Energy 
Energy is required for every single biological reaction in the body. Without adequate energy intake, vital organs do not operate properly; cells do not multiply, bones and muscle tissue do not grow, the mammary gland does not produce milk, and reproduction is halted. Energy is essential for maintenance, growth, health, lactation, reproduction, and gestation.
Maintenances are the metabolic processes, such as heartbeat, brain activity, respiration, and other vital functions. Physical activity is also classified as maintenance.