There’s an Atlantic article making the rounds about how Old Economy Steve, in addition to being able to earn a living wage right out of high school, could eat Big Macs every day and still stay skinny because people had different stomach bacteria in the 1980s. According to the Atlantic, the researchers found that “a given person, in 2006, eating the same amount of calories, taking in the same quantities of macronutrients like protein and fat, and exercising the same amount as a person of the same age did in 1988 would have a BMI that was about 2.3 points higher. In other words, people today are about 10 percent heavier than people were in the 1980s, even if they follow the exact same diet and exercise plans.”
That’s genuinely startling, if true. Is it?
The researchers drew their data from an ongoing, many-decades-long study done by the CDC called NHANES. The National Health and Nutrition Examination Survey has been gathering data since the 1970s on a wide range of health issues. They examine 20,000-30,000 people a year (checking blood pressure, cholesterol, and a number of other health and well-being measures) and also interview them about diet and exercise.
A sample question about diet from the survey used in 1999: “In the past 30 days, how often did you have milk to drink or on your cereal? Please include chocolate and other flavored milks as well as hot cocoa made with milk. Do not count small amounts of milk added to coffee or tea. Would you say never; rarely–less than once a week; sometimes–once a week or more but less than once a day; often–once a day or more?” http://www.cdc.gov/nchs/data/nhanes/spq-db.pdf
The problem with interviewing people about diet is that people lie to themselves, they lie to researchers, and they have terrible memories. They’re particularly bad at recalling incidental exercise. With the advent of fitness trackers we’re doing better at knowing how much people move, but even now we have to rely heavily on self-reporting to know how much people eat, and that’s recognized as a problem. A 1998 study asking, “Do unsuccessful dieters intentionally underreport food intake?” concluded that the answer was solidly yes. “Underreporting may be an intentional attempt to manage presentation to others in a society that is increasingly critical of overweight persons.”
For these reasons and more, there was a paper published this year in the International Journal of Obesity that attacked the practice of basing research on the NHANES dietary data. The authors wrote that “it is time to move from the common view that self-reports of EI [energy intake, i.e., calories consumed] and PAEE [physical activity energy expenditure, i.e., calories burned through exercise] are imperfect, but nevertheless deserving of use, to a view commensurate with the evidence that self-reports of EI and PAEE are so poor that they are wholly unacceptable for scientific research on EI and PAEE.”
But okay, even if we take the dietary data at face value, it says right in the summary that between 1971 and 2008, both BMI and total caloric intake increased 10-14%. In other words, people ate more and weighed more. The surprising result is only surprising when you add in the exercise data, which they only started gathering in 1988 and which is also entirely self-reported.
Apparently people report a lot more leisure-time physical activity now. And maybe they’re not lying. I feel like participation in softball leagues and other active adult recreational stuff has gone up since 1988, although in 1988 I was a high schooler and my perceptions of stuff adults did was probably wildly inaccurate. But if a health researcher asked me how frequently I engaged in physical activity I would probably round up, and I am pretty sure I am not alone, and as awareness that we really should be exercising has increased, that sort of improvement on the actual numbers has undoubtedly also increased. Also, I’m pretty sure that the incidental exercise people are getting has gone through the floor since the late 1980s.
Now. Was it easier to be skinny in (say) 1982? Well, obviously it was, because far, far more people were. It’s definitely not the case that adults in 1982 were just better and more disciplined people, because if they were, fewer of them would have smoked like unfiltered Victorian factory chimneys. Also, it’s not just the humans who’ve been getting fatter — so have the lab animals, and with lab animals, you have detailed records of what they were actually fed so you know the number of calories is the same.
So why? The study doesn’t really say, although a lot of people have speculated that we metabolized fewer calories back in the day thanks to the magic of our microbiomes. Wouldn’t it be awesome if people could turn skinny just by taking a handful of daily probiotics? Unfortunately there are probably a lot of reasons and some of them may also go back to Old Economy Steve. Chronic stress is linked to obesity and while I don’t want to downplay the stressors in Old Economy Steve’s life…he grew up in an era where you could pay your in-state college tuition by working a part-time job.
But are people actually getting fatter on fewer net calories? Maybe, but I’m really skeptical of dietary data that’s based on interviewing people.