Nutrition

The nutritional needs of animals vary greatly with their metabolic state. A rapidly growing, active young animal has much different nutri­tional needs from those of an older, more sed­entary animal.

Some essential nutrients are required in minute quantities and are toxic in large quanti­ties. For example, copper is an essential mineral, but excessive consumption over a short period leads to copper toxicity. Minerals that are needed in small quantities are often termed trace minerals. Vitamins function as coen­zymes for various biochemical reactions throughout the body and are also typically required in small amounts. Water-soluble vita­mins (B complex, biotin, C, folic acid, and niacin) are not stored in the body, but fat­soluble vitamins (A, D, E, and K) are stored in the liver and adipose tissue. Water-soluble vita­mins can be consumed in large amounts without significant toxicities because the excess is rapidly excreted in the urine. Because they accumulate in the body, excessive consumption of fat-soluble vitamins can be harmful.

The nutritional and metabolic status of indi­vidual animals may be evaluated by determin­ing the balance for a given nutrient or type of nutrient. Balance is determined by comparing the amount consumed by the animal to the amount used or lost from the body. For example, because proteins are the primary nitrogen­containing nutrient, nitrogen balance is often used as an indicator of the status of protein metabolism. if an animal consumes more nitro­gen than it excretes, it is said to be in a positive nitrogen balance, indicating that the animal is synthesizing more body proteins than are being degraded and lost from the body. Young, growing animals are typically in positive nitrogen balance. During prolonged starvation, an animal is in negative nitrogen balance, for body proteins are broken down to provide energy, and the result­ing nitrogen is excreted in the urine.