Underweight

Having underweight means that your body weight is below the range that’s healthy for you. This means your body may lack the nutrition it needs to maintain itself properly. For most people, the body mass index (BMI) is a good reference. A BMI below 18.5 would be underweight for most adults (over age 15). For children, underweight means having a BMI below the 5th percentile. (That’s less than 95% of children their age.)

BMI is based on population averages. It may not account for differences in muscle mass or bone mass due to age, genetics or other factors. If you think you might have underweight, a healthcare provider can evaluate you to find out. It’s possible to have a low body weight and still be healthy. But if your weight falls below the range normally found to be healthy, you should see a provider for testing.

Symptoms of underweight

A very low body weight may be visible in the mirror or on a scale. But if you tend to be skinny, you might not notice when your weight dips below a healthy range. Children with underweight may appear smaller overall than others their age. But if they’ve always been on the small side, this might not seem unusual.

Other symptoms of underweight are related to undernutrition. This can show up in many ways, like:

• Weakness and fatigue
• Dizziness or lightheadedness
• Low pulse and blood pressure
• Hair thinning or hair loss
• Getting sick a lot and taking longer to recover
• Irritability, apathy or depression
• Irregular periods or missing periods
• Trouble getting pregnant

Underweight causes

Some people tend to be thinner due to their genetics, what they eat or their lifestyle. Certain long-term health conditions can also play a part. These conditions don’t always cause underweight by themselves. But they can make it easier to fall below the healthy range. Other health conditions can cause more severe or rapid weight loss. If you’re sick, you might not notice when sudden weight loss leads to underweight.

Many factors can contribute to underweight, including:

• Limited access to food: If food is scarce or restricted, or you have aversions to the food offered, it may be hard to eat enough. Movement challenges and child or elder neglect can also play a role.
• Mental health conditions: Eating disorders and mood disorders like depression can cause you to deprive yourself of food. Substance use, severe stress or dementia may make you forget to eat, too.
• Physical difficulties with eating: Many illnesses and some medications can cause nausea or loss of appetite, making it hard to eat enough. Others can cause issues with chewing or swallowing.
• Malabsorption disorders: Conditions like EPI or IBD can make it hard to absorb enough calories from your food. Chronic diarrhea can also deplete calories.
• Needing extra calories: Overexercising or hypermetabolism may increase your body’s need for calories. Pregnancy, breastfeeding and childhood growth also require more calories than usual.

Risk factors

Risk factors for underweight include:

• Age: Older people are more at risk for various health conditions that can affect their calorie intake. Children have a greater need for calories overall and are more at risk of malnutrition.
• Economic status: Having limited access to food may mean you may not eat enough for your body’s needs. You may also be more at risk of certain infectious diseases that can cause calorie loss.
• Biological family history: In some cases, underweight can run in families. This may be due to your family’s genetics. It may also be due to inherited habits and attitudes related to food and eating.

Complications of underweight

Complications of underweight and undernutrition can include:

• Anemia
• Loss of bone mass (osteoporosis) and muscle mass
• Weakened immune system
• Growth delays in children
• Infertility and pregnancy complications
• Giving birth to infants with low birth weight

How doctors diagnose underweight

If your healthcare provider thinks you may have underweight, they’ll take measurements of your height and weight. The ratio between the two tells them your BMI. This formula can show if your weight is below the range normally found to be healthy. In some cases, your provider might use other methods to cross-check their findings. For example, they might measure your mid-upper arm circumference.

If your weight is below the normally healthy range, your provider will want to check:

• If you have nutrient deficiencies
• If you have complications, like osteoporosis or anemia
• If you have an underlying condition that needs treatment

This may involve various medical tests, like blood tests or imaging tests.

How is underweight treated?

If you don’t have nutrient deficiencies or other complications, you may not need treatment. Some people with a genetic profile for persistent thinness appear to maintain good health. But most people with underweight have some degree of undernutrition. You may also need treatment for another condition related to your underweight. Your treatment plan will depend on your test results.

It might include:

• Consultation with a registered dietitian
• Personalized eating plan and supplement recommendations
• Supervised refeeding (like IV fluids, IV nutrition or tube feeding)
• Other treatments for related conditions

When recovering from underweight and undernutrition, it’s important to add nutrient-dense foods to what you eat — not just empty calories. Healthcare providers recommend adding foods like avocados, hummus, nuts and nut butters, dairy products and protein shakes to help you gain weight safely.

What can I expect if I have underweight?

Having underweight can lead to serious health risks. It can also be a sign that you already have a serious condition that needs treatment. If you have a low body weight, it’s important to see a healthcare provider for testing. If they find nutrient deficiencies, treating them can help bring your body back to full strength and prevent complications. Left untreated, underweight is linked to a shortened lifespan.