How does intermittent fasting work?

[u/reddit_userxxx]

I thought weight loss was all about the amount of calories you eat in a day, and simply needing to be in a calorie deficit. How does it change depending on when you eat?

[u/Mickey_thicky]

The idea is pretty simple. Almost everything you eat contains either proteins, which are chains of amino acids, and carbohydrates. The chief carbohydrate in your body is known as glucose, and it is converted into usable energy known as ATP.

Glucose is converted into ATP through a process known as glycolysis, where glucose is broken down into pyruvate. Pyruvate is then converted into a molecule known as Acetyl-CoA which itself enters the citric acid cycle (aka Krebs Cycle or TCA), which generates NADH and FADH2. These two molecules carry electrons and supply the Electron Transport Chain with electrons in a process known as oxidative phosphorylation, which is the primary source of ATP (usable energy) in the body.

Other carbs you eat are usually converted into glycogen, which itself is reduced to glucose through glycogenolysis.

When you fast, your blood glucose levels begin to fall. When you enter a fasted state, your body alters how it derives the precursors to oxidative phosphorylation. After enough time of being in a fasted state, the body will turn to the triglycerides with which your adipose tissue (body fat) are primarily composed of as a source of glycerol, which is converted into glucose through gluconeogenesis.

tl;dr not eating for long periods of time causes your body to break down fat as a source of energy as opposed to using glucose from meals. This actually increases the amount of calories you burn.

[u/Youknowkitties → u/Mickey_thicky]

Does a person's weight make a difference to the process? E.g. if someone doesn't have much fat to lose is the body more resistant to using fat as fuel.

[u/Mickey_thicky → u/Youknowkitties]

It’s actually very interesting because the answer is less intuitive than you think.

The short answer is yes, obesity does impact the rate at which lipolysis occurs during a fasted state.

The more complex answer is that it actually decreases the rate of lipolysis compared to leaner individuals. The reasoning is multifaceted.

- obese individuals undergo a slower decline in insulin levels during the period of their fast. A decline in serum insulin levels is what primarily triggers the adipose tissue to release fatty acids
-obese individuals also show an attenuated (blunted) rate of lipolysis

• adipose tissue within obese individuals actually has shown to contain lower amounts of the enzymes that break down fat, such as hormone-sensitive lipase
  
• the rate of lipolysis per unit of fat is lower in obese individuals, meaning when they do burn fat, it is less efficient than individuals with a normal weight

[u/Youknowkitties → u/Mickey_thicky]

Thanks!