The body is always in a state of nutrition ggxx, whether this nourishment is from external or internal sources. On a water-only-fast conducted according to thirst, the body knows it has to rely on its internal reserves so conserves its internal nutritional supplies, but if we take anything other than water (and that includes supplements) the body recognizes it is on a reducing diet and does not conserve these food-reserves very well if at all. Malnutrition always occurs on a restricted diet eventually, but not so on water-only because natural hunger will appear before actual starvation sets in.
Many fasting authorities are against the use of any supplements because thay have been found to both unnecessary and sometimes injurious, and that would include Dr Joel Fuhrman MD, who has fasted thousands of his patients.