Sugars have no preventive or curative functions in dogs or cats. But, when found in excess in their food, sugar can cause obesity and diabetes.
In everyday language, when we talk about sugar, we refer to the sweet taste of processed sugar (from cane or beet) or fruit sugar (fructose). Sugar can also refer to glucose (grape sugar) or lactose (milk sugar). Unlike dogs, cats are not sensitive to sweetness a have taste buds more honed to detect meat.
While lactose provides immediate energy to unweaned puppies or kittens, a digestive enzyme, lactase, is necessary to make it biologically available; lactase disappears once the animal stops feeding on milk. Cats and dogs are able to synthesise glucose from proteins ingest through their diet. As a result, carbohydrates do not have much of a key role in their diet.
Most plants, fruit, berries, roots and/or tubers are energy stores of sugar. The only known source of this type found in the animal body is milk lactose.