Ketosis is a state where the body is burning fat for energy and generating high levels of ketone bodies in the blood as a result. These compounds are the result of the breakdown of fatty acids and while they are always present in the body, their levels are elevated in people who have reached the state of ketosis. Substantial research has been done on how people reach this state and what happens metabolically. There is some debate about whether or not ketosis is potentially dangerous or even beneficial for some people.

The body's metabolism works to balance the energy needs of the body by utilizing stored energy from food. One of the preferred forms of energy is glucose. In people who eat a low carbohydrate diet with limited glucose sources, the body starts to burn fat for energy instead. In this process, ketone bodies are produced as a byproduct. If the diet persists, the person will enter ketosis. People in this state usually have slightly fruity breath and dark urine, a reflection of the ketone bodies being expressed in their breath and urine.

The conversion of fat for energy means that once people enter ketosis and maintain it for several days, their reserves of fat begin to be burned away. In people who are trying to lose weight, this is a very desirable outcome. People interested in losing weight should consult a doctor about diet and exercise options to develop a plan, including permanent lifestyle changes, that will be effective for them.

Research on historic humans suggests that ketosis was probably a common state for many early human societies. Hunter-gatherer societies would have eaten a meat-heavy diet with fewer sources of carbohydrates than the modern diet. It's clear that the body adapted the ability to use fat for energy for a reason, and some researchers feel that this state is not necessarily harmful, even though it mimics starvation metabolically. As long as people are receiving all the nutrients they need and monitoring their health, this state can be created and maintained to lose weight or to control epilepsy, two common reasons people eat a ketogenic diet, a diet that induces ketosis.

In certain people, this state can turn into ketoacidosis. This is dangerous, as it means that the blood is becoming acidic and the person could be exposed to the risk of serious complications. Generally, people with a history of alcoholism or diabetes are at risk for ketoacidosis. These individuals are not good candidates for ketogenic diets and they must monitor themselves for the early warning signs of ketosis so that they can avoid ketoacidosis.