Why the surge in obesity?

The Weight of the Nation is a four-part series on obesity in America by HBO Films and the Institute of Medicine, with assistance from the Centers for Disease Control (CDC) and the National Institutes of Health (NIH). It’s been showing on HBO and can be viewed online. Each of the four parts is well done and informative.

Obesity is defined as having a body mass index (BMI) of 30 or more. For a person 6 feet tall, that means a weight of more than 220 pounds. For someone 5’6″, the threshold is 185 pounds. People who are obese tend to earn less and are more likely to be depressed. They are at greater risk of diabetes, heart disease, stroke, and some types of cancer, and they tend to die younger. The CDC estimates the direct and indirect medical care costs of obesity to be $150 billion a year, about 1% of our GDP.

The chart below, which appears several times in The Weight of the Nation, shows the trend in obesity among American adults since 1960, the first year for which we have good data. The data are from the National Health and Nutrition Examination Survey (NHANES). They are collected from actual measurements of people’s height and weight, rather than from phone interviews, so they’re quite reliable. After holding constant at about 15% in the 1960s and 1970s, the adult obesity rate shot up beginning in the 1980s, reaching 35% in the mid-2000s.

What caused the surge in obesity? The standard explanation is too much eating and too little physical activity, and The Weight of the Nation sticks with this story. But it shouldn’t, because the evidence suggests one of these two hypothesized culprits has been far more important than the other.

Here is the trend in eating, measured as average calories in the food supply (adjusted for loss and spoilage) according to data from the Department of Agriculture. This chart too is from The Weight of the Nation. The timing of change matches that for obesity; the level is flat through the 1970s and then rises sharply beginning in the 1980s. An alternative series, measuring energy consumption per capita, goes back to 1950 (see figure 6, chart F here); it too shows little or no change until 1980, and then a sharp jump. The rise in food consumption correlates closely with the rise in obesity.

That isn’t true of physical activity. We’re less active now than we were half a century ago, but the timing of the decline in activity doesn’t match up with the shift in obesity.

We don’t have good historical data for a comprehensive measure of activity, such as calories expended, so we have to look instead at individual components. We can begin with the most-often-cited culprit: television. Here too The Weight of the Nation presents data, shown below, with the suggestion that TV watching is a significant cause of rising obesity. But the trend doesn’t support that inference. Time spent watching television has increased steadily since 1950. There was no sudden rise in the 1980s.

What about video games, the internet, and smartphones? The internet and smartphones arrived on the scene too late to account for the rise in obesity in the 1980s and most of the 1990s. The timing doesn’t work for video games either; they’re played mostly by the young, beginning in the 1980s, but obesity rates rose sharply in the 1980s and 1990s among adults of all ages, even among the elderly (see table 2 here).

More Americans now have sedentary jobs and drive to work. Yet as David Cutler, Edward Glaeser, and Jesse Shapiro noted in a paper published nearly a decade ago, these shifts have been going on for a long time, with no acceleration in the 1980s.

“Between 1910 and 1970, the share of people employed in jobs that are highly active like farm workers and laborers fell from 68 to 49 percent. Since then, the change has been more modest. Between 1980 and 1990, the share of the population in highly active jobs declined by a mere 3 percentage points, from 45 to 42 percent. Occupation changes are not a major cause of the recent increase in obesity.

“Changes in transportation to work are another possible source of reduced energy expenditure — driving a car instead of walking or using public transportation. Over the longer time period, cars have replaced walking and public transportation as a means of commuting. But this change had largely run its course by 1980. In 1980, 84 percent of people drove to work, 6 percent walked, and 6 percent used public transportation. In 2000, 87 percent drove to work, 3 percent walked, and 5 percent used public transportation. Changes of this minor magnitude are much too small to explain the trend in obesity.”

Another reason to doubt the importance of declining physical activity is that the elderly probably have become more active over time, rather than less, and yet we observe a rise in obesity among the elderly too, similar in timing and magnitude to that of younger adults (again see table 2 here).

In short, the evidence suggests that reduced physical activity has not been a key cause of the surge in obesity in America (more here, here, here, here, here).

This doesn’t mean physical activity plays no role in determining which persons become obese. And it doesn’t mean an increase in activity won’t help reduce obesity’s prevalence. But it does suggest that a strategy focused on increasing activity — and The Weight of the Nation leans in this direction — may not get us as far as we’d like. To make serious progress in reducing obesity, we need to significantly reduce the number of calories many of us consume.

25 thoughts on “Why the surge in obesity?

  1. Hey Prof. Kenworthy, does there seem to be any correlation between obesity rates across the OECD and “institutional regime types”? For example, do the “Anglo-American,” “Continental,” “Mediterranean,” and “Nordic” countries have different health outcomes re: obesity, morbidity, etc.?

  2. What about a change in measurement methodology in 1980?

  3. Before TV, families would sit around listening to the radio in the evenings, or sitting out in the front porch. I thus doubt that TV is really all that much at fault.

    When I was a kid, I always walked or biked to school, as did all the kids that lived in town. The idea of a school bus transporting a kid that only lived a mile or less from school would have been unthinkable. This probably does have a very slight amount to do with childhood obesity.

    Lately, I’ve been going back to ordering the same Double Cheeseburger, small fries, and small drink at McDonalds that I did back in the early 60s. It was a full meal then, no good reason why it shouldn’t be now. I almost never notice anyone else ordering the same thing – they all order the more expensive and larger (and more caloric) options that are advertised with full color photos. Portion sizes have gotten bigger just about across the board, and that does clearly have a lot to do with it. Part of it too is that we’re just able to afford to buy more – and more expensive – food than we used to.

    Another part of it is that the number of smokers has dropped tremendously. (Thank God!) This started after the Surgeon General’s report in the early 60s, but you really didn’t start to see a noticeable change until the early 80s. It was then that “no smoking” prohibitions started being seriously enforced and expanded to include a lot of public spaces where it was previously considered socially acceptable to smoke. Cigarettes had been marketed in earlier days as an aid to losing weight and preventing weight gain (“Reach for a Lucky instead of a sweet” was one well-known advertising slogan), and there does seem to be something of an inverse correlation between smoking and weight. Thus, just as a smoker that quits frequently gains weight, perhaps we as an entire population have gained weight as the smoking has declined.

  4. Ken – I have read that the calories/food are underestimates, because our food is more processed, and hence more digestible – That is, the calories you actually extract when you cook 1 ounce of oatmeal varies if you are using old fashioned steelcut oats or rolled oats; similarly, “100 calories” of steelcut oats is , in reality, less then 100 calories of easy to digest snackfood.
    Also, can you give us a feel for the error in the numbers ? and how they vary across the population – like the TV/day, that could hide two or three different subpopulations in the USA.
    Finally, there is a disparity between your two data sets: the calorie intake dataset is pretty comprehensive; it includes ALL calories.
    On the other hand, the activity datasets, each by itself, is only a small fraction of activity, eg you have to add together the TV data and a whole bunch of other data sets together.

  5. Andrew: I haven’t (yet) looked carefully at the cross-country patterns.

    Doc: To my knowledge there was no change in BMI measurement or in the obesity threshold.

    Stefan: According to CDC data, smoking has decreased gradually since the mid-1960s (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5644a2.htm).

    Ezra: For more detail on physical activity, follow the links in the next-to-last paragraph of my post.

  6. It is not that there is anything special about HFCS, except that it is cheap – especially given US sugar tarrifs and subsidies that drove up the price of cane and beet sugar. Lower costs meant that more sweet foodstuffs could be sold at a lower price. This in turn made it more affordable for us to buy and eat more of the stuff.

  7. Stefan: agreed, I would never say that it was purely HFCS, but it’s not hard to see an interaction effect between the subsidized low cost you note and the need for cheaper food in a period of stagnating real wages.

  8. I am certain that there was a change in the definition of obesity in the early 80’s. I was in the Navy and weighed 222 which alright in 1982. In 1984 I weighed the same (still played rugby), but was declared obese and ordered to lose weight.

  9. Often the discussion on obesity rates considers calories in the diet to the exclusion of the components of the diet. Gary Taubes (and others) have done a nice job showing that it’s much more about the nature of the food and the particular affect on the body. Taubes looked at, among other populations, very poor people. The diet of these people, and many Americans today, is heavily tilted toward processed grains and sugar. Wheat and corn, stripped of nutrients and fiber, mixed with sugar and sweet drinks.

  10. The nanny state is responsible. Since the early 70’s seat belts & bike helmets have been required.

  11. Lane, health and nutrition is something I’ve studied heavily for many years, for myself and my family (By the way, I’ve put it into practice; since you last saw me I’ve lost about 70 lbs. I’m now a lean 180 at 6’3″)

    The scientific evidence does seem to be that diet is much more important than exercise for overweightness.

    But diet is much more than calories; it’s the quality of the food. Way too little whole plant food, and way too much processed and animal food, appears to be much more important than the level of calories. High quality nutritious food causes better management of the calories ingested, so too many that aren’t needed, aren’t stored.

    Ok, so what do I know, let’s get to some evidence. One of the top scientists in this area is T. Colin Campbell of Cornell. He headed the Cornell-Oxford China Study, the largest and best epidemiological study in history, and he has a long decorated career. Here’s what he writes in his 2006 book, The China Study:

    How can it be that even the least active Chinese [in the 1980s] consume even more calories [than Americans per pound of bodyweight], yet have no overweight problems? What is their secret?…

    …I have a more comprehensive interpretation that is based on our own considerable research and on the studies of others. It goes like this. Provided we aren’t restricting our calorie intake, those of us who consume a high fat, high protein diet simply retain more calories than we need…

    …The body employs a delicate balancing act and some very intricate mechanisms in deciding how to use the calories being consumed. When we treat our body well by eating the right foods, it knows how to partition the calories away from body fat and into the more desirable functions…or just disposing of any excess.

    – Pages 99-101.

    I recommend this book, Lane. It’s long and scientifically dense, not a fast read, but I think it’s worth it just for the long term benefits to you and your family. After that, I recommend, “Eat to Live” by Joel Fuhrman. It’s broader, and has a lot of practical things like recipes (the best ones, though, are on his website).

  12. A little more, the main idea of Campbell (and Dr. Fuhrman and others) is that we evolved to make use of the hundreds of phytochemicals and fibers in plants. When you predominantly eat refined foods, you strip the vast majority out (even if you later throw in a few vitamins and some fiber; you strip out hundreds, and add back in five or ten). And animal foods have little of this. So this deficient diet leads to all sorts of problems long term, in addition to retaining too many of the calories we eat.

  13. Professor Kenworthy: I would grant that energy consumption is probably more important than physical activity levels in accounting for growing rates of obesity. But one still needs to account for the epidemic of overeating this suggests; I think there’s pretty strong reason to believe that stress caused by economic insecurity is a significant factor in this; people eat heavily to respond to stressful lives. First of all, obesity really takes off in the eighties, when economists find a combination of stagnant wages and sharply rising inequality, factors that would have generated increased economic stressors in the lives of many, especially working class Americans. This explanation also accounts for the strong class gradient of obesity, with the poor having much higher levels than more economically secure groups. And this explanation also explains why the rates of obesity in the U.S. are among the worst in the world, given our weak support for the disadvantaged. Obviously, something as complex as obesity prevalence has many causal factors, and the weakness of public transportation and the corresponding heavy use of cars in transport no doubt is another factor, but economic insecurity is very probably a part of the solution. By the way, the study cited below appears to support this explanation. Finally, thanks for your excellent blog.

    Obesity under affluence varies by welfare regimes: The effect of fast food, insecurity, and inequality Original Research Article

    Economics & Human Biology, Volume 8, Issue 3, December 2010, Pages 297-308
    Avner Offer, Rachel Pechey, Stanley Ulijaszek View Abstract

  14. @Barnes. that’s got to be the stupidest thing I’ve heard in months. sure, if you focus on car-safety regulations, then you can make a case for the “nanny state.” but if you actually look at policies and laws traditionally associated with the nanny state, like welfare, unemployment sure, workplace regulations, you see that we’ve seen a massive cutback in those laws/policies. so the “nanny state” argument doesn’t have a leg to stand on. Think before you respond, Barnes

  15. @ 1980 or thereabouts:

    Heavily processed foods + microwaves + HFCS + ever larger standard portions at restaurants (and then at home).

    At the same time there is a corresponding shifting of the palate to sweeter and saltier, especially since many processed foods, particularly “diet” / “healthy” ones, would essentially taste like cardboard without sweeteners or salt.

    Add to the that the atrophy of cooking skills and the number of people who can’t prepare for themselves at home a satisfying, healthy meal falls to nearly zero, and it becomes a self-feeding cycle.

    The other interesting thing that isn’t emphasized enough is that it’s not just that more people are crossing that line between overweight and obese, but the obese keep getting bigger. The top line is still rising as the overall average weight goes up. That’s a much more worrisome trend.

    People need to learn to cook again. A few years ago, at 32, I finally learned how to really cook. I lost 40 pounds. And now I am teaching my mother so that she can help my father lose weight for the sake of his heart. For 40 years the main option for vegetables she gave him was steamed, which really do need to be choked down (trust me). Also, my father should learn to cook as well and not just rely on my mother, but that’s an argument for a different generation…

  16. “Families earning the median income now work about 3500 hours, on average, compared to 2800 hours in 1975. The 26 percent increase in hours worked mainly reflects increases in work outside of the home among women. In fact, among two-parent families with median earnings, the hours of men were relatively constant over time, while hours worked by women more than doubled from 1975 to 2009.”

    http://www.brookings.edu/up-front/posts/2011/07/08-jobs-greenstone-looney

    If you look at the chart in this link that details the increased hours worked by the average American family, you will see it correlates almost exactly with the calorie and obesity charts in this topic.

  17. Another omitted variable could be sleep. Average sleep duration has been declining, although I’m not sure if there was a major change around 1980. I couldn’t find data on this.

    While I agree that too many calories is probably the specific cause of obesity, I don’t agree that it’s the most important factor for overall health. BMI isn’t the only measure of health. If calorie supply had increased, but exercise had also increased, we would still expect to see obesity rise, but health outcomes wouldn’t change much. In other words, it’s possible to be fat and healthy. Think about professional discus throwers in track and field or bodybuilders. They would be considered obese by a simple BMI measure, but they’re healthy people. Obesity shouldn’t be the only health marker we are worried about.

  18. Professor, how familiar are you with the work on activity levels? I’ve read the linked study and searched for more. There isn’t much data. Much of it is unreliable: small studies, short time periods, data taken out of the original study context. For example, the activity study that says levels have not decreased had to search literature and only found for North America 393 subjects across 13 studies (which had been designed to test other things). The authors note they had a lot of problems with the data. BTW, their other main data was a short study done in one Dutch town. That’s not sufficient work to base conclusions on. It’s a nicely done paper, but it’s one minor brick in a large building.

    Another example is pedometer studies. If you look into them, you see they tend to cover few people for short periods. And there are only a few of them. The numbers have been widely quoted but there is little foundation in them.

    Another point, the study I talk about above, I didn’t feel like trying to take apart their analysis to see if they did what I expected: figure total energy expenditure including BMI factors. I mean, bluntly, they discuss how being fat increases energy output just to move around a bit and so I expect they include that factor. Is that sensible when talking about how rates of “activity” have changed? If you weigh 400 pounds, you may move a very little but that expends a lot of energy so your “activity” level hasn’t gone down … except you weigh 400 pounds and don’t move around much.

    Your basic premise in this post is that obesity is a supply function. There is obvious truth in that: without supply of calories, there can’t be fat. The implication you draw is the demand function generates extra fat, which is also true, but may be misleading. Why? Because activity has not only failed to adjust up to account for additional calories but it seems, even if we simply accept the limited work done on activity levels, that “activity” is measured in ways that don’t reflect actual meanings of activity but instead reflect how much extra energy it takes to move a fatter person through daily life. Have you really thought through the demand side of this function?

  19. What if there is a “tipping point” for lack of physical activity? Just as physicians advise us that even 20 minutes of walking or other light to moderate per day is sufficient for cardiovascular health and for substantial longevity benefits, perhaps reducing physical activity beyond some threshold or inflection point leads to outsized detriment in terms of body weight and health.

    Clearly, the increase in caloric intake has a lot (and maybe everything) to do with rising obesity. But I would not discount inactivity as being a major factor without more evidence than looking at graphs for direct correlation.

  20. If you want to know the true source of obesity focus on corn which is found in everything. That one ingredient–high fructose corn syrup–is the culprit. Look at the list of ingredients on any cereal box, guaranteed you will find it there. Professor, focus on govt subsidization of the corn industry plus the induction of this product into every day ingredients, and the correlation is apparent.

  21. Obesity is really one of the health problems which plaque our society these days, It is easily treatable by being more physical and of course by having a healthier diet. .’*:.

  22. I agree with high fructose corn syrup is likely to be a key factor in the rising rates of obesity. It’s a relatively cheap additive that appears to keep food “fresh” on store shelves for a really long time.

    Why not put a tax on high-fructose corn syrup to limit its prevalence in food?

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