Metabolism
When the nutrients in the bloodstream pass through the cellular membranes of the body, they enter into the metabolic processes of the cell. Metabolism may be defined as a process by which the cells convert nutrients from food into useful energy, which can be utilized for performance of work as well as for synthesis of new compounds vital for cellular structure and function. The process by which nutrient molecules are degraded, with concurrent release of energy and subsequent elimination of waste products, is generally known as catabolism, whereas anabolism refers to the synthesis of new compounds. The anabolic processes depend on energy from the catabolic processes, both proceeding simultaneously.
Some may be linked together through common intermediates. Metabolism is an ongoing process in every cell of the body, requiring a continuous supply of nutrients.
Wide variations exist among groups of people and individuals in their daily intake of foods. Some people have meals at stated times others cannot or do not. Fortuna tely, mechanisms exist that allow a steady flow of nutrients to the cells to continue for limited periods of time, even though no food is ingested.
In the period immedia tely following ingestion of food, the levels of most nutrients in the blood rise due to absorption from the intestine. The rate of absorption varies with the nutrient, the quantity ingested, and the person, but in general the peak level for carbohydrate (glucose) is reached in 1 hour and for fat (chylomicron triglycerides) in 4 to 6 hours. At the same time the uptake of nutrients by the tissues also is rapid and eventually exceeds the rate of flow from the intestine, resulting in a gradual decline in the blood nutrients to fasting levels. In most people fasting levels for triglycerides are attained in 8 to 12 hours and for glucose in 2 to 3 hours.
The rate of protein (amino acid) absorption falls somewhere behveen those of carbohydrate and fat. However, the changes in the amino acid concentration and pattern in the blood are relatively small after protein ingestion because the absorbed amino acids pass through the liver, which removes a large proportion of them and controls their release to the general circulation.
Upon reaching the cells, some ofthese nutrients enter the catabolic pathways to supply energy for immediate needs. Aside from small functional needs, the remaining nutrients are converted to various storage forms from which they can be recalled later when needed.
Glucose is converted to glycogen to replenish the tissue stores, but due to the body's limited ability to store gycogen, the remaining glucose is converted to fat and stored as triglyceride, mostly in the adipose tissue and, to a lesser extent, in the liver and muscle. Excess dietary fatty acids also are stored as triglyceride. Protein synthesis in the tissues is high after ingestion of a balanced mixture of amino acids the excess is either oxidized to provide energy or first converted to glucose or fat.
