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Can asparagus ease high blood pressure?

BACKGROUND

Diuretics are a class of drugs used in the treatment of high blood pressure. They promote the production of urine which helps alleviate fluid retention. This can decrease the volume of fluid in the body and thus reduce elevated blood pressure.

A number of herbs and foods have reputations as natural diuretics. Foremost among these is asparagus, frequently included in what are now being called diuretic diets. The name asparagus comes from asparag, the ancient Persian term for a sprout or shoot. These are the edible parts and are rich in several vitamins, minerals and dietary fibre, while at the same time being low in calories and sodium.

The medicinal use of asparagus has a long tradition. A second-century Roman physician described asparagus as a cleansing and healing herb. These benefits were believed to arise from its diuretic properties. In Galen’s medical system, regulation of body fluids was believed to be central to good health.

A connection between asparagus and urine has long been noted. Records dating to the 1700s observed that people’s urine sometimes smelled strongly after they ate asparagus. Benjamin Franklin wrote that a few asparagus stems gave “urine a disagreeable odour”, while Proust said that they “transformed my chamber-pot into a flask of perfume”. The odour appears remarkably quickly in urine, being detectable within 15-30 minutes of eating asparagus.

EVIDENCE FROM STUDIES

Much research has been conducted into how asparagus changes urine’s odour and whether this has any clinical relevance. Several sulphur-containing compounds are the source of the odour.

However, not only do people’s reactions to the odour differ, but many people cannot detect any odour. This has led to significant interest in determining whether only some people produce the odorous urine or whether everyone produces it but only some can smell it.

The source of all the variation has not been settled, but genetic differences in people’s ability to smell the odour have been identified. While the smelly urine is said to be evidence of asparagus cleansing the body and eliminating waste, there is little support for these claims.

Some of the ingredients isolated from asparagus have been found to have diuretic effects when tested in laboratory studies. Only one clinical trial was identified which examined the effectiveness of asparagus as a diuretic in people. Almost 200 patients were enrolled in the study. All of them either had high blood pressure or were taking medication for high blood pressure.

The study did not involve a control group, which is a significant limitation. All patients took a herbal remedy which contained dried asparagus and dried parsley. Over the six weeks of the study, no meaningful changes occurred in the patients’ blood pressure or body weight. No evidence of additional urine production was visible.

PROBLEMATIC ASPECTS

During the above study, adverse effects were carefully monitored. People took 12 tablets per day, which was equivalent to 6g of fresh asparagus. Many people reported gastrointestinal problems and a small number of people had kidney pain. Allergic reactions to asparagus have also been reported.

RECOMMENDATIONS

Asparagus is a highly nutritious vegetable, particularly high in folic acid. It has a long-standing reputation as a diuretic, particularly in Germany. However, even there, it is not included in lists of approved herbal medications because evidence of effectiveness is lacking. Given the small amount of research conducted to date, asparagus should not be used as a diuretic. It may make your urine more noticeable, but it doesn’t appear to increase its volume to any degree that would benefit those with high blood pressure.


Dónal O’Mathúna has a PhD in pharmacy, researching herbal remedies, and an MA in bioethics, and is a senior lecturer in the School of Nursing, Dublin City University.