How to Healthily Increase a Low BMI

Individuals with a body mass index (BMI) below 18.5 are classified as underweight according to World Health Organization (WHO) criteria. While low BMI may reflect a naturally lean physique in some people, it can also signal underlying medical concerns—including malnutrition, chronic illness, gastrointestinal disorders (e.g., celiac disease or inflammatory bowel disease), endocrine dysfunction (such as hyperthyroidism or adrenal insufficiency), psychiatric conditions like anorexia nervosa, or unintended weight loss due to malignancy or infection.

A comprehensive clinical evaluation is essential before initiating any intervention. This includes a detailed dietary history, assessment of energy and nutrient intake, physical examination for signs of muscle wasting or micronutrient deficiencies, and targeted laboratory testing—such as complete blood count, thyroid function tests, serum albumin, vitamin D, iron studies, and inflammatory markers. Imaging or endoscopic evaluation may be warranted if gastrointestinal pathology is suspected.

Nutritional rehabilitation should prioritize gradual, sustainable weight gain—typically aiming for 0.25–0.5 kg per week—to minimize metabolic stress and avoid refeeding syndrome. Calorie-dense, nutrient-rich foods—such as avocados, nuts, seeds, full-fat dairy, oily fish, and legumes—are recommended over empty-calorie options like sugary beverages or highly processed snacks. Small, frequent meals (5–6 per day) may improve tolerance in individuals with reduced appetite or early satiety. In select cases—particularly when oral intake remains inadequate despite counseling—registered dietitians may recommend oral nutritional supplements or, rarely, enteral feeding.

Addressing comorbidities is equally critical: optimizing thyroid hormone replacement in hypothyroidism, treating active infections, managing psychiatric conditions with evidence-based therapies (e.g., cognitive behavioral therapy for eating disorders), and discontinuing or adjusting medications that suppress appetite or increase metabolic rate. Regular follow-up with primary care providers or specialists ensures timely reassessment and adjustment of the management plan.

Importantly, weight restoration alone is insufficient; restoring lean body mass and functional capacity requires concurrent attention to protein intake (1.2–1.5 g/kg/day), resistance exercise, and adequate sleep and stress management. Multidisciplinary collaboration—among physicians, dietitians, mental health professionals, and physical therapists—is often key to achieving durable, health-centered outcomes.