Storm in a teacup: Iron levels unchanged but saliva proteome modified after drinking green tea

Polyphenol effects

The well-publicised benefits of tea are ascribed largely to the presence of a series of compounds known as polyphenols, which have antioxidative properties and mop up damaging free radicals in the body. They appear to provide protection against a number of conditions, including heart disease, atherosclerosis, liver disease and some cancers.

Green tea is particularly high in polyphenols, due to its production process. The leaves from Camellia sinensis are steamed or pan-fired, a procedure which deactivates the enzyme polyphenol oxidase, so reducing oxidation of these key compounds. The final levels of the principal polyphenols known as catechins are in the region of 30-42% dry weight of tea. In contrast, black tea is produced by an alternative process which encourages polyphenol oxidation by the enzyme, giving catechin levels of 10-12%.

Despite all of the good press given to green tea, there are also some health concerns. In particular there have been reports that it reduces the amount of non-haem iron that is absorbed by rats and humans. However, the picture is a little blurred because other studies suggest that this might be a short-term effect, or that it does not occur at all.

This conundrum has been investigated by a team of scientists in the USA, who probed the link between iron absorption from the diet and the levels of certain proteins expelled in saliva. Dennis Miller and colleagues from Cornell University, and Le Zhu from the University of Wisconsin-Green Bay, measured the so-called proline-rich proteins (PRPs) in saliva as well as the iron levels in blood.

Rats' tea party

The secretion of PRPs in saliva has been reported to increase after drinking green tea and it has been suggested that this is a defensive response against the anti-nutritional effects of the tea polyphenols. So, the researchers designed an experimental scheme involving weaning rats, which are more sensitive to iron availability due to their high iron demands and fast growth rate.

The rats were fed on an initial diet for seven days to acclimatise the animals before they were split into two groups for acute and chronic studies. Within each of these groups, a control set was fed a basal diet which contained 20 mg iron/kg diet in the form of ferrous sulphate.

A second cohort was fed with the basal diet supplemented by green tea powder at 28.6 g/kg and a sham gavage of phosphate-buffered saline, while a third set was fed the basal diet and a gavaged solution of concentrated tea at iron levels reflecting the amount of iron in the green tea.

The rat saliva was collected and the proteins present were separated by 2D gel electrophoresis for subsequent identification by mass spectrometry. Their quantities were estimated by differential gel electrophoresis using fluorescent dye labels.

In order to measure the iron levels, the animals were given meals containing iron-58 and its incorporation into haemoglobin was blood was determined by inductively coupled plasma mass spectrometry.

Saliva proteome modified by tea

The first major finding was that iron levels were unaffected by a diet containing green tea, for up to 24 days. This was in conflict with some previous reports, including one published by Miller in 2005 which found that black tea inhibited iron absorption in rats. Although it is not clear why the discrepancy occurred, it may be due to the different type of tea - green vs. black.

Another potential explanation might lie with the mode of administration. In the current study, the tea and iron were given separately one after the other, whereas the earlier study by Miller used a mixed solution of tea and iron. Prior mixing would allow the polyphenols and iron to interact more before administration, reducing the amount of free iron to be absorbed in the gut.

The amounts of the PRPs increased in both groups of rats compared with the controls, but were highest in the group that was fed tea. It was interesting that tea given by gavage still raised the PRP levels in the saliva. This effect could be due to circulating levels of absorbed polyphenols in the blood or to a remote systemic reaction, possibly via a neural or hormonal signal.

However, as Miller says, “since the animals receiving tea in their feed produced substantially higher amounts of PRPs than either the gavage or control groups, local signals are likely the predominate mechanism for this effect.” If this behaviour is mirrored in humans, it could explain why many studies have reported no influence of regular tea drinking affecting iron deficiency.

The researchers also found that the amounts of other proteins in saliva were affected by green tea, some increasing in abundance and some decreasing. They included chitinase, cystatin S and parotid secretory protein. Their precise functions in this context are unclear but some may have a protective action against polyphenols.

This is the first reported study comparing the expression of PRPs in saliva after the administration of green tea to rats by oral and gavage routes. It adds to the argument that rats, and probably humans, invoke special internal mechanisms to protect against loss of iron caused by the polyphenols in tea.