Eating nutritious foods that are high in calories is a good way to gain weight. It's also important to understand the reason why you're underweight.

Being underweight can be defined in a couple of ways. It can mean low weight for a person's height, defined as a body mass index (BMI) of less than 18.5. It also could be weight that is 15 to 20% below the typical weight for a person's age.

There are many reasons you may not be at your goal weight. Recovering from an illness or losing weight as you age are two examples. It's also common for athletes to want to gain weight as muscle.

It is best to see your health care provider if you lost weight unexpectedly. Your provider or a dietitian can help you gain weight in a healthy way. Together, you can create a plan based on your needs.

In general, your plan may include:

• Eating more frequently. Slowly begin to eat 5 to 6 smaller meals during the day. Try to tune into your body to recognize when you might be hungry. But you may need to plan times to eat even if you aren't that hungry.
• Choosing food with lots of nutrients. Set up a routine to eat and drink things you like and that have a lot of nutrients as well as calories. Talk with your health care provider or dietitian about how many calories to eat a day or in each meal. You also can ask how many servings you should eat of the different food groups.
• Top it off. Add extras to your dishes for more calories, such as cheese in casseroles or nut butter on whole-grain toast. You also can add dry milk or liquid milk to foods for extra protein and calories. Some examples are mashed potatoes or soups.
• Try smoothies and shakes. Avoid beverages with few nutrients or calories, such as diet soda. But a blend of high-calorie, nutritious ingredients in a smoothie or shake can help if you're eating on the go. Meal replacement drinks also may be part of your weight-gain effort.
• But watch what and when you drink. Beverages can make you feel full. If that's the case for you, avoid drinking during a meal or before. But make sure you are drinking enough throughout the day.
• Exercise. Exercise, especially strength training, can help you gain weight by building up your muscles. Exercise also may stimulate your appetite.

There also are medicines that help boost the feeling of hunger.