If you can't, put your friend on a diet. Reducing treats and switching to a low fat, high fiber diet is usually all that's necessary to shed extra pounds. For overweight cats, try a low calorie cat food and cut the treats out altogether. Adding more fiber to the diet can help an older dog better absorb needed nutrients. High fiber dog foods and treats are available commercially, as are fiber pills.
Most dogs enjoy white rice, which is easily substituted for dry food.
Cats are less likely to enjoy a change in diet and are most often finicky eaters. Most cats however, do enjoy sprouts. Try mixing them with their regular food once per day. Most vitamins are low in calories and are flavored so that dogs are eager to take them from you. Many pets naturally drink less as they age, which can lead to dehydration.