We humans sometimes lose our appetites when we grow old. Those of us who don't may find ourselves gaining weight, although we're eating exactly what we ate when we were younger, because we're less physically active. It shouldn't (you should excuse the expression) happen to a dog!

Unfortunately, weight gain does happen to dogs as well as humans. And since they, too, are living longer, feeding the geriatric dog has become a growing concern.

As dogs age and become less active, it's very important not to feed them as many calories as you did when they were younger so they don't become obese. One obvious approach is to feed them smaller amounts of food. Another is to select a senior-type diet formulated to have lower caloric density. Soft-moist foods can be a problem in that the volume necessary to meet daily nutritional requirements is generally lower than dry or canned foods. There is a tendency to think it can't possibly be enough food, and so you provide more. This could increase the probability that a dog will gain weight.

Another area of concern is the dog who has lost his appetite with age. Increasing age should not have a negative effect on appetite. If an older dog is not eating, take him or her to your veterinarian to rule out a medical cause.

If there is no underlying medical problem, anything that the owner can do to entice the animal to eat is fine. That includes letting the dog eat cat food. Cat foods are higher in fat and proteins than many foods, which increases the palatability for a dog.

The solution for restoring an older dog's appetite could also be as simple as changing from one form of food to another, particularly if the new food is of higher quality and palatability. The dog who has begun to turn up his nose at dry dog food, for example, may find canned food or a mix of dry and canned food more appetizing. Alternatively, you can make the dry food more palatable by using soup or gravy to moisten it.

Another problem that older dogs share with aging humans is constipation. When that happens, the solution can be to add more bulk to the diet. Some newer products have different types of fiber for that purpose. If you don't want to risk a drastic change of diet, relief can be as simple as adding some wheat bran to the dog's regular food. However, if your dog continues to have constipation problems, a visit to your veterinarian is in order.

The writers are Dr. Steven B. Holzman, Dr. William R. Haagenson and Dr. Kathleen Tapley of the Nassau-Suffolk Veterinary Hospital in Farmingdale.