Ketosis
|
|
Ketosis (key-toe-sis) is a stage in metabolism occurring when the liver has been depleted of stored glycogen and switches to a fasting mode such as occurs during sleep, during dieting, and during the body's response to starvation. Energy from fat is mobilized to the liver and used to synthesize glucose (a process called gluconeogenesis) from lactic acid, glucogenic amino acids, and glycerol carbon substrates. Ketones are also produced during this fasting state, and are burned throughout the body.
Ketones are a means of making the energy of fat available in water soluble form, and to displace as much burning of glucose as possible. Glucose must be conserved in the fasting state because parts of the brain, retina, kidney and red blood cells depend exclusively on it for energy, and in order to conserve muscle protein which must be catabolized to provide the glucogenic amino acid substrate for synthesis of glucose. During the initial stages of starvation the brain does not burn ketones, since they are an important substrate for lipid synthesis in the brain. But after several days of starvation, the brain transitions to burning ketones in order to more directly utilize the energy from the fat stores that are being depended upon, and to reserve the glucose only for its absolute needs, thus slowing the depletion of the body's protein store in the muscles. The brain retains a residual need for glucose, because ketones can only provide energy aerobically via mitochondria. In the long thin neurons, much of the metabolically active cellular membrane is too far from the nearest mitochondria and must derive its energy anerobically (without oxygen) from glucose without the assistance of mitochondria.
Ketosis, which is one of the body's processes for the metabolism of body fat, should not be confused with ketoacidosis, a medical condition induced by diabetes, that is usually accompanied by dehydration, hyperglycemia, and insulin deficiency. Since insulin is required to utilize glucose, lack of insulin causes an energy crisis and the body goes to extremes to produce more glucose and also provide ketones for energy. Glucose accumulates to the point that the kidney must use water to spill it into the urine, losing that water and causing dehydration in the process. The dehydration exacerbates the acidosis in a vicious cycle. Alcoholic ketoacidosis is also an energy crisis but of different origin. Because alcohol causes dehydration and blocks the first step of gluconeogenesis, the body is unable to synthesize enough glucose to meet its needs, thus the energy crisis begins.
There are a number claimed side effects associated with prolonged ketosis. Among those mentioned are heart palpitations, kidney stones, osteoporosis, calcium depletion, depleted glycogen stores, electrolyte imbalances, gout, dehydration, dizziness, constipation, irratibility, light-headedness, fatigue, depleted mineral stores, acidosis, coma, and death. Many of these claims are unverified or understudied, in some cases the claim may be based on ketosis during starvation.
The breath of people in a ketagenic state commonly contains acetone, detectable as a sweet smell that may be mistaken for alcohol.
Deliberately inducing Ketosis through the Ketogenic Diet is an approach to treating epilepsy.
See also
References
- Lipolysis and Ketosis (http://atkins.com/helpatkins/newfaq/categories/LipolysisKetosis.html)
- Diabetic Ketoacidosis (http://www.emedicine.com/emerg/topic135.htm)
- Fat metabolism (http://www.unisanet.unisa.edu.au/08366/h&p2fat.htm)
- Breath acetone is a reliable indicator of ketosis in adults consuming ketogenic meals (http://www.ajcn.org/cgi/content/full/76/1/65), American Journal of Clinical Nutrition.
Further Reading
The Merck Manual -
- Diabetic Ketoacidosis (http://www.merck.com/mrkshared/mmanual/section2/chapter13/13b.jsp)
- Alcoholic Ketoacidosis (http://www.merck.com/mrkshared/mmanual/section2/chapter13/13c.jsp)