10% Human: How Your Body’s Microbes Hold the Key to Health and Happiness. Alanna Collen
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 and young adults, and many affect more women than men. Third, these illnesses occur in the Western world, but are now on the rise in developing countries as they modernise. Fourth, the rise began in the West in the 1940s, and developing countries followed suit later.
And so we return to the big question: Why have these twenty-first-century illnesses taken over? What is it about our modern, Western, wealthy lives that is making us so chronically ill?
As individuals and as a society, we have gone from frugal to indulgent; from traditional to progressive; from lacking luxuries to being bombarded by them; from poor health care to excellent medical services; from a budding to a blooming pharmaceutical industry; from active to sedentary; from provincial to globalised; from make-do-and-mend to refresh-and-replace; and from prudish to uninhibited.
Amongst these changes, and in answer to our mystery, are 100 trillion tiny clues waiting to be followed.
Garden warblers are the very epitome of the birder’s greatest identification challenge – the LBJ, or ‘Little Brown Job’. Their most distinguishing feature is, in fact, the absolute lack of any distinguishing features, making recognition of this small bird through a pair of shakily held binoculars particularly difficult. But boring birds these are not. Just a few months after hatching, juvenile garden warblers embark on a 4,000-mile migration from their summer homes across Europe to their winter residences in sub-Saharan Africa. It is a route they have never taken before, and they do it without the help of either their more experienced parents, or a map.
Before these tiny birds head off on this incredible journey, they prepare themselves for the effort of flying and the lack of food en route by becoming fat. Over just a couple of weeks, the warblers double in weight, going from a slender 17 g to a distinctly portly 37 g. In human terms, they become morbidly obese. On each day of the pre-migration binge, a garden warbler gains around 10 per cent of its original body weight – the equivalent of a 10-stone man putting on a stone a day until he weighs 22 stone. Then, once the birds are plump enough, they undertake a feat of endurance beyond the imagination of most elite athletes – flying thousands of miles with just a handful of meals along the way.
Of course, to become that fat that fast, the warblers must gorge themselves on summer’s bounty of food. Practically overnight, the birds shift from a diet of insects to one of berries and figs. Although the fruit is ripe enough to eat for several weeks before their binge begins, the warblers leave it untouched until the time is right. It’s as if a switch flips inside them, and suddenly they dedicate themselves to eating.
For a long time, researchers assumed that the weight gain in warblers and other migratory birds was simply the consequence of hyperphagia – excessive eating. But the incredible speed of the shift in these birds from lean to morbidly obese suggested there was something else going on to help them store so much fat. Something that had less to do with how much food they ate, and more to do with how that food was stored in their bodies. By keeping a check on how many extra calories the warblers ate, and how many calories came out in their droppings, researchers realised that the additional food the birds were consuming did not fully account for the weight they were managing to gain.
The riddle continues when it comes to the birds losing the excess weight again. Of course, as the obese warblers make their journey across the Mediterranean Sea and the Sahara Desert, their fat supplies dwindle. By the time they have arrived and settled in to their African winter home, they have returned to a normal warbler weight. But here’s the strange thing: captive garden warblers are no different. During the pre-migratory period at the end of summer, these caged birds still gain weight, becoming thoroughly obese in preparation for a journey they will never make. And, at the exact point that wild warblers arrive at their destination, the captive warblers completely shed their excess fat. Despite not flying 4,000 miles, and having unlimited access to food, these captive birds still lose the weight again when the migratory period is over.
It