Doing anything else, I guess
Answer:
All natural energy in living creatures comes from the sun.
Hydrogenation of vegetable oils and animal fats is a standard industrial procedure. The more saturated a fat is, the longer its shelf-life. Why? Because a site of unsaturation, an olefinic bond, is potentially a reactive site that can be oxidized.
Of course, these days we are sold on the benefits of unsaturated fats. A priori, an unsaturated fat will yield less energy upon combustion, and the <span>C=C</span> double bond is harder to metabolize than <span>C−H</span> bonds.
