RE: "CICO doesn't apply, the human body is not a closed system"

Thread.

It's startling that whenever you point to the first law of thermodynamics to someone who denies "calories in calories out" they respond with some variant of "Well the body is not a closed/isolated system"

Obviously the body is not a closed or isolated system, if it were, no one would be able to gain or lose any substantial amount of weight as the total energy content within a body would have to remain a constant (isolated system) and the total mass could not change (closed system)

Saying the human body is an open system just means that it is possible for the human body to gain or lose mass/energy (ie: not requiring the human body to have a constant total energy content and allowing mass to be added or subtracted from the system)

I'm not sure what relevant point this proves exactly. This is such a bizarre response. Anyway, the first law of thermodynamics absolutely CAN be invoked to explain open systems, so long as it can be shown that said open system is within a closed system

As a side note, if you are tracking the mass, it can be invoked even without it being shown that said open system is within a closed system.

For example, if an open system A is within a closed system B, the total energy content of system B must remain a constant.

This means that caloric balance of A is completely predictable given enough data, since any output of energy from A to B or input of energy from B to A must be able to be predicted as they both add up to a constant value.

Any work performed by A on B is measurable, and any calories consumed by A from B is also completely measurable.

This can be expressed as follows:

Ai+Bi=C
Af+Bf=C
Therefore:
Ai+Bi=Af+Bf
Where Ai is the initial energy state of A
Bi is the initial energy state of B
Af is the final energy state of A
Bf is the final energy state of B

Thus change in the energy content of A *MUST* be offset by an equal and opposite change in energy content in B despite the fact that A is an open system. Also notice how you don't even have to know the value of C (the total energy content of A+B) to make these calculations.

Same thing applies to the human body: In an open system, the caloric expenditure from the system of the body to the system of the outside world and caloric intake from the system of the world to the system of the body entails a balance with respect to the body that always applies

This is known as conservation of energy it applies for open systems. We can simply treat the entire universe as a closed system, requiring the conservation of energy during energy transfers from the body to the universe and the universe to the body.

This doesn't mean the body will expend calories at a constant rate - and this may be the hardest part of the equation to predict, nor does it mean that metabolisms can't change over time, nor does it mean different foods can't result in different absorption/burning of calories.

But one thing is for certain - the fact that the human body is an open system in NO WAY implies that the first law of thermodynamics can not be invoked to explain weight loss and weight gain given enough data. It's CICO all the way regardless of the system being open or closed.