If your body mass index (BMI) falls below 18.5, you’re classified as underweight by the World Health Organization. A BMI below 17.0 signals moderate to severe thinness, and below 16.0 carries a markedly increased risk of serious health problems. The good news: with the right approach to eating, exercise, and medical guidance, most people can reach a healthier weight safely. Here’s how to get started.
Why Being Underweight Matters
Carrying too little weight isn’t just a cosmetic concern. Your body needs a baseline of energy reserves and lean tissue to function well. When it doesn’t have enough, several systems start to struggle.
Complications of being underweight include loss of bone mass (osteoporosis), reduced muscle mass, a weakened immune system, anemia, and infertility or pregnancy complications. In children, it can delay growth. Women who are underweight during pregnancy have a higher chance of delivering low-birth-weight infants. Even if you feel fine day to day, chronically low weight chips away at bone density and immune defense in ways you may not notice until a fracture or frequent illness forces the issue.
Rule Out a Medical Cause First
Before changing your diet, it’s worth figuring out why you’re underweight. Sometimes the answer is simple: a naturally fast metabolism, a period of high stress, or just not eating enough. But unintended weight loss can also signal a condition that needs its own treatment.
Common medical causes include hyperthyroidism (an overactive thyroid that speeds up your metabolism), celiac disease (where the intestines can’t properly absorb nutrients), inflammatory bowel disease, peptic ulcers, and diabetes. Mental health conditions like depression, mood disorders, and eating disorders such as anorexia nervosa or bulimia can also drive weight loss. Alcohol use disorder and certain medications are additional culprits.
A doctor can run blood work and other screening tests to check thyroid function, blood sugar, nutrient absorption, and other markers. If an underlying condition is driving your low weight, treating that condition is the most important first step. Piling on calories without addressing the root cause often doesn’t work.
How Many Extra Calories You Actually Need
Gaining weight requires eating more calories than your body burns, but the size of that surplus matters. A steady gain of one to two pounds per week is considered healthy and sustainable. To put that in numbers: gaining a pound of lean muscle takes roughly 2,000 to 2,500 extra calories over the course of a week, which works out to about 300 to 350 extra calories per day. A pound of fat requires about 3,500 extra weekly calories. For most underweight people, aiming for 300 to 500 extra calories daily is a realistic starting point.
Trying to gain too fast by eating 1,000-plus extra calories a day usually leads to fat gain concentrated around the midsection, digestive discomfort, and difficulty sticking with the plan. Slow, consistent surpluses give your body time to build muscle and adapt.
What to Eat: Calorie-Dense, Nutrient-Rich Foods
The goal isn’t to fill up on junk food. You want foods that pack a lot of calories and protein into reasonable portions so you’re not forcing yourself to eat enormous volumes. Here are some of the best options, with calorie and protein counts per serving:
- Nut butters (peanut, almond): 190 calories and 8 g protein per 2 tablespoons. Spread on toast, blend into smoothies, or eat straight from the jar.
- Nuts and seeds: 160 to 200 calories and 4 to 6 g protein per ounce. Easy to snack on between meals.
- Whole milk: 150 calories and 8 g protein per cup. Swap it in wherever you’d use water or skim milk.
- Full-fat Greek yogurt: 120 calories and 16 g protein per 6 ounces. One of the best protein-to-calorie ratios available.
- Cheese: 115 calories and 7 g protein per ounce. Add it to eggs, sandwiches, pasta, or snack on it alone.
- Eggs: 75 calories and 6 g protein each. Cheap, versatile, and quick to prepare.
- Avocado: 100 to 150 calories per half. Rich in healthy fats that support hormone production.
- Beans, peas, and lentils: 100 to 120 calories and 14 to 18 g protein per half cup. Among the most protein-dense plant foods.
- Olive oil, butter, or mayonnaise: about 100 calories per tablespoon. Drizzle oil on vegetables, cook with butter, or add mayo to sandwiches for an easy calorie boost.
If you struggle to eat large meals, eating five or six smaller meals throughout the day is often easier on your appetite and digestion than forcing three big ones. Liquid calories also help: a smoothie made with whole milk, a banana, nut butter, and a scoop of protein powder can easily hit 400 to 500 calories without making you feel overly full.
How Much Protein You Need
Protein is the building block of muscle, and getting enough of it ensures that the weight you gain is lean tissue rather than just fat. Research published in the Journal of the International Society of Sports Nutrition recommends 1.2 to 2.0 grams of protein per kilogram of body weight per day for people doing strength training. For a 130-pound (59 kg) person, that’s roughly 70 to 118 grams of protein daily.
Spreading your protein intake across all meals and snacks is more effective than loading it into one or two sittings. Your body can only use so much protein at once for muscle building, so three to four servings of 20 to 30 grams each works better than a single 80-gram meal.
Why Strength Training Beats Cardio
Exercise might seem counterproductive when you’re trying to gain weight, but the right kind of exercise directs those extra calories toward muscle rather than fat. Resistance training, like lifting weights, using resistance bands, or doing bodyweight exercises such as push-ups and squats, is the most effective way to increase lean mass.
In research comparing different exercise types, resistance training alone added an average of 0.8 kg (about 1.8 pounds) of lean mass compared to no training. Aerobic exercise like running, cycling, or swimming burns a significant number of calories without doing much to build muscle, which can actually make it harder to gain weight. That doesn’t mean you should avoid all cardio. Light walking, easy cycling, or recreational sports are fine. Just make resistance training the foundation of your routine.
If you’re new to lifting, starting with two to three sessions per week focusing on compound movements (squats, deadlifts, rows, presses) gives you the most muscle-building stimulus with manageable time commitment. Increasing weight or reps gradually over weeks is what signals your body to keep building new tissue.
Practical Habits That Help
Beyond the big-picture strategies, a few daily habits can make a noticeable difference when you’re trying to gain weight:
- Don’t drink water right before meals. It fills your stomach and blunts your appetite. Sip fluids between meals instead, or choose calorie-containing drinks like milk or juice with your food.
- Eat your protein and calorie-dense foods first. If you fill up on vegetables or salad at the start of a meal, you may not have room for the higher-calorie items.
- Keep snacks accessible. A jar of mixed nuts on your desk, cheese sticks in the fridge, or a bag of trail mix in your car removes the friction of having to prepare something when you’re not particularly hungry.
- Track what you eat for a week or two. Many underweight people overestimate how much they’re actually eating. A simple food diary or calorie-tracking app often reveals a gap between perception and reality.
Weight gain is a slower process than most people expect. Even with a consistent calorie surplus and regular strength training, gaining 4 to 8 pounds in a month is a solid result. Weigh yourself at the same time each day (morning, before eating) and look at weekly averages rather than daily fluctuations. If the trend isn’t moving upward after two to three weeks, increase your daily intake by another 200 to 300 calories and reassess.