Table 3. Beta coefficients^a for tryptophan and kynurenine metabolites derived from linear regression models on smoking status (current or former versus never smokers), cotinine, body mass index (BMI), and pyridoxal 5’-phosphate (PLP) among all controls (N = 362).

	Former smokers	Current smokers	Cotinine (per 100 nmol/L)	BMI (per 5 kg/m2)	PLP (per 100 ng/mL)
Tryptophan, μmol/L	0.024	-0.001	<0.001	0.026	0.013
Kynurenine, μmol/L	0.043	0.022	<0.001	0.041b	0.004
Anthranilic acid, nmol/L	-0.093	-0.103	-0.010	0.094	0.024
KA, nmol/L	-0.045	-0.040	-0.003	0.103b	0.064c
HK, nmol/L	0.029	0.082b	0.006	0.016	-0.067c
XA, nmol/L	-0.014	0.011	-0.002	0.084	0.020
HAA, nmol/L	-0.053	-0.051	<0.001	0.080	0.047b
KA:HK ratio	-0.091	-0.115b	-0.010	0.096b	0.131c
XA:HK ratio	-0.058	-0.075	-0.010	0.076	0.086c
HAA:HK ratio	-0.081	-0.133b	-0.010	0.064	0.114c
KTR (x 100)	0.019	0.022	<0.001	0.015	-0.009
Neopterin, nmol/L	-0.014	-0.102c	-0.010	0.062	0.012

Abbreviations: HAA, 3-hydroxyanthranilic acid; HK, 3-hydroxykynurenine; KA, kynurenic acid; KTR, kynurenine:tryptophan ratio; XA, xanthurenic acid.

^a All beta coefficients presented in the table were adjusted for age, gender, month of blood collection, year of baseline interview, and cohort location (Shanghai, Singapore).

^b 2-sided P value <0.05

^c 2-sided P-value <0.01