Defined as a metabolic state where fat is the main fuel for the body, rather than glucose.
Ketosis begins after 3-4 days on a restrictive diet, where your carbohydrate intake is less than 60gm of carbohydrate each day.
Adapting to this diet, may go on for up to three weeks. Breaking out of the diet, (Eating too many carbs.) delays the process.
During the transition, you may feel that you lack energy, or that you've got the flu.
If you stay with the diet, your body builds a new homeostasis point around your changed behaviour. Fully adapting takes 3-4 weeks.
One change you'll notice is that you need more salt, especially in the beginning. On a LCHF diet the kidneys excrete more salt than usual. Correct this by salting your food, and/or by drinking bone broth and the juice off cooked vegetables. And for me, I splash some hot water in the dirty fry-pan after breakfast, and make a small but delicious salted drink.
Ketones as Energy
After you have made the transition, your body will mostly burn the primary serum ketone beta-hydroxybutyrate, usually written β-OHB. The other ketone, is acetoacetate (AcAc).
When ketones are available glucose is conserved, and glycogen stores in the body are also conserved. The body is designed to use ketones first, and protect glucose levels in the blood.
Weight Loss Benefits
People who switch from a glucose metabolism to a ketone metabolism usually lose a lot of weight. However, on every diet there is a homeostatic point that the body will settle into. So weight loss won't go on forever.
Many people will be happy not to get too strongly into the science. They will use a low-carbohydrate diet to get back to desirable weight, and eat less stringently most of the time after that.
Once you understand that you can control insulin, keeping your weight stable is easy to do. You'll be keen not to see it get out of hand.
Ketones Burn Cleanly and Efficiently
This means that energy use produces far fewer free radicals (ROS). That's much better than trying to take anti-oxidants to control them.
Better still, histone deacetylase is produced, which activates your body's own genes in a way that combats oxidative stress. That's far better than trying to take anti-oxidants to combat oxidative stress in the body.