The source of energy is sun and if we cannot take energy from the source directly then the primary source is plants. It is easy to digest the simple cells of plants and release the energy from these cells.
We do not have to wait for long for this energy to be released in our body. Animals and birds are the secondary and tertiary sources of energy. So, they have a complex cell structure in which they store this energy, so they need more time to release this energy. They sit in the stomach and our bodies need to work extremely hard to obtain the energy. This phenomenon can be observed in the amount of time it takes us to cook non-vegetarian food versus vegetarian food.
If not totally becoming vegetarians, yogis should attempt to reduce non-vegetarian foods and increase vegetables in their diet.