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Chemical bonds are on the scale of eV---the cost to liberate an electron from a hydrogen atom is 13.6eV, for example. So the best you can hope for, in a chemical reaction, is a change in mass of the scale of 10s of eV / c^2.

In comparison, a single proton weighs 938 MeV = 9.38e+8 eV. So the fraction of the mass that is converted into energy is on the scale of 1e-7.

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This is the correct answer.

There's no such thing as conservation of mass, mass and energy are convertible into each other. But in the real of chemistry that conversion happens on a ratio of at most 1e-7, so when it comes to the human body we might as well say that the weight that goes in must equal what comes out, and that's close enough.

Typo. *in the realm of chemistry

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