What causes insulinoma in ferrets?
Does kibble or low quality diet cause insulinoma?
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There is significant evidence that insulinoma is caused by low protein, high carbohydrate diets. This includes the vast majority of kibbles, with very few exceptions.
But don't take my word for it, here are some sources from exotic vets:
Insulinomas appear to occur regionally. They are uncommon among ferrets in Europe, New Zealand and Australia. The majority of ferrets from these areas are fed a low-carbohydrate diet of meat scraps, poultry scraps, and/or fish scraps, and sugary treats are not provided. By contrast, insulinoma is common in the United States, where the majority of ferrets are fed dry kibble containing 10-45% carbohydrate, and many are fed treats that contain sugar.
– Dan Johnson, DVM 
Ferrets are obligate carnivores. The typical ferret diet in the US contains 22-42% protein, 15-28% fat, 10-45% carbohydrate, and 1.5-3.5% fiber. Such high levels of carbohydrate are unnatural and may have negative consequences for the ferret. Finkler advocates a nutritional approach to insulinoma prevention, and recommends a diet low in carbohydrates, and high in fat and protein.
– Dan Johnson, DVM 
As easy as it is to place blame on these sugary treats, they may not be the primary sources for carbohydrate overload in most ferrets. If not, then what is? Answer: their dry kibble diet.
– Mark E. Burgess, DVM 
Ferrets should eat a high-protein meat-based diet, [...] and should not eat high-carbohydrate diets or consume too many sugary treats.
– Anthony A. Pilny, DVM & Sue Chen, DVM 
A low carbohydrate diet is recommended ‑ sugary treats should be discontinued and high protein meals should be offered.
– Elisabeth Simone-Freilicher, DVM 
So there are 2 problems with kibbles: they are high in carbohydrates and low in meat-based proteins.
What types of foods contain carbohydrates? Carbohydrates include sugars, starches, and cellulose. Cellulose is found in paper so we mostly don't have to worry about that one (although incredibly, some kibbles do contain cellulose).
Simple sugars are not often found in pet foods, but they can be. Many ferret treats such as FerretVite/FuroVite also contain them. They are usually disguised as "malt", "fructose", or any number of names so that you don't realize they are sugars when looking at the ingredients label. Fortunately, this website can identify them.
Starches are found in many foods, but most commonly in grains (barley, rice, corn, etc.) and some root vegetables (potatoes, cassava aka tapioca). These are complex carbohydrates, which are good for humans, but ferrets can't process them.
Many "grain-free" foods replace the barley, rice, and corn with potatoes, which are just as harmful. Or with peas, which cause other, bigger problems.
As any diabetic knows, grains and starches are not created equally. These ingredients are rated on two scales: glycemic index and glycemic load. To put it very simply, glycemic index is a measure of how quickly a food will boost your blood sugar, and glycemic load is a measure of how much per serving it will boost your blood sugar. A good example of the difference between the two is a donut and a watermelon. Both of these foods have the same, high glycemic index, but donuts have a much higher glycemic load.
Meats have a glycemic index of 0. The only dry foods that should be fed to ferrets are ones that are entirely grain-free and starch-free (and pea-free). The only 2 kibbles that meet this requirement as of this publishing are Wysong and Dr. Elsey's cleanprotein.
If this can't be done, then dry foods with low glycemic ingredients should be used. I am working on incorporating glycemic index into this website so that we can give recommendations. However these foods will always be subpar and should only be used if there is no other option.
The other problem with ferret diets is that they are low in meat-based protein. Many kibbles guarantee a certain protein percentage, but they won't say how much of this protein comes from meat vs. non-meats. Quinoa, wild rice, millet, couscous, buckwheat, lentils, chickpeas, and of course peas are all grains and vegetables with high protein levels. These can be used to boost the "protein %" of your kibble while actually harming your ferrets.
One thing you will notice when switching to higher quality diets is that these foods boast about their meat percentage. Yes, sometimes you may see vegetable protein ingredients like lentils, but if the meat percentage is explicitly stated then you know that those ingredients aren't being used to boost anything. Low quality foods a) won't mention their meat percentage, and b) boost their protein % with grain and vegetable protein sources.
It should be noted that there may be a genetic predisposition to insulinoma, especially in American ferrets. We can't guarantee that feeding a quality diet will prevent insulinoma, but there is growing evidence and research from reputable exotic veterinarians that it will at the very least slow it down if not prevent it.
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