Being underweight can be just as concerning for your health as carrying too much weight. While much attention focuses on weight loss, those who struggle to maintain a healthy weight face their own set of challenges, from weakened bones to difficulties fighting off everyday infections.

What Does Being Underweight Mean?

Being underweight means your body weight falls below the range that research has shown is necessary for your body to function properly. This isn’t just about appearance. When your weight drops too low, your body may not be getting the nutrients it needs to maintain healthy bones, skin, hair, and internal organs.^[1]

The most common way to determine if someone is underweight is through a measurement called body mass index, or BMI. This calculation compares your weight to your height. For most adults, a BMI below 18.5 indicates underweight. For example, if you are 5 feet 4 inches tall and weigh 107 pounds or less, you would be considered underweight.^[2]

However, BMI is not perfect for everyone. Athletes with significant muscle mass, older adults, people from certain ethnic backgrounds, and pregnant women should not rely solely on BMI. Muscle weighs more than fat, so a very muscular person might have a high BMI but actually be healthy. For children, doctors use special growth charts rather than BMI to track whether they are growing as expected for their age and sex.^[1][10]

How Common Is Being Underweight?

Being underweight affects millions of people worldwide, though it receives far less attention than obesity. According to global health data, in 2022, approximately 390 million adults worldwide were underweight. While obesity rates continue to climb in many countries, underweight remains a significant concern, particularly in low- and middle-income countries.^[4]

Among children, the statistics are particularly troubling. Globally in 2022, about 149 million children under age 5 were stunted, meaning they were too short for their age due to chronic undernutrition. Another 45 million children were wasted, meaning too thin for their height. These conditions result from not getting enough calories and nutrients over time.^[4]

Certain groups are more vulnerable to becoming underweight. Older adults face higher risk because aging can bring health conditions that affect appetite, the ability to shop for or prepare food, or how the body absorbs nutrients. Children need extra calories for growth and development, making them particularly susceptible to the effects of insufficient nutrition. People with limited income may struggle to access adequate food. Women, particularly during pregnancy and breastfeeding, require additional calories and nutrients.^[13]

In developed countries like the United Kingdom, malnutrition and being underweight remain surprisingly common despite overall food abundance. Anyone can become malnourished, but it’s especially prevalent among those with long-term health conditions, people who are socially isolated, those with limited mobility, and individuals over 65 years old.^[5]

What Causes Someone to Become Underweight?

The fundamental cause of becoming underweight is taking in less energy than your body uses. This can happen in many different ways, and understanding the reason behind weight loss is crucial for finding the right solution.^[3]

Some people are naturally thin due to their genetics. They may have inherited a body type that tends toward slimness, or they may have a very high metabolism, meaning their body burns calories quickly even at rest. For these individuals, maintaining a healthy weight can be challenging even when they eat regularly.^[2][10]

Limited access to food is a major cause of being underweight worldwide. This might mean living in poverty where food is scarce or too expensive. It could also mean having difficulty shopping or cooking due to physical limitations, lack of transportation, or living alone without support. Older adults who struggle with mobility or those who are neglected by caregivers may not be able to obtain adequate nutrition.^[13]

Mental health conditions play a significant role in many cases. Eating disorders, such as anorexia nervosa and bulimia nervosa, cause people to severely restrict their food intake or purge after eating, driven by fear of gaining weight or distorted body image. Depression can eliminate appetite and make people forget to eat or lose interest in food. Severe stress, substance abuse, and dementia can also interfere with regular eating patterns.^[13]

Many physical health conditions can lead to being underweight. Long-term illnesses that affect appetite include cancer, chronic kidney disease, heart disease, and inflammatory bowel diseases like Crohn’s disease. Some conditions make swallowing difficult, a problem called dysphagia. Others cause nausea or vomiting that makes eating unpleasant. Certain medications can reduce appetite as a side effect.^[5]

Malabsorption disorders prevent the body from properly absorbing nutrients from food even when a person eats enough. Conditions like celiac disease, Crohn’s disease, ulcerative colitis, and chronic diarrhea can all deplete calories before the body can use them. Similarly, conditions affecting the pancreas can interfere with digestion.^[13]

Some people need more calories than usual, which can make maintaining weight difficult. Athletes who train intensely burn enormous amounts of energy. People with overactive thyroid glands, a condition called hyperthyroidism, have metabolism that runs too fast. Women who are pregnant or breastfeeding require hundreds of extra calories daily. Children and teenagers need additional energy to support their rapid growth.^[13]

Who Is at Higher Risk?

While anyone can become underweight, certain factors increase the likelihood. Age is a significant risk factor at both ends of life. Young children require so many calories for growth that they can quickly become malnourished if their diet is inadequate. Older adults face multiple challenges: they may have reduced appetite, difficulty accessing or preparing food, dental problems that make chewing painful, or health conditions that interfere with eating and digestion.^[13]

Economic status strongly influences weight. People living in poverty may not have consistent access to enough food or may have to choose between buying food and paying for other necessities like housing or medicine. They may also live in areas with limited access to grocery stores selling fresh, nutritious foods.^[13]

Family history matters too. If your biological relatives tend to be thin, you may have inherited genes that make you more likely to be underweight. Certain ethnic backgrounds are associated with different body compositions, which means BMI standards may not apply equally to all populations.^[13]

People with long-term health conditions are at higher risk, particularly those with digestive problems, cancer, HIV/AIDS, chronic lung diseases, heart failure, and chronic infections. Individuals taking medications that suppress appetite or cause nausea face additional challenges maintaining adequate weight.^[5]

Signs and Symptoms of Being Underweight

The most obvious sign of being underweight is a low number on the scale or a body that appears very thin in the mirror. However, if you’ve always been naturally slim, you might not recognize when your weight drops below a healthy range. For children who are underweight, they may simply appear smaller than other children their age, which parents might overlook if the child has always been petite.^[13]

Being underweight often comes with a constellation of other symptoms that reflect the body’s struggle to function without adequate nutrition. Persistent fatigue is extremely common. Without enough calories, which are literally units of energy, your body doesn’t have the fuel it needs to power daily activities. You might feel tired all the time, even after sleeping, and find it hard to concentrate or complete normal tasks.^[1]

Physical changes to your appearance can signal insufficient nutrition. Your hair may become thin, brittle, or start falling out. Your skin might become very dry, thin, or develop a pale color. Your teeth may weaken or develop cavities more easily. These changes happen because your body isn’t getting the vitamins, minerals, and protein needed to maintain these tissues.^[1]

Getting sick frequently and taking longer than usual to recover from common illnesses like colds is another telltale sign. Without adequate nutrition, your immune system cannot produce enough white blood cells and antibodies to fight off infections effectively. What might be a minor cold for someone else could leave you feeling ill for weeks.^[1][10]

Many people who are underweight feel cold all the time, even in warm environments. They may also experience dizziness or lightheadedness, particularly when standing up quickly. Some people develop headaches. These symptoms can indicate anemia, a condition where your blood doesn’t carry enough oxygen to your body’s tissues, which is more common in people who are underweight.^[1]

For women, irregular menstrual periods or the complete absence of menstruation is a significant sign. Teenage girls who are underweight may find their first period is delayed. Adult women may notice their periods become irregular or stop altogether. This happens because the body needs a certain amount of body fat to produce the hormones necessary for menstruation.^[1]

Mood changes are common when someone is underweight. You might feel irritable, anxious, or depressed. This makes sense when you consider that the brain needs adequate glucose and other nutrients to regulate mood properly. Some people also lose interest in activities they once enjoyed.^[13]

Health Risks Associated with Being Underweight

Being underweight carries serious health risks that can affect nearly every system in your body. These risks are not just theoretical concerns but real problems that can significantly impact quality of life and even survival.

Osteoporosis is one of the most significant risks, particularly for women. This condition causes bones to become brittle and porous, like a sponge, making them much more likely to break from even minor falls or bumps. According to research, being underweight substantially increases the risk of developing osteoporosis because bones need adequate calcium, vitamin D, and other nutrients to maintain their density and strength. Low body weight is also associated with lower estrogen levels in women, which further weakens bones.^[1]

The immune system suffers when the body lacks proper nutrition. Your body needs protein, vitamins, and minerals to produce immune cells and antibodies that protect against bacteria, viruses, and other pathogens. Without adequate nutrition, you’re more likely to develop infections, and when you do get sick, illnesses last longer and can become more severe. Even a simple cold can develop into complications like pneumonia in someone who is significantly underweight.^[1]

Anemia frequently develops in people who are underweight. This condition means your blood has too few red blood cells or too little hemoglobin, the protein in red blood cells that carries oxygen throughout your body. The result is constant fatigue, weakness, dizziness, headaches, and difficulty concentrating. Severe anemia can strain the heart and lead to other complications.^[1]

For women, being underweight can lead to serious reproductive problems. When body weight drops too low, the body essentially decides conditions aren’t right for pregnancy and shuts down reproductive functions. Menstruation may become irregular or stop entirely, a condition called amenorrhea. This can lead to infertility, making it difficult or impossible to become pregnant. Even if a woman who is underweight does become pregnant, she faces increased risk of premature birth, meaning the baby arrives before fully developed. These premature infants are more likely to have health problems and developmental delays.^[1]

Heart problems can develop when the body doesn’t receive adequate nutrition. The heart is a muscle that needs protein and other nutrients to maintain its strength. Severe malnutrition can cause the heart muscle to weaken, leading to low blood pressure, slow heart rate, and in extreme cases, heart failure. Some people who are very underweight also develop irregular heartbeats.^[10]

For older adults, being underweight is particularly dangerous. It increases the risk of bone fractures from falls, and recovery from any illness or injury takes much longer. Older people who are underweight have higher rates of hospitalization and death from common infections and other health problems. Wounds heal more slowly, and surgical recovery is more complicated.^[10]

Children who are underweight face risks that can affect their entire lives. Without adequate nutrition during critical growth periods, children may experience stunted growth, meaning they never reach their full height potential. Their brain development can be impaired, potentially affecting learning abilities and school performance. Their immune systems may never fully develop, leaving them vulnerable to infections throughout life.^[4]

Interestingly, research has also found that even people who are underweight can develop insulin resistance, a condition typically associated with obesity. This shows that body weight alone doesn’t tell the whole story about metabolic health, and thin people can still develop problems with blood sugar regulation.^[14]

How Can Being Underweight Be Prevented?

Preventing underweight involves ensuring your body receives adequate energy and nutrients through a balanced diet. The best approach is eating a variety of foods from all the major food groups: fruits and vegetables, whole grains, protein sources like meat, fish, eggs, and beans, and dairy products or their alternatives. This variety ensures you get not just enough calories but also all the vitamins and minerals your body needs.^[5]

For people at risk of becoming underweight, paying attention to hunger signals is crucial. Many of us ignore hunger due to busy schedules or stress, but these signals tell us our body needs fuel. When you feel hungry, make time to eat something nutritious. If you tend to forget meals, setting regular meal times and treating them as important appointments can help.^[3]

Early intervention matters enormously for medical conditions that might lead to weight loss. If you have a health problem affecting your appetite, ability to swallow, or digestion, work closely with your healthcare providers to manage it. Don’t wait until significant weight loss has occurred to seek help, as it’s easier to prevent underweight than to recover from it.^[2]

For older adults, maintaining social connections around meals can help prevent underweight. Eating alone can lead to reduced food intake, while sharing meals with others tends to increase how much people eat. If shopping or cooking becomes difficult, exploring services like meal delivery programs or community meal sites can ensure access to nutritious food.^[5]

Mental health care is an essential part of prevention for many people. If you’re experiencing depression, anxiety, or stress that affects your appetite, getting treatment for these conditions can help protect your nutritional status. For anyone struggling with thoughts about restricting food to control weight, eating disorder treatment can be lifesaving.^[2]

Regular health checkups can catch weight loss before it becomes severe. Your doctor can monitor your weight over time, spot downward trends early, and help identify any underlying causes. For children, regular well-child visits track growth patterns to ensure they’re developing appropriately.^[2]

How Does Being Underweight Affect the Body?

Understanding what happens inside your body when you’re underweight helps explain why it causes so many different problems. The changes occur at multiple levels, from individual cells to entire organ systems.

At the most basic level, every cell in your body needs energy to function. This energy comes from the food you eat, particularly from carbohydrates, fats, and proteins. When you don’t eat enough, your body first uses up glucose (sugar) stored in your liver and muscles. Once those stores are depleted, your body starts breaking down fat tissue to use for energy. If insufficient food intake continues, your body eventually begins breaking down muscle tissue, including the heart muscle, to keep vital organs functioning. This progressive breakdown explains the weakness and fatigue that underweight people experience.^[1]

Your skeletal system depends on adequate nutrition to maintain bone density. Bones are living tissue that constantly break down and rebuild. This remodeling process requires calcium, vitamin D, phosphorus, and protein. When these nutrients are lacking, bone breakdown exceeds rebuilding, causing bones to become progressively weaker. The hormonal changes that often accompany being underweight, particularly low estrogen in women, further accelerate bone loss.^[1]

The immune system becomes compromised because producing white blood cells and antibodies requires protein and various micronutrients. Your body prioritizes keeping your heart and brain functioning, so immune function gets less resources when nutrition is inadequate. The result is fewer and less effective immune cells, making it easier for pathogens to establish infections and harder for your body to clear them.^[1]

Hormone production throughout the body becomes disrupted. Fat tissue is not just energy storage but an active endocrine organ that produces hormones. When body fat drops too low, hormone production becomes abnormal. In women, this particularly affects reproductive hormones, which is why menstruation stops. In both men and women, low body weight can affect thyroid hormones, growth hormone, and stress hormones, creating a cascade of metabolic problems.^[1]

The digestive system can be affected in a vicious cycle. When someone isn’t eating much, the digestive tract actually slows down and may even shrink slightly. This can make it uncomfortable or difficult to eat normal amounts of food later, even when the person wants to gain weight. The stomach may empty slowly, causing feelings of fullness after small amounts of food. Digestive enzyme production may decrease, making it harder to digest what is eaten.^[15]

Temperature regulation becomes impaired because fat tissue provides insulation. People who are underweight often feel cold even in warm environments because they lack this insulating layer and because their metabolism may slow down to conserve energy. The body literally turns down its thermostat to preserve calories.^[10]

The skin, hair, and nails suffer visible consequences. These tissues need protein, vitamins, and minerals to maintain themselves. Hair growth slows and existing hair becomes brittle and falls out. Skin becomes thin, dry, and may heal slowly from cuts or injuries. Nails become brittle and may develop ridges or split easily. These changes reflect that the body is prioritizing vital organs over tissues it considers less essential for immediate survival.^[1]

In the brain, inadequate nutrition affects neurotransmitter production and function. Neurotransmitters like serotonin and dopamine, which regulate mood, require specific amino acids from protein and various vitamins to be produced. When these building blocks are scarce, mood regulation suffers, contributing to depression, anxiety, and irritability. Cognitive functions like memory and concentration also decline because the brain needs a steady supply of glucose and other nutrients to function optimally.^[13]

The cardiovascular system experiences multiple changes. Blood pressure often drops because there’s less blood volume and the heart beats more weakly. Heart rate may slow down as the body tries to conserve energy. In severe cases, the heart muscle itself wastes away, becoming smaller and weaker, which can lead to irregular heartbeats and, in extreme cases, heart failure.^[10]