Assessment of current nutritional status (CNS) in children and adults using weight-length/height standard deviation score (SDS) difference

Technical Summary: Assessment of Current Nutritional Status (CNS) Using Weight-Length/Height Standard Deviation Score Difference (WLSDS-d/WHSDS-d)

Problem Statement
Current anthropometric indices used to assess Current Nutritional Status (CNS) in children and adults—including weight-for-length/height, Body Mass Index (BMI), BMI-for-age, Tri-Ponderal Mass Index (TMI), and Ponderal Index (PI)—possess significant limitations. These indices often fail to account for variations in sex, age, and body stature (specifically shortness/cum-lightness and tallness/cum-heaviness).

• Children: Weight-for-length/height is age-dependent, while BMI-for-age is height-dependent. Both can misclassify individuals with identical standard deviation scores (SDS) for weight and height but different statures. For instance, shorter, lighter children may be over-diagnosed with wasting/thinness, while taller, heavier children may be over-diagnosed with overweight/obesity.
• Adults: The WHO BMI classification is not sex-specific, despite known differences in body fat percentages between men and women at identical BMIs. Furthermore, standard BMI cut-offs may underestimate the burden of overweight and obesity in populations with shorter statures (e.g., in low- and middle-income Asian countries).
• General: Existing indices struggle to provide a continuous, accurate assessment of CNS from childhood through adulthood that reflects sex- and age-based variations in body fat.

Methodology
The study proposes a novel index: the Weight-Length/Height Standard Deviation Score Difference (WLSDS-d/WHSDS-d).

• Conceptual Basis: The index is grounded in the observation that individuals with an Ideal Current Nutritional Status (ICNS) have weights and lengths/heights that correspond to the same rank scores (percentiles or SDS) within their sex and age group.
• Calculation:
  1. Observed weight and length/height are converted to Standard Deviation Scores (SDS) using standard growth references (e.g., WHO, CDC).
  2. The index is calculated as: WLSDS-d/WHSDS-d = Weight SDS – Length/Height SDS.
• Classification Criteria:
  • Children (0–5 years): A difference of '0' indicates ICNS. Values < −3, < −2, −2 to +1, > +1, > +2, and > +3 correspond to severe wasting, moderate wasting, normal CNS, risk of overweight, overweight, and obesity, respectively.
  • Children (5–20 years): Similar criteria apply, distinguishing between thinness and overweight/obesity.
  • Adults (>20 years): The study utilizes the median weights and heights of the oldest children (20 years) from growth references as a baseline to calculate adult SDS. WHSDS-d values of 0, < −3, < −2, −1 to +1, > +1, > +2, > +3, and > +4 correspond to ICNS, severe thinness, moderate thinness, mild thinness, normal CNS, overweight, and obesity classes I, II, and III, respectively.
• Validation: The study assessed 34 exemplary children and 64 exemplary adults using the proposed index and compared the results against weight-for-length/height, BMI-for-age, TMI, and PI. Statistical correlation coefficients ($r$) were calculated to determine the relationship between WLSDS-d/WHSDS-d and existing indices.

Key Contributions

1. Proposal of a Novel Index: The paper introduces WLSDS-d/WHSDS-d as a continuous metric for assessing CNS from childhood to adulthood.
2. Correction of Stature Bias: The index mathematically counters the limitations of existing indices regarding shortness and tallness. It posits that if weight and height SDS are identical, the nutritional status is ideal, regardless of whether the individual is shorter/lighter or taller/heavier than the median.
3. Sex-Specific Adaptation: By utilizing sex-specific growth references for SDS calculation, the index inherently accounts for sex-based variations in body fat, addressing a known limitation of the WHO BMI classification for adults.
4. Unified Framework: The study provides a single classification system that bridges the gap between pediatric and adult nutritional assessment, unlike current methods which often require switching between different indices (e.g., weight-for-length for infants, BMI-for-age for children, and BMI for adults).

Results

• Ideal Nutritional Status: WLSDS-d/WHSDS-d correctly identified ICNS (value of 0) for individuals whose weight and length/height matched standard medians, as well as for those with identical SDS ranks above or below the median (e.g., +3 SDS for both weight and height).
• Comparison with Existing Indices:
  • Weight-for-length/height and BMI-for-age: These indices systematically underestimated ICNS in shorter-cum-lighter and taller-cum-heavier individuals. They overestimated wasting/thinness in shorter individuals and underestimated overweight/obesity in the same group. Conversely, they underestimated wasting in taller individuals and overestimated overweight/obesity.
  • TMI and PI: These indices tended to overestimate overweight and obesity in shorter individuals and underestimate them in taller individuals. They also overestimated obesity in children compared to the proposed index.
  • Adult Reclassification: In a sample of 64 adults categorized by WHO BMI, WHSDS-d reclassified 30 as thin, 18 as normal, and 16 as overweight/obese. In contrast, BMI misclassified 46.7% of thin, 38.9% of normal, and 12.5% of overweight individuals when compared to WHSDS-d. Misclassification rates were higher in women than men for all indices.
• Correlations: WLSDS-d/WHSDS-d showed significant positive correlations with TMI and BMI-for-age (in children) and BMI (in adults), and a significant negative correlation with PI. The correlation was strongest with PI.

Significance and Claims
The paper claims that WLSDS-d/WHSDS-d offers a more accurate assessment of CNS than weight-for-length/height, BMI-for-age, BMI, TMI, or PI. Specifically, it asserts that the proposed index:

• Reflects sex- and age-based variations in body fat and relative weight status continuously from children to adults.
• Counters the underestimation of ideal status and the misclassification of wasting/overweight associated with shortness and tallness found in current indices.
• Aligns more closely with the concept of Ideal Body Weight (IBW) based on age- and height-appropriate percentiles.
• Has the potential to reduce the underestimation of the total burden of overweight and obesity in low- and middle-income countries, particularly in Asian populations where shorter statures are common.

The authors conclude that while WLSDS-d/WHSDS-d shares limitations with BMI (e.g., inability to distinguish between fat, muscle, and bone mass), it should replace current indices for CNS assessment to avoid incompatible malnutrition intervention programs.