[Peradine]

Basic thermodynamics:

If inputs > outputs, there is net energy storage (as fat or protein)

If inputs < outputs, there is net energy deficit (lost from fat and protein)

Inputs = food and drink consumed

Outputs = Basal metabolic rate (BMR) + energy used for activity + energy lost in urine and faeces

Faeces caloric content varies between 50-350 kcal / day, urine 91–117 kcal / day [1]

Basal metabolic rate is primarily decided by fat-free mass, mass, and age, 26% of BMR is not defined by these parameters. Mean BMR is 1500 kcal/day, and 26% of this represents a variability of 1305-1695 kcal per day.[2]

(350-50) + (117-91) + (1695-1305) = 716 kcal

Therefore two people of identical age, fat-free mass and mass who consume identical caloric intake and perform identical exercise will have, at most, a variance of 716 kcal / day in their energy expenditure from the variance in urinary and faecal caloric loss and BMR

[1]: Rose C, Parker A, Jefferson B, Cartmell E. The Characterization of Feces and Urine: A Review of the Literature to Inform Advanced Treatment Technology. Critical Reviews in Environmental Science and Technology. 2015;45(17):1827-1879. doi:10.1080/10643389.2014.1000761. Section 3.2.5 and Section 3.6.4

[2] Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. Johnstone AM, Murison SD, Duncan JS, Rance KA, Speakman JR. Am J Clin Nutr. 2005 Nov;82(5):941-8.

[pavel_lishin]

Neat. But what if your body is more prone to converting calories into fat than muscle, and/or breaking down muscle tissue when in starvation mode rather than fat?

[closeparen]

Aren't we relying on the body to be inefficient? That is, consume more energy from food than is actually exerted onto the treadmill (or whatever) during exercise.

Thermodynamics provides a lower bound on the amount of energy taken from food, but but couldn't it cost me 2 kcal to put 1 kcal into a stationary bike, while it costs someone else only 1.5?

All conservation of energy says here is that it has to cost at least 1.

[nodamage]

Thermodynamics does not account for the role of digestion with regards to nutrient absorption. For example, recent studies have shown that gut bacteria can have an affect on fat absorption in the body, and hence how many calories are extracted from the food you eat. Meaning that two people can eat the exact same thing but extract different amounts of calories from it.

[criddell]

> two people of identical age, fat-free mass and mass who consume identical caloric intake and perform identical exercise

His friends may be the same age, but have wildly different amounts of fat and muscle despite similar exercise regimens. I think that's what his point was, not that thermodynamics don't apply.

[lukeschlather]

Assuming that 9kcal of fat = 1g, that's about 80g of extra fat stored/lost per day. Over the course of the year, that's 3 kg, or about 6.5 pounds.

That kind of variance is enough to mean that if we trust the study, you could raise two different twenty-year-olds and one will die of starvation and the other will be clinically obese despite the same routine and diet.

[bluecalm]

It's not true. Even if they start with very different calorie needs for the same lifestyle that will even out once their weight changes. The more you weigh the more calories you need and the less you weigh the less you need. That's where the myth of metabolism slowing down after weight loss comes from btw: you just need less calories after some time of dieting so your progress will slow down or steak stall if you don't adjust.