Sugars have no preventive or curative functions in dogs or cats. But, when in excess in the food, they can cause obesity and diabetes.
In everyday language, when we talk about sugar, we refer to the sweetening power and taste of carbohydrates like saccharose (sucrose) or fructose. With no qualifier, this term usually refers to saccharose (beet or cane sugar), but it could just as well refer to glucose (grape sugar), fructose (fruit sugar) or lactose (milk sugar). Unlike the dog, the cat is not sensitive to sweet tastes.
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. Since later on cats and dogs cannot taste"sweet" things and since they can synthesise their own blood glucose from proteins autonomously,sugars have no nutritional value for them.
Sugars are simple forms of energy reserves for most plants with fruit, berries, roots or tubers. The only known source of this type found in the body is milk lactose.