WHAT IT IS: Most of us are familiar with the sweet potato as a tuberous root or storage root. But few may know about another tuber - the thick, black tuberous root of the plant Rehmannia glutinosa.
This perennial herb, native to China, Japan and Korea, is commonly used in traditional Chinese medicine (TCM).
The rehmannia root, also known as Chinese foxglove root or dihuang in Mandarin, can be used raw or cooked.
Cooking the herb is said to enhance its properties as a blood tonic. During winter, the raw herb, called shengdihuang, is set out in the sun to dry.
The cooked version, called shudihuang in Mandarin, is prepared by mixing the fresh root with rice wine, then steaming and drying it repeatedly, said Ms Jin Jin Hua, a consultant TCM acupuncturist at Raffles Chinese Medicine.
The third edition of Chinese Herbal Medicine Materia Medica noted that rehmannia roots of good quality are large, thick, soft, sweet and completely black.
The processed rehmannia root is sold at $3.10 for a tael (37.5g) at some medical halls here.
HOW TCM USES IT: The sweet rehmannia root in its cooked form is slightly warm in nature, but cold in its raw form.
The cooked herb is commonly used in TCM to treat blood disorders such as anaemia and thalassaemia, said Ms Jin.
These are conditions related to blood deficiency in the body, which is marked by symptoms such as dizziness, heart palpitations, insomnia, paleness, fatigue and irregular menstruation, she added.
Someone with a blood deficiency usually has a weak constitution exacerbated by insufficient iron intake, exhaustion or blood loss from surgery.
Ms Jin said women are especially prone to blood deficiency because of menstruation.
Rehmannia root and Chinese angelica may be prescribed together to promote blood production in the body.
In TCM, it is believed that the kidney stores jing (essence of nutrients), which generates blood-making bone marrow.
This essence declines with age and is depleted through excessive sexual activity. Rehmannia root can be used to replenish this essence.
A person deficient in jing in the kidneys would have memory problems, backache, premature greying or hair loss, weakness in the knees, a loss of libido and infertility.
Since the kidneys are thought to play an important role in reproduction in TCM, this deficiency can lead to painful sex for women and sperm leakage for men, Ms Jin said.
Who should avoid it
WHO IT IS FOR: This herb is ideal for women to consume after their periods, since it is a blood-promoting herb. It is also touted as being able to combat the signs of ageing - good for those keen to maintain a healthy complexion, bright eyes, a full head of hair and their sexual prowess.
WHO SHOULD AVOID IT: The raw form of rehmannia root is cold in action, so it should not be consumed by those with a weak spleen and a stomach that is prone to diarrhoea.
Others who experience bloatedness - a sign of "dampness" in the body - should also avoid aggravating their condition with cold herbs such as the raw form of the rehmannia root.
Anyone allergic to alcohol should steer clear of the prepared herb as it contains rice wine, said Ms Jin.
To make the processed rehmannia root suitable for a "heaty" person prone to symptoms such as a sore throat, constipation and dry mouth, other herbs have to be used at the same time, added Ms Jin.
WHAT RESEARCH HAS SHOWN: The herb has been tested for its anti-fatigue efficacy in a study published in the International Journal Of Biological Macromolecules last year.
A total of 80 mice were randomly allocated to four groups. For four weeks, three groups were given the herb at different concentration levels, while the control group was given distilled water.
The mice were then put through a swimming test with a thin wire loaded onto their tails and assessed to be exhausted when they failed to rise to the surface of the water to breathe within a 10-second period.
The herb had significant effects on the endurance of mice that took the herb as their swimming capacity increased compared with those in the control group.
Recipe to try
Chicken soup with rehmannia root
One chicken drumstick
25g processed rehmannia root
15g Chinese angelica
5g Sichuan lovage rhizome
10g white peony root
800ml to 1 litre (about six bowls) water
Salt, to taste
1. Clean the chicken, then cut it into smaller pieces.
2. Scald them in a pot of boiling water, then remove and rinse them.
3. Rinse the herbs.
4. Place the herbs and chicken in a pot filled with about six bowls of water.
5. Bring the water to the boil, then let it simmer for 30 minutes.
6. Add salt to taste just before serving.
Source: Ms Jin Jin Hua, a consultant traditional Chinese medicine acupuncturist at Raffles Chinese Medicine
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