What is Ketosis?

The Truth About Ketosis

urine ketone test
Using a urine test to detect ketosis. Ian Hooton/SPL/Science Photo Library/Getty Images

A lot of people are confused by the term "ketosis." You may read that it is a "dangerous state" for the body, and it does sound abnormal to be "in ketosis." But ketosis merely means that our bodies are using fat for energy. Ketones (also called ketone bodies) are molecules generated during fat metabolism, whether from the fat in the guacamole you just ate or fat you were carrying around your middle.

When our bodies are breaking down fat for energy, most of the it gets converted more or less directly to ATP. (Remember high school biology? This is the "energy molecule.") But ketones are also produced as part of the process.


When people eat less carbohydrate, their bodies turn to fat for energy, so it makes sense that more ketones are generated. Some of those ketones (acetoacetate and ß-hydroxybutyrate) are used for energy; the heart muscle and kidneys, for example, prefer ketones to glucose. Most cells, including the brain cells, are able to use ketones for at least part of their energy. But there is one type of ketone molecule, called acetone, that cannot be used and is excreted as waste, mostly in the urine and breath (sometimes causing a distinct breath odor).

If enough acetone is in our urine, it can be detected using a dipstick commonly called by the brand name Ketostix (though there are other brands, they are still often called Ketostix).

Even though everyone is generating ketones continuously, this detection in the urine is what is commonly called "ketosis."  (Less commonly but more accurately blood ketones can also be measured.)

The higher the concentration of ketones in the urine, the more purple the sticks will turn. The Atkins Diet in particular advises people to monitor ketosis as an indication of fat burning.

Other reduced-carbohydrate diets don't pay much attention to this or aren't low enough in carbs to make much of an impression on the sticks. The latter type of diet is sometimes called a "nonketogenic" low-carb diet, and there are many benefits to be had from low-carb diets, even if they are not ketogenic.

Why do some people think ketosis is a bad thing?

There are several reasons people point to as proving that ketosis is a state to be avoided.

1) Not enough glucose. There is an assumption among some advice-givers that if a body is burning a lot of fat for energy, it must not be getting "enough" glucose. However, there is no indication, from studying people on reduced-carbohydrate diets, that this is the case. Although it's true that our bodies can't break fat down into glucose (though, interestingly, they easily use extra glucose/carbohydrate to make fat, and not the good kind), our bodies can convert some of the protein we eat into glucose. Indeed, this works well for people who don't tolerate a lot of sugar, because this conversion happens slowly so it doesn't spike blood glucose.

2) Negative symptoms at first.  Another thing that people often point to as a negative thing about ketosis is that people can suffer symptoms such as headaches and dizziness in the first few days of a ketogenic diet.  Also, athletic performance can suffer until the body adapts to using fat for energy instead of glucose.  (This is called keto-adaptation.)  These symptoms are temporary, but they are often used as "proof" that it's bad to eat a very low-carb diet as if people are likely to have to tolerate headaches for the rest of their lives if they eat a low-carb diet.  (In fact, people often find that they have better mental function after keto-adaptation than they did when they were eating a lot of carbohydrate.)

3) Confusion: Ketosis vs Ketoacidosis - A dangerous condition called ketoacidosis can develop in those with type 1 diabetes, and it is sometimes confused with normal ketosis. The body usually avoids this state by producing insulin, but people with type 1 diabetes are unable to produce insulin. Even most people with type 2 diabetes who inject insulin usually produce enough insulin of their own to prevent ketoacidosis.

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