Hard working folks burn upward of 3000 calories per day. minimal weight loss diets bottom out at about 1000 calories per day,If intake is below the burn rate, we metabolize liver glycogen stores, then stored fat, then muscle tissue. After that runs out, we die. (I figure I'm good for about 6 months between the glycogen and muscle stages). As we metabolize fat and protein, we produce ketones as a waste product, and need to drink lots of fluid to flush ketones out of the system.
Vitamin tablets are pretty important in the long run to supplement a limited diet, but scurvy, pellagra and night-blindness will take a long time to kill you.