What makes us fat?

The standard answer, consistent with the first law of thermodynamics, is that we intake too many calories as food or we expend insufficient calories because of our sedentary lifestyle. We either eat too much or are slobs. There’s evidence that these factors alone however cannot explain it all – see here. New Scientist sets out 10 possible reasons for the obesity epidemic - the following is a summary of their discussion.

1. Not enough sleep. Several studies suggest people who sleep less than 7 hours a night have a higher body mass index (BMI) than people who sleep more. Obesity impairs sleep, so perhaps people get fat first and sleep less afterwards but it is also known that sleep loss may cause weight gain. Sleep deprivation alters metabolism because leptin, the hormone that signals satiety, falls while ghrelin, which signals hunger, rises.

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.

2. Climate control. Humans like to keep our core body temperatures pretty constant regardless of what's going on in the world. We do this by shivering or sweating. Keeping warm and staying cool take energy unless we are in the ‘thermoneutral zone’ of around 27 °C for a naked body - increasingly where we choose to live and work.

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% from 1978 to 1997. In the southern states - where obesity is highest - the houses with air con has shot up to 70% from 37% in 1978.

Studies of people in respiration chambers 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. Whether these factors significantly alter energy balance is not clear, but it's got to be worth investigating.

3. Less smoking. Smokers are thinner than the rest of us, and quitting packs on the pounds probably because nicotine is an appetite suppressant that ups your metabolic rate.
Between 1978 and 1990, the prevalence of obesity in the US increased by 9% . About a fifth of this increase can be attributed to people giving up smoking. 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 before you are even born. Children of obese mothers - especially those who develop gestational diabetes - are much more likely to become obese themselves. While this may be partly down to genetics, there is also some ‘intrauterine programming’.

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 extreme, energy restriction in the womb can lead to obesity later in life. This is likely if there is a period of rapid catch-up growth in the first 2 years of life. In the prosperous west, that might seem of little relevance, yet in the US the incidence of low birth-weight is now at its highest level for 30 years.

5 Fat equals fecund. Heavier people have more children. One study suggests women of normal weight or below had 3.2 children, while overweight or obese women had 3.5 children.
Does having more children make women gain weight, or does being overweight cause women to have more children? Having lots of kids can increase the chances of getting fat - if for no other reason than poor sleep as above. But people's BMI before they become parents is associated with the number of children they eventually have.

Explanations vary. Extreme thinness impairs fertility. Extreme obesity does too, but the effect is probably stronger at the thin end. Also, obesity can lower socioeconomic status, which in turn is associated with having more children.

This is relevant to the epidemic because obesity is heritable - twin studies indicate it's about 65% 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. Aging. Adults aged 40 to 79 are around 3X as likely to be obese as younger people. Non-white females fall at the plumper end of the spectrum: Mexican-American women are 30% more likely than white women to be obese, 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%. The proportion of Hispanic-Americans also grew, from under 5 to 12.5% of the population, while the proportion of black Americans increased from 11 to 12.3%. These demographic shifts may account in part for the increased prevalence of obesity.

7. More drugs. In the 1970s new antipsychotic medications (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. Furthermore, anticonvulsants to treat epilepsy, antihypertensives for high blood pressure, protease inhibitors to treat HIV and diabetes medications, including insulin, are associated with weight gain. Beta blockers add 1.2 kg to people using them, and taking contraceptive pills for 2 years will add an extra 5 kg. Even antihistamines can fatten you.

8. Pollution. People are exposed to thousands of industrial chemicals: pesticides, dyes, flavorings, perfumes, plastics, resins and solvents, to name but a few. There is evidence low levels of these chemicals can lead to weight gain. Mice given small amounts of the dieldrin, 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 interfere with 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 study found that the concentration of PBDE (polybrominated diphenyl ether, a now-banned fire retardant) in breast milk doubled every 5 years between 1972 and 1998.

9. Mature mums. Mothers are getting older. Mean age at first birth in the US has increased from 21.4 in 1970 to 24.9 in 2000. But having an older mother is an independent risk factor for obesity. The odds of a child being obese increase 14% for every 5 years of their mother's age.

Also first-born offspring have more fat than younger siblings. As family size decreases, firstborns account for a greater share of the population. In 1964, the British woman had 2.95 children; by 2005 that figure had fallen to 1.79. In the US in 1976, 9.6% of women in their 40s had had only one child; in 2004 it was 17.4%. This combination of older mothers and more single children could contribute to obesity.

10. Like marrying like. Just as people pair off according to looks, so they do for size. Lean people marry lean and fat marry fat. Hence there is a correlation between spouses of both BMI and ‘skinfold’ measures of flabbiness that can’t be accounted for by the fact that they live together.

On its own, this cannot account for increased obesity. But combined with the fact that obesity is partly genetic and that heavier people have more children - it amplifies the increase from other causes.