BMI Z-Scores and Nutritional Status in Children
A BMI of -1 to -2 standard deviations (z-score) below the mean does NOT represent good nutrition according to WHO standards—it indicates mild to moderate undernutrition that requires nutritional intervention.
WHO Classification of Pediatric Nutritional Status
The question refers to BMI z-scores, which are used in children and adolescents rather than adults. The WHO uses standard deviation scores (z-scores) to classify nutritional status in pediatric populations:
Standard BMI Z-Score Categories
- Normal nutrition: BMI z-score between +1 and -1 SD 1
- Mild undernutrition (at risk): BMI z-score between -1 and -2 SD 1
- Moderate undernutrition: BMI z-score between -2 and -3 SD 1
- Severe undernutrition: BMI z-score below -3 SD 1
Clinical Significance of -1 to -2 SD
A BMI z-score of -1 to -2 SD falls into the "borderline underweight" or "at-risk" category, which signals:
- Increased nutritional risk requiring monitoring and potential intervention 1
- Not optimal nutritional status—this range indicates the child is lighter than expected for their height and age 1
- Potential for progression to more severe malnutrition if dietary intake remains inadequate 1
Screening and Assessment Implications
When BMI Falls Below Normal Range
Children with BMI z-scores in the -1 to -2 SD range should undergo:
- Detailed dietary intake assessment to identify inadequate caloric or nutrient consumption 1
- Evaluation for recent weight loss (>5% over 3 months is significant) 1
- Assessment of underlying disease processes that may accelerate nutritional deterioration 1
- Monitoring of food intake patterns to determine if current intake meets requirements 1
Nutritional Risk Scoring
The Nutrition Risk Screening 2002 (NRS-2002) system assigns points for:
- BMI < 20.5 kg/m² in adults (equivalent concept to low BMI z-scores in children) 1
- Recent weight loss or reduced food intake 1
- Severity of disease as a reflection of increased nutritional requirements 1
Contrast with Adult BMI Standards
For context, in adults, the WHO defines:
- Normal/healthy weight: BMI 18.5-25.0 kg/m² 1, 2
- Underweight: BMI < 18.5 kg/m² 1, 2
- Borderline underweight: BMI 18.5-20.0 kg/m² 1
The adult equivalent of a pediatric BMI z-score of -1 to -2 SD would be a BMI in the borderline underweight to underweight range (approximately 18.5-20.0 kg/m²), which is NOT considered optimal nutrition 1.
Clinical Action Required
Nutritional Intervention Strategy
For children with BMI z-scores of -1 to -2 SD:
- Nutritional counseling should be provided to increase caloric and nutrient intake 1
- Monitoring of weight trajectory is essential to prevent progression to moderate or severe malnutrition 1
- Dietary modifications focusing on nutrient-dense foods to support catch-up growth 1
- Regular follow-up to assess response to nutritional interventions 1
Prevention of Further Deterioration
The goal is to prevent progression from mild undernutrition (-1 to -2 SD) to moderate or severe categories by:
- Early identification through systematic nutritional screening 1
- Prompt intervention before severe malnutrition develops 1
- Addressing underlying causes of inadequate intake or increased requirements 1
Important Caveats
- BMI z-scores are age and sex-specific in children, unlike adult BMI which uses fixed cutoffs 1
- Growth velocity matters—a child maintaining -1.5 SD consistently may be healthier than one rapidly declining from 0 to -1.5 SD 1
- Ethnic variations exist—some populations may have different optimal BMI ranges, though WHO standards are internationally applied 3
- Body composition assessment may be needed beyond BMI alone, as BMI doesn't distinguish muscle from fat mass 1, 4, 5