Table 1.

Key steps for validating a biomarker, and if alkylresorcinols meet these criteria as biomarkers of alkylresorcinol containing foods (adapted from [75, 82]).

(1) Present in wholegrain foods, but not refined foods, nor other food sources
Quantitative analytical methods for grains and food	GC	[9, 83]
	HPLC	[16, 84, 85]
	Colorimetry	[86–88]
Not present in other foods	In food plants, only found in wheat, rye and barley, and genetically related crops, and in low amounts in mango flesh. Very low amounts in beer and animal fat.	[9, 19, 70]
Not affected by food processing	AR stable during baking and pasta production	[9, 15]
	Limited effect of fermentation and germination in rye	[89, 90]
Variation in raw material	Wheat (350–900 μg/g)	[9, 15, 91, 92]
	Rye (500–1300 μg/g)	[83, 93, 94]
	Barley (30–100 μg/g)	[17, 18]
(2) Intake, absorption, distribution, metabolism, and elimination
Quantitative analytical methods for biological samples	GC-MS (plasma, erythrocytes, adipose tissue, urinary metabolites)	[41, 55, 57] [46, 59]
	GC-MS/MS (plasma, erythrocytes)	[58]
	LC-MS/MS (plasma)	[56]
	HPLC-CAED (metabolites)	[48, 49]
Intake	Average intake in the UK and Sweden estimated to be 12 and 23 mg/d, respectively	[20]
Absorption	Pigs: 60–79% depending on dose	[44]
	Humans: 58% ileal absorption	[43]
Distribution	Rats: negligible accumulation 100 h after a single dose	[44]
	Adipose: AR-measured in rat and human adipose	[29, 46]
Metabolism	Main AR metabolites in humans: DHBA and DHPPA	[47]
	DHBA and DHPPA also measured in human plasma	[49]
	DHPPA extensively glucuronidated in human urine	[59]
Elimination	61% and 31% of a single dose eliminated in faeces and urine in rats	[44]
	Urinary recovery 45–89% depending on dose	[45]
(3) Dose response and pharmacokinetics
Dose response	Increased dose of AR leads to decreased absorption in pigs	[44]
	Urinary recovery % lower with increased AR dose	[45]
Pharmacokinetics	Pigs: T max⁡: 3 h; T 1/2: 4 h	[50]
	Humans: T max⁡1: 2.8 h; T max⁡2: 6.7 h; T 1/2: 4.8 h	[23]
	Plasma metabolites: T max⁡: 6 h; T 1/2: 10–16 h Urinary metabolites: T max⁡: 6 h; T 1/2: 10–12 h	[52, 53]
(4) Determinants of biological concentrations, variation, and reproducibility
Determinants of plasma alkylresorcinol concentration	Gender: males have generally higher concentrations	[62, 63]
	Triglycerides and lipoproteins	[27, 63]
	Nonesterified fatty acids	[63]
Variation in different populations	Healthy subjects, fasting plasma
	Mixed results for females with hormone-related cancers	[37, 70]
Reproducibility and validity	Intervention studies: good-to-moderate ICC	[54, 63]
	Free-living studies: low ICC	[62]
(5) Application in clinical and epidemiological studies
Surrogate endpoint for WG intake	Endometrial cancer case-control study: no difference in nonfasting plasma AR	[38]
Validation of dietary assessment tools	WG FFQ: correlation with FFQ: 0.53	[76]
Biomarker of compliance to an intervention	WG interventions	[51, 63, 74]