Calculate BMI

What is BMI?

BMI (Body Mass Index) is a calculation that broadly categorizes an adult's body weight as underweight, normal, overweight, or obese, based on their height and weight. It serves as a general indicator of body fat and is correlated with an increased risk of certain health issues. To calculate BMI, you divide weight in kilograms by height in meters squared (kg/m²). For easier calculations, we also provide an Imperial unit calculator.

Health Risks of Low BMI (Body Mass Index)

A low BMI (typically defined as <18.5 for adults or below the 3rd percentile for age and gender in children/adolescents) can cause systemic health risks, varying by age, sex, and individual factors. Below are the key hazards:

Risks in Adults with Low BMI

1. Malnutrition and Metabolic Issues

• Protein-energy malnutrition: Muscle loss (sarcopenia), reduced physical strength, and weakened immunity, increasing susceptibility to infections (e.g., colds, pneumonia).
• Vitamin/mineral deficiencies: Inadequate intake of iron, calcium, and vitamin D raises risks of anemia (dizziness, fatigue), osteoporosis (fragile bones), and dental problems.
• Hormonal imbalances: Women may experience irregular menstruation, amenorrhea, or ovarian dysfunction; men may have low testosterone levels, impairing fertility.

2. Organ Function Impairment

• Cardiovascular system: Atrophy of heart muscle, low blood pressure, bradycardia (slow heart rate), and higher risk of arrhythmias. Palpitations or chest tightness may occur during exercise.
• Digestive system: Reduced gastric motility, indigestion, constipation, and long-term risks of gastric ulcers or malnutrition-related liver disease.
• Nervous system: Insufficient nutrient supply to the brain leads to memory decline, poor concentration, low mood, or even depression.

3. Other Health Risks

• Delayed wound healing: Protein deficiency slows tissue repair, prolonging recovery after surgery or injury.
• Muscle and joint injuries: Weakened muscles offer less joint protection, increasing falls and fractures (especially in older adults).
• Pregnancy complications: Underweight pregnant women face higher risks of fetal growth restriction, preterm birth, and low birth weight infants.

Risks in Children/Adolescents with Low BMI

(Below the 3rd percentile for age and gender, defined as "underweight")

1. Growth retardation: Slow height growth, poor bone development (e.g., delayed bone age), and delayed puberty (e.g., underdeveloped secondary sexual characteristics).

2. Impaired cognition and learning: Insufficient nutrients (e.g., protein, DHA) for brain development cause deficits in attention, memory, and logical thinking.

3. Weakened immunity: Frequent infections (respiratory or gastrointestinal) reduce school attendance and quality of life.

4. Psychological issues: Low self-esteem, social anxiety, or even eating disorders (e.g., anorexia nervosa) due to perceived "being too thin."

Common Causes of Low BMI

1. Dietary factors: Chronic calorie insufficiency, picky eating, crash diets, or malabsorption (e.g., gastroenteritis, celiac disease).

2. Medical conditions: Hyperthyroidism, diabetes, chronic infections (e.g., tuberculosis), cancer, or autoimmune diseases (e.g., Crohn’s disease).

3. Psychological factors: Stress, anxiety, or depression reducing appetite; or intentional weight loss (e.g., due to distorted body image).

4. Physiological factors: Genetically high metabolism or excessive physical activity without adequate calorie intake.

How to Improve Low BMI

1. Dietary adjustments:

• Increase high-calorie, high-protein foods (e.g., lean meat, eggs, milk, nuts, full-fat dairy).
• Eat small, frequent meals to avoid appetite suppression from prolonged fasting; use nutritional supplements (e.g., protein powder, vitamins) if needed.

2. Medical evaluation: If weight drops rapidly or symptoms like fatigue/anemia occur, consult a doctor to check for underlying issues (e.g., thyroid function, digestive absorption).

3. Moderate exercise: Prioritize strength training (e.g., weightlifting, push-ups) to build muscle mass, rather than excessive cardio that burns calories.

4. Psychological support: For BMI loss due to anxiety or restrictive eating, seek counseling to correct cognitive biases.

Summary of low BMI index

A low BMI doesn't necessarily mean "the thinner, the healthier"; it's a nutritional or health warning signal from the body. Adults should be wary of potential organ damage and reproductive risks, while children and adolescents should prioritize growth and cognitive development. If your BMI remains consistently below the normal range, it's recommended that you consult a doctor or nutritionist to investigate the cause and develop a personalized improvement plan to avoid blindly gaining weight and potentially causing health risks.

Health Risks of High BMI (Body Mass Index)

A high BMI (typically defined as ≥25 for overweight and ≥30 for obesity in adults, or ≥85th percentile for children/adolescents) is linked to a range of chronic diseases and health complications. Below are the key risks, categorized by body system:

I. Metabolic and Endocrine Disorders

1. Type 2 Diabetes

• Risk: BMI ≥30 increases diabetes risk by 7-fold compared to normal BMI. Excess fat (especially abdominal fat) impairs insulin sensitivity, leading to high blood sugar.
• Complications: Nerve damage, kidney failure, vision loss, and slow wound healing (e.g., foot ulcers).

2. Metabolic Syndrome

A cluster of conditions including:

• High blood pressure (hypertension)
• High triglycerides and low HDL ("good" cholesterol)
• Abdominal obesity (waist circumference: men ≥90cm/35in, women ≥85cm/33in in Asians)
• Increased risk: Heart disease, stroke, and diabetes.

II. Cardiovascular Diseases

1. Heart Disease

• Mechanism: Excess fat strains the heart, raises LDL ("bad" cholesterol), and promotes arterial plaque buildup (atherosclerosis).
• Conditions: Coronary artery disease, heart attack, heart failure, and arrhythmias (e.g., atrial fibrillation).

2. Hypertension (High Blood Pressure)

• Risk: Obese adults are 3x more likely to develop hypertension. Adipose tissue releases hormones that constrict blood vessels and retain salt.

3. Stroke

• Link: High BMI increases risk of ischemic stroke (blocked artery) by 50% due to blood clots or narrowed blood vessels in the brain.

III. Respiratory and Sleep Disorders

1. Sleep Apnea

• Symptoms: Pauses in breathing during sleep, loud snoring, daytime fatigue.
• Risks: Heart disease, high blood pressure, and reduced cognitive function.

2. Asthma and Reduced Lung Function

• Excess weight limits lung expansion and triggers inflammation, worsening asthma symptoms.

IV. Musculoskeletal and Joint Problems

1. Osteoarthritis

• Weight-bearing joints (knees, hips, spine) endure extra stress, leading to cartilage breakdown and chronic pain.
• Risk: BMI ≥30 doubles knee osteoarthritis risk.

2. Back Pain and Mobility Issues

• Weak core muscles and spinal strain from excess weight cause chronic back pain and reduced mobility (e.g., difficulty walking).

V. Cancer Risks

Obesity is linked to 13 types of cancer, including:

• Breast (postmenopausal), colorectal, endometrial (uterine), and pancreatic cancer.
• Mechanism: Fat cells release estrogen, insulin, and inflammatory molecules that fuel tumor growth.

VI. Mental Health and Quality of Life

• Psychological impact: Depression, anxiety, low self-esteem, and body image disorders (e.g., binge eating disorder).
• Social stigma: Discrimination in employment or relationships, leading to social isolation.
• Functional limitations: Reduced ability to perform daily activities (e.g., climbing stairs, exercising).

VII. Risks Specific to Children/Adolescents

Childhood obesity increases risks of:

• Type 2 diabetes (previously rare in youth)
• Fatty liver disease (non-alcoholic steatohepatitis, NASH)
• Early puberty (girls) and hormonal imbalances
• Persistent obesity into adulthood (70% chance if obese by age 10).

Key Causes of High BMI

• Energy imbalance: Consuming more calories than burned (e.g., high-sugar, high-fat diets).
• Sedentary lifestyle: Lack of physical activity (e.g., excessive screen time).
• Genetics: Family history increases susceptibility, but lifestyle plays a larger role.
• Environmental factors: Easy access to processed foods, urban design discouraging movement, and stress eating.

How to Reduce Health Risks

1. Weight loss goal: Even 5-10% weight loss (e.g., 5kg for a 100kg person) improves blood pressure, blood sugar, and cholesterol.

2. Lifestyle changes:

• Balanced diet (high fiber, lean protein, fruits/vegetables; limit added sugars).
• Regular exercise (150+ minutes/week of moderate activity like walking, cycling).
• Sleep management (7-9 hours/night) to regulate hunger hormones (leptin and ghrelin).

3. Medical support: For severe obesity (BMI ≥40), consult doctors for medications or bariatric surgery (e.g., gastric bypass).

Summary of high BMI index

High BMI is a major risk factor for chronic diseases, organ damage, and reduced lifespan. Early intervention—through diet, exercise, and lifestyle adjustments—can significantly lower risks. For children, preventing obesity in childhood is critical to avoiding lifelong health complications. If BMI is consistently high, consult healthcare providers to develop safe, sustainable weight management plans.