The New Scientist
UK Edition
Published 3 November, 2006
Bad diet and a lazy lifestyle will “supersize” you, but that may not be the only reason obesity is booming - New Scientist looks at the charts and tucks into the science
Ask anyone why there is an obesity epidemic and they will tell you that it's all down to eating too much and burning too few calories. That is undoubtedly true - you cannot get round the first law of thermodynamics. It's also true that we live in an "obesogenic environment": calorific food is plentiful and cheap and our lifestyles are increasingly sedentary. Most of us have to make an effort not to get fat.
That explanation appeals to common sense and has dominated efforts to get to the root of the obesity epidemic and reverse it. Yet obesity researchers are increasingly dissatisfied with it. Many now believe that something else must have changed in our environment to precipitate such dramatic rises in obesity over the past 40 years or so. Nobody is saying that the "big two" - reduced physical activity and increased availability of food - are not important contributors to the epidemic, but they cannot explain it all (see "Why the 'Big Two' just will not do", below).
Earlier this year a review paper by 20 obesity experts set out the 10 most plausible alternative explanations for the epidemic (International Journal of Obesity, DOI: 10.1038/sj.ijo.0803326). Here they are.
1 Not enough sleep
It is widely believed that sleep is for the brain, not the body. Could a shortage of shut-eye also be helping to make us fat?
Several large epidemiological studies suggest there may be a link. People who sleep less than 7 hours a night tend to have a higher body mass index (BMI) than people who sleep more, according to data gathered by the US National Health and Nutrition Examination Survey. Similarly, the US Nurses' Health Study, which tracked 68,000 women for 16 years, found that those who slept an average of 5 hours a night gained more weight during the study period than women who slept 6 hours, who in turn gained more than those who slept 7. Two recent studies, one in Canada and the other in Germany, show that sleep duration and weight are linked in children too.
It's well known that obesity impairs sleep, so perhaps people get fat first and sleep less afterwards. But the nurses' study suggests that it can work in the other direction too: sleep loss may precipitate weight gain. One factor that could be at work here is the way sleep deprivation alters metabolism. Leptin, the hormone that signals satiety, falls while ghrelin, which signals hunger, rises - and this boosts appetite. Sleep-deprived rats eat about twice as much as rested ones. "They're voracious," says Carol Everson at the Medical College of Wisconsin in Milwaukee.
The same goes for people. In men allowed only 4 hours' sleep for two consecutive nights in a sleep lab, leptin levels dropped 18 per cent and ghrelin levels rose 28 per cent, and the volunteers reported they felt a lot hungrier. A big study recently confirmed these trends outside the lab. "Sleep loss seems to alter the ability of leptin and ghrelin to accurately signal caloric need," says Karine Spiegel at the Free University of Brussels (ULB), who ran the lab study, "and could lead to excessive caloric intake when food is freely available."
Although getting figures is difficult, it appears that we really are sleeping less. In 1960 people in the US slept an average of 8.5 hours per night. A 2002 poll by the National Sleep Foundation suggests that the average has fallen to under 7 hours. According to Spiegel, the decline in sleep is mirrored by the increase in obesity (see Diagram).
2 Climate control
We humans, like all warm-blooded animals, can keep our core body temperatures pretty much constant regardless of what's going on in the world around us. We do this by altering our metabolic rate, shivering or sweating. Keeping warm and staying cool take energy unless we are in the "thermoneutral zone" - around 27 °C for a naked body - which is increasingly where we choose to live and work.
There's no denying that ambient temperatures have changed in the past few decades. Between 1970 and 2000, the average British home warmed from a chilly 13 °C to 18 °C. In the US, the changes have been at the other end of the thermometer as the proportion of homes with air conditioning rose from 23 to 47 per cent between 1978 and 1997. In the southern states - where obesity rates tend to be highest - the number of houses with air con has shot up to 70 per cent from 37 per cent in 1978.
Could air conditioning in summer and heating in winter really make a difference to our weight? Sadly, there is some evidence that it does - at least with regard to heating. Studies of people enclosed in respiration chambers for a few days show that in comfortable temperatures we use less energy. In one study of women exposed to 27 °C versus 22 °C, it amounted to a difference of about a megajoule (239 kilocalories) a day. That's the amount of energy in 27 grams of body fat.
Sweating burns up energy, however, and there's good evidence that high temperatures reduce the amount people eat, according to biostatistician David Allison of the University of Alabama, Birmingham. Whether these factors significantly alter energy balance is not clear, but it's got to be worth investigating.
3 Less smoking
Bad news: smokers really do tend to be thinner than the rest of us, and quitting really does pack on the pounds, though no one is sure why. It probably has something to do with the fact that nicotine is an appetite suppressant and appears to up your metabolic rate.
Katherine Flegal and colleagues at the US National Center for Health Statistics in Hyattsville, Maryland, have calculated that people kicking the habit have been responsible for a small but significant portion of the US epidemic of fatness. From data collected around 1991 by the US National Health and Nutrition Examination Survey, they worked out that people who had quit in the previous decade were much more likely to be overweight than smokers and people who had never smoked. Among men, for example, nearly half of quitters were overweight compared with 37 per cent of non-smokers and only 28 per cent of smokers.
Between 1978 and 1990, the prevalence of obesity in the US increased by about 9 per cent. Flegal reckons that about a fifth of this increase can be attributed to people giving up smoking (The New England Journal of Medicine, vol 333, p 1165). That's not to say that quitting smoking is a threat to public health - far from it. Smoking is so dangerous that you'd have to gain about 45 kilograms to justify continuing.
4 Prenatal effects
Your chances of becoming fat may be set, at least in part, before you are even born. Children of obese mothers - especially those who develop gestational diabetes - are much more likely to become obese themselves later in life. While this may be largely down to genetics, there is also evidence that some "intrauterine programming" goes on.
Offspring of mice fed a high-fat diet during pregnancy are much more likely to become fat than the offspring of identical mice fed a normal diet. Intriguingly, the effect persists for two or three generations. Grandchildren of mice fed a high-fat diet grow up fat even if their own mother is fed normally - so your fate may have been sealed even before you were conceived.
At the other end of the spectrum, we know from studying people born during the famine inflicted on the Netherlands during the second world war that energy restriction in the womb can lead to obesity later in life. This is especially likely if there is a period of rapid catch-up growth in the first two years of life. In the prosperous west, that might appear to be of little relevance, yet in the US the incidence of low birthweight has been rising since the mid-1980s and is now at its highest level for 30 years (National Vital Statistics Report, vol 53, p 1).
5 Fat equals fecund
Heavier people have more children. A study by Lee Ellis at Minot State University in North Dakota found "small but highly significant correlations" between BMI and reproductive rates. Women of normal weight or below had an average of 3.2 children, while overweight or obese women had an average of 3.5 children.
Does having more children make women gain weight, or does being overweight cause women to have more children? Probably both. Having lots of kids can increase the chances of getting fat - if for no other reason than poor sleep (see above). But Ellis also showed that people's BMI before they ecome parents is associated with the number of children they eventually have.
Explanations vary. Extreme thinness impairs fertility in both men and women. Extreme obesity does too, but Ellis thinks the effect is probably stronger at the thin end. Also, as David Allison of the University of Alabama at Birmingham points out, obesity can lead to lower socioeconomic status, which in turn is associated with having more children.
So why is this relevant to the epidemic? It's because obesity is heritable - twin studies indicate it's about 65 per cent genetic - so a tendency for this to be associated with having a large family will cause the proportion of overweight people to go up.
6 A little older...
Some groups of people just happen to be fatter than others. Surveys carried out by the US National Center for Health Statistics found that adults aged 40 to 79 were around three times as likely to be obese as younger people. Non-white females also tend to fall at the plumper end of the spectrum: Mexican-American women are 30 per cent more likely than white women to be obese, and black women have twice the risk (Journal of the American Medical Association, vol 295, p 1549).
In the US, these groups account for an increasing percentage of the population. Between 1970 and 2000 the chunk of the US population aged 35 to 44 grew by 43 per cent. The proportion of Hispanic-Americans also grew, from under 5 per cent to 12.5 per cent of the population, while the proportion of black Americans increased from 11 to 12.3 per cent. These demographic shifts may account in part for the increased prevalence of obesity.
7 More drugs
In the 1970s a new class of antipsychotic medication called neuroleptics came on the market, and millions of people worldwide now take these drugs. Alongside their undoubted success in treating psychosis, neuroleptics have a drawback: users typically gain 4 kilograms in the first 10 weeks, and another 4 or 5 kg in the year that follows.
Neuroleptics are not the only class of drugs to cause weight gain: anticonvulsants to treat epilepsy, antihypertensives for high blood pressure, protease inhibitors to treat HIV and diabetes medications, including insulin, have all been associated with packing on the pounds. Beta blockers add an average 1.2 kg to people using them, and taking contraceptive pills for two years will pad you out with an extra 5 kg. Even common, over-the-counter antihistamines can fatten you up.
So have pharmaceuticals contributed to the obesity epidemic? There is no firm evidence either way, but there is no doubt that the use of all these drugs has mirrored the rise in obesity over the past 30 years.
8 Pollution
In daily life, people are exposed to tens of thousands of industrial chemicals: pesticides, dyes, flavourings, perfumes, plastics, resins and solvents, to name but a few. We swallow them, inhale them and absorb them through our skin.
There is some evidence that low levels of some of these chemicals can lead to weight gain. Mice given small amounts of the pesticide dieldrin, for instance, more than doubled their body fat. Hexachlorobenzene, another pesticide, caused rats to gain significantly more than controls, despite eating half as much. Studies of humans exposed to PCBs by eating fish caught in North America's Great Lakes have found similar associations: the more the toxic load, the greater the body weight.
Some of these chemicals are endocrine disruptors that interfere with the functioning of hormones such as oestrogen. Numerous animal and human studies suggest that when oestrogen is not functioning properly, adiposity increases. Our exposure to such chemicals is on the rise: one Swedish study found that the concentration of PBDE (polybrominated diphenyl ether, a now-banned fire retardant) in breast milk doubled every five years between 1972 and 1998.
9 Mature mums
Mothers around the world are getting older. In the UK, the mean age for having a first child is 27.3, compared with 23.7 in 1970. Mean age at first birth in the US has also increased, rising from 21.4 in 1970 to 24.9 in 2000.
This would be neither here nor there if it weren't for the observation that having an older mother seems to be an independent risk factor for obesity. Results from the US National Heart, Lung and Blood Institute's growth and health study found that the odds of a child being obese increases about 14 per cent for every five extra years of their mother's age, though why this should be so is not entirely clear.
Could sheep hold a clue? Michael Symonds at the University of Nottingham, UK, has found that lambs born to older ewes lay down more adipose tissue in the first year of life. Symonds also found that first-born offspring have more fat than their younger siblings. As family size decreases, firstborns account for a greater share of the population. In 1964, the British woman gave birth to an average of 2.95 children; by 2005 that figure had fallen to 1.79. In the US in 1976, 9.6 per cent of women in their 40s had had only one child; in 2004 it was 17.4 per cent. This combination of older mothers and more single children could be contributing to the obesity epidemic. "It could skew the population to having fatter offspring," says Symonds.
10 Like marrying like
Just as people pair off according to looks, so they do for size. Lean people are more likely to marry lean and fat more likely to marry fat. In a study of 1341 Canadian families published in 1999, Peter Katzmarzyk at York University in Toronto found that there is a small but significant correlation between spouses of both BMI and "skinfold" measures of flabbiness that cannot be accounted for by the fact that they live together.
On its own, like marrying like cannot account for any increase in obesity. But combined with others - particularly the fact that obesity is partly genetic, and that heavier people have more children - it amplifies the increase from other causes.
From issue 2576 of New Scientist magazine, 03 November 2006, page 34-38
Why the Big Two just will not do
Two prime suspects are in the frame when it comes to explaining the obesity epidemic: slothful western lifestyles, and the manufacturing and marketing practices of the food industry. It is not hard to think of examples of how these have changed for the worse: less physical education in schools, kids playing computer games rather than ball games, aggressive marketing of junk food, ever-bigger portions in restaurants, the increased use of high-fructose corn syrup in processed foods, and so on.
Yet any causal link between these changes and obesity is proving hard to pin down. A recent paper authored by 20 obesity experts from the US, Canada and Italy (International Journal of Obesity, DOI: 10.1038/sj.ijo.0803326) concluded that any evidence that they are the main cause of the epidemic - or that halting them would reverse it - is "largely circumstantial".
For example, in 2000 Benjamin Caballero and colleagues at Pathways, a large obesity-prevention programme run through the Johns Hopkins School of Public Health in Baltimore, Maryland, recruited 1704 Native American children from 41 schools and randomly assigned them to experimental and control groups. The experimental group got a better diet, more physical activity and classes about healthy eating and lifestyle. The children were studied for three years. Astonishingly, they showed no significant reduction in body fat compared with the controls (The American Journal of Clinical Nutrition, vol 78, p 1030). "We threw tens of millions of dollars at the best investigators in the world - and they found absolutely no effect," says David Allison of the University of Alabama, Birmingham, one of the 20 authors of the review paper.
In a similar vein, attempts at linking children's obesity rates with how close they live to fast food outlets and parks have drawn a blank (Preventive Medicine, vol 38, p 57). The link between TV watching and obesity in young people has been shown to be statistically significant, though not large enough to make a clinical difference (International Journal of Obesity and Related Metabolic Disorders, vol 28, p 1238). And while obese people do eat bigger portions, there is no evidence to show that this is what causes their problem in the first place (Nutrition Today, vol 38, p 42).
Nobody is suggesting that these studies let the "big two" off the hook. But the case against them remains unproven, and many believe it is time to stop assuming that they are the only causes worth investigating.
http://www.newscientist.com/channel.../mg19225760.900
The 10 roads to Fatsville
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. 18% decrease in leptin from sleep deprivation? Sure, I buy it, as it is / was probably low leptin which made me always ravenous before (and the symptom, along with many others, went away with gaining weight and eating more). 
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