Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: February 25, 2000
Publication Date: N/A
Interpretive Summary: The imposition of stress on mammals results in compromised nutrient intake and efficiency of nutrient use for growth and lactation. The level that an animal's metabolism is affected by stress can be thought of as a two-part response that is determined by the severity of the stress. In low-level stresses, some disturbances in nutrient use can be made up for through the animal's increased food intake. At higher levels of stress, especially where food intake decreases, more and more nutrients are channeled away from growth to support immune functions. When the "break-point" in stress is reached, where nutrient intake can not match the need for calories and amino acids, tissues in the body are broken down to provide the resources needed for the immne response. Many forms of stress are additive and further complicate mechanisms that can rebalance to a normal state. In the process of responding to stress, the production of oxygen and nitrogen free radicals results in the further formation of compounds toxic to cells in organs or disruption of normal biochemical signaling processes. Identification of key sites at which stress affects metabolic controls will be helpfyl in developing ways to help animals combat the pathology of stress.
The priority with which nutrients are delivered to, used and turned over by different tissues in the body varies naturally according to the age, reproductive and lactation status, and activity needs of the animal. The overall impact of stress on metabolism revolves around a dynamic relationship between the level of challenge imparted on a physiological systems and the degree of host response that is mounted in the process of detecting and reacting to the stress. Typical of the young animal, the majority of milder stress encounters are manifest in terms of energetic inefficiencies and periods of reduced growth and anabolism. In contrast, severe stress is often characterized by frank catabolism and tissue wasting. In either scenario, there is a gradient hierarchy with which physiological processes are affected by stress (survival has a higher priority than adipose tissue accretion) and as such different tissues are metabolically affected by the level of stress. In some instances a level of stress (that might be termed a "stress breakpoint" ) is reached at which time the host response itself contributes to the cascade of negative effectors that further shift metabolic processes away from maintenance or anabolism towards catabolism. This summary points out some of the important relationships between stress and altered metabolism common to animals undergoing stresses from a variety of sources.