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
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UnderweightNormalOverweightObese
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
Mild Underweight
17.0 - 18.49
Moderate Underweight
16.0 - 16.99
Severe Underweight
Below 16.0
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.
Relative Health Risks by BMI Category
Severe Underweight
Very High
Moderate Underweight
High
Mild Underweight
Moderate
Normal Weight
Low
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.
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:
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Reduced Bone Density
Lower body weight means less mechanical stress on bones, leading to reduced bone mineral density over time.
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Calcium Deficiency
Underweight individuals often lack adequate calcium and vitamin D intake, essential for bone maintenance.
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Hormonal Changes
Low body fat affects estrogen production in women, accelerating bone loss similar to post-menopause.
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Muscle Loss
Reduced muscle mass means less protection for bones during falls and less stimulation for bone growth.
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
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:
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Cardiovascular Issues
Low heart rate (bradycardia), low blood pressure, and heart arrhythmias can occur with severe underweight.
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Temperature Regulation
Insufficient body fat makes it difficult to maintain body temperature, leading to chronic feeling of cold.
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Hair and Skin Problems
Hair thinning, dry skin, and brittle nails result from protein and essential fatty acid deficiencies.
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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 matters because the underlying cause is what a clinician needs to identify before anything else. Causes range from lifestyle factors to serious medical conditions.
Both sides — causes and consequences — are discussed in detail below with references to WHO, NIMH and other institutional sources.
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
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
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. A comprehensive clinical evaluation is warranted 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:
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
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.
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.
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.
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.
Medical Disclaimer
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.