/jcem/article/99/12/E2772/2833739

This is very interesting paper, but is it a right one? I do not see any mentioning of working in such conditions nor 5-6000 calories per day.

That paper isn't the right one. I can't find it now. Here is another interesting one though...

http://www.dtic.mil/dtic/tr/fulltext/u2/a240839.pdf

I will update this if I find it again.

A colleague who was into serious mountain climbing said that at 5km it was not enough to eat like 5000 calories per day to maintain weight. He even on purpose gained weight before his trips, and still returned leaner despite eating a lot.

You mean 5k elevation?

Yes, 5 km elevation

Do you have any sources for those numbers or did it just feel right to type them?

"Strong likelihood" only in cases of extreme dehydration. Many wrestlers/ boxers will drop >5 lbs in one day with no longer term impact. The majority of that being water weight they gain right back again.

Well, but you also breath in the O2 first, I think the C just binds to that. C and O weigh similar amounts so it is just 1/3 of the 2 pounds. But you have to count the water loss as well, it is part of fat cells.

How many calories can you max burn per day, around 8000? That would be ~1.1kg of fat loss at 7 kcal per g fat. At a normal day of not eating it is ~300g afaik (2100 kcal at 7kcal/g).

I can't tell you how happy it makes me that both methods for estimating weight lost in a day work out to be almost exactly the same.

> 2 pounds a day. That's how much CO2 you breath out

I bet I breath out more on Saturday's marathon than Sunday's recovery day.

What if someone has a deficit of over 7,000 calories/day for an extended period of time (suppose they eat 3k calories and bike for 14 hours/day)?

To have such enormous deficit you would have to exercise a lot. And of course you breathe faster during the workout so I think it is entirely possible to breathe out more than 2 pounds of CO2 per day. 2 pounds if probably average for a typical person.

That isn't physically possible. You would deplete the nutrients in your muscles within a few hours and then "bonk", unable to continue cycling.

But you also breathe in co2 with every breath so would need to subtract that..

the inhaled partial pressure of CO2 is 0.3 mmHg. The exhaled partial pressure is 35-45 mmHg. The inhaled CO2 would be lost in the rounding.

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