[Elte]

I'm a bit confused by this point in the article, because it also states:

> There seems to be a hard limit on how many calories our bodies can burn per day, set by how fast we can digest food and turn it into energy. He calculates that the ceiling for an 85-kilogram man would be about 4650 calories per day.

Given that "regular" people clearly do not burn 4650 calories per day, and it is possible to burn 4650 calories per day, there must be a point at which exercise _does_ increase energy expenditure. I'm guessing it just doesn't happen for regular doses of exercise (including, evidently, walking 14km per day).

Perhaps the body down regulates calorie-consuming processes to a point where it's just the bare minimum, and calorie expenditure increases from there. Or perhaps we should take the opposite view and say that our bodies up regulate unwanted processes (like inflammation) to use the energy of an engine designed to keep running at a certain level?

Either way I find this incredibly interesting. And either way I'm probably also going to keep stuffing my face on a day I run 30km :).

[tempestn]

Yeah, the article isn't very clear on this. ISTM the claim is not that exercise doesn't increase energy consumption at all, but just that it increases it by much less than the amount of energy expended in the exercise, because the body (partially) compensates elsewhere. Whether the degree of compensation varies based on amount of exercise I'm not sure, and I wish it had explained. I expect it probably would, as presumably there would only be so much 'low hanging fruit' for your body to use when compensating.

[alistairSH]

I can definitely see how that works. I'm a cyclist and often ride 60-100 miles on a weekend day, plus 3 days pf 20-30 miles during the week. That should be a lot of extra calories. But, after those long rides, I usually need a nap and don't do much else (maybe grocery shopping, but definitely not woodworking or other serious projects).

That said, at the peak of my training (12 hours/week), I can definitely consume more calories without putting on weight. Like, a giant bowl of ice cream most nights. But, if I continue that diet during recovery periods, I'll put on a few pounds. So, it's not that exercise has zero impact, but possibly quite a bit smaller an impact that one might assume.

[hgomersall]

Yeah, I thought this. There's clearly an amount of exercise that will cause you to lose weight. An interesting question is whether your body prevents you from achieving that.

Cycling was always interesting to me in this context because it seems easier to burn energy on a bike than other forms of exercise.

[alistairSH]

seems easier to burn energy on a bike than other forms of exercise

I'm not sure this is true. Cycling isn't weight-bearing (you're sitting down) and only engages the large leg muscles. Running and rowing likely provide more calorie burn for a given RPE (rate of perceived exertion) over a fixed time period. Running because it's weight-bearing; rowing because it engages more muscle groups. That said, you can probably cycle for more hours total, if you have nothing else to do.

[naasking]

Running doesn't burn as much as you think because your muscles store energy elastically on the eccentric phase of each step. This removes a lot of energy needed for the subsequent concentric part.

Contrast this with cycling which is pretty much all concentric contractions.

[alistairSH]

Interesting, hadn't considered that aspect. Just goes to show how complicated and counter-intuitive this can be.

[naasking]

Evolution is great at optimizing! Running is so efficient for humans that some human tribes use it to hunt (persistence hunting). They literally run their prey to exhaustion then just walk up and kill them.

[ajuc]

When cycling there's this "wall" you hit after a few hours. At least I do. After that it's much harder to keep pace or go uphill.

[nradov]

There's nothing special about cycling. You can deplete your glycogen stores and hit the wall with any aerobic activity. If you have a healthy metabolism and stay in heart rate zones 1-2 then you'll mostly burn fat stores and can cycle for many hours without hitting the wall. At higher zone 3+ efforts you'll need periodic carbohydrate supplements to keep going.

[hgomersall]

Indeed, but you can keep going just about. I've properly hit the wall once, and that was an unpleasant experience - nearly fainting and seriously in danger of falling off my bike, but I think that was largely a hard removal of all accessible glycogen, and it was years ago when I was relatively green. Now I just tend to eventually find it hard and unpleasant.

[throwawayboise]

> there must be a point at which exercise _does_ increase energy expenditure

Of course there is. The problem is that it's much easier to eat than it is to exercise. You can eat a Big Mac in 5 minutes, and you'd have to run or bike for hours to burn those calories off. A 30 minute jog on a treadmill won't do it.

Most people eat more calories than they need, and would have to exercise much more than they realize to burn them off. So losing weight over a reasonably short period of time almost always requires cutting calories. Few people have enough free time to do it with exercise alone.

[noogle]

The efficiency of our bodies (or the amount of calories in food) is astounding. Makes one think whether there is any acceptable use case for serving snacks and other high-calorie-density food.

[aixi]

out of context, that quote is really funny; what happens once you reach 4650 calories? you start violating the rules of thermodynamics?

(i know your body starts compensating for it and burning less calories, but still)

[KronisLV]

> what happens once you reach 4650 calories?

Well, we probably need to look at the direct quote again.

> There seems to be a hard limit on how many calories our bodies can burn per day, set by how fast we can digest food and turn it into energy.

Maybe i'm reading this wrong, but if one's body were to burn calories at 100% of this supposed possible rate, then by the time you'd reach 4650, a new day would start.

If digestion would top out at 4650 calories at day, with 24 hours per day, it would come down to 193.75 calories per hour. Or, in other words, it'd be about 3.23 calories per minute.

[dahart]

> what happens once you reach 4560 calories?

There are loads of videos on YouTube of what happens. You may or may not want to watch them. Spoiler alert: you vomit when you eat too much.

The vomit limit is probably higher than 4560 (which BTW has a suspicious number of significant digits), there might be a range between too many calories and vomiting where your body breaks down the food into waste without digesting any more nutrients/calories. Kinda like how if you eat too much vitamin C, you’ll just pee most of it out.

[InitialLastName]

At some point your body will also fail to exert energy; there are also metabolic safeguards ahead of the point where your body can no longer manage the energy to keep basic functions going where the lower-priority functions (immune system, cognitive function, motion muscles) start to degrade.

You don't need to be at the limit of human energy intake to see what happens to people when their energy expenditure greatly exceeds the energy available from food over a sustained time period; there is quite a lot of medical literature on the effects of such... er... malnutrition.

[tempestn]

Long term, yes. Short term you might be able to get some from fat stores. But the claim is that ~4650 is the most you can persistently get from food intake per day.

[Peeda]

Sort of makes sense, the body is a machine and will get worn down/depleted at some point and not some infinite bag of holding. Surprised its as low at 4650 calories though, figured marathoners + swimmers and such could burn more.