The found that volunteers who ate 80 grams of watercress a day, equivalent to a cereal bowl sized helping, had elevated levels of cancer-fighting molecules in their blood within hours of eating the salad leaves.
The research found that the compound, phenylethyl isothiocyanate (PEITC), which gives the salad leaf a peppery taste, is able to interfere with the function of a protein that plays a critical role in cancer development.
As tumours develop they rapidly outgrow their existing blood supply and further development isn't possible until they are able to obtain enough oxygen and nutrients to maintain the growth of cancer cells.
To get past this roadblock, the cancer cells send out signals which cause the surrounding normal tissues to grow new blood vessels into the tumour which then supply oxygen and nutrients.
The protein Hypoxia Inducible Factor (HIF) is at the heart of this process of inducing new blood vessel growth. However, PEITC, of which watercress is the richest natural source, was shown in laboratory tests to have the ability to block the function of HIF.
The two studies, which have been published in the British Journal of Nutrition and Biochemical Pharmacology, provide new insight into the potential anti-cancer effects of watercress, although more work still needs to be done to determine the direct impact watercress has on decreasing cancer risk.
The research, which was funded by the Watercress Alliance, provides new hope to the 45,000 women diagnosed with breast cancer each year.
Watercress Alliance member Dr Steve Rothwell says: "We are very excited by the outcome of Professor Packham's work, which builds on the body of research which supports the idea that watercress may have an important role to play in limiting cancer development."
Breast cancer is the most common cancer in women in the western world and currently affects approximately 1 in 9 women during their lifetime.