How should I evaluate and manage a patient with a body mass index below 18.5 kg/m²?

Evaluation and Management of Underweight Patients (BMI < 18.5 kg/m²)

Screen all underweight patients for malnutrition and underlying causes, then implement a personalized nutritional strategy to restore healthy body weight, as underweight status significantly increases mortality, hospitalization, and perioperative complications. 1, 2

Initial Risk Stratification

Underweight patients face substantially elevated health risks that demand immediate attention:

• Mortality risk is doubled to tripled compared to normal-weight individuals, with adjusted odds ratios of 2.4 in men and 2.0 in women for all-cause mortality 3, and up to 3.64-fold increased mortality risk in older adults 2
• Hospitalization rates increase by 64% (adjusted OR 1.64) and emergency room visits by 70% (adjusted OR 1.70) 2
• Perioperative complications are highest among all BMI categories, with underweight surgical patients showing 2.22-fold increased odds of any adverse event and 3.18-fold increased odds of serious adverse events within 30 days 4

The severity of underweight matters: BMI < 16 kg/m² represents severe thinness, 16-16.99 moderate thinness, and 17-18.49 mild thinness 1

Mandatory Screening Components

Nutritional Assessment:

• Document current BMI and calculate percentage below ideal body weight using modified Devine's formula: men = 51.65 kg + 1.85 kg/inch of height > 5 feet; women = 48.67 kg + 1.65 kg/inch of height > 5 feet 1
• Assess recent weight loss, particularly any loss > 5% in the preceding 3 months, which independently predicts one-year mortality 1
• Use validated tools like the Nutritional Risk Index to quantify malnutrition severity 1

Underlying Etiology Investigation:

• Screen for malignancy, hyperthyroidism, gastrointestinal malabsorption, psychiatric disorders (anorexia nervosa, depression), chronic infections (tuberculosis, HIV), and substance abuse 5
• Evaluate for respiratory disease, as U-shaped relationships exist between BMI and pulmonary conditions 5
• Assess activities of daily living and functional status, which show U-shaped associations with BMI 5

Demographic and Social Factors:

• Underweight individuals are more likely to be younger, current smokers, alcohol abstainers, physically inactive, and of lower socioeconomic status 5
• Men with BMI < 18.5 face particularly high hospitalization risk (OR 3.45) compared to underweight women 2

Management Strategy

Nutritional Intervention:

• Implement a personalized nutritional strategy immediately rather than delaying for further workup 1
• Calculate resting energy expenditure (REE): For men = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5; For women = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161 1
• Multiply REE by activity factor (1.5-1.6 for women, 1.6-1.7 for men) to determine daily caloric needs, then add 500-1000 kcal/day to promote weight gain 1
• Target restoration to BMI 18.5-24.9 kg/m² range 1

Preoperative Optimization:

• For surgical candidates, correct undernutrition preoperatively as BMI < 18.5 increases major complication odds ratio to 3.8 in thoracic surgery patients 1
• Consider delaying elective surgery if nutritional intervention can meaningfully improve BMI within a reasonable timeframe 1
• Recognize that underweight patients undergoing total shoulder arthroplasty have the highest perioperative complication rates of any BMI category, including the super morbidly obese 4

Cardiovascular Risk Management:

• Despite lower BMI, underweight status does not confer cardiovascular protection—maintain standard screening for atherosclerotic disease, hypertension, and lipid disorders 1
• Target LDL < 100 mg/dL in patients with vascular disease, using statins as first-line therapy 1
• Encourage physical activity (minimum 30 minutes, 3-4 days weekly) to build lean body mass rather than just adipose tissue 1

Critical Pitfalls to Avoid

• Do not assume underweight patients are "healthier" than overweight individuals—they face comparable or greater mortality and morbidity risks 6, 3
• Do not delay nutritional intervention while pursuing extensive diagnostic workups; implement feeding strategies concurrently with evaluation 1
• Do not underestimate perioperative risk in underweight surgical patients, who require the same heightened vigilance as morbidly obese patients 4
• Do not overlook underweight men, who face disproportionately higher hospitalization risk compared to underweight women 2

Monitoring and Follow-up

• Reassess BMI at every clinical encounter alongside waist circumference measurement 7
• Track weight gain trajectory weekly initially, then monthly once stable improvement is established
• Monitor for refeeding syndrome in severely malnourished patients (BMI < 16 kg/m²) when initiating aggressive nutritional support
• Address modifiable risk factors including smoking cessation, alcohol moderation if excessive, and optimization of physical activity 5