In vitro mineral availability from digested tea: a rich dietary source of managanese.

Tea is potentially a rich source of some dietary metals and approximately 70 l are drunk per capita per year in the UK. In particular, tea may be an important source of Mn, since leaf tea contains 350-900 micrograms g-1 of this essential element. However, the leaching and bioavailability of Mn from tea have been little studied, so a recently developed in vitro assay was applied to compare the bioavailability of Mn from tea infusions with that of other major and trace essential elements. Analysis of tea infusions before digestion showed that 1.0 l contained 115% of the average daily dietary intake of Mn but < 6% of all other minerals. Samples of these infusions were incubated with human gastric juice (37 degrees C, 1 h) and some were then adjusted to pH 6.5 to simulate intestinal pH. All were centrifuged through ultrafilters with molecular mass cut-offs of 3, 10 and 30 kDa. The percentages of ultrafilterable (< 3 kDa) elements following simulated gastrointestinal digestion were (n = 3; mean +/- s) Ca 47.7 +/- 10.7, Cu 45.3 (n = 1), Fe < 5, Mg 66.4 +/- 1.6, Mn 39.8 +/- 11.4, K 40.3 +/- 2.2, Na 100.0 +/- 5.3 and Zn 33.7 +/- 1.1. Hence the ultrafilterability of elements showed the general trend M+ > M2+ > M3+, which is probably the inverse of the order of their strengths of binding to tea polyphenols. However, Mn was the only element found in significant dietary amounts in tea, and under simulated intestinal conditions was still 40% bioavailable.