In a new article published in CA: A Cancer Journal for Clinicians, Roland Sturm (RAND Corporation) and Ruopeng An (University of Illinois at Urbana-Champaign, IL) review the research focusing on the role that the US economic and policy environment (eg, tax, subsidy, direct pricing, serving size regulation, nutrition labeling, etc.) may have played in the obesity epidemic.  In the process, they debunk several popular, but misleading, ideas about causes of the epidemic.

According to Sturm and An, changes to BMI are neither as recent, or as concentrated in specific socio-economic groups (identifiable by race/ethnicity, income, education, or geography), as one might think.  Data suggest that BMIs have been increasing since the 1950s (and perhaps earlier) for American children, since the 1920s for young men, and since about 1900 for men aged 40-49.  And although the prevalence of obesity tends to be highest among groups with lower income and education, all socio-demographics have seen their BMIs increase:

"This makes is very unlikely that the obesity epidemic is caused by environmental changes that affect certain socio-demographic subgroups disproportionally. [...] If we want to understand the role of the environment in the obesity epidemic, we need to understand a bit more of the changes over time affecting all groups rather than differences between subgroups at a given time."

Popular environmental explanations for the obesity epidemic include:

1. labor market changes that have resulted in longer workdays, less time spent preparing meals, reduced leisure time, and reduced levels of physical activity.  Sturm and An show, however, that these explanations may not be valid, because the past fifty years have actually been marked by a reduction in work hours, a decline in sedentary behavior, an increase in time spent in transportation, and an increase in leisure time.
2. the deterioration of diet quality (higher "junk food" consumption, lack of access to "healthy food"). Americans, as a whole, eat more fruits and vegetables now than they did in 1970.  But this is not to say that fruits and vegetables have substituted for more energy-dense foods (such as sugar-sweetened beverages), the consumption of which has also increased.  Distance to supermarkets (and access to "healthy food") does not appear to be a predictor of obesity or diet quality, and one study found that opening a new supermarket in a "food desert" had no effect on fruit and vegetable intake or BMI.
3. increased motorization/mechanization: the authors explain that "the largest changes due to motorization precede the obesity epidemic by decades and are primarily a phenomenon of the first half of the 20th century. [...] Occupation-related changes in physical activity should have affected mainly individuals in the labor force, but weight changes have been similar across groups regardless of employment status and the obesity epidemic affected children as well."

A more likely environmental explanation, according to Sturm and An, is that the US production-led agricultural policy "was very successful in producing cheap calories."  Food has become very cheap so Americans are spending less of their disposable income (from 1/4 in the 1930s, to 1/10th in 2010) feeding themselves, and per capita food availability has increased.  In other words, "this smaller share of disposable income now buys many more calories."  This was also accompanied by greater convenience and reduced time costs for obtaining meals--all of which have resulted in increased food consumption and weight gains.