Positive Health Online

"Eating an organic apple every day is 100,000 times more likely to give you cancer than eating food contaminated with residues of the pesticide lindane". This statement is meaningless, of course, as neither apples nor pesticide residues on food cause cancer. In fact, a diet rich in apples and other fruit and vegetables, protects against cancer.

But it is true that apples – as well as other plant-foods – contain natural compounds which when extracted and given to rats in abnormally high doses cause tumours. When thinking about chemicals in food, we cannot trust in the safety of natural compounds, nor necessarily assume the worst about synthetic compounds. We have to know:

• whether the chemical could cause poisoning or cancer,
• whether people are exposed to high enough levels,
• whether our bodies have defence systems to cope with it,
• whether there are other chemicals in the food or diet which could neutralise or counter its damaging effect.

Carcinogens and toxins: Apples contain a carcinogen called caffeic acid, a compound naturally present in many kinds of fruit and vegetables. The evidence that it is carcinogenic comes from animal tests. Caffeic acid causes tumours when very high doses (2% of the diet) are fed to rats or mice. The pesticide lindane is also carcinogenic by these tests.

About half of the synthetic pesticides tested in rodents are carcinogenic. Only 52 of the thousands of natural toxins present in plants have been tested for carcinogenicity. Of these, 29 or nearly half, are rodent carcinogens. They include caffeic add, psoralens in parsnips, estragole in basil, symphytine in comfrey, allyl isothiocyanate in brown mustard, and hydrazines in mushrooms.

Exposure: People are not usually exposed to high levels of caffeic acid or lindane. In food, the levels are extremely low. But exposure to caffeic acid from food is higher than to lindane residues. In one apple, there could be 24.4 mg of caffeic acid. The daily US average consumption of lindane residues in 1990 was 0.000032 mg.

The majority of pesticides to which we are exposed are natural compounds present as normal constituents of plants. Plants were not placed on Earth in order to be food for humans. The plants that were successful evolved ways to defend themselves from pests. One way plants do this is by producing chemical compounds which function as pesticides. Those used commercially include pyrethrum from Chrysanthemum flowers and nicotine from the tobacco plant. But even edible, nutritious and cancer-preventing plants contain natural pesticides. Natural toxins can make up as much as 10% of the plant's weight and are our largest dietary source of exposure to toxic chemicals. People have learned ways to detoxify food-plants by food preparation and cooking methods but it is not possible to purify food of all natural toxins.

Natural and synthetic chemicals in food were ranked according to their concentrations in food (based on USA data) and how potent they are as carcinogens to rodents. According to this ranking, daily consumption of apples, lettuce, parsnip and orange juice poses far greater risk of cancer than do DDT and lindane residues. This ranking is not proof that eating fruit and vegetables are an important cause of cancer. It is proof of the extremely low risk there is from pesticide residues in food and puts the risk of cancer from synthetic and natural compounds into perspective.

Defence systems: The main way humans have evolved to cope with the inevitable exposure to toxins and carcinogens in food is through our physiological defense mechanisms. The defences we have evolved are general systems, able to protect us against a broad range of toxins of different chemical structures and from a variety of sources. As long as the amount of toxin does not overwhelm our defences, our bodies can de-toxify, store and eliminate poisons.

Defense mechanisms in the body are induced by the arrival of a foreign chemical, whether from a natural or a synthetic source. When the potato was introduced into Europe from South America 400 years ago, our bodies had not evolved specific defences against the toxic glycoalkaloids in the potato – nor have we had time to evolve new defences since. Nevertheless, humans have been able to incorporate the potato into the diet as a nutritious staple food.

Interaction with other chemicals: A holistic approach to diet and health should include the interactions between compounds in foods. Food-plants are composed of thousands of chemicals. A few of these are nutrients, others are toxins or carcinogens, most have no known effect on human health and some protect against disease. Protective factors include anti-oxidant vitamins and minerals, flavonoids and indoles. Some chemicals, including caffeic acid, are carcinogenic at high doses and protective at low doses. At a concentration of 2% caffeic acid, rats develop stomach tumours. At a concentration of 0.0005%, caffeic acid reduces the number of tongue tumours induced by a known carcinogen in rats.

Very little is known about the amount of natural toxins and natural protective factors in different foods, nor about how much of our diet is made up of foods containing them. Nor is much known about the way natural toxins interact with the rest of our diet. A chemical extracted from a plant may be toxic or carcinogenic in laboratory studies, but the plant itself may not be. Protective factors may be present in the same plant or in the same meal or in the overall diet. For example, Vitamin C in spinach inhibits the formation of carcinogenic nitrosamines in the stomach from the nitrates also in the spinach.

A diet rich in fruit and vegetables protects against many kinds of cancer and against heart disease. Whatever carcinogenic and toxic contaminants and natural constituents may be present in food are clearly overpowered by the beneficial protective factors.

The common-sense view is that synthetic pesticide residues on food are inherently bad and that natural compounds are inherently good (or at least neutral). The fact is that plant-foods contain chemicals that are not only as toxic or as carcinogenic as synthetic pesticides, but in greater amounts. We cannot avoid potentially harmful chemicals in food but we can prevent many chronic diseases by eating a healthy diet. The apparent contradiction is resolved by taking a holistic approach to food and health and disregarding the reductionist theory that disease is caused primarily by unnatural "outside agitators".