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

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      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.

       TWO

       All Diseases Begin in the Gut

      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 is quite extraordinary that warblers deprived of cues in the weather, day length and seasonal food supplies are still able to rapidly gain enormous stores of fat for the migratory period and then slim down again apparently effortlessly, perfectly in sync with their wild cousins. These are birds, with brains the size of a pea. They don’t gain weight, then think to themselves: ‘I really must go on a diet.’ They don’t fast, or exercise madly, either. Their food intake does relapse after the binge, but again, not enough to account for losing that much weight, that fast. Imagine being able to drop a stone a day for seven days – that’s the degree of weight loss these little birds manage once the migratory period is over. Even eating nothing at all would not result in that kind of weight loss in humans.

      Although we don’t yet know exactly how this astounding degree of weight change is regulated in the warblers’ bodies, the fact that these shifts happen beyond what is expected from changes in caloric intake makes one thing clear: maintaining a stable weight is not always a simple case of balancing calories-in and calories-out. In humans, the scientifically accepted explanation for weight gain is this: ‘The fundamental cause of obesity and overweight is an energy imbalance between calories consumed and calories expended.’

      It seems obvious: if you eat too much and move too little, the extra energy must be stored and you will gain weight. And if you want to lose weight, you must eat less and move more. But the warblers are able to rapidly lay down fat reserves that appear to go far beyond the calories they eat, and then deplete those reserves far beyond the calories they burn. Clearly, there’s more to the weight-regulation game than meets the eye. If calories-in versus calories-out isn’t true for warblers, perhaps it’s not true for humans either?

      Attempting to treat over 10,000 cases of obesity made the Indian physician Dr Nikhil Dhurandhar wonder the exact same thing. His patients returned again and again, after regaining the small amounts of weight they’d lost, or failing to lose any weight whatsoever. Despite the difficulties, Dhurandhar and his father – another doctor specialising in obesity – ran one of the most successful obesity clinics in Mumbai in the 1980s. But after a decade of trying to help people to eat less and move more, he began to feel his efforts – and those of his patients – were futile. ‘After weight loss, you gain weight again: that is the big problem. And that has been my frustration.’ Dhurandhar wanted to know more about mechanisms behind obesity. If eating less and moving more didn’t permanently cure obesity, perhaps eating more and moving less wasn’t the only cause.

      It’s something we desperately need to work out. Our species is in the midst of a warbler-like collective weight gain. And just as in warblers, the amount of weight we have gained does not quite tally with changes in ‘calories consumed’ and ‘calories expended’. Even the biggest and most comprehensive of studies show that most of the weight we have gained as a species is not accounted for by the extra food we are eating, nor by our lack of physical activity. Some even indicate that we are eating less than we used to, and exercising just as much. The scientific debate about whether gluttony and sloth alone can fully account for the exponential rise in obesity over the past sixty years rumbles quietly on. It is a mere scientific undercurrent lapping at the foundations of research that’s seen as more relevant: which diets work best?

      At the time of Dhurandhar’s frustrations, a mysterious disease was spreading through India’s chickens, killing the birds and destroying livelihoods. Dhurandhar’s family were friends with a veterinary scientist who was involved in looking for the cause and finding a cure. The culprit was a virus, he told Dhurandhar over dinner, and the birds would die with large livers, shrunken thymus glands and a lot of excess fat. Dhurandhar stopped him. ‘The dead chickens are especially fat?’ he checked. The vet confirmed it.

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