Net gain of food
The body gets its energy through food by way of nutrients. The more energy the body must expend to digest, assimilate, and utilize the nutrients in the food we give it, the less energy we are left with. Unfortunately most food in the modern western diet require almost ass much energy to assimilate as they contain. They therefore have an extremely low net gain. The nutritional value of food stated on the packaging refers to what is in the food, not what your body gets from it. The digestion process requires energy, a large portion of which is expelled as heat.
The energy used digesting the food is turned into heat that is then expelled into the environment. As heat escapes the body, so does energy.
Had energy not been lost through this process, had it been conserved through greater efficiency in digestion, it might have been used as fuel for the body or to refabricate healthy cells.
Because high-net-gain foods are easier to digest, you may notice that your core body temperature drops slightly when you adopt a plant-based whole-food diet. While much healthier and a testament to a more efficient body, a lower temperature may take some getting used to. But your body will adapt and become accustomed to it and function more efficiently as a result. We can all benefit significantly from a body that retains energy by operating more efficiently, and the advantage for people with busy, stressed lives are especially great. Starting with a lower core temperature provides a greater window in which to operate.