What change in the nutritious regime are needed without organic limbs

Question

I got this idea of a society where for some reason where electric energy and a vast array of mechanical prostesis are basicly free, but food on the other hand is extremly expensive. The thing is I want to know how much some replacement parts would impact the kind & quantity of food required.

There would be three big kinds of body modification: one where you get just all of your limbs replaced, one where it's all of your muscle that would be phased out, and lastly but more rare, your brain would be the only remaining biological part of you.

Answer 1

This is a topic that is of interest in the real world. Amputees have different nutritional needs compared to what they had pre-amputation.

TL;DR after the whole healing process amputees generally need only to adjust for weight, but not for the actual weoght they have.

I found an article in the American Journal of Clinic Nutrition with a proposal for the means to estimate the proper body weight to use for diet adjustments for amputees. You can read it fully and for free at this link.

It already starts with:

Body weight is a good indicator of a person's size and is widely used in clinical assessment. However, health-status assessment based on observed body weight (W_O) is incorrect for persons with limb amputation.

It is also heavy on the math side, no pun intended. An excerpt:

(...) Thereafter, estimation of the remaining proportion of the limb may be made by calculating the proportion of the length of the stump to the knee height or buttock-knee length as follows:

ΔpWT=ΔWT−ΔWT×LStp/LKn=ΔWT(1−LStp/LKn) (11)
ΔpWF=ΔWF−ΔWF×LStp/LBtK=ΔWF(1−LStp/LBtK) (12)

where LStp = length of the stump, LKn = knee height, and LBtK = buttock-knee length.

Therefore, new corrected formulas for estimating the lost proportions of total body weight in the case of persons with amputations below or above the knee may be written as follows: Amputation below the knee (transtibial),ΔW=ΔWS+ΔWT(1−LStp/LKn) (13)

or

ΔW/WE=ΔWS/WE+ΔWT/WE×(1−LStp/LKn) (14) Amputation above the knee

(transfemoral),ΔW=ΔWS+ΔWT+ΔWF(1−LStp/LBtK)

If you are writing fiction, I suggest you drop the math and just simplify it by just saying that people eat less.

Answer 2

You shouldn't need different food so long as you can still actually eat food since your organic parts don't require anything different. To be able to continue to eat you would need the stomach and intestines just get replaced with artificial equivalents. If they were instead just removed, you would have to consume some special formula instead and still need something equivalent to intestines for it to exchange nutrients into your blood.

Much trickier is that the bone marrow that replenishes the blood which keeps all the other organs alive. At some point in the cyberization process, you will eventually no longer have enough bone to replenish the blood. Before that happens you would need to install a brand new organ to do the job (possibly also replacing the blood with artificial blood). And if you do use artificial blood, depending on how you want the artificial blood to work in your story, it may be incompatible with the spleen, bladder, and kidneys so they might have to be replaced with compatible equivalents at the same time too.

However, if food is so valuable those amputated and excised biological parts are probably worth quite a bit. Surely worth saving as food so that they can be eaten by the person they were taken from at least. So you could have a system where desperate people get cyberized in order to feed themselves with their own body parts.

Quantity of food? I'm no doctor or biologist but I imagine the easiest thing to look up would be the energy and nutrition requirements of the brain, and everything else would be much more difficult to look up.