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 isn’t 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% of non-smokers and only 28%of smokers.

　　4. Genetic effects

　　Yours chances of becoming fat may be set, at least in part, before you were even born. Children of obese mothers are much more likely to become obese themselves later in life. 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. Grand-children of mice fed a high-fat diet grow up fat even if their own mother is fed normally-so you fate may have been sealed even before you were conceived.

　　5. 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 fatter end of the spectrum: Mexican-American women are 30% more likely than white women to be obsess, 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 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 changes may account in part for the increased prevalence of obesity.

　　6. 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 study found that the odds of a child being obese increase 14% for every five extra years of their mother’s age , though why this should be so is not entirely clear.

　　Michael Symonds at the University of Nottingham, UK, found that first-born children have more fat than younger ones. As family size decreases, firstborns account for a greater share of the population. In 1964, British women 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% of woman in their 40s had only one child; in 2004 it was 17.4%. This combination of older mothers and more single children could be contributing to the obesity epidemic.