Ketones are a beneficial product of fat metabolism in the body. When the body breaks down stored fat, it creates fatty acids, which are then burned in the liver in a process called beta-oxidation. This process results in the creation of ketone bodies, which are then used as fuel by the muscles and brain cells.
There are three major types of ketone bodies present in the human blood stream when the metabolic process of ketosis is in progress:
In times of starvation and 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. Although glucose is the main source of fuel for neurons when the diet is high in carbohydrates, these fatty acid fragments are used preferentially by brain cells when carbohydrate or food intake is low.
During fasting or low carbohydrate intake, normal levels of ketone bodies in the blood stream can rise to levels between .5mM and 5 mM, depending on the amount of protein and carbohydrates consumed.
There is some evidence for enhanced athletic (endurance type sports) performance using ketones as sole fuel.
After a few weeks of adapting to a ketogenic diet 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 beta-hydroxybutyrate (BHB) and returning it to circulation, as the brain prefers to utilize BHB for fuel.
So as time goes on, and the muscles convert more acetoacetate to BHB, the levels of beta-hydroxybutyrate ketones increase, and the acetoacetate levels decrease. This is important to know because the Ketone stix you buy in the store only test for Acetoacetate, not beta-hydroxybutyrate ketones.
So the longer you are eating low carb, the less MEASURABLE ketone bodies will show up on the Ketostix. In other words, it will seem like ketosis is slowing. However, at this point, your brain will be happily burning the beta-hydroxybutyrate ketones for fuel, and as long as you stay under your carb sensitivity levels, you will be burning fat for fuel (and burning the pathological visceral fat around your gut and waist).
STAYING IN KETOSIS:
For most people, taking in adequate calories (30 calories per kg body mass) and eating 1.5 grams of protein per kg per day; keeping carbohydrate reduced to ~ 50-60 g/day will keep you in ketosis provided the balance of daily calories are ingested as FAT. Therefore 60% of daily calores per day should come from fat; 30% calories from protein and ~ 10% from carbohydrates (if you are trying to lose weight then cut daily calories by 1000-1500 calories daily).
It is important NOT to overdo the protein intake as this is TOXIC to the liver and kidneys and will also short circuit the ketosis as too much protein intake will produce glucose (via the Cori Cycle). Finally those with carbohydrate sensistivity will need to restrict daily carbohydrate to < 40 g/d to maintain ketosis.
Ultimately you have to know in detail the nutritional value of each foodstuff including the gram content of protein; fat and carbohydrate and also the total energy content of the product eaten. For 1g of fat = 9 calories; 1g of protein = 5 calories and 1g carbohydrate = 4 calories. Keeping daily records of exact intake; total energy; exercise and daily body mass will make your journey more interesting and productive.
Enjoy the ketones.