For the health of ferrets, I have and always will recommend slow food changes, whether it is kibble or what is referred to as natural. Ferrets won't necessarily eat 'whatever is available' when they are hungry. I have seen the deadly results of the is misconception. I have heartbreaking times in trying to find a food that a new ferret will eat when I have no idea of what they ate before. Because ferrets are sensory animals, they imprint on the foods they have been fed and do not recognize other types of foods as food. This is why I serve multiple kibble choices.
With the fast metabolism of the ferret, I would never let a ferret go 24 hours without eating unless ordered by a veterinarian skilled in ferret health. There are times when a ferret is so malnourished that hydrating takes precedence and feeding is limited until the ferret is able to take nourishment. A skilled veterinarian would need to make that decision so when in doubt always check with your veterinarian.
I take care of old and ailing ferrets and many times it is a syringe of recipe every hour or so when they are not eating on their own. Ailing ferrets may eat less which may contribute to poor health, low glucose, and a myriad of other health issues. When it comes to feeding ferrets, I do what is best for the ferret. Ferrets depend on us for everything.
I've read that in the wild mustelids live only 2-3 years. The majority of domestic ferrets live much longer, even when being fed kibble their entire lives. Wild mustelids eat prey or raw from the beginning. I would guess if they were given kibble as a food source they wouldn't know it was food either. The domestic ferrets that are raised on kibble view prey or raw food the same, not knowing it is a food source.
With any feeding regime, the species must have specific nutrient needs met, such as minerals and vitamins, and in the right amounts. When an animal doesn't get the right amount of vitamins, minerals, and other nutrients, they become malnourished. As an example - Taurine. Studies indicate that a deficiency of taurine contributes to certain heart ailments. In Bob's Chicken Gravy, he added ingredients to complete the nutritional balance that wasn't met by the whole chicken. Wild mustelids wouldn't naturally have access to these additional ingredients. Maybe the ferretvite and honey trees are where the kibble tree is?
The body needs energy to accomplish things like growth and repair. The body needs fluids to keep the blood thin enough to flow, help with the process of digestion, transport wastes, and help rid toxins and other undesirable or no longer needed by-products of digestion and daily functioning. When ferrets are deprived of food, or what they perceive of as food, they will sleep more and drink less which can lead to dehydration in addition to the malnutrition.
Malnutrition occurs over time and develops in stages. First, nutrient levels in blood and/or tissue change, then intracellular changes in biochemical functions and structure. Malnutrition can retard growth and may allow underlying medical problems to surface. Inadequate food intake causes a rapid decrease of bacteria and protozoa and in the volatile fatty acid concentration in the stomach. In ferrets, this may lead to an overgrowth of bad bacteria.
If an animal is forced into an inadequate plane of nutrition, in ferrets being switched to a different diet this would include inadequate food intake, there are many physiological changes as the body tries to meet energy requirements. At the cellular level, catabolism (breaking down of complex chemical compounds into simpler ones) continues to supply the substances required for anabolism (usage of nutritive matter and conversion to living substance) and to continue vital functions. Reserve stores of nutrients in the individual are utilized to compensate for the lack of nutritional intake. Energy is generated from the utilization of proteins, fats, and carbohydrates. The most readily usable material, the carbohydrate glycogen, is utilized first. This is derived from glycogen stored in the liver and is exhausted within a few hours. This is followed by stored fat from subcutaneous deposits, around the kidney, and in the mesentery and omentum tissue. Fat deposits in the parenchymatous organs are utilized next. The last area of the body to lose its fat deposits is the marrow of the bones. The final source of energy available is the protein comprising the cytoplasm of the cells. It is at this time that ketosis and an increase in nitrogen excretion may occur. Ketosis (a condition in which ketone substances appear in the blood and urine) is commonly seen in malnourished animals. This is because it is necessary for the animal to derive its energy from the stored fat and protein. After all the fat reserves have been exhausted, nitrogen excretion rises due to the protein catabolism which occurs just prior to death. The animal will eventually reach a point where the cells of the body are unable to perform the functions necessary for life. Death results from lack of sufficient blood glucose to provide the energy needs of the brain and hypoglycemic shock occurs.
I don't have the exact time-line for ferrets but I that information is available for dogs. Remember that ferrets have faster metabolism action than dogs so the time-line would need to be adjusted.
When food is deprived, biochemical function changes occur. In the dog, this process begins within twenty-four hours. The highest priority of the metabolic processes is to keep the blood glucose concentration at a normal level. If the blood glucose (blood sugar) level drops too low for any reason, the brain, heart, muscles and kidney function shuts down rapidly and death comes quickly. When the animal has no opportunity to eat, in ferrets this would mean no opportunity to eat the food they perceive as food or are familiar with, the body will take stored glucose from the liver first, then from muscle reserves, This is done by changing the biochemical processes to different chemical pathways that make glucose readily available.
After about two days the liver reserves of glycogen (glucose) will be depleted in the dog. In order to keep the blood level of glucose in the normal range, new chemical pathways open, called gluconeogenesis, where the liver and kidneys create molecules from complicated biochemical reactions so that fats and proteins are extracted from adipose tissue and muscle. As the glucose reserves are tapped and diminished, chemical reactions kick in to create glucose internally from those protein and fat reserves. Energy to run the body's machinery (muscle, brain, kidney, heart and other organ functions require energy to fuel their activities) is now fueled less by glucose and more by fatty acid extracted from fat reserves.
On the third day of food deprivation the metabolism (metabolism refers to all the chemical reactions going on to maintain life) slows down. (This occurs in people when not enough calories are consumed as in dieting and not consuming what the body needs - our metabolism slows down.) This lower, or slowed metabolic rate continues as long as no food is consumed. The lowered metabolism is a survival mechanism to decrease the utilization of body fat and muscle for energy. Lowered blood sugar levels changes insulin secretion by the pancreas (with ferrets being prone to insulinoma, this is hazardous) which in turn lowers thyroid hormone levels; and it is the thyroid gland function that ultimately dictates the metabolic rate.
During starvation the liver releases chemicals called ketones into the blood stream; ketones are then used as a source of energy for the body cells. By creating ketones and fatty acids to be used as energy sources, the body conserves what little glucose is circulating so that glucose-dependent red blood cells and important kidney tissues can continue to access glucose. Red blood cells and kidney tubule cells cannot utilize anything other than glucose for cell energy needs.