In cases of extreme starvation, the energy substrate most likely to be used as a last resort would be

During hunger or starvation, the body reacts to periods of low energy intake by burning and draining different energy substrates. During short times of hunger or energy abstinence, the body burns fatty acids from body fat stores, and utilizes glycogen that are stored in liver cells. However, during extreme cases of starvation, the body runs out of fat, which results to the body using proteins and burning muscle tissue as an energy source.

then that particular organisms become extinct.