eTable 1. Overview of the methods used to determine the biochemical parameters.

Sample material	Measured by:	Biomarker	Method	Additional information
Serum	Saarland University, Homburg, Germany	Vitamin B12 Holotranscobalamin Folate	Electrochemiluminescence immunoassay Chemiluminescent microparticle immunoassay Competitive immunoassay
EDTA plasma	Martin-Luther University, Halle, Germany	Parathyroid hormone	Commercial ELISA test kits	The test measures intact, i.e. biologically active PTH.
Serum	Potsdam University, Nuthetal, Germany	Selenium Selenoprotein P	Inductively coupled plasma—tandem mass spectrometry; For quality control, reference serum and Seronorm were also tested. ELISA kit	Kopp JF, Mülle, SM, Pohl G, Lossow K, Kipp AP, Schwerdtle T: A quick and simple method for the determination of six trace elements in mammalian serum samples using ICP-MS/MS. J Trace Elem Med Biol 2019; 54: 221–5.
EDTA plasma	Medical laboratory Labor 28 GmbH, Berlin, Germany	Hemoglobin Hematocrit Erythrocytes Leucocytes Lymphocytes Platelets HbA1 c	SLS Photometric Hemoglobin detection (XN) Impedance measurement with hydrodynamic focusing; Fluorescence flow cytometry (XN) Turbidimetric inhibition immunoassay (XN)	www.labor28.de
NaF blood	Medical laboratory Labor 28 GmbH, Berlin, Germany	Glucose Homocysteine	Hexokinase/G6p DH testing High performance liquid chromatography with fluorescence detector	www.labor28.de
Serum	Medical laboratory Labor 28 GmbH, Berlin, Germany	GGT GOT GPT Creatinine hsCRP Ferritin Zinc	Enzyme International Federation of Clinical Chemistry and Laboratory Medicine Enzymatic method, nephelometry Electrochemiluminescence immunoassay Flame atomic absorption spectrometry	www.labor28.de
EDTA-Plasma	Bevital AS, Bergen, Norway	Methylmalonic acid	Gas chromatography–mass spectrometry	Windelberg A, Arseth O, Kvalheim G, Ueland PM: Automated assay for the determination of methylmalonic acid, total homocysteine, and related amino acids in human serum or plasma by means of methylchloroformate derivatization and gas chromatography–mass spectrometry. Clin Chem 2005; 51: 2103–9.
		Vitamin D3 and D2 Vitamin A Alpha-tocopherol Gamma-tocopherol Vitamin K	Liquid chromatography–mass spectrometry/ mass spectrometry	Midttun Ø, Ueland PM: Determination of vitamins A, D and E in a small volume of human plasma by a high-throughput method based on liquid chromatography/tandem mass spectrometry. Rapid Commun Mass Spectrom 2011; 25: 1942–8. Midttun Ø, McCann A, Aarseth O, et al.: Combined measurement of 6 fat-soluble vitamins and 26 water-soluble functional vitamin markers and amino acids in 50 μL of serum or plasma by high-throughput mass spectrometry. Anal Chem 2011; 88: 10427–436.
		B1 Thiamin	Liquid chromatography–mass spectrometry/ mass spectrometry	McCann A, Midttun Ø, Whitfield KC, et al.: Comparable performance characteristics of plasma thiamine and erythrocyte thiamine diphosphate in response to thiamine fortification in rural cambodian women. Nutrients 2017; 9: pii: E676.
		B2 riboflavin B3 nicotinamide B6 pyridoxal 5‘-phosphate	Liquid chromatography—mass spectrometry/ mass spectrometry	Midttun Ø, Hustad S, Ueland PM: Quantitative profiling of biomarkers related to B-vitamin status, tryptophan metabolism and inflammation in human plasma by liquid chromatography/tandem mass spectrometry. Rapid Commun Mass Spectrom,2009; 23: 1371–9.
24-hour urine collection	Medical laboratory Labor 28 GmbH, Berlin, Germany	Calcium Creatinine	Flame atomic absorption spectrometry (AAS NovAA®350) Photometry	www.labor28.de
24-hour urine collection	ETH Zurich, Zurich, Switzerland	Iodine	Modified Sandell–Kolthoff reaction	Pino S, Fang S, Braverman L: Ammonium persulfate: a safe alternative oxidising reagent for measuring urinary iodine. Clin Chem 1996; 42: 239–43.

GOT, glutamic oxaloacetic transaminase; GPT, glutamic oxaloacetic transaminase; hsCRP, high-sensitivity C-reactive protein; SLS, sodium lauryl sulfate