Watercress is a super-duper food. Along with kale, collards, mustard greens, and turnip greens, watercress is one of the most nutrient-dense foods in the world. Most importantly, watercress is a specialist at preventing cancer.
Watercress belongs to the family of cruciferous vegetables, uniquely high in glucosinolates, which are precursors to cancer-fighting molecules called isothiocyanates (ITCs). Watercress is rich in a specific glucosinolate called gluconasturtiin, which is a precursor to the ITC phenethyl isothiocyanate (PEITC). Epidemiologic associations between cruciferous vegetable intake and reduced cancer risk have sparked a surge in studies on the anti-cancer effects of specific cruciferous vegetables and their constituent isothiocyanates.
Anti-cancer properties of watercress had previously been established in cell culture experiments: In human breast cancer cells, watercress extract blocked the degradation of structural proteins, an early step in preparation for migration and subsequent invasion, which eventually leads to metastasis.  PEITC in watercress was also found to reduce tumor cell survival and decrease the action of hypoxia-inducible factor (HIF), which is a molecule that stimulates angiogenesis (blood vessel development), allowing a tumor to obtain a blood supply. 
A study investigated the effects of watercress on HIF activity in human subjects. Hypoxia (low oxygen levels) is a key stimulus for tumor growth – as a tumor grows, its oxygen and nutrient needs exceed those that it can receive by diffusion from adjacent healthy tissue. When tumor cells sense hypoxia, they send angiogenic signals to surrounding normal tissue in order to obtain a direct blood supply. HIF is an essential part of this process, activating the production of angiogenic proteins, consequently promoting tumor growth. 
Since the current study tested the effects of ingesting watercress on HIF activity in cells of human subjects, the data provided is more physiologically relevant, and strengthens the earlier cell culture results. Four breast cancer survivors ingested 80 grams of watercress (about 2 cups). Six and eight hours later, blood was drawn; PEITC levels were found to be elevated, and the effects of the watercress on white blood cells were measured. HIF activity was indeed reduced in these cells, confirming in humans the anti-cancer effects of watercress previously observed in cultured cells. [3, 5]
In short, PEITC from watercress prevents tumors from sending the signal to the body that requests a blood supply. Without a blood supply, the tumor cannot continue to grow. Watercress is a potent anti-cancer food!
For more information on the anti-cancer effects of cruciferous vegetables, read Newsletter #32.
1. Higdon, J., et al., Cruciferous vegetables and human cancer risk: epidemiologic evidence and mechanistic basis. Pharmacological Research, 2007. 55(3): p. 224-236.
2. Rose, P., et al., Broccoli and watercress suppress matrix metalloproteinase-9 activity and invasiveness of human MDA-MB-231 breast cancer cells. Toxicol Appl Pharmacol, 2005. 209(2): p. 105-13.
3. Syed Alwi, S.S., et al., In vivo modulation of 4E binding protein 1 (4E-BP1) phosphorylation by watercress: a pilot study. Br J Nutr, 2010: p. 1-9.
4. Chen, L., A. Endler, and F. Shibasaki, Hypoxia and angiogenesis: regulation of hypoxia-inducible factors via novel binding factors. Exp Mol Med, 2009. 41(12): p. 849-57.
5. Watercress may 'turn off' breast cancer signal. 9/14/2010 9/30/2010]; Available from: http://www.soton.ac.uk/mediacentre/news/2010/sep/10_94.shtml.