Fasting and Insulin

Fasting does great things for our insulin levels. But it might help to first talk about what eating does to our insulin levels first.

What Happens to Insulin When We Eat?

When we eat our insulin levels rise. It is an important hormone for storing and burning food that we eat. Carbs and protein spike insulin. Fat does too at a much lower degree.

Insulin lets the body immediately start utilizing food’s energy. Carbs absorb quickly and convert into glucose increasing our blood sugar. Most cells are opened up by insulin for immediate use of the energy. Amino acids are broken down proteins that are absorbed in the cells. Any excess amino acids may be stored as fat. Protein can raise insulin but does not raise blood glucose.

Fats absorb into the cells as fat and have very little effect on insulin levels.

Insulin and Energy

Insulin stores excess energy in a couple ways. In a limited amount insulin stores glucose as glycogen in the liver. This lasts until the livers limit is reached. Once the liver is maxed out the body will turn glucose into fat. Some fat may be stored in the liver as well and an unlimited amount of stored as body fat.

What Happens When You Fast?

Basically the process described above goes in reverse when we fast. Insulin levels decrease causing the body to burn stored food (energy). The first of the energy to begin to burn is glycogen, which is the glucose stored in the liver. This liver energy storage lasts around 24 hours before the body begins to burn body fat.

This is the 24 hour mark hunger pain I get when I fast. Once I get past that hour or so I have no more hunger and am burning body fat painlessly.

For some people the liver glycogen stores may not run out until closer to 48 hours. In a non-diabetic people the glucose drops but maintains normal levels. Gluconeogenesis is the process where the liver manufactures new glucose from amino acids.

When Does Ketosis Start?

For an average person ketosis would begin 24 to 48 hours after beginning a fast. Fat is broken down when insulin levels drop triggering lipolysis. The body’s fatty acids produce ketone bodies. The brain begins to use these for energy. The longer the fast the more the ketones supply the brain. Two major ketones are beta-hydroxybutyrate and acetoacetate. These will be explained in another post.

Fatty acids are used for energy for most of the body once ketosis begins.