To those who think "herbal" means safe, natural and nurturing, I like to point out that deadly nightshade (belladonna), opium, hemlock and digitalis are all herbs – but very dangerous indeed!

In fact Nature has seen fit to endow a number of plants with the capacity to synthesize substances that are toxic to humans and other animals. Ingesting them may produce unpleasant consequences which are not allergic but may become confused with an allergy. As I reported in THE FOOD ALLERGY PLAN (Unwins, London, 1985), humans are probably able to tolerate the majority of foods only because of the discovery of fire, which cooks away toxins (although several plant toxins are heat-stable).

Farmers and veterinarians, who are more advanced in clinical ecology than many doctors, have known for years that animals become sick if they graze on certain types of plant (for example, bulls become enraged if they eat loco weed – 'loco' being Spanish for crazy). Many plant substances are toxic to humans in quite small quantities, including deadly nightshade, acorns and hemlock. Ricin, the toxic principle in caster seeds (Ricinus communis), is one of the most poisonous substances known: a minute drop on a needle at the tip of an umbrella was used in an infamous political assassination on the streets of London in 1978.

The fact is that all plants, including edible ones, contain quantities of poisons. Carrots, for example, contain a nerve toxin: caratotoxin. And someone once pointed out that if cabbage had to undergo the tests that drugs are now subjected to before being pronounced fit for humans, it wouldn't pass. Obviously, most often the amounts of poison in foods are tolerable. Toxicity is a matter of degree.

There are a number of interesting groups of plants toxins in our food supply. To understand them a little may help you work out some mystifying food reactions that defy even the advice given here in this book!

Lectins are large protein molecules; they are toxic and also mimic allergies. Lectins are widespread and may be up to 20 per cent of the protein content of plants, especially of seeds and pulses. They have the curious property of imitating antigen-antibody reactions without actually sensitizing the immune system.

These interfere with body enzymes such as trypsin (a protein digestive enzyme). In experimental animals this interference has been shown to cause retarding of growth, abnormal hypertrophy (enlargement) of the pancreas and, in the case of prolonged feeding, even the formation of cancer of the pancreas. Soya-bean protein derivatives have been shown to retain some of this effect, leading to concern that infants fed on soya milk might suffer growth retardation.

Goitrogens are substances causing goiter or thyroid enlargement. Soya-bean extract is in this category and goiters have been seen in human infants fed with soya milk. Iodine appears to counter act this effect, so infant soya milks are fortified with iodide as a precautionary measure.

Goitrogens are a common constituent of plans belonging to the Crucifer family (cabbage, turnip, swede, broccoli, cauliflower, kale brussel sprout, rape and mustard seed). An epidemic goiter seen in Tasmania is probably due to milk from cows fed on kale and turnips.

There are naturally occurring oestrogenic compounds in many plants. These heat-stable compounds are capable of eliciting an oestrogenic response (feminization) in experimental animals. Recently much interest has focused on so-called "phyto-oestrogens", some women like to use these substances in the belief that they are somehow a natural alternative to medication for hormone imbalances. This is partly deluded, in that pseudo-oestrogens in plants are not the human hormone. It is possible to manufacture natural human hormone, such as progesterone from plant sources (such as diascorea from the wild yam) but this requires a whole factory manufacturing process and chemical formulations: not what I personally accept as a "natural" substances.

Moreover excess oestrogens are bad for males and equally harmful to women already suffering from oestrogen dominance, a condition where oestrogen is not balanced by opposing progesterone, causing bloating, water retention and extreme mood changes. Therefore oestrogens, naturally occurring in plants or not, are potentially toxic.

Also known as cholinesterase inhibitors, these affect chiefly animals, causing paalysis and sometimes death, though humans are occasionally afflicted. Lathyrism, a condition associated with high intake of lathyrus bean (chickpea family), is a kind of paralysis.

These include prussic acid and its precursors, nicotine, solanin, atropine and a host of others.

Antinutrients are substances that interfere directly with the absorption of vitamins, minerals and other nutrients.

Phytate occurs in several plant groups, particularly grains and also the pulses. These are known to chelate, that is, combine with and remove, valuable minerals such as calcium and magnesium and trace elements such as zinc, copper and iron, which are vital for health.

The pulses (peas and beans) are especially noted for this effect. The cause is low molecular weight oligosaccharides (simple sugars), namely raffinose and stachyose. Flatulence is generally attributed to the fact that humans do not possess the enzyme alpha 1,6-galacto-sidase necessary for breaking down these sugars. Blowing off? see flatulence section

There is growing interest in drug-like substances in plans. Well-known are the psychedelic substances such as those in marijuana and peyote cactus; the coca plant gives rise to cocaine and the opium poppy is notorious for its forbidden juices. But there have been opium-like alkalodis called exorphins, and many other pharmacologically active substances, found in plants. These may have beneficial effects as well as unwanted ones.

These are small organic molecules, usually comprising several carbon rings with side chains, one or more of the carbon atoms being replaced by a nitrogen (which confers the alkalinity). About 7 to 10 per cent of all plants contain alkaloids, of which several thousand are now known.

Famous alkaloids include nicotine, quinine, strychnine, ergotamine and atropine. The less toxic ones, such as caffeine, are used for pleasant social effects. The powerful ones are hallucinogens (cannabis, LSD and mescaline).

The well-known food allergy effect of addiction, where withdrawal from the food causes unpleasant symptoms, may be due at least in part to the addictive properties of alkaloids present in the food.

The action of alkaloids on the nervous system is generally to disrupt electrochemical transmission at nerve junctions (synapses), either preventing transmission (as in the case of the plant poison curare) or enhancing it inappropriately (as, for example, physostigmine). Locoism, referred to above, is of this latter class.

Outbreaks of food poisoning due to solanine (from potatoes), tomatine (tomatoes) and dioscorine (yams) have all been reliably observed in either humans or domestic animals. Death due to alkaloid overdose is fortunately uncommon in humans; in Socrates' case (hemlock) it was deliberate murder by the state. But subclinical alkaloid intoxication occurs all the time. The 'edible' nightshades (potatoes, tomatoes, capsicums, pepers) are especially rich sources, but cabbage, peppercorns and many other foodstuffs are not far behind.

These are morphine-like peptides derived from partially digested grain, milk and legume proteins. Pharmacologically they behave, when tested on isolated tissues, very much like morphine, hence the name. It is reasonable to propose that in people whose intestinal digestion of these foodstuffs is incomplete, exorphins are absorbed and have the effect of a small dose of an opiate drug-for example, patients who take wheat bran and find their constipation gets worse. On the plus-side, the well-known effect of pleasant somnolence after a meal is probably also due to morphine-like activity.

A disease known as milk sickness, characterized by weakness, nausea and collapse, has occasionally reached epidemic proportions in certain parts of the US. It probably caused the death of Abraham Lincoln's mother. The name derives from the fact that the disease is brought on by drinking milk from cow made ill with a disease known as the trembles. This was eventually tracked down to the consumption, by cattle, of a plant known as snake root (Eupatorium rugosum), containing the chemical tremetone.

Along the same lines, lupin alkaloids have been known to be transferred to human beings via goat's milk. Birth abnormalities have been reported and, significantly, lupin alkaloids have the same effect on goat offspring.

It is commonly forgotten that caffeine and theobromine (which occur in tea and coffee) are toxic substances. Taken in sufficient quantities they can cause cerebral oedema (so-called 'water on the brain), convulsions and even death, though no one has ever been able to establish tissue damage caused by chronic ingestion at normal levels.

Salicylates are aspirin-like chemicals that occur in many fruits and vegetable. They tend to cause pharmacological rather than allergic reactions. Adverse reactions are dose-related and only occur in sensitive individuals who have a constitutional predisposition.

These are aromatic amino compounds such as serotonin and norepinephrine (noradrenalin), which constrict blood vessels and thereby elevate the blood-pressure. Such substances occur in chocolate, pineapple juice, avocado, alcohol and cheese.