Our bodies do amazing things, but we cannot make or change something else into certain nutrients that the are essential for proper body function. Such things, known as required nutrients, are required in the diet. The following site gives a brief overview of major categories of required nutrients
https://www.healthline.com/health/food-nutrition/six-essential-nutrients#water
Laking one or more of the required nutrients over a prolonged period leads to a nutritional disease. Examples are described here.
https://www.stylecraze.com/articles/diseases-caused-by-nutritional-deficiency/#gref