Carbohydrates in pet foods include low- and high-molecular-weight sugars, starches, and various cell wall and storage nonstarch polysaccharides or dietary fibers. The four carbohydrate groups functionally are absorbable (eg, monosaccharides such as glucose, galactose, and fructose), digestible (eg, disaccharides, some oligosaccharides), fermentable (eg, lactose, some oligosaccharides), and poorly fermentable (eg, fibers such as cellulose, which is an insoluble fiber).
Although there is no minimum dietary requirement for simple carbohydrates or starches for dogs, certain tissues, such as the brain and RBCs, require glucose for energy. If inadequate amounts of dietary carbohydrates are available, the body will synthesize glucose from glucogenic amino acids and glycerol. Dogs usually synthesize glucose from dietary carbohydrates. The use of dietary protein to synthesize energy in dogs diverts amino acids away from functions such as synthesis of nonessential amino acids and building muscle. Carbohydrates can become conditionally essential when energy needs are high, such as during growth, gestation, and lactation. Different carbohydrate sources have varying physiologic effects. Properly cooked nonfibrous carbohydrates are utilized well by dogs. In dogs, if starches are not cooked, they are poorly digested and may result in flatulence or diarrhea. Except for the occasional case of lactose or sucrose intolerance, most cooked carbohydrates are well tolerated in both dogs.
Source: https://www.merckvetmanual.com/management-and-nutrition/nutrition-small-animals/nutritional-requirements-and-related-diseases-of-small-animals
