Let’s say someone is obese and stops eating completely to lose weight. He consumes next to no calories, only all the vitamins and minerals (in pill form), water, salt, etc. he needs to survive. Of course, it isn’t healthy, but would this person be able lose the weight before starving to death?
“Lose weight”? Sure. Lose all excess body fat and achieve a healthy BMI? Unlikely, even impossible.
At Weight Watchers they warn you to keep the weight loss at an average of 1-2 pounds a week*. Any faster, and it’s likely that you’re losing muscle (ie, protein) and not fat. If your protein runs out before your fat, you’ll die of malnutrition while still obese.
5 of the 28 heaviest people in the world actually died from malnutrition or starvation.
*I believe men can lose weight a little faster with no ill effects, but not a lot faster.
Your brain needs glucose to survive. You get glucose from two sources, one, from carbohydrates, and two, by converting protein to glucose. If you don’t get either carbohydrates or protein your body will consume muscle mass to produce the glucose your brain needs to survive. Consuming your own muscle mass is not healthy, and can lead to death.