Ketosis is a biochemical process where our body uses fat as its primary source of fuel/calories/energy, when the liver converts either ingested (eaten) or stored (visceral) fat into ketones which are the bodies fuel, as opposed to using glucose derived from carbohydrates (carbs) in standard diets.
To understand ketosis, we must first look at how we use calories in our body.
The brain is the largest user of calories in the body (nerve signalling and cell health maintenance). It is only 2% of our total body weight yet uses 20-25% of energy consumed based on a balanced diet with normal daily activity, i.e. at least 1/5 of all calories consumed are used by the brain, and the fuel requirement increases when we do mental tasks such as studying, playing musical instruments, or critical thinking.
From this we can see that when looking at fuel consumption (i.e. burning calories) that it is important start with the brain.
Carbs, fats or protein? Which of these fuels the brain?
The brain has a protective barrier between the bodies blood vessels and itself called the blood brain barrier. Only 2 types of fuels can pass through this barrier: glucose, which is the simplest sugar from digested carbohydrates, and ketones which are created in the liver from fat.
From this we can see that the body is a hybrid engine capable of using two different types of fuel just like a hybrid engine car.
But how can we get our body into ketosis, where we use fat as fuel instead of glucose?
The body will always use the cheapest source of fuel available. Glucose provides 4 calories per gram, while fat provides 9 calories per gram. As glucose is a poorer /cheaper fuel providing less energy per gram than fat, the body will use this fuel first.
So if we have glucose in our body that is what will fuel our brain, and we will not be in ketosis.
When we eat carbohydrates (bread, pasta, rice, oats, fruit, potatoes, vegetables, or anything with sugar in it) they are broken down in the body into glucose, the simplest form of sugar, which then enters the blood stream. Human blood vessels can hold approximately 1-1.5 teaspoons of sugar (glucose) which is about 5 grams. Anything in excess of this is toxic and will lead to hyperglycemia, which in the long term this can cause many health problems of the eyes, nerves, kidneys, as well as heart disease and cancer.
To prevent this occurrence the pancreas creates and secretes the hormone insulin into the blood which brings the glucose to the cells that need it for energy at the times we are active or working (Glycolysis). If we have more glucose than our energy requirements at that time, insulin then brings the glucose to the liver where it is converted to non-toxic glycogen (Glycogenesis) which is then stored in our muscles for later use i.e. our muscles are our fuel tanks for glucose.
Therefore, if we do not eat carbohydrates our body will use this stored glycogen, convert it back into glucose in the liver and use it to fuel the body.
The average person's muscles store about 2,000 calories worth of glucose/glycogen. This can last 1-3 days depending on our energy requirements. Once the glycogen in the muscles is used up (I.e. the glucose fuel tanks are empty) the body needs to adapt to fuel the itself. It does this by taking fat (either ingested or stored) and converts it into ketones in the liver to fuel the brain as well as the rest of the body.
This is how the body gets into ketosis, and as long as we keep glucose levels low in the blood (below 5g to avoid an insulin spike) we will remain in ketosis where our body is burning fat as a fuel. When we use up the eaten fats our body will find the next cheapest/poorest/easy-to- access source of fuel, which is the stored flabby visceral white fat.
To maintain this biochemical state of ketosis we would need to follow a ketogenic diet/lifestyle.