February 2026 • 18 min read
Underweight BMI Risks: Complete Guide to Health Concerns, Causes & Solutions
Being underweight, defined as having a BMI below 18.5, affects approximately 1.8% of adults in the United States and carries significant health risks that are often overlooked. This comprehensive guide explores the medical consequences of being underweight, common causes, and evidence-based strategies for healthy weight gain.
- Underweight Definition: BMI below 18.5 kg/m2 is classified as underweight by the WHO
- Severity Matters: Underweight is categorized as mild (17.0-18.49), moderate (16.0-16.99), or severe (below 16.0)
- Major Health Risks: Weakened immunity, osteoporosis, fertility issues, nutrient deficiencies, and increased mortality
- Common Causes: High metabolism, eating disorders, chronic illness, medications, or inadequate nutrition
- Treatment Approach: Gradual, healthy weight gain through nutrition, lifestyle changes, and medical support
- Check Your BMI: Use our BMI calculator to determine your current weight status
- What Defines Underweight BMI?
- Underweight Classifications
- Health Risks of Being Underweight
- Weakened Immune System
- Osteoporosis and Bone Health
- Fertility and Reproductive Issues
- Causes of Being Underweight
- When to See a Doctor
- Healthy Weight Gain Strategies
- Mental Health Considerations
- Frequently Asked Questions
What Defines Underweight BMI?
Body Mass Index (BMI) is a screening tool that uses your height and weight to estimate body fat and categorize your weight status. According to the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC), a BMI below 18.5 kg/m2 is classified as underweight.
The BMI formula is simple: weight in kilograms divided by height in meters squared (kg/m2). For example, a person who weighs 50 kg (110 lbs) and is 1.70 m (5'7") tall would have a BMI of 17.3, placing them in the underweight category.
BMI Scale: Where Underweight Falls
While BMI is a useful initial screening tool, it has limitations. It does not directly measure body fat or account for factors like muscle mass, bone density, age, sex, or ethnicity. Learn more about these nuances in our guide to BMI categories.
Underweight Classifications: Severity Levels
The World Health Organization further divides underweight into three severity categories based on BMI ranges. Understanding these classifications helps healthcare providers assess risk levels and determine appropriate interventions.
| Classification | BMI Range | Risk Level | Typical Concerns |
|---|---|---|---|
| Mild Underweight | 17.0 - 18.49 | Moderate | Fatigue, minor nutrient deficiencies |
| Moderate Underweight | 16.0 - 16.99 | High | Immune weakness, hormonal issues |
| Severe Underweight | Below 16.0 | Very High | Organ dysfunction, hospitalization risk |
The distinction between these categories is clinically significant. While mild underweight may cause subtle symptoms, severe underweight (BMI below 16) is a medical emergency that requires immediate professional intervention. Research published in medical journals has shown that mortality risk increases substantially as BMI drops below 18.5, with the highest risks observed in severely underweight individuals.
Example Weights by Height
To help you understand what underweight means in practical terms, here are example weight thresholds for different heights:
| Height | Underweight (BMI <18.5) | Normal Range Start (BMI 18.5) | Ideal Weight Mid-Range |
|---|---|---|---|
| 5'0" (152 cm) | < 95 lbs (43 kg) | 95 lbs (43 kg) | 108 lbs (49 kg) |
| 5'4" (163 cm) | < 108 lbs (49 kg) | 108 lbs (49 kg) | 124 lbs (56 kg) |
| 5'8" (173 cm) | < 122 lbs (55 kg) | 122 lbs (55 kg) | 140 lbs (64 kg) |
| 6'0" (183 cm) | < 137 lbs (62 kg) | 137 lbs (62 kg) | 157 lbs (71 kg) |
| 6'4" (193 cm) | < 152 lbs (69 kg) | 152 lbs (69 kg) | 175 lbs (79 kg) |
Use our ideal weight calculator for personalized weight recommendations based on your height, age, and body frame.
Health Risks of Being Underweight
While much public health focus is placed on overweight and obesity, being underweight carries its own set of serious health consequences. Research consistently shows that very low BMI is associated with increased mortality and morbidity across multiple body systems.
Weakened Immune System
One of the most significant consequences of being underweight is a compromised immune system. Adequate nutrition is essential for immune function, and when the body lacks sufficient calories and nutrients, its ability to fight infections diminishes substantially.
According to research from the National Institutes of Health (NIH), underweight individuals experience:
- Increased infection susceptibility: Higher rates of respiratory infections, urinary tract infections, and skin infections
- Slower recovery times: Wounds heal more slowly and illnesses last longer
- Reduced vaccine effectiveness: The immune response to vaccinations may be diminished
- Higher surgical risks: Post-operative complications are more common in underweight patients
The immune system requires adequate protein, vitamins (especially A, C, D, and E), and minerals (zinc, iron, selenium) to function properly. Chronic undernutrition depletes these resources, leaving the body vulnerable to pathogens.
Osteoporosis and Bone Health
Low body weight is a well-established risk factor for osteoporosis and bone fractures. This connection exists for several reasons:
Reduced Bone Density
Lower body weight means less mechanical stress on bones, leading to reduced bone mineral density over time.
Calcium Deficiency
Underweight individuals often lack adequate calcium and vitamin D intake, essential for bone maintenance.
Hormonal Changes
Low body fat affects estrogen production in women, accelerating bone loss similar to post-menopause.
Muscle Loss
Reduced muscle mass means less protection for bones during falls and less stimulation for bone growth.
The Cleveland Clinic notes that women with BMI below 18.5 have significantly higher rates of osteoporotic fractures, particularly in the hip, spine, and wrist. This risk is especially pronounced in those with eating disorders, where nutritional deficits are severe and prolonged.
Fertility and Reproductive Issues
Being underweight can significantly impact reproductive health in both women and men. The body requires adequate energy stores to support pregnancy and produce reproductive hormones.
Effects on Women
- Irregular menstruation: Low body fat disrupts the hypothalamic-pituitary-ovarian axis, leading to irregular or absent periods (amenorrhea)
- Anovulation: Failure to release eggs regularly, reducing chances of conception
- Increased miscarriage risk: Underweight women have higher rates of early pregnancy loss
- Pregnancy complications: Higher risks of preterm birth and low birth weight babies
Effects on Men
- Reduced testosterone: Low body weight can decrease testosterone production
- Lower sperm count: Nutritional deficits affect sperm production and quality
- Decreased libido: Hormonal changes can reduce sexual desire
Research indicates that achieving a healthy BMI (18.5-24.9) significantly improves fertility outcomes. Women who gain weight to reach normal BMI often see their menstrual cycles normalize and fertility improve. Learn more about healthy weight ranges for women in our guide to BMI for women.
Nutrient Deficiencies and Their Consequences
Underweight individuals frequently experience multiple nutrient deficiencies that compound health problems:
| Nutrient | Deficiency Symptoms | Long-Term Consequences |
|---|---|---|
| Iron | Fatigue, weakness, pale skin, cold hands/feet | Anemia, impaired cognitive function |
| Vitamin D | Bone pain, muscle weakness, mood changes | Osteoporosis, immune dysfunction |
| Vitamin B12 | Tingling, memory problems, balance issues | Nerve damage, megaloblastic anemia |
| Zinc | Hair loss, slow wound healing, taste changes | Immune impairment, growth delays |
| Protein | Muscle wasting, edema, brittle nails/hair | Sarcopenia, organ dysfunction |
These deficiencies create a cycle: poor nutrition leads to fatigue and reduced appetite, which further decreases food intake and worsens nutritional status.
Additional Health Concerns
Beyond the major risks discussed above, being underweight is associated with numerous other health issues:
Cardiovascular Issues
Low heart rate (bradycardia), low blood pressure, and heart arrhythmias can occur with severe underweight.
Temperature Regulation
Insufficient body fat makes it difficult to maintain body temperature, leading to chronic feeling of cold.
Hair and Skin Problems
Hair thinning, dry skin, and brittle nails result from protein and essential fatty acid deficiencies.
Chronic Fatigue
Without adequate calories, the body lacks energy for daily activities, leading to persistent tiredness.
Causes of Being Underweight
Understanding why someone is underweight is crucial for developing an effective treatment plan. Causes range from lifestyle factors to serious medical conditions.
Medical Conditions
Several medical conditions can cause unintentional weight loss or make weight gain difficult:
- Hyperthyroidism: An overactive thyroid increases metabolism, burning calories faster than they can be consumed
- Celiac disease: Damage to the small intestine prevents proper nutrient absorption
- Inflammatory bowel disease: Crohn's disease and ulcerative colitis interfere with digestion and absorption
- Diabetes (Type 1): Uncontrolled diabetes can cause significant weight loss
- Cancer: Many cancers cause cachexia (wasting syndrome) and appetite loss
- Chronic infections: HIV, tuberculosis, and parasitic infections can lead to weight loss
Eating Disorders
Eating disorders are serious mental health conditions that significantly impact weight and overall health. The National Eating Disorders Association (NEDA) estimates that 20 million women and 10 million men in the U.S. will experience an eating disorder at some point in their lives.
- Extreme restriction of food intake
- Intense fear of gaining weight
- Distorted body image
- Excessive exercise
- Withdrawal from social eating situations
- Preoccupation with food, calories, or dieting
If you or someone you know shows signs of an eating disorder, professional help is essential. Contact the NEDA helpline at 1-800-931-2237 or visit nationaleatingdisorders.org for resources.
Lifestyle and Other Factors
- High metabolism: Some people naturally burn calories faster, requiring more food to maintain weight
- Genetic factors: Family history of low BMI may indicate a genetic predisposition
- Medication side effects: Certain medications reduce appetite or increase metabolism
- Stress and mental health: Depression, anxiety, and chronic stress can suppress appetite
- Substance use: Alcohol and drug use can interfere with nutrition and appetite
- Economic factors: Food insecurity prevents adequate nutrition for many people
When to See a Doctor
It is important to consult a healthcare provider if you experience any of the following:
Unintentional Weight Loss
Losing more than 5% of your body weight within 6-12 months without trying warrants medical evaluation.
BMI Below 17
Moderate to severe underweight requires professional assessment to identify causes and prevent complications.
Persistent Fatigue
Ongoing tiredness that interferes with daily activities may indicate nutritional deficiencies or underlying conditions.
Menstrual Irregularities
Missing periods for 3+ months (amenorrhea) or irregular cycles may signal hormonal imbalances from low weight.
Difficulty Gaining Weight
If you are eating adequately but cannot gain weight, underlying medical issues should be investigated.
Your doctor may order blood tests, check thyroid function, screen for celiac disease, or refer you to a registered dietitian or specialist based on your symptoms. The Mayo Clinic recommends a comprehensive evaluation for anyone with unexplained weight loss or persistent underweight status.
Healthy Weight Gain Strategies
Gaining weight healthily requires a thoughtful approach that focuses on nutrient-dense foods and gradual increases in caloric intake. The goal is to build lean muscle mass and restore nutrient stores, not simply add body fat.
Nutrition Guidelines
Focus on these principles for healthy weight gain:
| Strategy | Recommended Approach | Example Foods |
|---|---|---|
| Increase Calories Gradually | Add 300-500 calories/day to gain 0.5-1 lb/week | Extra snacks, larger portions |
| Eat More Frequently | 5-6 smaller meals rather than 3 large ones | Scheduled snacks between meals |
| Choose Nutrient-Dense Foods | Foods high in calories AND nutrients | Nuts, avocados, whole grains, olive oil |
| Prioritize Protein | 1.2-1.7g protein per kg body weight daily | Eggs, fish, chicken, legumes, dairy |
| Include Healthy Fats | 30-35% of calories from healthy fats | Salmon, nuts, seeds, olive oil |
| Add Calorie Boosters | Extra ingredients that add calories without bulk | Nut butter, cheese, dried fruit, honey |
Exercise for Weight Gain
While it may seem counterintuitive, exercise is important for healthy weight gain. Strength training helps build muscle mass, which weighs more than fat and improves overall health.
- Focus on resistance training: Weight lifting, bodyweight exercises, or resistance bands 2-3 times per week
- Limit excessive cardio: Too much aerobic exercise burns calories that could be used for weight gain
- Eat before and after workouts: Fuel exercise and support muscle recovery with protein and carbohydrates
- Allow adequate rest: Muscles grow during recovery, so avoid overtraining
For detailed guidance on reaching your target weight, see our guide to improving your BMI and ideal weight calculator guide.
Sample Daily Meal Plan for Weight Gain
Breakfast: Oatmeal with banana, almond butter, walnuts, and honey (550 cal)
Mid-morning snack: Greek yogurt with granola and berries (300 cal)
Lunch: Whole grain sandwich with avocado, turkey, cheese, and soup (650 cal)
Afternoon snack: Trail mix with dried fruit and chocolate (350 cal)
Dinner: Salmon, quinoa, roasted vegetables with olive oil (600 cal)
Evening snack: Smoothie with milk, peanut butter, and banana (350 cal)
Mental Health Considerations
The relationship between weight and mental health is bidirectional. Mental health conditions can contribute to being underweight, and being underweight can worsen mental health symptoms.
How Mental Health Affects Weight
- Depression: Can cause appetite loss, lack of motivation to prepare food, and changes in eating habits
- Anxiety: Chronic stress hormones can suppress appetite and increase metabolism
- Eating disorders: Anorexia nervosa, bulimia, and other disorders directly impact weight and nutrition
- Body dysmorphia: Distorted perception of body size can drive restrictive eating
How Being Underweight Affects Mental Health
Malnutrition affects brain chemistry and can worsen psychological symptoms:
- Inadequate nutrition affects neurotransmitter production, potentially worsening depression and anxiety
- Low blood sugar from insufficient calories can cause irritability and mood swings
- Social isolation due to body image concerns or difficulty eating with others
- Cognitive impairment including difficulty concentrating and memory problems
Addressing weight concerns often requires treating mental health conditions simultaneously. A comprehensive approach involving mental health professionals, dietitians, and physicians produces the best outcomes.
Check Your BMI Status
Use our free calculator to determine your current BMI and weight category.
Calculate My BMIFrequently Asked Questions
A BMI below 18.5 kg/m2 is classified as underweight according to the World Health Organization and CDC. This is further divided into mild underweight (17.0-18.49), moderate underweight (16.0-16.99), and severe underweight (below 16.0). Use our BMI calculator to check your current BMI.
Being underweight increases the risk of weakened immune function, osteoporosis and bone fractures, fertility problems and menstrual irregularities, nutrient deficiencies (iron, vitamin D, B12, zinc), anemia, fatigue, hair loss, and delayed wound healing. Severe underweight can also affect heart function and significantly increase mortality risk.
Healthy weight gain involves eating nutrient-dense foods more frequently (5-6 meals/day), adding 300-500 extra calories daily, choosing calorie-rich healthy options like nuts, avocados, whole grains, and olive oil, including adequate protein (1.2-1.7g/kg body weight), adding strength training to build muscle mass, and addressing any underlying medical conditions with a healthcare provider.
Yes, being underweight can significantly affect fertility in both women and men. In women, low body fat disrupts hormone production, leading to irregular or absent periods and failure to ovulate. It also increases miscarriage risk and pregnancy complications. In men, it can reduce testosterone levels and sperm quality. Reaching a healthy BMI often improves fertility outcomes.
Consult a doctor if you experience unintentional weight loss of more than 5% of body weight in 6-12 months, have a BMI below 17, suffer from persistent fatigue, have menstrual irregularities (3+ missed periods), or are unable to gain weight despite eating adequately. Your doctor can test for underlying conditions and refer you to appropriate specialists.
Research shows that being severely underweight carries mortality risks comparable to or even greater than severe obesity. Both extremes of BMI are associated with increased health risks. While the specific conditions differ (osteoporosis vs. heart disease, for example), both underweight and overweight status warrant medical attention. Learn more about overweight BMI risks and obesity risks for comparison.
Common causes include high natural metabolism, medical conditions (hyperthyroidism, celiac disease, diabetes, cancer, chronic infections), eating disorders (anorexia, bulimia), medication side effects, mental health conditions (depression, anxiety), substance use, genetic factors, and economic factors like food insecurity. A thorough medical evaluation can help identify the specific cause.
For children and adolescents (ages 2-19), underweight is defined using BMI-for-age percentiles rather than fixed BMI numbers. A child below the 5th percentile for their age and sex is considered underweight. This accounts for normal growth patterns and developmental changes. Being underweight during growth years can affect development, bone formation, and puberty timing. See our guide to BMI for children for more information.
This article is for informational purposes only and does not constitute medical advice. BMI is a screening tool, not a diagnostic measure. If you are concerned about your weight, have experienced unintentional weight loss, or show signs of an eating disorder, please consult a qualified healthcare provider. Always seek professional medical guidance before making significant changes to your diet or exercise routine.