Local Government Elections – The Island
by Professor Panduka Karunanayake
I was interested in two recent articles in The Island on the issue of the safety of green vegetables in our diets. The first was “Shed more light on green foods” by M. Gamini Pieris (September 26) and the second was a response to the first, “Are murunga and kathurumurunga leaves poisonous?” by the eminent scientist Dr Parakrama Waidyarathna (October 5).
Sri Lanka is a country with a rich biodiversity of flora. Naturally, then, fruits and vegetables, including greens, are an important part of our average diet – as they should be. They contain many vitamins, minerals and trace elements, as well as fiber and sometimes even protein. Due to their abundance and low cost (in most cases), they are a gift of nature, especially for our rural populations. There is no doubt that their use should be generally encouraged.
But both of these articles were specifically about their toxicity, which is a rare but important issue. As Dr. Waidyarathna pointed out, plants also contain toxic substances. Scientists like him should all help us decide on the safe use of plant foods, not only with regard to their choice, but also with regard to aspects such as their growing conditions, harvesting practices, storage , preparation and storage, all of which can affect whether their toxicities increase or decrease. In this regard, we have to thank Dr. Waidyarathna for his kind reply of October 5th.
When it comes to toxicity, there’s a lot that these scientists already know, but there may also be a lot that remains to be scientifically elucidated. The usual practice, especially among doctors, in the face of any uncertainty, is to adopt the precautionary principle: avoid any possible danger if there is a choice. Plants can be poisonous in two main ways: one is dose-dependent (as Paracelsus so elegantly put it) and the other is not.
Non-dose-dependent toxicity occurs due to an idiosyncrasy of the person, who then cannot tolerate even a small, so-called “safe” dose of a substance. A good example of this is how the kuppamenia plant can affect people with a genetic predisposition called G6PD deficiency by causing dangerous destruction of their red blood cells (hemolysis).
Interestingly, G6PD deficiency is more common in human populations that have co-evolved with malaria, as it confers a survival advantage to recover from malaria. As a result, kuppamenia is likely to be more troublesome in malaria-endemic areas, as G6PD deficiency may be more common there. Indeed, it is common knowledge among the traditional people of Rajarata – which was once highly endemic for malaria for millennia – that kuppamenia is a “harmful” plant.
The dose-dependent form of toxicity is generally considered to occur if a person takes a larger dose of the substance than usual. But there is a trick here. We are usually used to thinking of a high dose as something taken in large quantities in one meal. But there may be another situation. A person may take a small dose repeatedly, and although a dose may not be toxic, if taken repeatedly and continuously, even in a small dose, it may gradually accumulate in the body and reach dangerously high levels. This too can lead to damage. In other words, the “dose” that Paracelsus refers to may have been the dose in a single intake, but we should actually consider the dose in any form of intake that creates a dangerous plasma level – whether that either in a single dose or in several repeated doses, frequent doses. The latter situation occurs when there is a problem with the elimination of the substance from the body.
When the body absorbs such substances, it usually gets rid of them from the body (which is technically called “elimination”) through the liver (through enzyme metabolism) or the kidneys (through excretion in the urine). The rate at which this occurs is called the “elimination half-life”: it is the time (in hours or days) required to eliminate the substance to such an extent that the plasma concentration is reduced by half compared to the initial level.
Theoretically, if the elimination half-life is >24 hours and the substance is absorbed every day, elimination can never be complete and accumulation can occur – with the possibility of reaching a toxic concentration. One example is how repeated ingestion of plants containing aristocholic acid (as a “natural weight loss medicine”) has led to kidney failure and cancer. If several similar herbal substances share the same elimination mechanism (such as the metabolizing enzyme), they may decrease the elimination of the other (by competition) and cause an extension of the elimination half-life – leading to a additional risk of accumulation.
There are also substances (such as food constituents and drugs) that specifically inhibit these elimination mechanisms (by inhibiting the metabolizing enzyme), and if taken concurrently, accumulation may occur.
Naturally, if the elimination is faulty for another reason, the accumulation occurs more easily and more quickly. An example of this is how to eat kamaranga (which is promoted by “natural therapists” to “lower cholesterol”, “prevent cancer”, “help digestion”, “regulate blood pressure” and “provide weight loss” – none of which has a scientific basis) produces toxicity (confusion, uncontrollable convulsions and even death) in patients with kidney failure.
These are of course theoretical considerations that may only be important in occasional or rare situations, but the precautionary principle is important to avoid such calamities. If we did this, the simple advice is to eat a variety of plant foods every day and avoid eating the same plant food frequently. When a substance is absorbed infrequently, it cannot accumulate. In a country like ours that is full of plant food varieties, this should be no problem and would even be desirable. I wonder if it is this type of reasoning and the adoption of the precautionary principle that led the doctor quoted by Mr. Pieris to give this advice. This wouldn’t be murunga or kathurumurunga specific advice – it would apply to all greens.
Interestingly, a natural phenomenon that highlights the possible dangers of plant foods is the onset of morning sickness in early pregnancy. Anthropologists generally agree that the evolutionary benefit of morning sickness is that it ensures that pregnant women reduce their food intake early in pregnancy so that the fetus is not exposed to any toxins in food. during its crucial period of organ formation (organogenesis). This reduces organ malformation and gives our species the evolutionary advantage. Indeed, I think that the precautionary principle vis-à-vis green foods must be respected even more seriously at the beginning of pregnancy.
It’s also important to keep in mind that we shouldn’t promote any particular type of green food for scientifically unproven benefits. It is usually this type of promotion that leads to repeated and frequent consumption of a particular plant food. The recent horrors that have resulted – stories of kamaranga and aristocholic acid nephropathy – should remind us of the dangers of such promotions. Unfortunately, with the rich biodiversity of our flora also comes a greater abundance of stories to tell about their “benefits”!
(The author is a professor of clinical medicine at the University of Colombo.)