Fasting and starvation mode
Hi – I was wondering if I have to fast (its usually 12 hours a day) – does my body enter into starvation mode? Also – when stopping my fast – what is the best way to start eating?
Blood sugar, called glucose, is needed for brain and nervous system function. Many tissues including red and white blood cells are also dependent on glucose.
To maintain proper function, the body’s blood glucose levels need to be maintained within normal range (70 – 90 mg/dL). Glucose can be obtained from carbohydrate intake, or from the breakdown of stored sugar, glycogen, within the body.
Once glycogen stores have been exhausted, glucose must be synthesized from proteins. During starvation or fasting, hormones stimulate the breakdown of glycogen and cause the release of muscle protein and other substrates for gluconeogenesis (the process of glucose synthesis.)
Muscle and brain are unable to release free glucose, but they can release pyruvate and lactate, compounds that can be released to the Cori Cycle for gluconeogenesis. Glutamine may also be released by the muscle.
As a source of energy during fasting, the body can utilize fat stored in adipocytes (fat cells) to produce ATP. (ATP is a molecule that provides energy in the body to allow chemical reactions to take place.)
Fat is broken into glycerol and fatty acids. The fatty acids travel to the liver where they can be cleaved into two-carbon units (acetate). The acetate binds to coenzyme A to form acetyl-CoA.
Acetyl-CoA can enter the citric acid cycle, but excess acetyl-CoA molecules bind into four-carbon units called ketones. Ketones may be used in place of glucose as an alternate fuel source for the brain and nervous system.
Because the brain is using an alternate fuel source, the demand for gluconeogenesis declines, reducing the rate of muscle breakdown. The starving individual who has adapted by using ketones as fuel will have lean body mass (muscle) spared.
As long as water is available, a normal weight person can fast for one month maintaining relatively normal system and immune function. Once fat stores are exhausted though, protein is used, leading to death once all protein stores are utilized.
Re-feeding must be done slowly and will vary depending upon the degree of starvation. Slow re-feeding may include introducing easily digested foods in smaller amounts, then increasing food amount and variety over time, until the individual has returned to normal eating.
For example, a person who would normally need to consume a 2000 calorie per day diet who has been in starvation mode for a period of time could be re-fed beginning with a 1200 calorie diet. Additional calories could be added each day until the individual was again eating at a 2000 calorie per day level.
Mahan K, Escott-Stump S. Krause’s Food, Nutrition, and Diet Therapy. 11th ed. Saunders. Phila, Pa. 2004
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