If a person is underweight, their body may not be getting the nutrients it needs to build healthy bones, skin, and hair. Related symptoms or signs can include osteoporosis, anemia, feeling tired, and more.

While some people may have a genetic background or a medical illness that prevents them from putting on weight, there are interventions doctors can recommend to help a person gain weight.

In this article, we look at ways to tell if you are underweight, causes, treatments, and when to see a doctor.

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The Centers for Disease Control and Prevention (CDC) recommend people use a body mass index (BMI) to calculate if they are underweight, at a healthy weight, or overweight.

Using the BMI is considered a good measure of a person’s weight because it compares their weight to their height. For example, a 170-pound person may not be overweight if they are very tall but could be overweight if they are very short.

A person can calculate their BMI by visiting the CDC’s Adult BMI Calculator. Ranges for BMI include:

  • Underweight: less than 18.5
  • Normal/healthy weight: 18.5 to 24.9
  • Overweight: 25.0 to 29.9
  • Obese: 30 or higher

These calculations may be slightly inaccurate for a person who is an elite or endurance athlete whose body has a significant amount of muscle. This is because muscle weighs more than fat.

Being underweight can cause health problems, just as being overweight can.

Not all people who are underweight experience adverse side effects or symptoms from being underweight. However, some people, experience the following symptoms related to being underweight:

  • Osteoporosis. According to a 2016 study, being underweight increases a woman’s risk of osteoporosis, which is where the bones are brittle and more prone to breaking.
  • Skin, hair, or teeth problems. If a person does not get enough nutrients in their daily diet, they may display physical symptoms, such as thinning skin, hair loss, dry skin, or poor dental health.
  • Getting sick frequently. If a person does not get enough energy from their diet to maintain a healthy body weight, they may also not be getting enough nutrients to fight off infections. As a result, a person may get sick more frequently, and common illnesses, such as a cold, can last longer than they usually would.
  • Feeling tired all the time. Calories are a measurement of the energy a particular food can give a person. Not getting enough calories to maintain a healthy weight can make a person feel fatigued.
  • Anemia. A person who is underweight is more likely to have low blood counts, known as anemia, which causes dizziness, headaches, and fatigue.
  • Irregular periods. Women who are underweight may not have regular periods, they may find menstruation stops, or an adolescent’s first period may be delayed or absent. Irregular or absent menstruation can cause infertility.
  • Premature births. According to a study published in An International Journal of Obstetrics & Gynaecology, a woman who is pregnant and underweight is at a higher risk for pre-term labor, which means having a baby before 37 weeks.
  • Slow or impaired growth. Young people need nutrients to grow and develop healthy bones. Being underweight and not getting enough calories could mean a person may not develop as expected. Doctors call this a ‘failure to thrive.’

According to a study published in the journal BMC Public Health, being underweight is associated with an increased risk for mortality when compared to people with an average BMI. The researchers suggested that being underweight may impair a person’s healing processes following an accident or trauma compared to a person with an average BMI.

There are a variety of reasons why a person may be underweight. Sometimes, multiple underlying causes may be related. Causes of being underweight include:

  • Family history. Some people have a naturally low BMI due to physical characteristics that run in their family.
  • A high metabolism. If a person has a high metabolism, they may not gain much weight even when eating high-energy foods.
  • Frequent physical activity. Athletes or people who engage in high levels of physical activity, such as runners, may burn significant amounts of calories that result in low body weight.
  • Physical illness or chronic disease. Some disease types can cause regular nausea, vomiting, and diarrhea, making it difficult to gain weight. Other conditions may decrease a person’s appetite, so they do not feel like eating. Examples include cancer, diabetes, thyroid disorders, and digestive conditions, such as Crohn’s disease or ulcerative colitis.
  • Mental illness. Poor mental health can affect a person’s ability to eat, including depression, anxiety, obsessive-compulsive disorder (OCD), and eating disorders, such as anorexia and bulimia. Each of these conditions can affect a person’s body image and appetite.

A doctor can help a person identify the cause of their low BMI and recommend a treatment plan that allows them to gain weight healthfully.

If a person is underweight, there are various healthful weight-gain methods that they can try.

A person can gain weight by following a healthful diet that incorporates nutritious calorie-dense foods. A doctor may recommend a person tries a specific diet for weight gain or refer them to a dietitian, who can help a person develop a diet plan that works for them.

Some key components of a diet for weight gain may include:

  • Adding snacks. High-protein and whole-grain carbohydrate snacks can help a person gain weight. Examples include peanut butter crackers, protein bars, trail mix, pita chips and hummus, or a handful of almonds.
  • Eating several small meals a day. Sometimes a person may be underweight because they cannot tolerate eating large meals. Instead, a person can eat several small meals throughout the day.
  • Incorporating additional foods. A person can add calorie-dense food sources to their existing diet, such as putting slivered almonds on top of cereal or yogurt, sunflower or chia seeds on a salad or soup, or nut butter on whole-grain toast.
  • Avoiding empty calories. Eating high-calorie foods may cause a person to gain weight, but they also have excess fats that could affect a person’s heart and blood vessels. A person should avoid foods that are high in sugar and salt.

Doctors may also prescribe anti-nausea medications or appetite stimulants to help a person who is underweight gain weight whenever possible. Doctors will usually only prescribe these treatments when at-home treatments have not worked.

A person should see their doctor if they have tried to gain weight but have not been able to. Anyone who is experiencing any effects of ill health due to being unable to gain weight, such as missed periods or infertility, should also see a doctor.

If a person struggles with mental illness or an eating disorder, it is essential they seek professional help. Unfortunately, a person may not always recognize their behavior is a problem.

Some of the symptoms associated with eating disorders include:

  • secretive behavior
  • sudden, unexplained weight loss
  • refusal to attend family or social events
  • appearing fatigued
  • refusing to eat in front of others

If a person has these symptoms, their friends or family members should encourage them to seek professional help from a doctor or therapist.

A person who is underweight may be at an increased risk of developing complications, including bone, teeth, and fertility problems.

A person should aim to maintain a healthy BMI. Working with a medical professional can help a person achieve and sustain a healthy weight.