[dmurray]

We use cows for the same purpose - we eat them instead of shoveling grass straight into our mouths, even though in some strict sense the grass contains more caloric energy.

Or, we eat vegetables even though the energy in them is less than all the solar energy that falls on them.

But of course the calories in grass and sunlight aren't accessible to us. It's not impossible to imagine some parallel situation where human biology - perhaps genetically modified - is useful to transform some source of energy into a more usable form.

[nottorp]

For meat you could argue that the energy density is much higher than in veggies, so the cows do the job of concentrating it for you.

[WalterBright]

Carnivores are essentially thieves - stealing the energy concentrated by plants instead of doing the work ourselves.

[hinkley]

Herbivores just hate plants.

[hinkley]

Synthetic organisms would have a very different opinion of consumption than we do. Sustenance is much more than calories for us. Not so much for them.

[dmurray]

Use your imagination. Maybe some biological process in genetically engineered humans can sort uranium isotopes more efficiently than a centrifuge. That would make them a net producer of "energy" by a reasonable definition.