Phytochemicals & Antioxidants

What are phytochemicals and what is their function?

In addition to nutrients, watercress is a rich source of a variety of phytochemicals. Those most clearly identified include glucosinolate derivatives such as phenylethyl isothiocyanate (PEITC) and methylsulphinylakyl isothiocyanates (MEITCs), flavonoids such as quercetin, hydroxycinnamic acids, and carotenoids such as beta-carotene and lutein. Studies indicate different mechanisms of action for different phytochemicals (Reference 19), which may help to explain the observed health benefits associated with regular consumption of fruit and vegetables, including cruciferous vegetables such as watercress.

Phytochemical Content of Watercress

Table 2: Flavonoid content of Watercress (Reference 20)

Table 3. Rutin and Phenolic acid content of Watercress (Reference 24)

Hydroxycinnamic acids (HCAs) are phenolic acids, which are a major class of phenolic compounds.

Table 4. Carotenoid content of Watercress (References 4, 5)

Glucosinolates

Watercress is one of the most concentrated sources of glucosinolates. Glucosinolates are sulphur-containing, water soluble phytochemicals that are widely distributed throughout cruciferous vegetables. When the vegetables are cut, crushed or chewed, the enzyme myrosinase is released from the plant cell compartment to hydrolyze glucosinolates into a number of breakdown products including aglucones. These aglucones are very unstable and spontaneously convert to isothiocyanates, such as PEITC, sulphoraphane and indole 3-carbinol. Isothiocyanates are absorbed from the upper gastrointestinal tract (Reference 21). Cooking destroys myrosinase, but colon microflora appear to hydrolyse glucosinolates and allow their absorption, albeit less efficiently than myrosinase (References 21 , 22).

Isothiocyanates are metabolised in vivo by conjugation with glutathione, followed by conversion through the mercapturic acid pathway to N-acetylcysteine conjugates which are excreted in the urine. The total urinary level of isothiocyanate equivalents is an excellent biomarker of human consumption of different glucosinolates (Reference 23). Watercress is nature's richest source of PEITC, which is derived from the hydrolysis of the glucosinolate gluconasturtiin.

Cooking can reduce glucosinolate levels by 30-60% depending on the cooking method, cooking intensity and on the type of compound, so light and short cooking methods are recommended to conserve them (Reference 21). Since watercress is often consumed raw this helps to maximise glucosinolate intake.

Table 5. Isothiocyanate content of Watercress (Reference 24)

Isothiocyanates are active derivatives of glucosinolates

(4-Methylsulphinylbutane (sulphoraphane): levels on average are 1/20 those of PEITC)

Antioxidants

Diets rich in fruit and vegetables are associated with a reduced risk of a number of chronic diseases. Protection has often been at least partly attributed to the antioxidant nutrients in fruit and vegetables, such as vitamins C and E and selenium (needed to form glutathione peroxidase, an antioxidant enzyme). However, many phytochemicals such as phenolics and carotenoids, also have antioxidant activity and appear to work additively and synergistically (References 25, 26, 27, 28).

What is an antioxidant and why is it important?

Antioxidants help to protect cells by scavenging highly reactive free radical molecules (by donating electrons) before they cause cell and tissue damage. The body constantly reacts with oxygen as part of the energy producing processes of cells. As a consequence, free radicals are formed and can be increased by infection, pollution, cigarette smoke, alcohol and ultraviolet (UV) radiation exposure. The body has an endogenous (internal) system of antioxidant enzymes (such as glutathione peroxidase, superoxide dismutase, quinone reductase, catalase) to counterbalance free radicals. However, overproduction can cause an imbalance, leading to oxidative stress, which can damage lipids, proteins, and DNA and is implicated in the development of cardiovascular disease, Alzheimer's disease and other chronic diseases (Reference 27).

Antioxidants provided by the diet may support endogenous antioxidant activity. Watercress has significant antioxidant potential in vitro according to ferric-reducing antioxidant potential (FRAP) analysis (see Table 6). A number of antioxidants or more likely, a combination of its bioactive components, might contribute to this (References 24, 29). In addition, isothiocyanates have been shown to stimulate endogenous antioxidant activity (Reference 30).

Table 6. Ferric - Reducing Antioxidant Potential (FRAP) (References 25, 31 **, Direct Laboratories analysis of watercress 2006 *)

The FRAP assay directly measures antioxidants in a liquid sample. FRAP values are the combined concentrations of all electron donating antioxidants.

Potential health benefits of watercress

Hippocrates was one of the first to recognise the potential health benefits of watercress. Today, good intake of nutrient and phytochemical-dense green leafy and/or cruciferous vegetables has been associated with a decreased risk of conditions such as coronary heart disease, age-related macular degeneration and skin photodamage. A prospective study of over 6000 adults found that a higher intake of cruciferous vegetables was linked to a lower risk of all cause mortality (Reference 32), however, epidemiological associations do not show causality. Ongoing laboratory, animal and human intervention studies are helping to increase our understanding of why vegetables, including watercress, offer potential health benefits.

Antioxidant potential of watercress It is hypothesized that consuming foods rich in antioxidants may help to prevent or slow the oxidative stress induced by free radicals. Since there is still much to understand about phytochemicals, interest is moving towards assessing the antioxidant activity and in vivo effects of whole foods.

The in viva evidence that fruit and vegetables actually reduce markers of oxidative damage has been limited. Null findings may be influenced by many factors including absorption and metabolism of phytochemicals, study length and design, and individual variation in antioxidant status and responsiveness to dietary antioxidants (References 33, 34).

Nevertheless, studies have shown that eating fruit and/or vegetables can reduce the amount of damage free radicals cause to the blood. For example, it has been associated with markers such as a reduction in serum oxidizability, enhanced resistance of plasma lipoproteins to oxidation and increased red blood cell glutathione peroxidase (Reference 33). Specifically, fruit and vegetable consumption can reduce DNA damage in blood cells (References 24, 36, 37). The effects in blood cells is an indicator of what is happening to tissues less accessible for sampling and testing. Responsiveness may be greater for people who are under a higher level of oxidative stress, such as smokers (Reference 34). The University of Ulster study (Reference 24) found that daily watercress consumption resulted in a 100% increase in plasma lutein, and 33% increase in beta-carotene and a significant decrease in oxidative damage to white blood cell (lymphocyte) DNA, especially amongst smokers.

Urease Inhibitors

Exeter University & Torbay Medical Research Fund

New horizons for watercress research are already well underway, with some impressive contributions coming from the South West of England.  Dr Kyle Stewart, a GP trainee in Torbay, and a collaboration of Torbay Medical Research Fund and The University of Exeter, are currently extracting and classifying clinically useful urease inhibitors from watercress. Dr Stewart commented... 

His work is extending further into other aspects that affect the body’s production of ammonia.  This in itself will be researched to help reduce a plethora of unintended/associated ailments contributing to further additional medical intervention and costs.  These properties also have the ability to impact on the control and effect of a particular pathogen called H-Pylori which is linked to stomach ulcers and other diseases.