Watercress is rich in vitamin A (from beta-carotene) and vitamin C, and is a source of folate, calcium, iron and vitamin E. It also contains useful amounts of vitamin K, thiamin, vitamin B6, potassium and iodine and is naturally low in sodium. Due to its high water content (93%) it is low in calories. It contains very little carbohydrate and fat but provides some protein.
Watercress is a rich source of vitamin A with 100g providing 420μg (53% of the RDA) and an 80g portion providing 42% of the RDA. Watercress provides vitamin A via beta-carotene, which has Provitamin A activity, where 6μg beta-carotene is equivalent to 1μg vitamin A activity. Vitamin A is an essential fat-soluble vitamin and is necessary for normal vision (including night vision), structure and function of the skin and mucous membranes, reproduction, embryonic development, growth and cellular differentiation, and for the maintenance of immune function.
Watercress is a rich source of vitamin C with 100g providing 62mg (103% RDA) and an 80g portion providing 83% of the RDA. Vitamin C is necessary for the normal structure and function of blood vessels and connective tissues (as required for normal gums, skin, bones, cartilage and wound healing). It increases the gastrointestinal absorption of non-haem iron (the form of iron found in plant foods), and as an antioxidant helps to prevent the cell and tissue-damaging effects of free radicals. Vitamin C is needed to synthesize neurotransmitters making it essential for normal neurological function. Higher plasma vitamin C levels may benefit cardiovascular health.
Watercress is a source of folate with 100g providing 45μg (23% RDA) and a 80g portion providing 18% of the RDA. Folate is a B vitamin naturally occurring in food. Folate is often referred to as folic acid, which is a manufactured form of the vitamin. The National Diet and Nutrition Survey (NDNS) has indicated poor folate status amongst some young women and older people. Watercress also provides useful amounts - around 11% RDA per 100g - of thiamin (B1) and vitamin B6, which are required for the metabolism of carbohydrate and protein. Folate (and vitamin B6) is also involved in the maintenance of normal blood levels of the amino acid homocysteine. Elevated levels have been associated with modestly increased risk of coronary heart disease and stroke. The main nutritional cause of raised plasma homocysteine in most healthy populations is folate insufficiency. There is ongoing debate as to whether this is a causal relationship. Folate is also needed for normal cell division, blood formation (including hemoglobin) and structure of the neural tube in developing embryos. To reduce the risk of neural tube defects, women who are planning pregnancy are advised to take a daily 400μg supplement of folic acid until the 12th week of their pregnancy and to eat more good sources of folate such as green leafy vegetables, and foods fortified with folic acid. Folate is involved in the synthesis, repair, and functioning of DNA, and a deficiency may result in damage to DNA, with subsequent tumor initiation. This has recently been demonstrated in an animal model study related to colorectal cancer. A metaanalysis found that folate from food sources may offer some protection against colorectal cancer. However, positive associations may be confounded by nutrients such as fiber, which accompany a diet naturally rich in folate.
Watercress is a source of iron providing 2.2mg iron per100g (16% RDA) and 13% of the RDA per 80g portion. Watercress is also rich in vitamin C, which is known to increase the absorption of non-haem iron. Iron is essential for energy production and the normal transport of oxygen in the body via its role in the formation of hemoglobin in red blood cells and myoglobin in muscle cells. It is also needed for normal immune function, blood formation, and neurological development in embryos. The NDNS reported that 24% of women (40% aged 19-34) and 3% of men aged 19-24 have iron intakes below the Lower Reference Nutrient Intake (LRNI) which puts them at risk of iron deficiency. Iron deficiency can lead to anemia (a deficiency of red blood cells).
Potassium is an essential mineral, which helps regulate our blood pressure and nerve muscle functions. Watercress has one of the highest natural levels of potassium
Watercress is a source of calcium containing 170mg (21% RDA) per 100g or 17% RDA per 80g portion. Calcium is needed for the normal structure of bones and teeth, nerve and muscle function, blood coagulation and the function of digestive enzymes. It can help to maintain healthy blood pressure and contribute to the release of hormones such as insulin. Many green leafy vegetables, spinach in particular, contain high levels of oxalates, which can significantly reduce calcium bioavailability. However, like kale, watercress is a low oxalate vegetable containing <2.5mg/100g watercress (14). Fairweather-Tait et al (15) have shown that the fractional absorption of calcium from watercress soup is 27.4%, which compares well to the 35.7% fractional absorption from calcium-enriched skimmed milk in the same study. The NDNS survey indicated that 8% and 5% of 19-24 year old women and men respectively, have calcium intakes below the LRNI, which puts them at risk of deficiency. This is of particular concern since the twenties is the last window of opportunity for increases in bone density before peak bone mass is attained.
Watercress is naturally low in fat. But the little polyunsaturated fat it does contain has a high proportion of the essential fatty acid alpha-linolenic acid (18:3n-3), the parent of the omega-3 fatty acid family. Regular consumption can contribute useful amounts of omega-3 fatty acids to the diet. While heart health benefits have been more closely associated with the long chain omega-3 fatty acids (20:5n-3 and 22:6n-3) - and conversion of 18:3n-3 to long chain omega-3 fatty acids is inefficient - alpha-linolenic acid is still classified as ‘essential’ and has key physiological effects. For example, it helps to maintain the function and integrity of cell membranes and optimizes the ratio of omega-3 and omega-6 fatty acids in the diet. Over the past few decades there has been a shift in the ratio between omega-3 and omega-6 fatty acids in the diet, to favor omega-6, which may interfere with key functions in the body, for example, optimal prostaglandin and leukotrine formation.
In addition to nutrients, watercress is a rich source of a variety of phytochemicals. The 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 (19), which may help to explain the observed health benefits associated with regular consumption of fruit and vegetables, including cruciferous vegetables such as watercress. For example PEITC and MEITC have been shown to play a role in the suppression of carcinogenesis in a number of in vitro studies, and flavonoids and carotenoids have potent antioxidant activity. Recent in vitro and human intervention studies provide more specific evidence probably as a result of synergistic activity between the components of watercress.
Phytochemical Content of Watercress
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. Cooking
destroys myrosinase, but colon microflora appear to hydrolyse glucosinolates and allow their absorption, albeit less efficiently than myrosinase. Isothiocyanates are metabolized 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. 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. Since watercress is often consumed raw this helps to maximize glucosinolate intake.
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. 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 cancer, cardiovascular disease, Alzheimer’s disease and other chronic diseases.
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. In addition, isothiocyanates have been shown to stimulate endogenous antioxidant activity.
Research has highlighted how different nutrients and phytochemicals, including different carotenoids, work best as a team. Vitamins C and vitamin E, which also have an antioxidant role, help enhance the benefits of phytochemicals too. This is one very important reason why eating a variety of different fruit and vegetables is so beneficial to our health. It may also help to explain why regularly eating nutrient-packed green leafy vegetables, has been strongly linked with maintaining good health. Is it any wonder that watercress has long been hailed as a superfood?