Why Do Americans Keep Getting Fatter?

Olga’s latest piece examines “why it was easier to be skinny in the 1980s.” Researchers found that Americans in 2006 had an average BMI 2.3 points higher than those in 1988, even when food intake and exercise are controlled. The leading theories as to why: more environmental chemicals today, more prescription drugs, and our microbiomes have evolved to retain more weight.

But a reader questions the methodology of the researchers: “My main concern is that the calories are almost assuredly self-reported, which is notoriously unreliable.” Another reader agrees:

In the 1980s, we weren’t walking around with a computer in our pockets to look up accurate calorie counts for everything that allowed us to store an accurate list of everything we’ve eaten and compute the calories based on that database. It was 100% self-reporting and calorie lookups “from memory” or done manually (complete with calculations) long after the fact. So the reports based on that old data might be suspect.

But another reader notes:

The study authors addressed that point somewhat:

Whether self-reported dietary intake accurately reflects an individual’s true dietary intake has been questioned. Indeed, doubly-labelled water studies typically show that individuals underreport their energy intake, and that the magnitude of the underreporting may be larger in people who are obese.

National self-reported dietary survey data, such as the NHANES, is the primary source of information on the U.S. populations food and beverage consumption, and is used to assess dietary trends, adherence to dietary recommendations and to inform public policy. Notwithstanding the known errors of dietary assessment, it is interesting that we observe consistent trends over time in terms of how dietary intake relates with obesity and how this relationship has changed over time. This lends more confidence to our primary findings and suggests that there are either physiological changes in how diet relates with body weight or differences in how individuals are reporting their dietary intake over time.

Another reader raises a good point:

People are getting a lot less “passive exercise” than in the past. Sure, I can still go to the gym or run or bike, but I’m working at home now, which means I no longer walk 20 minutes a day to and from the subway, and 10 minutes a day to and from work. A lot of the stairs in public spaces I frequent now have escalators. Almost everything is electrified on my current car. I no longer have to pull the seat forward and back—just push a button.

These examples are small things on their own, but, over time, they add up.

Another reader is on the same page:

BBC’s Horizon science show did an episode about diet several years ago showing a US doctor studying weight gain very closely. One subject confused him because he took no regular exercise and ate badly but remained thin. After much analysis of the guy, they discovered the reason was his level of activity outside structured exercise; he paced back and forth and fidgeted all day. He wasn't going to the gym or out jogging, but he was active.

I would hazard a guess that it what is at play here. Industry has declined since the 1980s, more tasks automated, and more people are working in a chair all day. I had a friend who went from a call centre job to a tech support role, where she had to walk from desk to desk helping people, and she lost a lot of weight with no other change to her lifestyle or diet.

But the study authors addressed that factor somewhat:

[W]e observed that the BMI associated with a given leisure time physical activity frequency was still higher over time in men. This may be attributed to changes in non-leisure time physical activity such as reductions in occupational physical activity or increasing screen time. However, a study using doubly labelled water demonstrated that there is no difference in total energy expenditure between traditional hunter-gathers, subsistence farmers and modern Westerners. Thus, numerous other factors in addition to energy intake and physical activity may be important to consider when trying to explain the rise in obesity, and should be further evaluated in further studies.