There’s so much talk about losing weight — hello, diet culture — that it’s easy to forget that sometimes people actually need to gain weight.
From pregnancy to struggling with infertility to overexercising, there are reasons why gaining weight can benefit someone’s health. But that doesn’t mean the best strategy for someone who is underweight is to just eat whatever they want.
How do I know if I’m underweight?
Since body mass index, or BMI, is often used to judge whether someone is underweight or overweight, you would think that would be a good place to start, right? Think again.
BMI isn’t as accurate as we’ve been taught. It can provide helpful, if limited, information about your body but isn’t a complete picture of your health.
Instead of relying on something like BMI, check in with your body. People who are underweight or undernourished often experience symptoms such as fatigue and lethargy as well as low blood pressure, heart rate and blood sugar levels, which can cause night sweats, having cold fingers and toes and muscle problems, says Judy Simon, a registered dietitian nutritionist at UW Medicine who treats patients who are underweight and who specializes in nutrition for people experiencing infertility and eating disorders.
“These things can happen if someone doesn’t eat enough to match their physical activities and energy needs. That results in the body budgeting its available energy carefully,” Simon explains.
For women and people assigned female at birth, this lack of an energy source can also mean that your hormone levels drop, leading to missed periods and infertility.
What causes undernourishment?
There are many situations and medical conditions that can lead to someone being underweight or undernourished.
Some of these medical conditions include inflammatory bowel disease, cancer, neurological diseases that cause loss of appetite, and malabsorption disorders. Other common causes of undernourishment are overexercising and eating disorders, including relative energy deficiency in sport (aka RED-S), where someone doesn’t eat enough to fuel their athletic activities.
Mental health conditions like anxiety or depression can prevent someone from eating as much or as nutritiously as they should. Certain medications, such as those that are prescribed to people who have ADHD, can also lead to lack of appetite and insufficient nutrition.
In pregnant people, the weight gain needed to support the growing baby can sometimes be interrupted by things like morning sickness. Growing children and teenagers can be undernourished or underweight for any of the reasons listed above, too.
Undernourishment is serious. If left untended it can lead to chronic conditions such as a depressed immune system, anemia, osteoporosis or other serious conditions.
If you think you’re underweight, it’s always a good idea to talk with a doctor about it, as they can help figure out why and what to do next.
Tips for healthy weight gain
Rarely can you tell if someone needs to gain weight just by looking at them. Some people are just naturally small, while people in larger bodies may be dealing with eating disorders or other issues that lead to undernourishment.
If you think you might need to gain weight, here are Simon’s top tips.
1. Work with a dietitian
Along with talking with your doctor about your desire to be more nourished, it is a good idea to work with a registered dietitian. They can help you figure out exactly how much — and what — you should be eating.
“It sounds logical to just eat more, but an approach may be individualized depending on someone’s appetite and satiety level,” Simon explains.
Working with a professional is especially important if you have other medical conditions or if you are recovering from an eating disorder and need to gain more weight than the average undernourished person.
2. Be realistic with your timeline
“For most adults, gaining 1-2 pounds per week may be realistic, but it is so individual. People in eating disorder treatment may gain 1-4 pounds a week with professional support,” says Simon.
There is no one-size-fits-all approach, however. What works for one person may not work or be feasible for another. It’s important to tailor your goals to your individual needs.
For example, rather than simply eating more, it might be better to swap out what you’re currently eating for foods that are more calorie-dense, Simon suggests.
“So if someone were to add an additional 500 calories per day, theoretically they should gain 1 pound per week,” she says.
3. Choose the right foods
It may seem like eating more of whatever you want will help you gain weight. This may be true in some cases, though it probably isn't the healthiest approach. Plus, if celery is your food of choice to eat more of — well, that’s not going to help much.
What you should eat to gain weight in a healthy way depends on your specific needs and food preferences (not to mention taking into account any food allergies or sensitivities you may have).
If your goal is to eat an additional 500 calories a day, a simple peanut butter sandwich or smoothie may do the trick.
“I encourage people to add foods that are not only calorically dense but also nutrient dense,” explains Simon.
Calories provide your body with energy, while nutrients — such as proteins, vitamins and minerals, and yes, fats — supply your body with the essentials it needs to function properly.
Some of Simon’s favorite options for calorie-dense, nutrient-dense foods include nut butters, yogurt, smoothies, fruit and avocados.
4. Expect setbacks
Just because you may need to gain weight doesn’t mean the process will always go smoothly. In fact, sometimes it may be downright frustrating.
Stomach problems are one common issue. If you haven’t been eating as much as you should, your gastrointestinal tract may not work as efficiently.
That means that you may experience short-term issues like gas, constipation, diarrhea or cramping when you start eating more.
If you’ve been dealing with an eating disorder or have anxiety around eating certain foods, you may have feelings of fear as you try to gain weight.
“Many people know they need to gain back lost weight but fear gaining too much weight, afraid they will lose control or afraid they will be unhealthy despite being severely underweight,” says Simon.
It’s important to be patient and kind with yourself as you go through the process of trying to get healthier. Focus on your ultimate goal and the small steps it will take to get there. Give yourself credit for every small step you make and recognize that any setbacks are temporary.
5. Address mental health issues
If anxiety, depression, an eating disorder, or another mental health issue is the underlying reason for your weight loss or undernourishment, it’s important to seek help for that as well to set yourself up for success.
“Seeking therapy or relaxation techniques may help weight gain by treating the root cause of the problem,” says Simon.
This is also true for people who have gastrointestinal conditions, especially irritable bowel syndrome (IBS). IBS is often linked to anxiety or depression, so working with a therapist will likely help improve your mental health as well as your gut health, while working with a doctor or nutritionist can help you find foods to eat that don’t irritate your stomach.
It can be difficult to purposefully gain weight — even when it’s very needed — in a society that treats all weight gain as unhealthy and shameful, says Simon.
If this is tripping you up and preventing you from accomplishing your goals, remind yourself that your body needs fat and energy in order to fuel itself, and there is nothing wrong with taking steps to make sure it can do just that.