This description of the dangers of Very Low Calorie Diets (VLCD) from the University of Michigan describes some of the metabolic issues that are associated with sustained diets under 1000 calories a day.
https://www.uofmhealth.org/health-library/aa144561
(from the above link)
The following are the changes your body goes through during a VLCD:
Your metabolism slows to conserve energy because the body thinks it is starving. A slower metabolism burns fewer calories.
To get the carbohydrate it needs, your body breaks down protein. You lose lean body tissues (muscle and organ tissue). It is important to preserve lean tissue, because it increases your basal metabolic rate. Losing too much lean tissue increases the percentage of fat in your body. The result is a reduced metabolism. This is one reason why it is so easy to regain weight after you lose weight quickly.
In a VLCD (or during starvation), about half the weight you lose is fat and the other half is lean tissue, such as muscle. On a more moderate diet, you lose 3 times more fat than lean tissue. It is important to preserve lean tissue, since it increases your resting metabolic rate.
Mineral and electrolyte imbalances can occur. These imbalances can be life-threatening. This is the reason these VLCDs must only be used under a health professional's supervision.
Bone mass is lost. This is more risky for women, because they diet more often than men, and they are also at higher risk for osteoporosis.