# Age- and sex-specific reference intervals and determinants of plasma vitamin B6 metabolites in healthy Chinese adults

## TL;DR

This study establishes age- and sex-specific reference ranges for vitamin B6 metabolites in healthy Chinese adults and identifies factors influencing these levels.

## Contribution

The study provides novel age- and sex-specific reference intervals for vitamin B6 metabolites in a Chinese population.

## Key findings

- Females had higher PLP and PLP/PA levels and lower PAr compared to males.
- Age-related declines in PLP and increases in PAr were observed in males but not in females.
- ALP, albumin, eGFR, and serum phosphorus were significant determinants of vitamin B6 metabolite levels.

## Abstract

Reference intervals (RIs) for vitamin B6 biomarkers remain underexplored in Chinese adults. We aimed to establish age- and sex-specific RIs of pyridoxal 5′-phosphate (PLP), pyridoxal (PL), 4-pyridoxic acid (PA), and their ratios (PLP/PL, PLP/PA, and PAr), and to identify independent determinants of these markers.

Vitamin B6 metabolites were measured by liquid chromatography–tandem mass spectrometry (LC–MS/MS). The distribution changes were illustrated by sex-stratified age percentile curves (P10/P50/P90). Robust 95% RIs (P2.5–P97.5) were obtained by sex and age (<50 vs. ≥50 years) using the Horn–Pesce method. Sex-stratified multivariable linear regression was conducted to identify the independent determinants of vitamin B6 biomarkers.

This study included a community-based sample of 367 healthy adults (197 males and 170 females) with a median age of 49.0 years (range 20.0–80.0 years). Females had higher PLP, PL and PLP/PA, and lower PAr than males (all p < 0.01). In males, percentile curves and age-group contrasts showed age-associated declines in PLP, PLP/PL and PLP/PA and an increase in PAr (all p < 0.05); corresponding indices in females were largely stable. The RIs of PLP (nmol/L) were 10.36–145.87 (males <50 years, n = 103), 13.03–103.46 (females <50 years, n = 87), 9.27–142.96 (males ≥50 years, n = 94), and 16.83–173.06 (females ≥50 years, n = 83), respectively. Higher ALP levels were associated with lower PLP, PLP/PL and PLP/PA, and higher PAr; albumin was positively related to PLP/PL and PLP/PA, and negatively to PAr; eGFR was positively correlated with PLP/PA, and negatively with PAr. Notably, serum phosphorus was positively associated with PLP, PLP/PL and PLP/PA, and negatively with PAr.

We established population-specific, age- and sex-stratified RIs for vitamin B6 biomarkers in healthy Chinese adults, providing a baseline for clinical and laboratory assessment of vitamin B6 metabolism. We also found that ALP, albumin, renal function, and serum phosphorus were closely related to the vitamin B6 status in both males and females.