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.
Dhurandhar was curious. Animals dying of a viral infection would normally be skinny, not fat. Was it possible that a virus could induce weight gain in chickens? Could this be the explanation behind his patients’ difficulties in losing weight? Dhurandhar, excited to know more, set up an experiment. He injected one group of chickens with the virus, and left another group alone. Sure enough, three weeks later, he found that the infected birds were far fatter than the healthy ones. It seemed as if the virus had made them gain weight as they fell ill. Could it be that Dhurandhar’s patients, and countless other humans around the world, were also infected with the virus?
What is happening to our species is on such an enormous and unprecedented scale, that in the distant future, when humanity looks back on the twentieth century, they’ll remember it not just for two world wars, nor solely for the invention of the internet, but as the age of obesity. Take a human body from 50,000 years ago and one from the 1950s, and they will look more similar to one another than either does to the average human body today. In just sixty years or so, our lean, muscular, hunter-gatherer-like physiques have been encased inside a layer of excess fat. It’s something that has never happened to humans on this scale before, and no other animal species – apart from the pets and livestock we care for – has succumbed to this anatomy-changing disease.
One in every three adults on Earth is overweight. One in nine is obese. That’s the average across all countries, including those where under-nutrition is more common than being overweight. Looking just at the figures for the fattest of countries is even harder to believe. On the South Pacific island of Nauru, for example, around 70 per cent of adults are obese, and a further 23 per cent are overweight. Just 10,000 people live in this tiny country, and only about 700 of them are a healthy weight. Nauru is officially the fattest nation on Earth, but it is closely followed by most of the other South Pacific islands and several Middle Eastern states.
In the West, we have gone from being skinny enough that no one thought to comment on, worry about, or count the number of overweight people, to being fat enough that it would be quicker to count those that remain skinny. Roughly two in three adults are overweight, and half of those are not just overweight, but obese. The United States, despite its reputation, is seventeenth in the world rankings, with a mere 71 per cent of the population overweight or obese. As for the UK, it ranks thirty-ninth, with 62 per cent of adults overweight (including 25 per cent obese): the highest figures in Western Europe. Even among children in the Western world, being too fat is shockingly common, with up to one-third of under-twenty-year-olds overweight – half of them obese.
Obesity has crept upon us in a way that makes it seem almost normal. Yes, there is a steady stream of articles and news pieces about the obesity ‘epidemic’ to remind us that it is actually a problem, but we have very quickly adapted to living in a society where most people are overweight. We are quick to assume that fatness is the next step along from greediness and laziness, but if that’s the case, it’s quite an indictment of human nature. Looking at our other achievements as a species over the past century or so – the inventions of mobile phones, the internet, aeroplanes, life-saving medicines and so on – suggests we are not all just lying around, stuffing ourselves with cake. The fact that lean people are now in the minority in the developed world, and that this change has happened in just fifty or sixty years, after thousands upon thousands of years of human leanness, is shocking – just what are we doing to ourselves?
On average, people in the Western world have gained roughly a fifth of their own body weight in the last fifty years alone. If your allotted time on Earth had fallen so that your ‘today’ fell in the 1960s, not the 2010s, you would, in all likelihood, be considerably lighter. People who are 11 stone in 2015 might well have been just over 9 stone in 1965, no special efforts required. Today, to regain a pre-1960s weight, tens of millions of people are perpetually on a diet, attempting to deprive themselves of foods for which their brains have a deep-rooted desire. But despite the billions of dollars spent on fad-dieting, gym-going and pill-taking, obesity levels rise inexorably.
This rise has taken place in the face of sixty years of scientific research into effective weight-maintenance and weight-loss strategies. In 1958, back when being overweight was still relatively rare, one of the pioneers of obesity research, Dr Albert Stunkard, said: ‘Most obese persons will not stay in treatment for obesity. Of those who stay in treatment, most will not lose weight. And of those who do lose weight, most will regain it.’ He was broadly right. Even half a century later, success rates in trials of weight-loss intervention strategies are extremely low. Often, less than half of participants achieve weight loss, and for most it’s just a few kilos over a year or more. Why is it so very hard?