More on eating green foods safely

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 diet. The first was ‘Throw more light on green foods’ by Mr Gamini Pieris (September 26) and the second was a response to the first, ‘Are murunga and kathurumurunga leaves toxic?’ by the eminent scientist Dr Parakrama Waidyarathna (October 5).

Sri Lanka is a country with a rich biodiversity of flora. Naturally therefore, fruits and vegetables, including green vegetables, constitute a significant proportion of our average diet – as they should. They contain many vitamins, minerals and trace elements, as well as fibre and sometimes even protein. Because of their abundance and low cost (in most instances), they are a gift of nature especially to our rural folk. There is no question that their use should be generally encouraged.

Toxicity

But these two articles specifically addressed their toxicity, which is a rare but important issue. As Dr Waidyarathna pointed out, plants do also contain toxic substances. Scientists such as he should help us all decide on the safe use of plant foods, not only with regard to choosing them but also with regard to aspects such as their growth conditions, harvesting practices, storage, preparation and preservation, all of which may have a bearing on increasing or decreasing their toxicities. In this regard, we should be thankful to Dr Waidyarathna for his kind response of October 5.

Regarding toxicity, there is a lot that these scientists know already, but there may also be a lot that is yet to be scientifically elucidated. The usual practice, especially among medical doctors, in the face of any uncertainties is to adopt the precautionary principle: avoid any possible dangers if there is a choice.Plants can be toxic in one of two main ways: the first is dose-dependent (as stated so elegantly by Paracelsus) and the other is not.

Idiosyncratic reactions

The non-dose-dependent toxicity occurs due to an idiosyncrasy of the person, who cannot then tolerate even a small, so-called ‘safe’, dose of a substance. A good example of this is how the plant kuppamenia can affect persons with a genetic predisposition called G6PD deficiency by causing a dangerous destruction of their red blood cells (haemolysis).

Interestingly, G6PD deficiency is commoner in human populations that have co-evolved with malaria, because it confers a survival advantage to recover from malaria. As a result, kuppamenia is likely to be more troublesome in malaria-endemic areas, because G6PD deficiency may be commoner there. Indeed, it is common knowledge among the traditional folk in Rajarata – which used to be highly endemic for malaria for millennia – that kuppamenia is a ‘harmful’ plant.

Dose-dependent reactions

The dose-dependent form of toxicity is generally considered to occur if any person happens to take a larger-than-usual dose of the substance. But here, there is a trick. We are usually accustomed to thinking of a high dose as something taken in a large quantity in one meal. But there can be another situation. A person may take a small dose repeatedly, and while one intake may not be toxic, if it is taken in repeatedly and continuously in even a small dose, it could gradually accumulate in the body and reach dangerously high levels. That too can lead to harm. In other words, the ‘dose’ that Paracelsus referred to may have been the dose in one intake, but we should actually consider the dose in whatever form of intake that creates a dangerous plasma level – be it as one intake or as several repeated, frequent intakes. This latter situation occurs when there is a problem with the elimination of the substance from the body.

Problems with elimination

When the body takes such substances, it usually gets rid of them from the body (which is technically called ‘elimination’) through either the liver (by enzymatic metabolism) or the kidneys (by excretion in urine). The rate at which this happens is called ‘the elimination half-life’: this is the time (in hours or days) it takes to eliminate the substance to such an extent that the plasma concentration is reduced to half of the initial level.

Theoretically, if the elimination half-life is >24 hours and the substance is taken in every day, the elimination can never be complete and accumulation can occur – with the possiblity of reaching a toxic concentration. An example of this is how the repeated ingestion of aristocholic acid-containing plants (as a ‘natural slimming medicine’) led to kidney failure and cancer.If several similar plant substances share the same elimination mechanism (such as the metabolising enzyme), they can decrease each other’s elimination (by competition) and cause a lengthening of the elimination half-life – leading to further risk of accumulation.

There are also substances (such as food constituents and medications) that specifically inhibit such elimination mechanisms (by inhibiting the metabolising enzyme), and if these are taken in simultaneously too, accumulation can occur.

Naturally, if the elimination is faulty for some other reason, accumulation occurs more readily and rapidly. An example of this is how eating kamaranga (which is promoted by ‘natural therapists’ for ‘reducing cholesterol’, ‘preventing cancer’, ‘promoting digestion’, ‘regulating blood pressure’ and ‘producing weight loss’ – none of which has a scientific basis) produced toxicity (confusion, uncontrollable seizures and even death) in patients with impaired kidney function.

Precautionary principle

These are of course theoretical considerations that can be important only in occasional or rare situations, but the precautionary principle is important to avoid such calamities. If we did that, the simple advice is to eat a variety of plant foods every day, and avoid eating the same the plant food frequently. When a substance is taken in only infrequently, it cannot accumulate. In a country such as ours with its abundance of varieties of plant foods, this should not pose any 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 physician mentioned by Mr Pieris to give that advice. That would not be an advice specific to either murunga or kathurumurunga – it would apply across the board to all green vegetables.

Interestingly, a natural phenomenon that highlights the possible dangers of plant foods is the occurrence of morning sickness in early pregnancy. Anthropologists generally agree that the evolutionary advantage of morning sickness is that it ensures that pregnant mothers reduce their food intake during early pregnancy, so that the fetus is not exposed to any toxins in food during its crucial period of organ formation (organogenesis). That reduces malformation of organs, and gives our species the evolutionary advantage. Indeed, I think that the precautionary principle with regard to green foods should be adhered to even more seriously during early pregnancy.

It is also important to keep in mind that we should not promote any particular type of green food for scientifically unsubstantiated benefits. It is generally that type of promotion that leads to the repeated and frequent consumption of one particular plant food. The recent horrors that had resulted – the stories of kamaranga and aristocholic acid nephropathy – should remind us of the dangers of such promotions. Unfortunately, with the richer biodiversity of our flora comes also a greater abundance of tall stories about their ‘benefits’!

(The writer is a professor in clinical medicine in the University of Colombo.)