Underweight

Being underweight means your body weight falls below the healthy range needed for your body to function properly. This can lead to various health problems, from weakened bones to fertility issues, and may require medical attention to address underlying causes and restore healthy weight.

Table of contents

• What is underweight?
• Health risks of being underweight
• Causes of being underweight
• When to see a doctor
• Treatment and weight gain strategies

What is underweight?

Being underweight means that your body weight is below the healthy weight range for your height and age. When you are underweight, your body may not be getting the nutrients it needs to maintain healthy bones, skin, hair, and other tissues.^[1]

The most common way to determine if someone is underweight is by calculating body mass index (BMI), which compares weight to height. For most adults, a BMI of less than 18.5 is considered underweight.^[1] A normal or healthy weight range is a BMI between 18.5 and 24.9.^[2]

For example, if a woman is 5 feet 4 inches tall and weighs 107 pounds or less, she would be considered underweight with a BMI of 18.4. A healthy weight range for that height would be 108 to 145 pounds.^[2]

It’s important to note that BMI calculations may not apply equally to all people. For example, athletes with significant muscle mass, people from certain ethnic backgrounds, older adults, and pregnant women should not rely solely on BMI to determine if they are underweight.^[2] For children, weight is usually measured using growth charts rather than BMI, which compare how a child is growing against others of the same sex and age.^[10]

However, BMI is just one way to measure healthy weight. Some people may have a low but still healthy weight. Not everyone who is underweight will experience health problems, but it’s important to speak with a healthcare provider about what is a healthy weight for you.^[2]

Health risks of being underweight

Being underweight can cause various health problems, just as being overweight can. While not all people who are underweight experience negative effects, many face serious health risks.^[1]

One major risk is osteoporosis, a condition where bones become brittle and more prone to breaking. Research shows that being underweight increases a woman’s risk of developing this bone disease.^[1] Being underweight can be especially dangerous for older people since it increases the risk of breaking bones and becoming ill.^[10]

When a person doesn’t get enough nutrients from their diet, they may develop physical problems with their skin, hair, or teeth. These can include thinning skin, hair loss, dry skin, or poor dental health.^[1] Hair may become thinner or fall out, and skin may become very dry.^[10]

People who are underweight often get sick more frequently and take longer to recover from common illnesses like colds. This happens because the body may not be getting enough nutrients to fight off infections effectively.^[1] The immune system becomes weakened, making infections harder to fight off.^[10]

Feeling tired all the time is another common problem. Calories are a measurement of the energy food provides. Not getting enough calories to maintain a healthy weight can make a person feel constantly fatigued and weak.^[1]

Anemia, a condition of low blood counts, is more likely in people who are underweight. This causes dizziness, headaches, and fatigue.^[1]

For women, being underweight can affect reproductive health. Women who are underweight may not have regular periods, menstruation may stop completely, or an adolescent’s first period may be delayed or absent. These irregular or absent periods can lead to infertility, making it difficult to become pregnant.^[1] Pregnant women who are underweight face a higher risk of premature labor and giving birth to babies who are too small.^[1]

Children who are underweight may not grow as expected for their age.^[10] The developmental, economic, social, and medical impacts of malnutrition (when the body doesn’t get the right amount of nutrients) are serious and lasting, affecting individuals, families, communities, and entire countries.^[4]

Causes of being underweight

There are many different reasons why someone might become underweight. Some people may be born naturally small, with their low BMI due to their genes or family history.^[10] Others may have a very high metabolism (the way the body uses energy from food) and find it hard to gain weight, even when eating foods with many calories.^[2]

Limited access to food is a common cause. If food is scarce or restricted, or if you have strong dislikes of available foods, it may be difficult to eat enough. Movement challenges and neglect of children or elderly people can also contribute to not getting enough food.^[13]

Mental health conditions can play a significant role. Eating disorders and mood disorders like depression can cause people to restrict their food intake severely.^[2] Substance use, severe stress, or dementia (memory loss) may make people forget to eat.^[13]

Physical difficulties with eating can prevent proper nutrition. Many illnesses and some medications can cause nausea or loss of appetite, making it hard to eat enough. Other conditions can cause problems with chewing or swallowing.^[13] Dental and oral problems, as well as digestive issues, can affect how food is digested and absorbed.^[16]

Some medical conditions make it difficult for the body to absorb nutrients properly. Malabsorption disorders and conditions like inflammatory bowel disease (IBD) can make it hard to absorb enough calories from food. Chronic diarrhea can also deplete calories.^[13]

Certain situations increase the body’s need for calories beyond normal levels. Excessive exercise or hypermetabolism (when the body burns energy faster than normal) can increase calorie needs. Pregnancy, breastfeeding, and childhood growth also require more calories than usual.^[13] People with thyroid problems or certain types of cancer may experience rapid weight loss.^[16]

Older people are particularly at risk for being underweight because they may face multiple challenges. These can include long-term health conditions that affect appetite, difficulty leaving the house to buy food, social isolation, limited mobility, or reduced income.^[1] Recovering from illness or surgery can also lead to weight loss as you age.^[3]

When to see a doctor

You should see a healthcare provider if you have lost weight suddenly without changing your diet or exercise routine.^[5] Sudden, unintended weight loss can be a sign of a serious medical problem such as cancer, digestive disorders, or neurological diseases.^[1]

It’s important to consult a doctor if you have unintentionally lost a lot of weight over the last three to six months, if you have other symptoms of malnutrition, or if you’re worried that someone in your care (such as a child or older person) may be malnourished.^[5]

A healthcare provider can check if you’re at risk of malnutrition by measuring your weight and height and asking about any medical problems you have or recent changes in your weight or appetite. If they think you could be malnourished, they may refer you to a specialist such as a dietitian (a professional who advises on nutrition) to discuss treatment.^[5]

It’s best to see your healthcare provider if you lost weight unexpectedly. Your healthcare team or dietitian can help you gain weight in a healthy way. Together, you can create a plan based on your individual needs.^[3]

Treatment and weight gain strategies

Treatment for being underweight depends on your overall health, how underweight you are, and the underlying cause. A healthcare provider will work with you to develop a personalized plan.^[15]

The first step usually involves making changes to your diet. A dietitian can advise you about dietary changes that can help and may create a tailored diet plan to ensure you get enough nutrients.^[15] Initial advice typically includes eating foods high in calories and nutrients, snacking between meals, and having drinks that contain lots of calories.^[15]

To gain weight gradually and healthily, try adding around 300 to 500 extra calories per day. If you want to gain weight faster, you might aim for around 700 to 1,000 calories above your normal needs.^[17] Adults could try adding these healthy calories gradually to avoid overwhelming the body.^[12]

The key is to focus on nutrient-dense foods rather than empty calories from junk food. Eating junk food may result in weight gain, but it won’t satisfy the nutrition your body needs and can still harm your health.^[24] Choose foods that are rich in nutrients, such as high-protein meats, nutritious carbohydrates like brown rice and whole grains, and foods containing healthy fats.^[3]

Practical strategies for gaining weight include eating more frequently throughout the day. Try to have five to six smaller meals during the day rather than three large ones. Add healthy snacks between meals, such as nuts, cheese, trail mix, protein bars, yogurt, or rice pudding.^[3] You can increase calories in your meals by adding toppings like cheese, nuts, and seeds.^[12]

Include plenty of protein in your diet to help build muscle. Good protein sources include milk, eggs, beans, fish, and lean meat. Aim for about 1.5 to 2.2 grams of protein per kilogram of body weight.^[16] High-calorie drinks between meals, such as milkshakes, can also help.^[12]

Have a balanced diet by choosing from a variety of food groups, including fruits and vegetables, starchy carbohydrates, dairy products or alternatives, and protein sources. Make sure to eat foods that you enjoy and that provide both nutrients and calories.^[12]

Exercise, particularly strength training and activities like yoga or weight lifting, can help you build muscle mass rather than just fat. This increases your overall weight and helps achieve a healthy body composition. Exercise can also improve your appetite.^[12] However, avoid exercising for very long periods, as this can burn too many calories.^[26]

If dietary changes alone are not enough, your healthcare provider may recommend nutritional supplements. These might include vitamin and mineral supplements or special nutritional drinks with extra calories and protein. These should only be taken on the advice of a healthcare professional.^[15]

For people who cannot eat enough due to swallowing problems or other medical conditions, alternative feeding methods may be necessary. These can include feeding tubes placed through the nose into the stomach, tubes placed directly through the skin into the stomach, or nutrition given directly into the bloodstream through a vein. These treatments are usually started in the hospital but can sometimes be continued at home.^[15]

Some people may need additional support for underlying issues. This might include home care assistance with shopping or cooking, occupational therapy to help with daily activities, or speech and language therapy for swallowing difficulties.^[15]

For children who are underweight or malnourished, treatment may involve dietary changes, support for families to manage factors affecting the child’s nutrition, treatment for underlying medical conditions, and nutritional supplements if needed. Severely malnourished children need special care in the hospital before gradually returning to a normal diet at home.^[15]

Regular monitoring is important to make sure treatment is working. Weight and height measurements will be taken, and adjustments to the treatment plan will be made as needed.^[15]