Underweight? Learn How to Gain Weight the Healthy Way With These Food Ideas & Tips From Doctors

Julia Savacool
·13 min read
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If you’re reading this article, you’re likely part of the one percent club. That is, the one (and a half) percent of Americans who are clinically underweight. The number, no surprise, has shrunk over the past several decades as more and more of people pack on pounds and head in the opposite direction—more than 71.6 percent of adults in the U.S. are overweight or obese, according to the CDC.

It’s not easy being a minority in the world of weight. “It’s important to understand there are people out there who are underweight and find it as difficult to gain weight as others do to lose it,” says nutrition and weight expert W. Scott Butsch, M.D., the director of obesity medicine in the Bariatric and Metabolic Institute at the Cleveland Clinic in Ohio. “There is a stigma on both sides.”

Moreover, whether your low weight is due to lifestyle, illness or physiological factors, falling below a healthy number on the scale has its own set of health risks. “Under-fueling your body can impact immune function, bone health, skin and muscle strength,” says Julie Stefanski, R.D.N., a registered dietitian in Morrisville, North Carolina, and spokesperson for the Academy of Nutrition and Dietetics. “Individuals who don’t meet their minimum calorie needs may feel fatigued, moody and pick up colds more frequently.” In fact, she says, people who carry a few extra pounds as they age are likely to live longer than those who are underweight.

If you want to put on pounds the healthy way, you’ve come to the right place. The first step is to figure out the causes of your weight issues—the next step is to address them with healthy lifestyle habits and foods that can help you gain weight. Start here.

First, determine: Are you underweight?

Regardless of the reason, if your body mass index (BMI)—or your weight-to-height ratio—dips below a certain point, doctors will consider you to be medically underweight.

The exact formula to calculate your BMI is: weight in pounds/(height in inches)² x 703

“A BMI of 18.5 or less is considered underweight,” says Alicia Romano, R.D., a registered dietitian at Tufts Medical Center in Boston, Massachusetts. “Of course, BMI does not take into account a number of factors, including age, muscle mass or bone structure, so it should not be the only marker used to assess weight status.” (Your doctor will likely also take into account your weight history and body composition.)

Assess why you’re underweight

So what might cause your weight to dip below an 18.5 BMI? Everything from lifestyle choices to your individual physiology could play a role. These are some variables doctors will consider:

Diet changes

A switch from a meat-based diet to veganism or another type of restricted food plan might cause you to not consume enough calories in the day.

Eating disorders

Deliberately restricting calories based on a desire to drop pounds—even if you are already at or below a normal weight—can result in a low BMI.

High levels of physical activity

Elite athletes, especially those who compete in endurance events like marathon running and cycling, may weigh less or have lower BMIs than the general public guidelines because of the physical demands of their sport.

Cancer

Serious illnesses such as cancer can result in unexpected weight loss for several reasons, including decreased appetite and smell/taste due to chemo, mouth sores that make chewing and swallowing painful, stomach/intestinal pain due to tumor location or medication and more.

Autoimmune diseases

This group of disorders causes your body to mistakenly attack itself, wreaking havoc on your gut, joints and overall energy levels.

  • Celiac disease: In this autoimmune condition, your body can’t tolerate gluten, leading to GI discomfort and absorption issues.

  • Type 1 diabetes: Here, an inability to correctly metabolize glucose makes it hard to use food for fuel, resulting in fatigue.

  • Rheumatoid arthritis: Inflammation in the body may increase your metabolism (a factor in weight loss) and the drugs used to treat RA, such as methotrexate, can lead to nutritional deficiencies, according to the Johns Hopkins Arthritis Center in Baltimore, Maryland.

Irritable bowel disease (IBD)

Sometimes characterized as an autoimmune or immune-mediated disease, chronic conditions like Crohn’s and ulcerative colitis affect your GI tract, with symptoms that can include ulcers in your intestinal lining, diarrhea and difficulty absorbing the nutrients from the foods you eat. “IBD and IBS (irritable bowel syndrome) both may reduce the body's ability to digest and absorb food properly—plus, uncomfortable symptoms related to eating and dietary restrictions may pose an increased risk of weight loss,” says Romano.

Physiological constraints

While “eat more calories than you burn” is a valid strategy for the majority of underweight people, it doesn’t work for everyone. “There are people who are underweight and find it difficult to gain because of the way their body regulates fat,” explains Dr. Butsch. “They have a physiological response to weight gain, where their body increases its metabolism and decreases a person’s interest in food, all in an attempt to drive weight back down to where it was.”

Health risks of being underweight

What you’ve likely heard about: Obesity leads to heart disease, stroke, diabetes, high blood pressure and more. What you may not know: Being underweight can cause bone loss, osteoporosis, low energy, constant fatigue, irritability and trouble conceiving. “It can also lead to a loss in muscle mass and trouble with your immune system, raising your risk for things like colds and the flu,” says Dr. Butsch.

For women, the fallout from being underweight is highlighted with a handful of fertility issues, including amenorrhea (missed or irregular periods) and trouble getting pregnant. Blame it on an appetite-regulating hormone called leptin. “Leptin is secreted by fat cells in the body and communicates with your brain to tell it how much body fat you have,” says Dr. Butsch. “When women become too thin, fat cells shrink and secrete less leptin, signaling to the brain that there is less fat stored, which is not optimal for reproduction.” Realizing that a woman may not have enough body fat to safely carry a baby to term, the body essentially shuts down the ability to get pregnant.

Related: 66 Ways to Boost Your Immune System Right Now

How many calories should you eat to gain weight?

Your goal when you’re trying to gain weight isn’t to house a whole fruitcake in one sitting, but rather, to sneak in a couple hundred extra calories here and few more there, in a way that feels manageable and sustainable.

A little bit of calorie math: One pound of body weight is equal (more or less) to 3,500 calories. So if you eat 500 additional calories a day, you’ll gain about a pound a week. “As a general rule, start by adding an extra 250 to 500 calories per day on top of your current intake,” says Stefanski. “If this doesn’t lead to weight gain, calories should continue to be increased.” It can be a little overwhelming trying to calculate your caloric needs for every meal, and every person is slightly different. A registered dietitian can help you work out a weekly meal plan for optimal weight gain.

Best foods to gain weight

A bag of potato chips may tip the scale, but it lacks the quality nutrition you’d get from foods that are part of a more balanced approach, like the so-called Mediterranean diet (see: whole-wheat pasta tossed with olive oil). Overall, says Romano, your goal should be to include foods that are both nutrient- and calorie-rich.

The foods here can serve as building blocks or add-ins to increase the caloric content of any meal, which can help you gain weight:

  • Avocados

  • Butter/margarine

  • Cream cheese

  • Full-fat yogurt and milk

  • Hard cheeses

  • Mayonnaise

  • Nuts and seeds

  • Oils and salad dressings

  • Peanut butter (and other nut butters)

  • Sour cream

Healthy weight-gain snacks

Certain snacks, or ingredients you add to existing snacks, can help you push your daily calorie total up. “Think about it like this: If you add one tablespoon of oil, two tablespoons of peanut butter and an ounce of full-fat cheese to your daily snacks, you will incorporate an extra 400 calories per day from those small changes alone,” says Romano.

These healthy snack choices for weight gain, suggested by the Cleveland Clinic, weigh in around 400-600 calories:

  • Blueberry muffin

  • Bagel and cream cheese (2 tbsp.)

  • Egg salad sandwich, half

  • Grilled cheese (2 slices bread, 2 slices cheese, 2 tbsp. butter)

  • Pita and hummus (one pita stuffed with ¾ cup hummus, sliced avocado, tomato and olives)

  • Powdered milk (Add 2 to 4 tbsp. per cup of milk to recipes, or mix into pudding, mashed potatoes, soups, cooked cereal and yogurt.)

  • Trail mix (1/2 cup) + 8 oz. juice

  • Yogurt (6 oz) + granola (¾ cup)

Related: Delicious Peanut Butter & Bacon Burger Recipe

Drinks to help you gain weight

High-calorie drinks can be an effective strategy for weight gain, says Dr. Butsch, but liquids can make you feel full, so be sure your choices are not interfering with your appetite for your actual meal. Look for drinks that have at least 250 calories and 10 grams of protein per bottle, Romano suggests. Here are some options:

  • Frappe

  • Ice-cream shake

  • Juice

  • Kefir smoothie

  • Whole milk

While sodas can have 250 or more calories a serving, they are high in sugar and low in protein, so they aren’t ideal from a nutritional standpoint.

Can protein powder help you gain weight?

When your weight drops below a healthy level, you don’t just lose fat, you lose muscle mass. That can be dangerous—your muscles support your bones, and without enough muscle strength, the load on your bones can lead to issues like stress fractures.

Protein is the building block for muscle growth, and protein powder, a nutritional supplement, can help support your muscles when you are low on calories. You can add it to shakes, fold it into pancake batter, stir it into mashed potatoes or sprinkle it on vegetable dishes.

But buyer beware: Protein powders are not FDA-regulated, meaning the quality of contents may vary greatly from one manufacturer to the next. What’s more, certain protein powders, which are often made from whey, soy or casein protein, can contain excessive sugar (avoid those), while a 2019 study from the national consumer safety group Clean Label Project found some powders contain high levels of toxins, including pesticides, arsenic and mercury.

If you do decide to add protein powder to your daily diet, talk with a dietician about which ones are best. And remember, while protein powder can be beneficial for building muscles, “if minimal calorie needs aren’t met, protein powder will do nothing special to promote weight gain,” says Stefanski.

Related: Should You Take Collagen Supplements?

Best protein shakes to put on some pounds

Let’s assume you’ve maxed out on your whole food options for gaining weight. If the needle’s still not moving, it’s time to break out the blender. “Making a homemade shake or smoothie with high-calorie ingredients and potentially a protein powder can be helpful,” acknowledges Romano.

Prioritize calories over protein: Research shows your body can only absorb around 30 grams of protein at a time, anyway. “Milk, yogurt, soy milk and peanut butter are already rich sources of protein and great ingredients to incorporate in a homemade shake,” says Romano. Once you’ve added those, protein powder could be overkill.

If you don’t have time to whip up your own beverage, you can buy these ready-made protein shakes at the store to gain healthy weight. They are easy to pack and drink on the go:

  • Boost VHC (Very High Calorie)

  • Ensure

  • Huel

  • Muscle Milk

  • Real Gains Weight Gainer

Healthy meal prep ideas

You don’t need to make major changes to your favorite dishes in order to gain weight—small tweaks can increase calories. “For any meal, think about how you can boost the calorie content by adding a nutrient-dense option to it,” says Stefanski. Start with these twists on popular meals:

  • Breakfast: Take your basic bowl of oatmeal, then stir in two tablespoons of nut butter, heavy cream, nuts, dried fruit or seeds.

  • Lunch: Choose full-fat dairy products such as yogurt, kefir, milks and cheeses. Eat them on their own, or throw them in the blender with fruit and protein powder to go with your midday meal.

  • Lunch/dinner: Add avocado to sandwiches, mash it up for a dip or smash it on top of a plate of rice and beans.

  • Dinner: Pureed legumes are a great addition to soups and stews.

  • Dinner: Add two tablespoons of olive oil, avocado oil, coconut oil or sunflower oil to pasta, slow-cooked rice, fried chicken, sautéed vegetables or most other dishes.

Related: How to Improve Your Gut Health

Exercises to gain weight

It might sound counterintuitive that you should workout to gain weight. Isn’t that what people trying to drop pounds do? But here’s the thing: Because muscle loss is such a worry with people who are underweight, it’s key that as you put on pounds, you also focus on strengthening your body to support it.

You may have heard that to build bigger muscles, you need to lift bigger weights. But recent research suggests that may not be the case: A study in the Journal of Applied Physiology found that exercisers were able to build the same amount of muscle mass, regardless of how heavy their weights were, as long as they did each move to exhaustion.

Moderate cardiovascular exercise is also important in an overall healthy diet plan, as it helps your body digest food and keeps systems regular. Activities like walking or swimming will get your heart rate up and blood pumping. If you’re looking for inspiration, there are numerous fitness apps that suggest workouts you can do at home.

With any luck, following these suggestions will put you on the path to healthy weight gain. If you’re still struggling, don’t give up. “Every person responds differently,” says Dr. Butsch. “Maybe you need to eat smaller meals, but more frequently. Or maybe you need to take in more calories through liquid nutrition.” Trial and error—and communicating with your healthcare provider—will eventually help you find a strategy that works.

Looking for a place to start? Try your hand at these healthy ground chicken recipes.

Sources

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