Food is Fuel - keep your tank full.

Don't start, continue or finish your workout on E just because you feel you don't "need" any fuel. Training requires an expenditure of energy above resting levels as oppose to sitting around by your TV or computer and feeling the "need" to eat. This required mechanical energy is provided through the conversion of metabolic fuels into ATP, the base currency of chemical energy. The sources of chemical energy that fuels exercising skeletal muscles are available through endogenous sources (intramuscular glycogen and triglycerides) or exogenous sources (plasma glucose and free fatty acids). Rather than worrying about extra calories put into your body while you are expending energy, consider how important it is that these exogenous and endogenous fuel sources are replenished through dietary intake. Next time you think about needing "energy" around mid afternoon because you are tired from sitting all day, consider important relationship between diet and fuel metabolism in skeletal muscle before, during and after training.