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

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СКАЧАТЬ use to be. To us now, looking back at black and white photographs of the skinny young men and women of the Thirties and Forties enjoying a spell of hot weather in shorts and swimwear, these healthy people appear emaciated, with prominent ribs and lean bellies. But they are not – they are simply not carrying our modern baggage. At the start of the twentieth century, human body weights were uniform enough that few thought to keep records. But, prompted by a sudden rise in weight gain in the 1950s at the epicentre of the obesity epidemic – America – the government began keeping track. In the first national survey in the early 1960s, 13 per cent of adults were already obese. That is, they had a Body Mass Index (weight in kilograms divided by height in metres squared) of over 30. A further 30 per cent were overweight (a BMI between 25 and 30).

      By 1999, the proportion of obese American adults had more than doubled to 30 per cent, and many previously healthy adults had piled on the pounds, keeping the overweight category at a plump 34 per cent. That’s a total of 64 per cent overweight or obese. Trends in the UK followed the same pattern, with a bit of a lag: in 1966, just 1.5 per cent of the adult population were obese and 11 per cent were overweight. By 1999, 24 per cent were obese and 43 per cent overweight – that’s 67 per cent of people now heavier than they should be. Obesity is not just about excess weight, either. It can lead to type 2 diabetes, heart disease and even some cancers, all of which are increasingly common.

      You don’t need me to tell you, this is not normal.

      Tummy troubles too are on the rise. Your cousin may be awkward for trying out a gluten-free diet, but she’s possibly not the only one at the table who suffers from irritable bowel syndrome, which affects up to 15 per cent of people. The name implies a similar level of discomfort to a midge bite, and belies the ruinous impact of the condition on the quality of life of its sufferers. Proximity to a toilet takes priority over more meaningful pursuits for most sufferers, and a near-absence of need for one makes pursuit of anything but colonic relief worthless for the remaining patients. Inflammatory bowel diseases like Crohn’s disease and ulcerative colitis too are on the rise, leaving the worst affected with a bowel so damaged it has to be replaced by a colostomy bag outside the body.

      This is definitely not normal.

      And finally we come to mental health conditions. Your dentist’s autistic son has more company than ever before, as 1 in 68 children (but 1 in every 42 boys) are on the autistic spectrum. Back in the early 1940s autism was so rare it hadn’t even been given a name. Even by the time records began in 2000, it was less than half as common as it is now. You’d be right in thinking that at least some of these extra cases are due to increasing awareness and perhaps some over-diagnosis, but most experts agree that the rise in autism prevalence is genuine – something has changed. Attention deficit disorders, Tourette’s syndrome and obsessive–compulsive disorder are all also on the rise. Depression and anxiety disorders too.

      This increase in mental suffering is not normal.

      Except these conditions are now so very ‘normal’, you might not even have realised that they are new illnesses, rarely encountered by our great-grandparents and those before them. Even doctors are often unaware of the histories of the conditions they treat, having received their medical training only in the context of today’s doctors’ experiences. As with the rise in cases of appendicitis, a change forgotten by today’s medics, what matters most to front-line carers is the patients in their charge and the treatments available to them. Understanding the provenance of illness is not their responsibility, and as such, changes in prevalence are incidental to them.

      In the twenty-first century, life is different thanks to the four public health innovations of the nineteenth and twentieth centuries, and so disease is different too. But our twenty-first-century illnesses are not simply another layer of ill-health, hidden beneath infectious disease, but an alternative set of conditions, created by the way we live now. At this point you might be wondering how these illnesses can possibly have something in common, such a disparate group do they seem. From the sneezing and itching of allergies, to the self-destruction of autoimmunity, the metabolic misery of obesity, the humiliation of digestive disorders and the stigma of mental health conditions, it’s as if our bodies, in the absence of infectious diseases, have turned on themselves.

      We could accept our new fate and be grateful that we will, at least, live long lives free from the tyranny of the pathogen. Or we could ask what has changed. Could there be a link between conditions that seem unrelated, like obesity and allergies, irritable bowel syndrome and autism? Does the shift from infectious diseases to this new set of illnesses indicate that our bodies need infections to stay balanced? Or is the correlation between declining infectious disease and rising chronic illness merely hinting at a deeper cause?

      We are left with one big question: Why are these twenty-first-century illnesses happening?

      At the moment, it’s fashionable to look to genetics for the source of disease. The Human Genome Project has unearthed a whole heap of genes that, when mutated, can result in illness. Some mutations guarantee disease: a change to the code of the HTT gene on chromosome 4, for example, will always result in Huntington’s disease. Others mutations simply increase the likelihood: misspellings in the genes BRCA1 and BRCA2 raise a woman’s risk of breast cancer to up to an eight in ten chance in her lifetime, for instance.

      Although this is the era of the genome, we cannot blame our DNA alone for the rise in our modern diseases. While one person might carry a version of a gene that makes them, say, more likely to become obese, that gene variant could not become dramatically more common in the population as a whole inside a single century. Human evolution just does not progress that rapidly. Not only that, but gene variants only grow more common through natural selection if they are beneficial, or their detrimental effects are suppressed. Asthma, diabetes, obesity and autism bring few advantages to their victims.

      With genetics excluded as the cause of the rise, our next question must be: Has something changed in our environment? Just as a person’s height is a result of not only their genes, but their environment – nutrition, exercise, lifestyle and so on – so is their disease risk. And this is where it gets complicated, as so very many aspects of our lives have changed in the last century, and pinpointing which are causes and which are mere correlations requires the patient process of scientific evaluation. For obesity and its related illnesses, changes in the way we eat are clear to see, but how this affects other twenty-first-century illnesses is less obvious.

      The diseases in question offer up few clues as to their joint origin. Could the same changes in our environments that lead to obesity also generate allergies? Can there really be a common cause of mental health conditions like autism and obsessive–compulsive disorder, and gut disorders like irritable bowel syndrome?

      Despite the disparities, two themes emerge. The first, clearly binding allergies and autoimmune diseases, is the immune system. We are looking for a culprit which has interfered with the immune system’s ability to determine our bodily threat level, making overreactions all too common. The second theme, often hidden behind more socially acceptable symptoms, is gut dysfunction. For some modern illnesses, the link is clear: IBS and inflammatory bowel disease have bowel disturbances at the core of their presentation. For others, although it is less overt, the connection is still there. Autistic patients struggle with chronic diarrhoea; depression and IBS go hand in hand; obesity has its origin in what passes through the gut.

      These two themes, the gut and the immune system, might also seem unrelated, but a closer look at the anatomy of the gut provides a further clue. Asked about their immune system, most people might think of white blood cells and lymph glands. But that’s not where most of the action is. In fact, the human gut has more immune cells than the rest of the body put together. Around 60 per cent of the immune system’s tissue is located around the intestines, particularly along the final section of the small intestine and into the caecum and the appendix. It’s easy to think of the skin as the СКАЧАТЬ