Just write the equation like this:<p><i>Energy intake - Energy expenditure - Change in energy stores = 0</i><p>There. Just f(x,y,z)=0. That's what the first law really says. There is no causality implied, it just a first integral of the "motion" of metabolism that the body, naturally, has to follow.<p>It does NOT say what variables are free to independently modify. It's just a relation the three of them have to obey.