Ferrets must eat frequently due to their unusual anatomy, therefore, constant availability of food and water is important for ferrets, especially in warm weather.  It is particularly essential for pregnant, old, or chronically ill pets, and for growing ferrets.  Ferrets that fast more than six hours will become irritable and much more likely to bite. 
Ferrets require a high-calorie, high-protein, low-fiber diet.  These requirements may be met by commercial ferret diets or premium quality dry cat food.  Dog food of any kind in inadequate for growth and good health in ferrets.  Even the best quality premium dog food will not support ferret growth or reproduction, although pets have been maintained on dog food supplemented with dairy products or meat scraps.
Grocery store brands of dry cat food are commonly coated with animal fat, making them more  palatable to ferrets, but feeding your ferret a steady diet of these cheaper brands can produce health hazards.  Even though the product ingredients may be listed as 30 to 35% protein, if the main source of this protein is ground yellow corn, your ferret will not get the nutrition he needs to stay healthy.  Premium brands available through veterinarians or pet shops also contain 30 to 35%  protein, but the primary ingredient is usually poultry, red meat meal, or meat by-products which meet a ferrets nutritional needs.

Like cats, many ferrets, especially older ones, will refuse certain diets even when nothing else is offered.  They may develop a taste for one specific flavor and may be extremely stubborn to the point of starvation when presented with a distinctly different diet.  The more recently formulated  pelleted ferret diets are well-balanced and palatable to most pets.

Ferrets require a higher caloric diet, and their main source of calories should be from fat.  Premium cat foods contain 15 to 23% fat, which is better for ferrets than the grocery store brands.   Adding fat to cheaper foods to increase the caloric concentration will give mature animals shinier coats temporarily.  However, because ferrets eat to meet their caloric requirement, they eat less of the higher fat diet and soon become deficient of protein and other essential nutrients.  The coat then becomes thin and bristly.  Growing kits will be stunted or die, adults will be think, reproduction will fail, and some animals will develop diseases or ulcers.

Housed under natural light conditions, ferrets will eat more in the fall and will become noticeably fatter (adding as much as 40% or more to their weight).  This is a normal, healthy response and is not a cause for concern.  They will, in turn, reduce their intake in the spring and lose the excess weight.

Obesity is rarely a problem in ferrets unless they are fed large quantities of high fat snack foods daily (such as packaged, liver flavored cat treats) in addition to their regular diet. The best way to return obese pets to a more normal body weight is to reduce their snack food consumption.

Ferrets drink more from water dishes than water bottles.  Because ferrets rest their feet on the bowl when they drink, a heavy crock is the ideal water container.