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3 July 2000

Dish the dirt and keep healthy

Bureaucrats always want to introduce more stringent hygiene regulations.James Le Fanu argues that cl

By James Le Fanu

Provocative ideas in science are not common, but when they come along it is a fair bet that established, “self-evident” truths will, deservedly, be in trouble. There has been no truth more self-evident over the past 100 years than the virtues of hygiene. A clean home is a healthy home where Mother, armed with Harpic and Dettol, has banished bacteria from the kitchen and the bathroom. But it is not so – exposure to dirt, germs and even pollutants is good for us.

This discovery may not be in the same league as Galileo’s insight into the movement of the planets, or Einstein’s Theory of Relativity, but shares with them the twin features of being both profoundly heretical and undoubtedly correct.

The notion that there may be a serious downside to hygiene has been discussed in medical journals and at scientific meetings for some time, but it has taken the intervention of our leading restaurateur, Terence Conran, to give it the public attention it deserves. The rules and regulations that require food to be prepared in a “bacteria-free environment” prevent the immune system from producing the antibodies that fight off infection, he told the Independent last month. As a result, we have become more vulnerable to illnesses of all sorts.

Another high-profile personality from the food world, Anthony Worrall Thompson, the presenter of the BBC’s Food and Drink programme, then expressed similar sentiments. “A little dirt in your organic vegetables is not going to hurt you,” he observed. He, too, speculated that “squeaky-clean” food could be harming the immune system: “We have to ask ourselves why there is now so much more food poisoning and allergies.”

No doubt such views will be echoed by many in the food business looking for ways to counter the ever more vigorous activity of the food safety lobby. But there is more to this than special pleading. The point they are making touches on a genuine feature of all biological systems, known as hormesis.

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This phenomenon, from the Greek “to excite”, is defined by Dr Gordon Lithgow of Manchester University as “a process whereby low doses of an otherwise harmful agent may result in a stimulating or beneficial effect”. The scientific reasoning behind hormesis is simple enough: low-grade stresses of any sort tend to strengthen rather than weaken the human body. Exercise makes us fit and strong by stressing and thus improving the efficiency of the heart and lung muscles. So those who must spend a lengthy convalescence in bed will see their legs become thin and weak, as they are no longer exposed to the everyday stresses of walking about.

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Similarly, regular low-dose exposure to pollutants stimulates the enzymes in our cells that detoxify and keep the genetic repair mechanism in trim. And, as everybody knows, exposure to germs stimulates the immune system to produce antibodies that keep serious infections at bay. “If you expose human cells to mild stresses, they do fantastically better when they are subsequently more severely stressed,” writes Lithgow, who has recently summarised the scientific evidence on hormesis. This shows – in animal experiments, at least – that chronic, low-dose exposure to chemicals and radiation can increase the lifespan by up to 40 per cent. “This has tremendous implications for human health,” he says.

The practical applications of hormesis are not immediately obvious – Kellogg’s is unlikely to start fortifying its cornflakes with small doses of arsenic or cyanide instead of vitamins and minerals. But it undoubtedly represents what historians of science would call a major “paradigm shift” in medical thinking about the causes of ill health and, in particular, allergies such as asthma, whose incidence has risen dramatically in recent years. As Professor Anthony Seaton of Aberdeen University points out: “There is no evidence of the significant rise in pollutants that would have been necessary to have caused this increased frequency.” Indeed, the evidence is all the other way. Asthma is commonest in the least-polluted parts of the UK such as Skye, where the only man-made airborne pollutant is the smoke from household fires, which is rapidly dispersed by the Atlantic winds. “Air pollution as the cause of asthma has the seeming merit of appealing to common sense,” comments Professor Newman Taylor of London’s Royal Brompton Hospital, “yet the relationship has not been observed.”

Asthma has much in common with two other increasingly prevalent allergic illnesses, eczema and hayfever. They all run strongly in families, suggesting that the same genes are involved. All three are also “diseases of the advantaged”, being more common in children from professional rather than working-class families. They are also more frequent in small, rather than large, families.

Clearly, then, something to do with the body’s immune system has changed to accelerate the rise of these three allergic illnesses in tandem. The candidate that fits with all the other observations to do with social class and family size is the decline in exposure to germs. The increased prevalence of these illnesses also coincides with a precipitous decline in the major childhood infections of the postwar years, and, as Professor Newman Taylor points out, the “fall in family size reduces the chances that toddlers and infants encounter infections sufficiently early”. Asthma illustrates in an interesting and complex way how “better” hygiene weakens the hormetic mechanism of the immune system and increases susceptibility to other illnesses.

A similar explanation may account for the escalation in cases of food poisoning in recent years – although this is partly a statistical artefact arising from more systematic reporting. But, here again, perceptions are changing on the valuable role of germs in keeping us healthy. The notorious E coli organism, for example, plays a crucial – if underappreciated – beneficial role in our lives. Everyone has more than 100 billion of these microbes in his or her gut, where they convert large quantities of me-thane and hydrogen generated by digestion into non-volatile substances. Without E coli in our guts, we would never stop farting. Similarly, billions of bacteria on the surface of the skin protect against infection by preventing other more virulent organisms from gaining a foothold. So if we need bugs to keep us healthy, then an obsession with hygiene is harmful. The bugs that can cause food poisoning tend to be so mercurial as to elude attempts to eradicate them.

Neither the recognition of the beneficial role of bacteria nor the hormetic mechanisms that keep our immune systems ticking over are likely to cut much ice with bureaucratic regulators committed to introducing ever more stringent “hygiene” measures. Last January, the European Commission launched a three-year action plan to ensure “the safety of the food chain from the farmyard to the fork”. It runs to 80 separate recommendations to be added to existing EU legislation and can be predicted, with certainty, to have zero impact on promoting human health.

The essence of combating food poisoning lies in understanding the specific nature of the threat posed (or whether there is a threat at all). This is a degree of detail that organisations such as the European Commission cannot cope with. Its large and expensive sledgehammers miss the nut, while causing endless aggravation to those like Conran and Worrall Thompson, whose crime is to serve enjoyable food in pleasant surroundings.

Little wonder that they have chosen this moment to launch their counter-offensive. The next time the newspapers are full of another hidden threat to health in the food supply, just whisper the word “hormesis”.