The action of preferentially using carbohydrate and fat to provide energy allows protein to be saved for other purposes. The protein sparing effect conserves protein for muscle tissue by using alternate sources of energy, such as carbohydrates, fatty tissues and dietary fats. By consuming an adequate amount of carbohydrate daily, you can prevent the use of protein for energy. This role of carbohydrate is known as its protein-sparing action. The carbohydrates of fruits, vegetables, grains and legumes provide most of the energy in a healthful diet. Excess proteins help supply energy or build reserves of glucose and lipids.
Storing Glucose
Your body needs glucose as its primary nutrient and source of energy. Your liver stores glucose in the form of glycogen and releases it into your bloodstream as needed. Muscle cells can also store glucose as glycogen, but they hoard it for their own local use in muscle contraction and exercise. Glycogen stores can provide energy for a matter of hours, not days. More glucose needs to be provided from carbohydrate, if available, or from protein and other non-carbohydrate sources.
Using Glucose
Glucose fuels the work of most of your body's cells. In the absence of dietary carbohydrate, you can survive by drawing energy from fat and protein through complex metabolic processes. Glucose alone, however, can meet the nutrient requirement of your brain and nervous system, developing red blood cells and white blood cells, according to "Krause's Food, Nutrition, & Diet Therapy" by Kathleen Mahan, M.S., R.D., CDE and Sylvia Escott-Stump, M.A., R.D., L.D.N. For these specialized cells, your body must make new glucose from non-carbohydrate sources, such as protein, if no carbohydrate is available.
Making New Glucose
When you do not replenish glucose from your depleted glycogen stores by eating carbohydrate, your body dismantles protein from your muscle tissues to convert it to glucose. When it must, your body can convert its own protein tissues to glucose for survival, but protein has other jobs of its own that no other nutrient can do, so sparing protein for its main role is a priority for your body.
Ketone Bodies
Another consequence of an inadequate supply of carbohydrate is a breakdown of fat into fragments that combine to form ketone bodies, the products of an incomplete breakdown of fat. Muscles and some tissues can use ketone bodies for energy, but they can accumulate in the blood and cause ketosis, a condition that disturbs your normal acid-base balance. To spare protein and avoid ketosis, you need 50 to 100 g of carbohydrate per day.
References
- "Understanding Nutrition, Ninth Edition"; Eleanor Whitney, Ph.D. and Sharon Rolfes, M.S., R.D.; 2002
- Elmhurst College Virtual Chembook: Protein Metabolism; Charles Ophardt; 2003
- "Krause's Food, Nutrition, & Diet Therapy"; Kathleen Mahan, M.S., R.D. and Sylvia Escott-Stump, M.A., R.D.; 2000
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