In a comment to my previous post about how weight is genetic, Dennis Mangan wrote:
The environment is just as, if not more, important than genetics. The gene pool hasn't changed over the last thirty years, but the number of overweight and obese has massively increased.
Well, if fat people were having more children than skinny people, then this would easily explain why the number of overweight and obese people is increasing.
Doing some research on the web, I discovered that the question was answered by a Duke University study:
The researchers examined data from the Health and Retirement Study (HRS), a national survey designed to study health, social, and financial issues in middle-aged Americans. The HRS study includes information on more than 12,600 Americans, collected from 1993 to 2000. Most respondents were age fifty-one to sixty in 1992. The study is a cooperative agreement between the In-stitute for Social Research at the University of Michigan and the National Institute on Aging. For the Duke analysis, the researchers used data only from a baseline survey completed in 1992. Only married respondents and spouses between the ages of forty and seventy with children were included in the analysis, which comprised a total of 9,046 men and women (4,523 couples). Single or divorced individuals with children were excluded. The number of children reported by the couples ranged from one to nineteen, and included both biological and adopted children. Of the sample, 79 percent were white, 12.5 percent were African American, and 8.5 percent were Hispanic.
Because the researchers focused their analysis exclusively on the connection between number of children and obesity, they controlled for age, household income, race and ethnicity, work status, physical activity, and tobacco and alcohol use. "After adjusting for all these factors, the number of children played a statistically significant role in the obesity of both men and women," says Truls Ostbye, senior author on the paper and professor of community and family medicine at Duke.
Ostbye says that, while the increased risk for obesity in women was 7 percent for each additional child and only 4 percent for men, the difference between these two figures was not statistically significant. "Obesity associated with number of children is not just a problem linked to physiological changes in women during pregnancy," he says. "There are social, cultural, or psychological mechanisms that bring about this weight gain, and this is illustrated by our results that showed men were also at a greater risk of obesity."
The researchers are implying that having children somehow causes obesity, but I strongly suspect that they have the cause and effect wrong, and actually what is happening is that heavier people have more children.
After posting the above, I found the more detailed study, and the authors cite many other studies showing the link between weight and the number of chidren:
Several investigators have found an association between reproductive history and obesity among women and, in one cohort, among men but we did not locate any studies that have examined the association in couples. A study published in 1956, which included 583 women attending a diabetic clinic in Oxford, England, found an increasing prevalence of overweight women with increasing numbers of pregnancies. In a Swedish study of twins, an average 2 kg increase in weight after pregnancy was observed in women with two or more children compared with childless controls. A population study from Finland found that number of children among women aged 25–84 was closely related to the prevalence of obesity, independent of marital status, occupation, and smoking habits. The strongest relationship between number of children and BMI was in the youngest age group (25–34), although the relationship persisted even among the women aged 75–84. In a more recent study from Sweden, number of children was also associated with obesity among 5464 women ages 45–73.
Similar associations between number of children and obesity have been observed in the United States. In the Nurses' Health Study, an increase in BMI was found with increasing number of children among women aged 42–67. Among 41,000 Iowa women, BMI increased with number of children in a linear fashion after adjustment for age, education, marital status, and smoking status. In a case-control study, women (n = 1716) from Massachusetts (age range 45–69) with five or more births were more frequently obese (defined as a BMI of >/=29) compared with women with no births. Among U.S. women aged 45–74 who participated in the first NHANES follow-up study, as well as among women aged 35–68 who participated in the Framingham Heart Study, investigators found a significant increase in BMI with an increase in number of children.
The Rancho Bernardo Study found an association between number of children and obesity among women and men many years after child-bearing.[29,30] In this cohort study of women and men ages 55–84, the mean age-adjusted BMI was positively associated with number of biological children. Among men 50–89 years of age (n = 1039), those who had five or more biological children were significantly more obese (as estimated by BMI and waist/hip ratio) than men with no biological children. Several investigators have proposed socioeconomic status (SES) as a significant confounder in the number of children and obesity relationship.[31,32] Men and women with less education or of lower social class are at higher risk for obesity, and this effect may be related to personal habits, such as excessive caloric intake, as well as number of children.
The evidence is pretty clear. Heavier people have more children. Weight is genetic. Thus the reason for the "obesity epidemic."