If you think you're too thin and want to gain weight, don't just sit on the couch and stuff yourself with food. Weight gain should always be in the form of muscle, not fat. To build muscle, start a weight-bearing exercise program. Go to a gym and learn how to do the weight training circuit. Build up those arms and legs! As you exercise, your appetite will respond to meet your needs. It only takes 15 extra grams of protein a day to build a pound of muscle a week -- so you really won't need to eat a lot more. Muscle weighs more than fat.
It's never too late to start a weight training program. Underweight older people look and feel frail because they have lost most of their muscles, not because of lack of fat. If you are inactive, you lose muscle mass to the point where you are unable to carry out daily activities -- climbing stairs, getting up out of a chair -- because your muscles are not strong enough to move the weight of your own body. Don't try to add fat to a weak body. Overweight older people often have the double burden of weak muscles AND 20, 40 or more extra pounds to lug around with them every day.
Once you are exercising regularly and gaining muscle, your appetite will probably increase and you will eat more without any conscious effort. Most muscular people and heavy exercisers will eat plenty to meet their calorie needs. The training tables for football teams are piled high with every kind of food.
Don't worry if you lose a lot of weight because of a temporary illness. You will recover your appetite and return to your normal weight, gradually, without overeating. However, unexplained weight loss is a serious concern and should be reported to your doctor.
