No other group of nutrients gets more grief than carbohydrates, especially in diets for carnivores. Much of the misunderstanding is simply in the fact that carbohydrate compounds are lumped into one huge category and we don't have a specific requirement for them. Indeed, when you look through the NRC for Dogs and Cats, there is no requirement for carbs like there is for fats and proteins. This leads people to believe that dogs and cats don't need them. I completely DISAGREE! In fact, I use carbohydrate blends all the time in raw formulations for carnivores and they are invaluable to me in most of my clinical formulations. Even our formulations for big cats at the zoo include some carbohydrate; however, we are very specific about the types and amounts.
Unfortunately, the term carbohydrate includes everything from sugars and starches to dietary fibers and prebiotics. Confusing? Absolutely. Even dietary fiber isn't analyzed the same. The fiber included on the label of a pet food is what we call "Crude Fiber" and it means absolutely not one thing to a dog or a cat. It's not even the value we see on human food products which is referred to as Total Fiber. Oh yes, they are totally different analyses and give you completely different results. There are even specific fiber assays that we use for herbivores that recover different fibrous compounds.
The crude fiber assay only recovers a very small fraction of true fiber found in food as portions (not even all) of lignin, hemicellulose and cellulose (structural carbohydrates in plants). Those fabulous soluble dietary fibers that we are encouraged to consume and the wonderful long chain polysaccharides that act as prebiotics in the gut are destroyed in that assay; therefore, not included. So where do those get recovered in terms of pet food labeling? They don't. They wind up being lumped along with starch and sugar in the total carbohydrate calculation minus the Crude Fiber...or what we call Nitrogen Free Extract (NFE).
The calculation is based on subtracting out protein, fat, crude fiber and ash (total minerals) from the total dry matter in the diet and looks something like this: NFE = 100% - protein % - fat % - ash% (total minerals) - crude fiber %. In other words, we are accounting for the minerals, fat, protein and the tiny bit of fiber; however, EVERYTHING else gets lumped into the "NFE" package including vitamins, starch, long chain polysaccharides, all soluble dietary fibers, most insoluble dietary fibers, FOS & MOS (prebiotics), and sugars...yes, the good the bad and the ugly all get lumped together. I have come across many websites and blogs that refer to this NFE fraction as "carbohydrate" which is misleading.
It is frustrating to hear the misconceptions about carbohydrates because they have a purpose in pet foods. For example, if you enjoy the convenience of a kibble pet food, then you are enjoying the benefits of starch because it must be there in substantial concentrations to hold the kibble together (including those grain-free foods). Although there is no specific requirement for carbohydrates, they do provide benefit and provide nutrition. That said, some types of carbohydrates are frequently overused in pet foods and that poses issues and concerns.
Over the summer I'll be writing a series on carbohydrates to hopefully answer some of your questions about their advantages, disadvantages and functions in foods. Hopefully, we'll also get to some vitamin issues as well (the group of nutrients I love to hate!).