When it comes to the relationship between weight and health, there's no one-size-fits-all ideal you can call on to make sure you're on the right track. If your BMI is 18.5, you're on the low end of what's considered healthy, but you may or may not be at a healthy weight. If you're naturally very slim, a BMI of 18.5 might be where you're supposed to be, but if you have a naturally larger frame, it might mean your weight is too low. If you're interested in gaining weight, diet and lifestyle choices can help you reach your goals. Just make sure you see a doctor before you start, so you can get personalized recommendations based on your body type.
Set Goals for Good Health
Since you're right on the verge of being underweight, you should consult your doctor to figure out if gaining weight might improve your health. How much you'll need to gain depends on your frame and current health; if you're naturally very thin, your doctor might only recommend gaining a few pounds. On the other hand, if you have an average-size frame, you might need to gain more weight to get to a healthy weight.
Regardless of your weight goals, you should take the opportunity to examine your lifestyle. Unhealthy lifestyle habits pose a health risk for anyone -- not just people who are overweight -- so making a few lifestyle tweaks might improve your health, as well as help you add pounds.
Increase Calories to Up Your BMI
If your doctor recommends gaining weight, you'll need to add calories to your diet, so that you're eating more calories than you burn each day. Use an energy calculator -- like the one provided by the Baylor College of Medicine -- to get a sense of your daily calorie needs.
For example, a 21-year-old woman who is 5 foot, 7 inches tall and weighs 119 pounds has a BMI of 18.5, and gets about an hour of activity a day, needs roughly 2,150 calories daily. To gain weight, this woman would need to eat an extra 250 to 500 calories to gain 0.5 to 1 pound each week, and she should up her intake to 2,400 to 2,650 calories each day. She'd only have to gain 3 pounds -- up to 122 pounds -- to get a healthier BMI of 19.1, which would take 3 to 6 weeks.
Don't worry if it takes some trial and error to gain the recommended 0.5 to 1 pound weekly. While a calculator provides estimated calorie needs based on your body size and activity, it's not perfect, and your actual calorie burn might deviate as much as 16 percent from the estimate, according to a study published in Current Opinion in Clinical Nutrition and Metabolic Care in 2004. If you're having trouble gaining weight, consult your doctor for help.
Strength Train to Gain Muscle
Whether you need to gain weight or not, make sure you're strength training to maintain a lean body composition. Even if you're on the border of being underweight according to BMI, you can still have too much body fat, and often it's located in your abdomen. A high body fat means you're "metabolically obese," which means you'll be at a greater risk of illnesses like heart disease and diabetes.
You're most prone to a high body fat percentage if you live a sedentary lifestyle, so hit the weight room two to three times weekly to build lean, healthy muscle. Full-body workouts help you tone up all over your body, and they'll also make you look fitter and healthier. Support your workouts with a diet rich in nutritious foods -- like whole grains, fruits and veggies -- as well as high-quality protein from nuts, beans, dairy, eggs, lean meat and fish.
Consider Other Body Measurements
While BMI often provides some insight into whether you're underweight, at a healthy weight, or overweight or obese, it's not the only worthwhile measurement. Consider consulting a professional for a body composition test to see how much of your weight comes from lean mass, like muscle, and how much comes from fat. Ask for skinfold caliper measurements, a BodPod scan, underwater weighing or DEXA scans; these measurements can give you an accurate read on your body fat levels.
A professional can also conduct tests to measure your strength and aerobic fitness to get more insight into your health. These tests can identify potential health issues -- like metabolic obesity or poor cardiovascular health -- you won't get by looking at BMI alone, so you can set appropriate goals to improve your health.