Nudge the fat; satiety & the implicit mind
Megan McArdle has has been talking about the high heritability of BMI again. I have expressed concern about her putting the high heritability numbers out there when it comes to its relevance for public policy, though I do tend to agree with her general stance that glib assertions about the importance of will-power are probably non-starters. And, rather than point to arguments such as “I have a slow metabolism,” it is probably more critical emphasize the complexity of the chain of events and framing of how we make decisions, much of which occurs “under the hood” and outside the purview of conscious explicit control. Interestingly, the reality that choice is highly conditioned by details of our environment combined with innate predispositions, and proximately is driven by many implicit factors, has pushed me in a less libertarian direction.
In any case, the whole discussion got me interested in the topic of obesity & heritability, and I found this review, Human Obesity: A Heritable Neurobehavioral Disorder That Is Highly Sensitive to Environmental Conditions. You can read the full text, it’s Open Access now, but this part caught my attention:
…He hypothesizes that random natural variation in “hypothalamic energy balance set points” has occurred over millions of years of primate evolution. Whereas variants that would tend to produce a state of low energy stores would have been systematically selected against, at least in part because of their adverse impact of reproductive success, upward drifts in such set points would have been allowed to persist (rather than being positively selected for, as the “thrifty gene†hypothesis would have it). This upward drift would be particularly prominent because the formation of organized social groups and the discovery of fire, both of which occurred around 2,000,000 years ago, made our ancestors less susceptible to predation. Not particularly emphasized by Speakman, but likely to be important, is the probability that such natural tendencies toward an upward drift in adipose stores may rarely have actually manifested themselves as obesity because of the high energy cost of obtaining food during most of human evolution. It is only in the past 50 years or so, when for the first time in human history the majority of people in the developed and developing world can readily access sufficient daily calories to exceed the calories expended in acquiring them, that those with intrinsically higher set points have manifested their “obesity potential” on a grand scale. Unlike the “thrifty gene†hypothesis, this scenario provides a credible explanation for the fact that even in places where obesity is very common, a substantial proportion of the population remains lean.
This is an old hobby horse of mine: if you see a quantitative trait which can be conceived of as normally distributed with a high degree of heritability, such as body mass index, then its fitness implication can’t have been too stark. In other words, if a very heritable trait still has a great deal of extant genetic variation, then it is either in transient, or, more likely the fitness implication of any particular trait value was low or there is balancing dynamics preserving the variance. Like IQ, body weight has been increasing over the past century. Many people think that they know the reason why this is occurring. If the reasons are ever established to a high degree of certitude, is it possible to reverse the slouch toward obesity without coercion?
Labels: Genetics, Origin of obesity
It is only in the past 50 years or so, when for the first time in human history the majority of people in the developed and developing world can readily access sufficient daily calories to exceed the calories expended in acquiring them, that those with intrinsically higher set points have manifested their “obesity potential” on a grand scale.
Obesity occurs even among people who are living on a pitiful amount of calories. The reason is that most of those calories are coming from empty carbs, usually government rations (sugar, bread / flour, cereal, rice, or potatoes). Taubes recounts all of the examples in Good Calories, Bad Calories, as well as in his lectures.
And in fact, the obesity epidemic began in the US in the late 1970s or early 1980s, according to NHANES data — not “in the last 50 years” — and we were pretty much the trend-setter there. We had access to plenty of calories before then, as did the rest of the first world, yet there was no skyrocketing obesity rate. Americans in the ’50s and ’60s were damn well-fed but didn’t look like walruses.
The switch in the late ’70s was to a diet lower in fat and therefore higher in carbs. USDA data on availability and consumption confirm that. Carbs in the diet raise glucose in the blood long-term, which raises insulin long-term. Insulin causes you to store fat rather than burn it, so a high-carb diet causes the mean of the BMI distribution to move up.
is it possible to reverse the slouch toward obesity without coercion?
Sure, just have the government and other health experts tell the public that fat is good (aside from trans fats or most vegetable fats), high levels of carbs are bad, and you can go back to eating the food that you like and that tastes good, except for sweets.
Let’s see, bread by itself or pastrami by itself… rice by itself or gruyere cheese by itself… it’s a no-brainer. High-carb food, unless it has lots of sugar, only tastes good if you add a lot of fat to it, so most people won’t miss it.
I rolled out heritability in response to the notion that this is a matter of culture. It’s clearly a matter of environment, but given the adoptive studies, I don’t see much evidence that it can be a matter of who your friends are. It seems to me that they’re mostly picking up some selection effect.
“The switch in the late ’70s was to a diet lower in fat and therefore higher in carbs.” That was a switch caused, I assume, by the preaching of government, the media and the medical profession. Well done, boys.
Mcardle shouldn’t use the word “heritable” if she doesn’t undertand what it means. I never use it.
i do think it is useful to put the high heritability number out there. but, it needs to be explained in a little more detail lest the public assume that “80% heritable = 80% genetic.” if, for example, assman is right then the obesity epidemic is reversible without too much onerous switch of lifestyles.
I generally agree with Taube that the whole “low fat” crusade was based on opinion, not scientific evidence, but I dont think its possible to blame everything on carbs, or at least, not on some kind of national switch to eating carbs. First of all, the actual decrease in fat intake is not that great. Most obese people are still eating a good deal of fat. The kind of low fat diet recommended by low fat fanatics is rarely followed by the general population. Some other factors may also have played a role, like the increased consumption of soft drinks sweetened with high fructose corn syrup. But the bottom line is, we really dont understand why well fed americans in the last 20 years have become fatter than well fed americans of the fifties or sixties. It could be more carbs, it could be more fructose, it could be more screen-time, it could even be the delayed effect of being a generation whose PARENTS and grandparents were well fed…and its probably all of the above. The so-called “epidemic” may also be plateauing, just as the obesity prevention industry gets into high gear and starts collecting new taxes and imposing new restrictions on mostly healthy people who happen to be modestly overweight….and does so with scant regard for evidence of effectiveness or causation.
agnostic: the late 1970s or early 1980s, according to NHANES data — not “in the last 50 years”
The late ’70s was 30 years ago.
I agree that the pro-carbs/anti-fat push has no real scientific justification, but I question the asserted linkage between orthodox dietary preaching and the increase in obesity.
For one thing, how many of those who became obese were even aware of the official diet gospel? How many actually followed it seriously? There was some substantial effect on diets, obviously, as shown by the consumption of “lite” and “low-fat” food products. But again, how many of the obese were actually following the recommended diet?
ISTM that the increase in obesity correlates with the rise in incomes, making excess amounts of food available cheap. Rising incomes also allowed more people to afford convenience foods and restaurant meals – especially the working classes and the semi-poor. It’s easier to eat a lot when one doesn’t have to cook it.
(It would be very interesting to compare obesity levels with the home-cooked proportion of diet – and not only home cooked but scratch-cooked.)
I know that the 70s were 30 years ago, so clearly I did not mean what you imputed. Obviously I meant that the authors were obfuscating by saying “last 50 years,” as though the cause might go back that far or earlier, when it was something much more recent.
dscience.blogspot.com/2009/06/did-following-experts-diet-advice-make.html
dscience.blogspot.com/2009/07/more-data-on-changing-consumption.html
There is data that we followed the experts’ advice:
http://lowcarbartan
http://lowcarbartan
The American Heart Association has revised its guidelines about sugars (e.g. sucrose and HFCS) added to the diet.
Added sweeteners — sugars that aren’t naturally part of the food we eat — shouldn’t account for more than 100 calories a day for women or 150 calories for men, the AHA said in a scientific statement…
Americans’ current intake averages 22 teaspoons, or 355 calories, per day — largely from soft drinks and other sweetened beverages — according to the 2001?2004 NHANES. A single can of cola contains about eight teaspoons of sugar.
[snip]
From a practical standpoint, outside nutrition experts noted that turning the AHA recommendations into action by consumers may not be easy. And some said the AHA concentrated too much on added sugar instead of total sugar and total carbohydrate intake…
“Also, it is important to keep in mind that refined starches like white bread, white rice, and potatoes have adverse metabolic effects similar to sugar because they are rapidly converted to glucose in the body.”
And sugar consumption has been on the rise, up 19% (about 76 calories per day) since 1970, according to one USDA report.