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Obesity Epidemic
Ask anyone why there is an obesity epidemic and they will say that it’s all down to eating too much and burning too few calories. That is undoubtedly true. But it’s also true that we live in an "obesogenic (肥胖基因的) environment": calorific food is plentiful and cheap and our lifestyles are increasingly sedentary.
Now, obesity researchers are increasingly dissatisfied with such explanations. They believe that something else must have changed in our environment to cause 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.
Earlier this year a review paper by 20 obesity experts set out the 9 most plausible alternative explanations for the epidemic. Here they are.
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 found that those who slept an average of 5 hours a night gained more weight during the study period than those who slept 6 hours, who in turn gained more than those who slept 7.
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 cause weight gain. One factor that could be at work here is the way sleep deprivation alters metabolism (新陈代谢). Leptin, the hormone that signals satiety (过饱), falls while ghrelin, which signals hunger, rises -- and this boosts appetite.
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.
There’s no denying that surrounding temperatures have changed in the past few decades. 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.
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 nonsmokers and only 28 per cent of smokers.
Prenatal Effects
Your chances of becoming fat may be set, at least in part, before you are even born. Children of obese mothers 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 (子官内的) 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. And 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.
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. But Ellis also showed that people’s BMI before they become parents is associated with the number of children they eventually have.
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 9f overweight people to go up.
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 obesity, and black women have twice the risk.
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.
More Drugs
In the 1970s a new class of antipsychotic (安定药) medication called neuroleptics came on the market, and millions of people worldwide now take these drugs. Alongside their undoubted success in treating psychosis, they 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: There are many drugs which have all been associated with packing on the pounds.
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.
Pollution
In daily life, people are exposed to tens of thousands of industrial chemicals: pesticides, dyes, perfumes, plastics, 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, for instance, more than doubled their body fat. Hexachloro-benzene, 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.
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.
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.
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. This combination of older mothers and more single children could be contributing to the obesity epidemic.
Less sleep can gain us weight, because it raises our leptin, which triggers us to eat more.