10% Human: How Your Body’s Microbes Hold the Key to Health and Happiness. Alanna Collen

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      Food allergies, eczema, asthma and skin allergies often begin at birth or in the first few years of a child’s life. Autism typically presents itself in toddlers, and is diagnosed before the age of five. Autoimmune diseases can hit at any time, but many show themselves at a young age. Type 1 diabetes, for example, typically reveals itself in childhood and the early teens, though it can also crop up in adulthood. Multiple sclerosis, the skin condition psoriasis, and inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis, all typically attack in the twenties. And lupus usually affects people between the ages of fifteen and forty-five. Obesity too, is a disease that can start young, with around 7 per cent of American babies considered over the normal weight at birth, rising to 10 per cent by the time they are toddlers, and about 30 per cent becoming overweight later in childhood. Older people are not immune to twenty-first-century illnesses – almost all of them can strike suddenly at any age – but the fact that they so often affect the young suggests it is not the ageing process itself that triggers them.

      Even among those diseases that kill people in the West in ‘old’ age – heart attacks, strokes, diabetes, high blood pressure and cancers – most have their roots in weight gain that begins in childhood or early adulthood. We can’t attribute deaths from these conditions to our longer lifespans alone, as even those people in traditional societies who make it to eighty or ninety years old very rarely die of this set of ‘age-related’ illnesses. Twenty-first-century illnesses are not limited by the burgeoning top tier of our demographic ranks, but rather are hitting us, like the 1918 flu, in what should be the prime of our lives.

      On to race. The Western world – North America, Europe and Australasia – is a largely white place, so are our new health problems actually a genetic predisposition among white people? In fact, within these continents, whites do not consistently have the highest rates of obesity, allergies, autoimmunity or autism. Blacks, Hispanics and South Asians tend to have higher incidences of obesity than whites, and allergies and asthma disproportionately affect blacks in some areas and whites in others. No clear pattern emerges for autoimmune diseases, with some, such as lupus and scleroderma, affecting blacks more, and others, including childhood diabetes and multiple sclerosis, tending to prefer whites. Autism does not appear to affect races differently, though black children are often diagnosed later.

      Could what seem like racial differences actually be largely due to other factors, such as wealth or location, rather than to the genetic tendencies of each race? In an elegantly designed statistical study, the higher rate of asthma in black American children than in other races was found to be due not to race itself, but the greater tendency of black families to live in inner-city urban locations, where asthma is more common in all children. Rates of asthma among black children growing up in Africa are, as in most less developed regions, low.

      A neat way to untangle the effects of ethnicity and environment in bringing on twenty-first-century illnesses is to look at the health of migrants. In the 1990s, civil war led to a large exodus of families from Somalia to Europe and North America. Having escaped turmoil in their own country, the Somali diaspora faced a fresh battle. Whereas rates of autism are extremely low in Somalia, the incidence in children born to Somali migrants rapidly jumped to match that of non-migrant children. Among the large Somali community of Toronto, Canada, autism is referred to as ‘the Western disease’, as so many migrant families are affected by it. In Sweden too, children of immigrants from Somalia have three or four times the rate of autism as Swedish children. Race, then, seems far less important than location.

      So what about the final aspect of Who?: sex. Do women and men suffer equally? That women have stronger immune systems may not come as a surprise to anyone who has witnessed a bout of ‘man flu’. But unfortunately, in this immune-mediated epidemic of chronic ill-health, women’s immune superiority proves a disadvantage. While men seem to succumb to the most benign of colds, women battle demons that only their immune systems can see.

      Autoimmune diseases show the widest divergence, with the vast majority of disorders affecting more women than men, though several affect both sexes equally, and a couple show a preference for males. Allergies, though more common in boys than girls, affect more women than men after puberty. Gut disorders too affect more women than men – just slightly so for inflammatory bowel disease, but twice as many women have irritable bowel syndrome.

      Perhaps surprisingly, obesity also seems to affect women more than men, particularly in developing countries. But when measurements other than BMI are used, such as waist circumference, these suggest that men and women actually suffer equally from dangerous levels of excess weight. Likewise, although it appears that some mental health conditions, including depression, anxiety and obsessive– compulsive disorder, affect more women than men, part of this difference may be down to the male reluctance to admit to feeling blue. In autism, it is males who carry the burden, with five times as many boys affected as girls. Perhaps in autism, as with allergies, which tend to strike young, and those autoimmune diseases that begin in childhood, the pre-pubertal onset makes all the difference. Without the influence of adult sex hormones, these illnesses are not subject to the same female bias.

      Women’s strong immune systems are likely to be behind the female preponderance of several twenty-first-century illnesses. For conditions that involve overreactions of the immune system, such as allergies and autoimmunity, a stronger starting point is likely to lead to a greater response. Sex hormones, genetics and lifestyle differences could all play a role too – the jury’s out on exactly why women are worse affected. Whatever it is, the female bias in these modern plagues emphasises the immune system’s underlying role in their development. Twenty-first-century illnesses are not diseases of old age. They are not diseases of genetic inheritance. They are diseases of the young, the privileged, and those of immune fortitude, especially women.

      We have reached the final question of our epidemiological mystery: When? Arguably, this is the most important question of all. I have been calling the modern chronic disease epidemic one of twenty-first-century sickness, though its root is not in this young century, but the last. What a century it was, the twentieth, bringing some of the greatest innovations and discoveries of all of human history. But over the course of its one hundred years, following the near-elimination of serious infectious disease in the developed world, came a new set of illnesses which went from being exceptionally rare to remarkably common. Among the many developments that took place in the last century lies the change, or cluster of changes, that have caused this rise. Pinpointing the moment that the rise began could provide our greatest clue as to its origin.

      You may have got a feel for the timings already. In the US, a sharp upturn in type 1 diabetes cases began around the mid-century. Analysis of conscript data in both Denmark and Switzerland placed it in the early 1950s, in the Netherlands in the late 1950s, and in slightly less developed Sardinia in the 1960s. Rises in asthma and eczema started in the late Forties and early Fifties, and increases in Crohn’s disease and multiple sclerosis took off in the Fifties. Trends in obesity were first recorded on a large scale in the Sixties, making it difficult to determine the start of the epidemic as we see it now, but some experts point to the end of the Second World War in 1945 as a likely turning point. A sharp upturn in cases of obesity took off in the 1980s, but the origin of the rise certainly occurred before then. Similarly, the number of children diagnosed with autism each year was not recorded until the late Nineties, but the condition was first described in the mid-1940s.

      Something changed around the middle of the last century. Perhaps more than one thing changed, and perhaps it continued to change in the decades that followed. That change has spread around the world since, enveloping ever more countries as the decades go by. To find the cause of our twenty-first-century illnesses, we must look at the changes centred on one extraordinary decade: the 1940s.

      From asking What?, Where?, Who? and When?, we have established four things. First, our twenty-first-century illnesses often arise in the gut, and are associated with the immune system. Second, they strike young, often in children, teenagers СКАЧАТЬ