Underweight (BMI < 18.5)
Having a BMI below 18.5 is classified as underweight according to global health authorities like the WHO and CDC. This range suggests that your body weight is lower than what is considered healthy for your height — potentially putting you at risk for various nutritional deficiencies, low energy, and weakened immunity.
What Is an Underweight BMI?
Body Mass Index (BMI) is a quick measure calculated using your height and weight to estimate body fat. An underweight BMI is defined as:
Less than 18.5 for adults aged 18–65
Based on CDC and WHO international standards
Indicator of nutritional deficiency or health risk
How to Reach a Healthy BMI
Gaining weight in a healthy way takes time and consistency. The focus should be on nutrient-rich, calorie-dense foods and building muscle mass safely. Rapid weight gain through junk food can lead to fat accumulation and health issues. Instead, aim to gradually increase your daily calorie intake with balanced meals and snacks. Tracking your progress, staying consistent with exercise, and supporting your body with rest and hydration are essential for sustainable and healthy weight gain. Here are the key strategies:
Nutrition for Weight Gain
Eat more frequently — 5 to 6 small meals per day
Focus on whole foods: nuts, seeds, avocados, dairy, legumes
Add healthy fats like olive oil, peanut butter, or coconut
Exercise to Build Strength
Do resistance training (weights or bodyweight exercises) 2–3 times/week
Avoid excessive cardio — it can increase calorie burn
Focus on muscle-building, not fat accumulation
Sleep & Stress
Sleep 7–9 hours per night to aid in muscle growth and recovery
Manage stress levels with yoga, meditation, or journaling
Avoid skipping meals during stressful periods
Why You Might Be Underweight
Several underlying causes can contribute to an underweight BMI:
High metabolism or genetically lean build
Chronic illness or untreated medical conditions
Mental health issues, including stress, anxiety, or eating disorders
Poor diet or nutrient absorption
Excessive physical activity without proper fueling
If you’re underweight unintentionally, it’s important to consult a healthcare provider to identify and treat any root causes.
Health Risks of Being Underweight
While much focus is placed on the risks of being overweight, being underweight also carries significant health concerns, including:
Weakened immune function and increased illness frequency
Nutrient deficiencies, especially iron, calcium, and vitamin D
Hormonal imbalances and menstrual irregularities in women
Fatigue, weakness, and poor concentration
Bone loss and osteoporosis due to low body mass
What to Do If You’re Underweight
If your BMI is under 18.5, take the following steps:
Use our BMI Calculator to confirm your score
Consult a healthcare provider for a full health evaluation
Start a personalized meal and fitness plan for healthy weight gain
Track progress weekly and make adjustments as needed
✅ Example: A person 5’7″ (170 cm) weighing 105 lbs (47.6 kg) has a BMI of 16.3 — considered underweight.
Summary: Key Takeaways
A BMI below 18.5 is considered underweight
Can lead to health problems if left untreated
Healthy weight gain is best achieved through nutrition, exercise, and lifestyle changes
Always consult a healthcare provider for tailored advice
Use BMI as a starting point — not the sole measure of health
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