Fuels are the energy substrates carried in blood plasma that cells burn for energy.
So far this group holds:
- Glucose — the body’s default fuel
- Ketones — the backup fuel the liver makes from fat when glucose is scarce
(Free fatty acids belong here too, once noted.)
This group is defined by role, not by origin: glucose is also something you eat (a dietary nutrient), while ketones are made inside the body. What unites them is that cells can burn them — so they sit together as fuels, even though only some are nutrients.