Your body mass index, or BMI, is calculated using your height and weight to help your doctor estimate the amount of fat on your body. If your BMI dips below 20, he may show concern that you're underweight, and if it's below 18.5, he may diagnose you as severely underweight. Being too thin can interfere with your hormone function, immunity, energy and self-esteem. To address a BMI that's too low, add a moderate number of calories from healthy, high-quality foods and strength-train to pack on a little extra muscle mass.
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How to Raise Your BMI
Increase your BMI by adding mostly healthy muscle mass through proper diet and exercise. Too many calories from junk foods without physical activity is likely to pile fat onto your body -- and excess fat comes with its own set of health problems.
Have your doctor help you estimate how many calories you need daily to maintain your current slight frame by factoring in your age, activity level and gender. Add 250 to 500 calories to that number to determine how many you should eat daily to put on about 1/2 to 1 pound per week. Muscle takes time to develop, while fat is easier to store, so gaining weight faster than that means the bulk of your added weight will be fat. Schedule at least two days per week to start a strength-training routine. This will support your muscle-gain efforts, but don't worry that it will bulk you up like a bodybuilder. A healthy approach to strength training simply improves your daily function and appearance of tone; it won't make you bulky.
Strategies for Raising Your BMI
Gaining weight when you have a low BMI can be as challenging as losing weight when it's too high. Plan on eating at least three meals and two to three smaller snacks every day to get all the calories you need. Skipping meals or snacks is a missed opportunity for calories and nutrients. Good snack times are between breakfast and lunch, between lunch and dinner and just before bed.
If you find you feel full at meals, drink minimal water while you're eating, as the liquid can fill you up. You could also adopt a grazing pattern of eating -- having multiple small meals every hour or two so you don't feel stuffed at mealtime.
Meal Planning for Weight Gain
Focus on adding the 250 to 500 calories through increased portions of quality foods at meals, such as starchy vegetables, whole grains, lean proteins, dairy and fruits. Just an extra egg at breakfast, another cup of brown rice at lunch and a glass of whole milk with dinner ups your daily calorie count by 455 calories.
If you're too full at meals to add more food, consider snacking often on high-calorie, nutrient-dense foods to fit in the extra calories. Opt for foods with extra protein as this will support your weight-training efforts. For example, at one snack have 1 cup of cottage cheese with 183 calories and 24 grams of protein; 1/2 cup of nuts with 400 calories and 13.5 grams of protein; or a smoothie with a serving of whey protein, 1 cup of milk and 1/2 of a banana for 350 calories and 34 grams of protein.
Muscle Gain to Raise a Low BMI
Put the added calories to work building muscle mass by strength-training. At each of your minimum of two workouts per week, address all of the major muscle groups -- including the hips, legs, chest, back, arms, shoulders and abs. One exercise for each muscle group performed for at least one set of four to eight repetitions should suffice. Use a weight that makes the last couple of reps in that set feel very challenging.
Your muscle fibers grow thicker and stronger when they're overloaded, so over time add more weight and additional sets to keep challenging them. Although strength training is your focus, don't give up on cardio activity altogether. A brisk walk or swimming laps, for example, for 20 to 30 minutes four or five times per week keeps your heart healthy and your joints mobile.