Ketones are a beneficial product of fat metabolism in the body. When carbohydrate intake is restricted, it lowers blood sugar and insulin levels. As insulin levels fall and energy is needed, fatty acids flow from the fat cells into the bloodstream and are taken up by various cells and metabolized in a process called
beta-oxidation.
The end result of beta-oxidation is a molecule called
acetyl-coA, and as more fatty acids are released and metabolized, acetyl-coA levels in the cells rise. This causes a sort of metabolic “feedback loop” which triggers liver cells to shunt excess acetyl-Coa into ketogenesis, or the making of ketone bodies.
Once created, the liver dumps the ketone bodies into the blood stream and they are taken up by skeletal and heart muscle cells at rates of availability. In addition, the brain begins to use ketones as an alternate fuel when blood levels are high enough to cross the blood brain barrier.
There are three major types of ketone bodies present in the human blood stream when the metabolic process of
ketosis is dominant:
- Acetoacetate (AcAc) is created first
- β-hydroxybutyrate (BHB) is created from acetoacetate
- Acetone is a spontaneously created side product of acetoacetate
In times of starvation, or a low carbohydrate intake resulting in low insulin levels, ketone bodies supply up to 50% of the energy requirements for most body tissues, and up to 70% of the energy required by the brain.
Glucose is the main source of fuel for neurons
when the diet is high in carbohydrates. But when carbs are restricted, ketogenesis becomes the primary fuel process for most cells.
During fasting or low carbohydrate intake,
levels of ketone bodies in the blood stream can rise to levels
between 0.5mM and 5 mM, depending on the amount of protein and carbohydrates consumed.
This state is called nutritional ketosis.
After a few weeks of adapting to a ketogenic diet, most people's blood ketone levels go above the 1 mM level. As the levels of ketone bodies rise, the brain begins to use more than half of them for fuel.
In addition, the muscles of the body use all of the ketone body types. But after a few weeks of keto-adaptation, the muscles start converting the acetoacetate into β-hydroxybutyrate (BhoB) and returning it to circulation. Although BoHB is the more stable molecule, when it is taken up by muscle tissues, it has to be converted back to acetoacetate so that cells can metabolize it. The conversion between BHoB and Acetoacetate goes back and forth depending on where in the metabolism it is being used. The third type of ketone, acetone is very volatile and released in the breath and urine. However it also has some effects on brain signaling and it prevents neuron hyperexcitability, which may explain why being in ketosis can calm neurological conditions such as epilepsy.
As time on a ketogenic diet progresses, the body becomes more efficient at using ketones for fuel. The kidneys adapt and get better at reabsorbing larger amounts of acetoacetate and the brain and muscles get more efficient in using circulating BoHB.
So the longer you are eating low carb, the less MEASURABLE ketone bodies will show up in
your urine and on the Ketostix because the kidneys get better at absorbing them. In other words, it will seem like ketosis is slowing. However, at this point, your brain and muscles will be happily burning ketones for fuel, and as long as you stay under your carb sensitivity levels, you will be burning stored fat as your main energy source.
The good news is that just recently, several companies have developed a blood meter to check for blood levels of BHoB at home. I've included links to these new products in the side bar at right. Note that the strips are pretty expensive. Other companies are working on a breath ketone analyzer, which may be cheaper, and have a better correlation to blood ketones as acetone is not filtered or reabsorbed by the lungs.
