Table 1:

Physiological Parameters

Parameter	Description	Value	Units	Source
Volumes
BW	Body Weighta	70	kg	[Brown et al., 1997]
Vadi	Fraction of body weight as adipose	21.4	Unitless	[Brown et al., 1997]
Vk	Fraction of body weight as kidney	0.4	Unitless	[Brown et al., 1997]
Vl	Fraction of body weight as liver	2.6	Unitless	[Brown et al., 1997]
Vrp	Fraction of body weight as rapidly perfusedb	9.6	Unitless	[Brown et al., 1997]
Vsp	Fraction of body weight as slowly perfused c	58.0	Unitless	[Brown et al., 1997]
Vap	Fraction of body weight as arterial blood	2.0	Unitless	[Brown et al., 1997]
Vvp	Fraction of body weight as venous blood	6.0	Unitless	[Brown et al., 1997]
Flow Rates
Qco	Cardiac output	312	L/hr	[Brown et al., 1997]
BP	Blood:plasma ratio	0.55d	Unitless	[Ramakrishnan et al., 2016]
Qadi	Fraction cardiac output to adipose	5.2	Unitless	[Brown et al., 1997]
Qk	Fraction cardiac output to kidney	17.5	Unitless	[Brown et al., 1997]
Ql	Fraction cardiac output to liver	22.7	Unitless	[Brown et al., 1997]
Qrp	Fraction cardiac output to rapidly perfused	25.5	Unitless	[Brown et al., 1997]
Qsp	Fraction cardiac output to slowly perfused	29.1	Unitless	[Brown et al., 1997]
Qap	Fraction cardiac output to arterial blood	1.5	Unitless	[Brown et al., 1997]
Qvp	Fraction cardiac output to venous blood	4.4	Unitless	[Brown et al.,1997]
Partition coefficients
$P_{adi:p}^D$	D adipose/plasma partition coeffi-cient	2.4	Unitless	[Heaney et al., 2009]
$P_{k:p}^D$	D kidney/plasma partition coefficiente	1.02	Unitless	[Heaney et al., 2009]
$P_{l:p}^D$	D liver/plasma partition coefficient	1.02	Unitless	[Heaney et al., 2009]
$P_{rp:p}^D$	D rapidly perfused/plasma partition coefficientf	0.44	Unitless	[Heaney et al., 2009]
$P_{sp:p}^D$	D slowly perfused/plasma partition coefficient	0.42	Unitless	[Heaney et al., 2009]
$P_{adi:p}^{25}$	25(OH)D adipose/plasma partition coefficient	0.11	Unitless	[Heaney et al., 2009]
$P_{k:p}^{25}$	25(OH)D kidney/plasma partition coefficient	0.23	Unitless	[Heaney et al., 2009]
$P_{l:p}^{25}$	25(OH)D liver/plasma partition coefficient	0.23	Unitless	[Heaney et al., 2009]
$P_{rp:p}^{25}$	25(OH)D rapidly perfused/plasma partition coefficient	0.08	Unitless	[Heaney et al., 2009]
$P_{sp:p}^{25}$	25(OH)D slowly perfused/plasma partition coefficient	0.05	Unitless	[Heaney et al., 2009]
Kinetic Parameters
$K_m^{27A1}$	Michelis-Menten constant for CYP27A1	3.2	μmol/L	[Sakaki et al., 2005]
$V_{\max }^{24A1}$	In vitro maximum velocity for CYP27A1	0.269	mol/min/mol p450	[Sakaki et al., 2005]
$K_m^{2R1}$	Michelis-Menten constant for CYP2R1	0.45	μmol/L	[Shinkyo et al., 2004]
$V_{max}^{2R1}$	In vitro maximum velocity for CYP2R1	1.2	mol/min/mol p450	[Shinkyo et al., 2004]
$K_m^{27A1}$	Michelis-Menten constant for CYP27B1	0.9	μmol/L	[Tang et al., 2012]
$V_{max}^{27B1}$	In vitro maximum velocity for CYP27B1	1.3	nmol/min/mg p450	[Tang et al., 2012]
$K_m^{24A1}$	Michelis-Menten constant for CYP24A1	0.16	μmol/L	[Sakaki et al., 2005]
$V_{max}^{24A1}$	In vitro maximum velocity for CYP24A1	0.088	mol/min/mol p450	[Sakaki et al., 2005]
P450liv	Conversion factor for CYP enzymes in liver	11	nmol p450/g liver	[Wilson et al., 2003]
P450kid	Conversion factor for CYP enzymes in kidney	12.8	mg p450/g kidney	[Al-Jahdari et al., 2006]
CYP27A1	Enzyme fraction for CYP27A1	0.010	Unitless	optimized
CYP2R1	Enzyme fraction for CYP2R1	0.075	Unitless	optimized
CYP27B1	Enzyme fraction for CYP27B1	0.016	Unitless	optimized
CYP24A1	Enzyme scalar for CYP24A1	0.012	Unitless	optimized
Additional Parameters
Kabs	Absorption constantg	0.88	hr−1	estimated
UnbD	Unbound percentage of vitamin D	0.083	Unitless	optimized
Unb25	Unbound percentage of 25(OH)D	0.03	Unitless	[Bikle et al., 1986]
h	Hill coefficient	4.0	Unitless	optimized
$E{C}_{50}^D$	Effect level for vitamin Dh	4.08	nmol/L	optimized
$E{C}_{50}^{25}$	Effect level for 25(OH)D	4.45	nmol/L	optimized

D, Vitamin D; 25, 25(OH)D

^aReference body weight. Organ density is assumed to be 1 kg = 1 L

^bRichly perfused tissues included adrenal, blood , brain, gastrointestinal tissues, heart, lungs and thyroid. This compartment served as a mass balance compartment, namely 100 — ∑_c V_C for other compartments c

^cPoorly perfused tissues included muscle, skin, and bone

^dPlasma flow rates assumed a hematocrit value of 0.45 as vitamin D and its metabolites are primarily bound to plasma compounds

^eKidney measurements were not taken in Heaney et al. [2009]; this partition coefficient was set equal to the rapidly perfused tissue

^fMeasurements were set to the “remainder” compartment as given in [Heaney et al., 2009]

^gEstimated using the half-life of vitamin D [Zerwekh, 2008] and the time to maximum concentration [Heaney and Armas, 2015]