Considering primary sources of energy, we find that coal replaced wood and hay, and oil in turn beat coal for the lead position in the world power game. (phe.rockefeller.edu)
You don't get the chance to access them because you eat carbs that are once again primary source of energy. (balancedbyana.com)
The diet is based on the simple premise that when carbohydrate intake is drastically lowered, or stopped entirely, the body must find a new primary source of energy. (healthline.com)