Nutritional composition of watercress
Watercress Rorripa nasturtium acquaticum is a member of the Cruciferae (or Brassicaceae) family, and therefore related to broccoli, cabbage, brussel sprouts, cauliflower, rocket and radish.
Cultivated in pure spring water, its health benefits have been known since ancient times. It is believed to have originated in Greece and remains an integral part of Mediterranean diets. In 500BC, Hippocrates, the father of medicine, is said to have located his first hospital close to a stream to ensure fresh watercress to help treat his patients. As a longstanding British favourite, it has been used in herbal remedies from the 1600s, and has been commercially cultivated since the I800s.
Watercress is part of the fruit and vegetable food group, with 80g (one cereal bowl full) providing one of the 'at least five a day' portions recommended by the Department of Health to help reduce the risk of some cancers, cardiovascular disease and many other chronic conditions.
Research indicates that the observed health benefits of diets rich in fruit and vegetables may come not just from their individual nutrients or phytochemicals, but from the interactions between them (Reference 3). Watercress is naturally low in calories, virtually fat free and contains a wide range of essential vitamins and minerals. It is also a source of a number of phytochemicals with potential health benefits.These include lutein, quercetin, phenolic acids, and glucosinolates. The latter release isothiocyanates, including phenylethyl isothiocyanate (PEITC), of which watercress is the richest known source. PEITC is a key contributor to the distinctive peppery flavour of watercress and in a large number of in vitro and animal studies has been shown to have a range of anti-cancer effects.
Watercress is sold as a fresh salad vegetable, by itself or mixed with other salad leaves, and is readily available from greengrocers and supermarkets. It has been enjoying a renaissance of late with annual sales increasing to more than £55 million.
Watercress is very versatile, and can be enjoyed as a salad vegetable, in soups and smoothies or as an ingredient in cooked dishes such as stir-fries, sauces for pasta and other meals, pizza and fish dishes.
What is the nutritional composition of watercress?
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.
Table 1. Nutrient content of watercress
* EC RDA - EC Recommended Daily Allowance for selected vitamins and minerals. DRV - UK Dietary Reference Values - used when there is no RDA. GDA - UK Guideline Daily Amounts for adults for key nutrients. **Calculated from beta-carotene content ***(F) = female, (M) = male DRV.The DRV for potassium is the same for men and women.
Watercress is a rich source of vitamin A with IOOg providing 420ug (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 6pg beta-carotene is equivalent to I pg 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 (Reference 8).
Watercress is a rich source of vitamin C with IOOg 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 synthesise neurotransmitters making it essential for normal neurological function (Reference 8). Higher plasma vitamin C levels may benefit cardiovascular health.
Folate - and other B vitamins
Watercress is a source of folate with IOOg providing 45pg (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 (Reference 9). Watercress also provides useful amounts - around I 1% RDA per I OOg - of thiamin (B I) 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 (Reference 10). There is ongoing debate as to whether this is a causal relationship.
Folate is also needed for normal cell division, blood formation (including haemoglobin) 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 400pg 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 tumour initiation. This has recently been demonstrated in an animal model study related to colorectal cancer (Reference 11). A meta-analysis found that folate from food sources may offer some protection against colorectal cancer (Reference 12). However, positive associations may be confounded by nutrients such as fibre, which accompany a diet naturally rich in folate (Reference 13).
Watercress is a source of iron providing 2.2mg iron per 100g (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 haemoglobin in red blood cells and myoglobin in muscle cells. It is also needed for normal immune function, blood formation, and neurological development in embryos (Reference 8).
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 (9). Iron deficiency can lead to anaemia (a deficiency of red blood cells).
Watercress is a source of calcium containing 170mg (21% RDA) per IOOg 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 (Reference 8). 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. Fairweather-Tait et al (Reference 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 (Reference 9). 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 (Reference 16).
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 (I 8:3n-3), the parent of the omega-3 fatty acid family. Regular consumption can contribute useful amounts of omega-3 fatty acids to the diet (Reference 17).
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 I 8:3n-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 (Reference 18) and optimises 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 favour omega-6, which may interfere with key functions in the body, for example, optimal prostaglandin and leukotrine formation (Reference 18).