The "too clean" theory of asthma rises again. It even makes the Wall St. Journal!

The germ theory has been in some eclipse in recent years because of some awkward epidemiological facts. For instance, Australian Aborigines often live in extraordinary squalor but don't seem to be protected from anything because of that. In fact they have quite high rates of autoimmune diseases such as diabetes

So how do we evaluate the findings below? It's a bit difficult as the article in NEJM has not yet appeared but there are at least two possibilities. The most favourable to the theory is that it is not the overall bacterial load that matters but rather just some bacteria. So southern German farmhouses might have the helpful bacteria but Aboriginal camps may not. That is not inherently absurd but would be very much in need of proof, considering that both populations have extensive contact with all the world's infective agents via the modern-day "global village".

The second much more skeptical possibility derives from the fact that we are only looking at epidemiology here -- so all causal links are speculative. For instance, it has recently been found that Paracetamol (Tylenol) use in children under 15 months doubles their chance of getting asthma. So maybe the "dirty" farms were less health conscious in general and so used fewer medications, including paracetamol. Isn't epidemiology wonderful?

The possibilities are endless, in fact. It was found last year, for instance, that that receptors for bitter tastes are not confined to the tongue but are also are found in the smooth muscles of the lungs and airways. And bitter tastes RELAX those airways. So in doing any epidemiological comparisons of asthma incidence, we would have to ask whether the different groups used in the research differed in their preferences for bitter drinks, including, of course, beer!

OK. I could go on but I will have mercy at that point

Children living on farms have a lower risk of asthma than children who don't because they are surrounded by a greater variety of germs, according to two large-scale studies published Wednesday.

The prevalence of asthma in the U.S. has doubled over the past 30 years, and one theory for the increase blames urban and suburban living environments that are too clean. The latest findings, published in the New England Journal of Medicine, bolster what is often known as the hygiene theory, which says that contact with bacteria and other microbes is necessary to building a normal immune system.

The key appears to be exposure to a diversity of bugs, not just more of them, according to Markus Ege, an epidemiologist at the Children's Hospital of Munich and first author on the paper that covered both studies. "Bacteria can be beneficial for asthma," said Dr. Ege. "You have to have microbes that educate the immune system. But you have to have the right ones."

Previous research, including some conducted by Dr. Ege's group, has found that children raised on farms exhibit substantially reduced risk for asthma and allergies—lower by 30% or more—than those raised elsewhere. Though scientists had hypothesized that the difference was linked to germs, they also had to determine whether it could be due to other elements of farm life such as fresh air, exposure to farm animals, or dietary factors like drinking raw milk.

The latest study helps untangle that question by providing evidence that the reduction in risk is indeed significantly related to the variety of bacteria and other bugs a child is exposed to, according to James Gern, a professor of pediatrics and medicine at the University of Wisconsin-Madison who wrote an editorial to accompany the paper in the journal but wasn't involved in the study.

In Wednesday's paper, the researchers surveyed and collected samples of house dust in two studies of children from Southern Germany, Austria and Switzerland. One study comprised 6,800 children, about half of whom lived on farms, and the other studied nearly 9,700 children, 16% of whom were raised on a farm. Researchers then examined the dust for presence and type of microbes.

Those living on farms were exposed to a greater variety of bugs and also had a lower risk of asthma. There was evidence that exposure to a particular type of bacteria, known as gram-negative rods, was also related to lower rates of allergic responses.

Identifying which microbes are beneficial to the immune system is important because those germs could help the development of new treatments or vaccines to prevent asthma, Dr. Ege said. His group is now studying some of the microbes in greater detail.

The findings don't yield much in the way of practical suggestions, however. Dr. Ege said it wouldn't help for parents to take their children to a farm two or three times a year or to get a dog or other pet for the purpose of exposing their children to microbes, since the biggest effect appeared to be related to prolonged exposure to cows and pigs.

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Could your blood group determine your health?

Since different blood types carry different antigens, it is not surprising that they might vary in their ability to fight different diseases. They may well have evolved to fight the threats most common in their original local environments. In modern populations, however, differences in disease resistance would appear to be small

Could your blood group determine your risk of major cancers, infertility and stomach ulcers, as well as diseases such as cholera and malaria? For years, the idea that blood groups had any medical significance beyond blood transfusions was dismissed by scientists.

It hasn’t been helped by the celebrity’s favourite, the ‘blood group diet’, which claims your blood type determines how your body responds to certain food.

But a growing number of studies is revealing how our blood groups may make us more prone to lethal illnesses — or even protect us from them.

The latest research into blood types shows that having group O blood can lower your risk of heart attacks. Researchers at Pennsylvania University discovered this benefit in a study involving 20,000 people. Their research, to be published in The Lancet, found that most people who have a gene called Adamts7 face a significantly raised risk of suffering a heart attack. But in people with blood group O who have the Adamts7, there is no raised risk.

Dr Muredach Reilly, the lead researcher, says this knowledge may help to develop new therapies for people at risk of heart attacks. Such drugs may mimic the beneficial effect of the O blood group gene.

Only 40 per cent of people in Britain know what their group is, according to the National Blood Service. But in future, we may be far more keen to learn it — and to understand its life-saving implications. Our blood group is determined by genes inherited from our parents.

Millennia of evolution have split human blood into four types: A, B, AB and O — around 44 per cent of Britons are type O, 42 per cent are type A, 10 per cent type B and 4 per cent are AB.

What distinguishes each type are their antigens (the immune defence systems) on the surface of the red blood cells. Each blood group type evolved to provide defences against lethal diseases.

But each has its own weaknesses, too. People with type O blood are at less risk of dying from malaria than people with other blood groups. But they are more vulnerable to cholera and stomach ulcers caused by viruses and bacteria.

For a long time, the study of blood groups and disease was discredited — thanks to the Nazis. Otto Reche, a Nazi German ‘professor of racial science’, claimed in the Thirties that pure Aryans all had blood type A. The main ‘enemy’ blood group was, he said, B type. He used this to identify ‘inferior’ races for persecution during Hitler’s rise to power.

While such claims are scientifically absurd, in Japan there is still widespread discrimination on the grounds of blood group. In the Twenties, Japanese scientists claimed blood groups produced different personalities. The idea became so ingrained that in World War II, the Imperial Army formed battle groups based on blood type.

The idea resurfaced in the Seventies and a rash of Japanese best-sellers has spread the belief that type As are sensitive but anxious; Type Bs are cheerful but focused; Os are outgoing but stubborn; and ABs are arty and unpredictable.

This theory has a dark side. Bura-hara (blood-group harassment) is common in Japan. Company chiefs often consider candidates’ blood types when picking staff. Children at some kindergartens are also divided by blood type. Matchmaking agencies provide blood-type compatibility tests.

Nevertheless, there is serious science behind the idea that blood groups can hold the secret to fighting deadly diseases.

In the Fifties, research at four London hospitals found the risk of developing gastric cancer was much higher for people with blood group A than for those with blood group O. But people with group O had a greater risk of peptic ulcers.

This month, those findings have been confirmed by investigators at Sweden’s Karolinska Institute, which studied more than a million people over a period of 35 years. The lead researcher, Dr Gustaf Edgren, says people with group A may be more susceptible to gastric cancer risks such as smoking, alcohol and use of non-steroidal anti-inflammatory drugs. Type O people may be more vulnerable to a bacterium that can cause peptic ulcers, Helicobacter pylori.

Last October, U.S. scientists showed that a woman’s blood group can affect her chances of becoming pregnant. The study of more than 560 women undertaking fertility treatment found that those with blood type O were up to twice as likely to have a lower egg count and poorer egg quality, which could affect the chances of conceiving. Women with blood group A seemed to be better protected against their egg counts falling over time.

Researcher Edward Nejat, from New York’s Albert Einstein College, says the exact reasons for a link between blood group and ovarian reserve was not clear.

Blood groups have been linked to other reproductive troubles. Last month, a study at Harvard University found that women with AB or B group blood have a raised risk of developing ovarian cancer.

There are also fears that AB blood may double or even treble the risk of pregnant mothers suffering from the potentially lethal blood pressure condition pre-eclampsia. This finding could be harnessed to identify women at higher risk.

Other research has found that people with type AB and B blood have a much higher risk of developing pancreatic cancer.

Meanwhile, people with type O might be less at risk of cancer, but research shows they are also more vulnerable than others to norovirus, the potentially lethal vomiting and diarrhoea bug.

And men with type O might be more prone to piling on the pounds, say Danish researchers. They have found that type O males who are exposed routinely to pollution at work have a significantly raised risk of obesity compared with men of other blood types.

The researchers, at Copenhagen’s Bispebjerg University Hospital, speculate that the pollution sets off chronic inflammatory responses in the men’s bodies that can result in them becoming overweight. It’s a good excuse anyway.

Taken overall, such a weight of medical evidence might prompt us to question why we are not told of the health threats we might face due to our blood type. But in the UK, there is little work in this field.

Professor Mike Murphy, of the NHS Blood and Transplant authority, says: ‘Our colleagues in the U.S. have become increasingly involved in this type of research, particularly in trying to harness the power of blood types to fight infectious diseases. But the interest in Britain is sparse.’

Meanwhile, a lone group of British researchers is trying to turn blood-group science into a bona-fide lifesaver in one area: malaria. The effort is being led by Alex Rowe, an infection specialist at Edinburgh University’s School of Biological Sciences. Her work shows that people with blood group O are resistant to the tropical disease, which kills millions every year.

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