Naturally Occuring Folate Bioavailability

Low intake of the B vitamin folic acid, or folate, causes the disruption of homocysteine homeostasis, which is an independent risk factor of neural tube defects, Alzheimer’s disease, cardiovascular disease, stroke, colorectal cancer and prostate cancer. In the United States, folic acid fortification was launched at 1998. The recommended daily allowance (RDA) for folate is 400μg/day. It is not known to what extent folate in the diet is bioavailable, i.e. is absorbed and used or stored in the body. In food, folate is often conjugated to a polyglutamate chain, which has to be removed enzymatically byγ-glutamyl hydrolase (GGH), before absorption can take place. Limitation or inhibition of this process may limit the bioavailability of polyglutamate folate.Green leafy vegetables are abundant sources of folate occurring mainly as folate polyglutamates. Despite numerous clinical studies, the effect of the polyglutamate chain on folate bioavailability is poorly understood, especially in the food matrix. The aim of the present research was to quantify the effect of the polyglutamate chain on folate bioavailability. In order to better understand the role of the polyglutamate chain on folate bioavailability, a crossover clinical study is presented on comparing the effect of the linking of dietary folate to a polyglutamate chain on bioavailability under the controlled conditions. To represent the extent to which natural folates are conjugated in foods, 3 folate-rich sources were used including steamed broccoli, crushed broccoli and high pressure processed broccoli, in which folates are present as 90% hexa-octaglutamyl folate, 90% triglutamyl folate and 90% monolgutamyl folate, respectively. To eliminate the effect of folate species, broccoli is chosen as the rich source of folate since 5-methyltetrahydrofolate (5MTHF) is the predominant folate speices (~90%) in steamed, crused and high pressure processing (HPP) broccoli. To eliminate the effect of nutrition status and host-related factors, a cross-over study was applied. For distinguishing the exogenous folate from the endogenous folate in human plasma, folate was intrinsically-labeled with 13C stable isotope. Using the new ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) assay developed as part of this dissertation, folate species were measured in each plasma sample from all the human subjects. To the best of our knowledge, this is the first report of comparison the bioavailability of native food folate with different polyglutamyl chain length under the control of other factors. The dual-isotope-labeling (one isotope is intrinsically-labeled) that we describe here provides a sensitive, accurate and efficient method of measuring folate bioavailability, provided that doses of labeled compounds can be reliably assessed. At relevant dietary doses, the relative bioavailability of food folate in HPP processed broccoli is 89%, in crushed broccoli is 65% and in steamed broccoli is 62%. Thus application of high pressure treatment could be applied to engineer healthy processed foods.So far, this is the first comprehensive study for monitoring the in vivo bioavailability of food folate with different poylgutamyl length in similar food matrix under the control condition.