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. Author manuscript; available in PMC: 2016 Jun 28.
Published in final edited form as: Br J Nutr. 2015 Apr 28;113(12):1965–1977. doi: 10.1017/S0007114515001142

Table 3.

Spearman correlation between various folate biomarkers and selected physiologic and lifestyle variables for the US population ≥1 year, NHANES 2011–2a

Serum RBC
Variable categories 5-MethylTHF UMFA Non-methyl folate MeFox Total folate without MeFox Total folate with MeFox Total folate
Fasting time
 Spearman r −0.02 −0.19 0.01 −0.34 −0.04 −0.06 0.02
 P value 0.10 <0.0001 0.39 <0.0001 0.0093 0.0005 0.07
eGFRb
 Spearman r −0.07 −0.17 −0.07 −0.23 −0.07 −0.08 −0.19
 P value <0.0001 <0.0001 0.07 <0.0001 <0.0001 <0.0001 <0.0001
BMIc
 Spearman r −0.22 −0.08 −0.02 0.08 −0.22 −0.22 0.07
 P value <0.0001 0.0021 0.31 <0.0001 <0.0001 <0.0001 0.0020
BSAd
 Spearman r −0.27 −0.10 −0.05 0.04 −0.27 −0.26 0.04
 P value <0.0001 0.0005 0.0241 0.0040 <0.0001 <0.0001 0.0344
Serum cotininee
 Spearman r −0.25 −0.14 −0.02 −0.06 −0.25 −0.24 −0.18
 P value <0.0001 <0.0001 0.71 0.0097 <0.0001 <0.0001 <0.0001
Alcohol intakef
 Spearman r −0.10 −0.14 −0.04 −0.13 −0.10 −0.11 −0.03
 P value 0.0022 <0.0001 0.05 <0.0001 0.0022 0.0012 0.21

NHANES, National Health and Nutrition Examination Survey; 5-methylTHF, 5-methyltetrahydrofolate; UMFA, unmetabolized folic acid; MeFox, pyrazino-s-triazine derivative of 4α-hydroxy-5-methylTHF; RBC, red blood cell; eGFR, estimated glomerular filtration rate; BSA, body surface area.

a

Serum folate forms and serum total folate (sum of folate forms excluding or including MeFox) were measured by HPLC-MS/MS; RBC total folate was measured by microbiologic assay; non-methyl folate represents sum of three minor forms: tetrahydrofolate, 5-formyl-tetrahydrofolate, and 5,10-methenyltetrahydrofolate; for sample sizes, see Table 1

b

Used to assess renal function; available for persons ≥12 years; impaired renal function was defined as eGFR <60 mL/(min×1.73 m2)

c

BMI definitions (kg/m2): <18.5 (underweight); 18.5–>25 (normal weight); 25–<30 (overweight); and ≥30 (obese)

d

Calculated as square root of [(height in cm × weight in kg)/3600] or square root of [(height in inches × weight in pounds)/3131]

e

Biomarker of tobacco smoke exposure; concentrations >10 μg/L are considered to represent smokers

f

Calculated for participants ≥18 years as average daily number of “standard” drinks [(quantity × frequency)/365.25]; 1 drink ≈ 15 g ethanol