Humans are remarkable in having the ability to switch between two different dominant fuel streams. We are like a hybrid car.
If carbohydrates are available, they will be utilized first and the metabolism will be geared to this pathway. If there is a very low intake of carbohydrates (and not a high intake of protein), the human body has the particular talent of being able to switch over it’s metabolism to be geared to burning fats and ketones.
By eating in this very specific way, you switch into this natural metabolic state, which is often referred to as “nutritional ketosis” or “therapeutic ketosis” or being on a “ketogenic diet”.
There is growing interest in nutritional ketosis because there are some ways that it may help people’s health and be a useful medical intervention.
What Situations Result in Nutritional Ketosis?
Fluctuations of daily life – if you go for a while without eating or without eating much, such as overnight, your blood ketone levels will go up a little above the usual low background levels. Similarly this will happen if you have substantial sustained exercise without much food intake. This is not what we normally mean by “ketosis” because it is at a low level, but it is worth mentioning. A trace amount of ketones may show up if a urine test is done in this situation. For example, it is considered normal if a trace amount of ketones is found in a routine first-void morning urine specimen.
Starvation – as the length of time with no (or very low) food intake gets longer, the level of ketones in the blood climbs higher. This is an essential feature of our metabolism. We have to be able to survive periods of low or absent food intake. If we were not able to start producing and using ketones as a major fuel source, we would have to break down our muscle tissue at a rapid rate to supply the high amounts of glucose we would need. (Protein can be broken down in the liver to supply glucose.)
A ketogenic diet – If you eat a diet that is very low in carbohydrates while also being low or moderate in protein intake, you can be in a state that to some degree mimics some of the effects of starvation. Yet you can avoid the downside of waisting away by eating high amounts of fat. There are situations in human history where people have lived on diets like this. Ketogenic diets are also used as a medical intervention. The most common medical use for ketogenic diets has been in control of epilepsy and in the treatment of obesity.
“Ketosis” means there are “ketones” in your blood. People have at least a very small amount of ketones in their blood all the time. Generally, the word “ketosis” is only used when the ketones are more than just the usual very low day-to-day background level.
“Ketones” are a type of molecule you produce in your metabolism. They are produced in the liver during the breakdown of fat molecules. These ketones do not show up in the blood at more than trace levels unless there is a lot of fat being broken down in the presence of low supplies of glucose. Insulin stops the production of more than trace amounts of ketones, and insulin levels are higher with increasing amounts of glucose.
Ketones are a valuable fuel molecule and an essential part of our metabolism. They are not directly harmful in any way except if they reach extremely high levels, which can only happen in diabetes (see “ketoacidosis”). Ketones can be used for fuel by almost all the organs and cells of the body. One of the especially important things about them is that they are the only fuel besides glucose that the brain can use. The brain uses ketones for a fuel very readily, but ketones can not be used in all the ways that the brain uses glucose, so there is always a need for an adequate, normal level of blood glucose (some other cells also need glucose, such as the retina and the red blood cells).
“Ketone bodies” – In chemistry, there are many, many molecules that are of the type classed as ketones. When we refer to “ketones” in your body, by common usage we are specifically meaning three molecules named acetoacetate, acetone and beta-hydroxybutyrate. Technically beta-hydroxybutyrate does not fit the definition for ketones. Functionally, beta-hydroxybutyrate acts like a ketone in that it can readily be metabolized back to acetoacetate, thus acting as sort of a transit or storage form of ketones. The term “ketone bodies” is useful in that it has been used as a catch-all phrase to refer to these specific three molecules (one of which is a ketone supplier, rather than a ketone itself). (Note: this description amended Sep 29/12)
“Ketoacidosis” is a very serious, potentially fatal condition that can occur in people with diabetes. Having insulin in the blood to stops ketones from going very high, so people who produce enough of their own insulin will not develop very high ketone levels no matter how they change their food intake. Even people in starvation do not get ketoacidosis (unless they are diabetic).
“Nutritional ketosis” or “dietary ketosis” means that, because of what you have or haven’t eaten, your body is making more of the type of molecule called “ketones”. This distinguishes ketosis caused by reasons outside the body from ketosis produced by abnormal function of the body (deficiency of insulin).
“Depth of ketosis” or “degree of ketosis” is an informal term that people often use. It refers to how much the level of ketones in the blood rises. You are “deeper in ketosis” as your blood level rises higher.
“Keto-adaptation” is a term coined by researcher Dr. Stephen Phinney M.D. to refer to the fact that it takes some weeks and up to 2-3 months for a person’s body to fully adapt to functioning in a state of nutritional ketosis. That is, to become adapted to using fats and ketones as the predominant fuel, instead of the usual situation, where glucose is used as the predominant fuel. Part of keto-adaptation is that there is a normal, steady blood level of glucose, with the glucose coming mostly from sources within the body, rather than coming mostly from the digestion and absorption of glucose from food.