A kitten needs to eat a more calorie-dense food than does your typical adult cat. A high quality kitten food might have 520 kcal per cup while an adult food in the same line could have 500 kcal per cup. That might not seem like such a big disparity, but the extra calories are very important over the long haul.
And the differences don’t just stop with calories. Take a look at some of the Association of American Feed Control Officers (AAFCO) minimum nutrient requirement for kittens and cats: You can see that kittens need more of many important vitamins, minerals, and amino acids (and more protein in general) than do adult cats. Kittens are at risk for nutritional deficiencies when they eat foods designed for adult cats. Nutrients not regulated by AAFCO are also important. For example, better kitten foods contain DHA (docosahexaenoic acid, a type of omega-3 fatty acid) to optimize brain and eye development.
In the interest of simplicity and convenience, is it okay to feed adult cats kitten food? The answer is "no." An excess of a nutrient can be just as dangerous as a deficiency. In this case, the most important excess that we have to worry about is calories. Obesity is a huge health problem facing cats in the United States today. As we all know, once the pounds are on, they can be awfully hard to get rid of. Don’t put your adult cats at risk for weight gain by feeding them kitten food.
Now, common sense should tell us that the world will not come to an end if your kitten occasionally eats some adult food or your adult cat raids the kitten’s bowl from time to time. Just do your best. Feed adults and kittens separately and pick up the food bowls in between meals.
Taking care of a kitten is a lot like having a baby in diapers — fun; but you also find yourself looking forward to the next phase of life. Kittenhood will be over before you know it. Enjoy it while it lasts, separate foods and all.
Dr. Jennifer Coates