Folate is absorbed across the colon of adults: evidence from cecal infusion of (13)C-labeled [6S]-5-formyltetrahydrofolic acid.
BACKGROUND
Folate deficiency increases the risk of several human diseases. Likewise, high intakes of folate, particularly synthetic folic acid intake, may be associated with adverse health outcomes in humans. A more comprehensive understanding of the "input side" of folate nutrition may help to set dietary recommendations that strike the right balance between health benefits and risks. It is well known that the microflora in the colon produce large quantities of folate that approach or exceed recommended dietary intakes; however, there is no direct evidence of the bioavailability of this pool in humans.


OBJECTIVE
The objective was to determine whether, and to what extent, the natural folate vitamer 5-formyltetrahydrofolic acid is absorbed across the intact colon of humans.


METHODS
During screening colonoscopy, 684 nmol (320 microg) [(13)C]glutamyl-5-formyltetrahydrofolic acid was infused directly into the cecum of 6 healthy adults. Three or more weeks later, each subject received an intravenous injection of the same compound (172 nmol). Blood samples were collected before and after each treatment. The ratio of labeled to unlabeled folates was determined in plasma by tandem mass spectrometry.


RESULTS
The apparent rate of folate absorption across the colon of a bolus dose of [(13)C]5-formyltetrahydrofolic acid infused into the cecum was 0.6 +/- 0.2 nmol/h, as determined by the appearance of [(13)C(5)]5-methyltetrahydrofolic acid in plasma. In comparison, the rate of appearance of [(13)C(5)]5-methyltetrahydrofolic acid after an intravenous injection of [(13)C(5)]5-formyltetrahydrofolate was 7 +/- 1.2 nmol/h.


CONCLUSIONS
Physiologic doses of natural folate are absorbed across the intact colon in humans.
Department of Nutritional Sciences, University of Toronto, Toronto, Canada.
Susanne
S
Aufreiter
Jesse F
JF
Gregory
Christine M
CM
Pfeiffer
Zia
Z
Fazili
Young-In
YI
Kim
Norman
N
Marcon
Patarapong
P
Kamalaporn
Paul B
PB
Pencharz
Deborah L
DL
O'Connor
D000328:Adult; D002247:Carbon Isotopes / Q000493:pharmacokinetics*; D002432:Cecum / Q000502:physiology*; D003106:Colon / Q000378:metabolism*; D003113:Colonoscopy; D005260:Female; D005492:Folic Acid / Q000031:analogs & derivatives* / Q000378:metabolism*; D005575:Formyltetrahydrofolates / Q000493:pharmacokinetics*; D005838:Genotype; D006801:Humans; D007408:Intestinal Absorption; D002955:Leucovorin / Q000378:metabolism*; D008297:Male; D042965:Methylenetetrahydrofolate Reductase (NADPH2) / Q000235:genetics; D008875:Middle Aged
D002247:Carbon Isotopes; D005575:Formyltetrahydrofolates; C020410:5-methyltetrahydrohomofolic acid; D005492:Folic Acid; D042965:Methylenetetrahydrofolate Reductase (NADPH2); D002955:Leucovorin