There are many misunderstandings among low carbers that I hear from time to time. One is that when we go on a low carb diet, we are burning ketones for fuel. The idea was probably born due to the fact that the heart and a few other body tissues prefer to use ketones over any other fuel source. But preference doesn't mean they get to.
Initially, when we restrict carbohydrates, we are using glucose predominantly for fuel. But when the blood glucose level falls, the body converts the glycogen it’s stored in the liver to glucose. It does this by using dietary fats and stored body fat to fuel the process of Gluconeogensis.
Normally, the body has a high requirement for glucose. Dr. Eades claims we need somewhere in the area of 200g of glucose per day. But in the absence of carbohydrates, the liver is able to get the glucose the body needs by breaking down muscle tissue and dietary protein into amino acids. Plus a little bit of glycerol. Fifty-eight percent of all dietary protein is broken down in the liver to make glucose.
Ketones comes from this process of Gluconeogenesis. When someone is said to be “in” Ketosis, that doesn’t mean they are burning ketones for energy. That means the liver is using fatty acids to fuel the process of making glucose, because glucose availability is low. The result is the synthesis of ketone bodies. The liver can’t use ketones, which is actually a safety measure to make sure the liver dumps them into the bloodstream for the use of other body tissues.
For the first few weeks of a low carb diet, muscles and all other body tissues do use ketones for fuel. This is a preservation mechanism, and the purpose is to save any available glucose for the brain. But this is only a temporary situation, until the brain becomes fully adapted to using ketones for the greater majority of its needs.
Once this adaption takes place, ketone production drops because the body doesn’t need to make as much glucose as it did before. At that time, muscles and other body tissues that don’t need glucose for fuel switch to using fatty acids, in order to save the body’s precious ketones for the brain.
The average length of time it takes for the brain to adapt to using ketones is three weeks. But it can take as long as six or more. Therefore, ketones are only an alternative energy source when there isn’t enough glucose available. Once the body adapts to being in Ketosis, the brain is the major body organ that uses ketones. All other body organs and cells, including those that prefer to use ketones, use glucose or fatty acids for fuel. Unless there are extra ketones floating around that the brain doesn't need.
Not a very likely occurrence, especially if you've been low carbing for awhile. The body gets more and more efficient in using ketones, and tends to make just enough to fuel the brain.