In 1965, an extremely obese 27-year-old male, weighing in at 456 pounds fasted for 382 days (one year and 17 days) and lost 275.5 pounds.
Doctors from the University of Dundee monitored his condition throughout his fast. During his fast, he obese man recorded low glucose levels for 100 days and defecated every 40 to 50 days. He was given potassium tablets to keep his heart healthy and multi-vitamins every day.
According to ABC.au, the man was able to fast for over a year due to these factors:
Once you stop eating, your body gets its energy from the glucose in your bloodstream and liver, thanks to your last meal. You carry a semipermanent 0.5 to 1 kilogram of solids in your gut. The glucose from this runs out after about eight hours.
Then you start burning up a chemical called glycogen. Glycogen is simply a whole bunch of glucose molecules loosely stuck together. It’s stored in your liver and muscles. Glycogen is really easy to break down into the individual glucose molecules from which it was made. You can burn glycogen to get the glucose you need for about another 36 to 48 hours.
After two or three days of fasting, you get your energy from two different sources simultaneously. A very small part of your energy comes from breaking down your muscles — but you can avoid this by doing some resistance training, otherwise known as pumping iron. The majority of your energy comes from breaking down fat.
But very soon, you move into getting all your energy from the breakdown of fat. The fat molecules break down into two separate chemicals — glycerol (which can be converted into glucose) and free fatty acids (which can be converted into other chemicals called ketones). Your body, including your brain, can run on this glucose and ketones until you finally run out of fat.
So if you’re extremely overweight, it’s possible to fast and lose weight. The man gained only 11 pounds five years after his fast.