Global Prevalence: A Shifting Landscape
While global attention often focuses on the obesity epidemic, millions of individuals are still medically underweight. World Health Organization (WHO) data from 2022 revealed that approximately 390 million adults globally were underweight, compared to 2.5 billion who were overweight. For school-aged children and adolescents (5-19 years), a UNICEF report from 2025 noted that obesity surpassed underweight for the first time, though a 9.2% prevalence of thinness (a form of underweight) persisted among this age group. The question of how common are underweight people varies significantly across demographics and geographic regions, with South Asia and Sub-Saharan Africa carrying a disproportionately high burden of undernutrition. The persistence of undernutrition alongside rising obesity rates creates a "double burden of malnutrition" in many low- and middle-income countries. In the United States, by comparison, the prevalence of underweight is much lower. For example, a CDC report on 2015–2016 data estimated that 1.5% of U.S. adults were underweight.
Key Causes and Contributing Factors
Becoming underweight can result from a complex interplay of genetic, medical, and environmental factors. It is not always a simple case of inadequate food intake.
Common Causes Include:
- Genetics and High Metabolism: Some individuals have a naturally high metabolic rate that makes it challenging to gain weight, regardless of their calorie consumption. This is sometimes referred to as having a fast metabolism, and it can be a primary factor for some naturally thin people.
- Underlying Medical Conditions: A wide range of health issues can cause or contribute to being underweight. These include an overactive thyroid (hyperthyroidism), diabetes, cancer, and gastrointestinal diseases that affect nutrient absorption, like Crohn's disease. Infections such as tuberculosis and persistent gastroenteritis can also lead to significant weight loss.
- Mental Health Issues and Stress: Psychological factors like severe stress, anxiety, and depression can lead to a loss of appetite and subsequent weight loss. Eating disorders, such as anorexia nervosa, are also a major cause of being severely underweight.
- High Physical Activity: Athletes or individuals with very physically demanding jobs may burn more calories than they consume, leading to a low body mass. In women, excessive strenuous exercise can lead to amenorrhea and low bone mineral density.
- Socio-economic Factors and Food Insecurity: Food insecurity and poverty are primary drivers of undernutrition in low-income countries, where access to adequate, nutritious food is limited due to economic or climatic factors.
Serious Health Risks of Being Underweight
Being underweight is not a sign of good health and carries several serious risks, impacting immune function, bone density, and reproductive health.
Potential Health Consequences:
- Weakened Immune System: Insufficient nutrient intake can compromise the immune system, making individuals more susceptible to infections and slowing down healing from injuries or surgery.
- Osteoporosis and Bone Issues: Low body weight, often accompanied by inadequate calcium and Vitamin D, is a significant risk factor for reduced bone mineral density and osteoporosis.
- Anemia and Fatigue: A deficiency in essential vitamins and minerals like iron, folate, and B-12 can cause anemia, leading to symptoms like fatigue, dizziness, and low energy levels.
- Fertility Issues: In women, a very low body mass index (BMI) can cause hormonal imbalances that disrupt menstrual cycles (amenorrhea) and lead to infertility.
- Developmental Delays in Children: Undernutrition in children can permanently hinder physical and mental development, a condition known as "failure to thrive".
- Increased Surgical Complications: Underweight individuals face a higher risk of complications during and after surgery, including infections and delayed wound healing.
Underweight Prevalence vs. Overweight/Obesity: A Global Comparison
| Feature | Underweight | Overweight/Obesity |
|---|---|---|
| Global Adult Numbers (2022) | ~390 million | ~2.5 billion |
| Global Trend (Past Decades) | Generally declining, but persistent in specific regions. | Rapidly increasing across most regions and income levels. |
| Primary Geographic Burden | Historically and currently highest in low- and middle-income countries, particularly in South Asia and Sub-Saharan Africa. | Affects all regions globally, including high-income countries, with sharp increases in low- and middle-income nations. |
| Health Risks | Compromised immunity, osteoporosis, anemia, fertility issues, developmental delays, surgical complications. | Type 2 diabetes, heart disease, high blood pressure, certain cancers, poor mental health. |
| Associated Mortality Risk | While less prevalent, being underweight can increase mortality rates, sometimes comparably to morbid obesity, especially when linked to underlying illness. | Associated with a myriad of non-communicable diseases that increase mortality risk. |
Effective and Safe Strategies for Gaining Weight
For individuals who are medically underweight, gaining weight in a healthy way is crucial. It’s not about eating junk food but rather focusing on nutrient-dense options. Learn more here.
- Increase Calorie Intake Safely: Focus on nutrient-dense foods rather than empty-calorie options. For gradual, healthy weight gain, adults can try adding around 300 to 500 extra calories per day through healthy means.
- Eat Frequently: Opt for 5-6 smaller meals and snacks throughout the day instead of three large ones, which can be more manageable for those with a small appetite.
- Choose Nutrient-Dense Foods: Incorporate nuts, seeds, avocados, dried fruit, healthy oils, and full-fat dairy products to boost calories and nutrition in meals and snacks.
- Build Muscle with Strength Training: Exercise, particularly resistance or strength training, helps build muscle mass rather than just fat. It can also stimulate appetite.
- Seek Medical Guidance: Consult a doctor or registered dietitian to identify any underlying causes for being underweight and create a safe, personalized plan. This is especially important if weight loss is unexplained.
Conclusion: Addressing the Full Spectrum of Malnutrition
The question of how common are underweight people reveals a persistent and serious global health challenge, even as rising obesity rates dominate headlines. While the prevalence of underweight has generally decreased, it remains high in vulnerable populations, particularly in low-income regions and among certain demographics. Understanding that being underweight is not harmless and carries significant health risks is crucial. A comprehensive public health approach must address the full spectrum of malnutrition, tackling both undernutrition and obesity through improved food security, education, and personalized medical care. Efforts to combat underweight must be culturally sensitive and address the specific socio-economic and medical factors driving it, ensuring all individuals have access to the nutrition they need to thrive.
