Watercress: the original King of the Greens
Foreword by Professor Ian Rowland, The University of Ulster
What's so special about watercress? This series of pages aims to provide the answers. Watercress is a very underrated vegetable. It is a good source of many important nutrients such as vitamin C, calcium, iron and folate. We will summarise the role of these vital substances and show just how valuable watercress is in contributing to our daily requirements for them.
Perhaps more importantly, watercress is packed with a wide range of natural, bioactive plant compounds (phytochemicals) for which there is increasing evidence for beneficial effects on human health. These include antioxidants such as beta-carotene, lutein and quercetin, which may help protect the cells of the body from damage from reactive free radicals (lutein specifically is considered to be very important for eye health). But watercress is also a particularly rich source of a group of phytochemicals called glucosinolates which, when eaten, break down to isothiocyanates. It is these compounds that give watercress its distinctive peppery taste. In addition, a wide range of experiments using human cells in culture and studies have demonstrated that isothiocyanates have anti-disease potential. Sensibly, however, we don't eat the purified compounds, we eat the whole food, so this report also covers the evidence relating to disease and watercress per se.
Epidemiology studies, which look at dietary patterns of large populations and attempt to correlate these with disease incidences, certainly provide us with important indications that cruciferous vegetables, like watercress, are particularly effective in helping to reduce risk. But epidemiology is a blunt tool, it rarely allows us to identify the role of specific foods and crucially it provides information only on associations between diet and disease risk not causal relationships.
At The University of Ulster we wanted to identify causal effects e.g. whether or not watercress actually has the potential to help reduce disease risk. To do so we needed to perform a dietary intervention, and therein lay another problem. Ideally we would like to have used development of disease as the end point, but many diseases can take decades to develop, so we had to take some short cuts.
The first of these was to employ a test tube approach. When we applied an extract of watercress to cultures of human cells we were able to show that it slowed the growth of diseased cells, stimulated the ability of cells to resist DNA damaging agents thought to induce disease, and inhibited the ability of diseased cells to invade tissues in a model of disease spread.
These positive results encouraged us to move to the second stage — to conduct a large scale watercress dietary study in healthy volunteers.
Happily, our 60 volunteers seemed to have no problems in eating a bag of watercress each day for 2 months!
The main endpoint of the study was DNA damage in blood cells. DNA damage is considered to be an important event in various stages of development of disease, so our finding that eating watercress not only reduces the level of DNA damage in blood cells, but also increases the ability of those cells to resist DNA damage caused by free radicals, is highly significant.
It is also important to note that our subjects were not a specially selected group on a restricted diet: they were very mixed — both men and women, some were smokers, and they ranged in age from 19-55 years, so the results are very much applicable to the general population eating a normal diet.
As I stated earlier, these types of study are not easy to do and the results represent a huge amount of work by my colleagues, particularly Chris Gill, Adele Boyd, and Sam Haldar. Special thanks also must go to our volunteers, without whose commitment, patience (and blood!) the study would not have been possible. Finally, the Watercress Growers, for having the foresight to fund the study and for their enthusiastic support.