Yesterday, an email landed in my Shapely Prose inbox with the subject line “Question (not a flame or troll I promise!)” These are almost inevitably flames or trolls, so it was with a certain amount of eye-rolling that I opened the message. As it turns out, though, reader Kathryn was not in fact trolling, and had a serious and astute question with an important answer, one that I’d just been thinking should be made explicit on the blog.
Kathryn’s question was about why our weight as a population has changed, if weight is largely genetic. A caveat to my answer: I am not a scientist. My training is in the history and philosophy of science. I understand genetics, I am intimately familiar with Mendel’s life and rediscovery, I’ve had tea with Jim Watson, and I’ve done gel electrophoresis — but I’m not a scientist, I am a writer. Furthermore, this is an informal email to a reader. So I don’t have citations here, and even if I added them, they’d be dredged up through Google-fu and not through encyclopedic knowledge of the literature. I know some of our readers, though, are doctors and biologists and geneticists, and I would love to hear your responses in comments. (ETA: Here’s a related blog post by the inimitable MissPrism, who is a scientist!)
Kathryn wrote (reprinted with permission):
What is hard for me to understand is the increase in human weight in certain parts of the world. I read so many blogs that purport genetic predisposition to weight. That is all well and good but how has genetics changed so drastically in the past century? I read one argument that compared the increase in weight to the increase in human height.
Auxology studies have shown that human height is not entirely genetic. It has much to do with environment. For example, impoverished North Koreans, on average, are shorter than their South Korean counterparts. Europeans’ average height declined up until the Industrial Revolution, during which general health practices began to improve.
Nutrition, auxology experts say, is a very important factor in population height.
That said, why should population weight be any different? Our environments have changed drastically and, as a result, our average population weight has increased.
Weight increases on the Western world’s scale have not been seen in populations who have not experienced the sudden surplus of fast food, overprocessing, and synthetic ingredients.
Please, if you can explain this to me I would be grateful. I just can’t seem to wrap my head around it.
Nutrition has directly affected height. How could it not be the reason for our recent increase in weight and the present “obesity epidemic”?
I responded (edited slightly, because indulging l’esprit d’escalier is my prerogative as the blogger):
Kathryn, if you’ve read The Triple Helix, you know that environment affects gene expression and genetics affect reaction to environment. Weight predisposition is largely genetic (about 77%, last I heard), while expression is affected by environmental factors — but only within a certain range. Just as a dwarf pea plant with great nutrition won’t rival the height of the tallest pea plants, so environment affects our weights and heights within the range set by our genetics. Both better nutrition and worse nutrition (i.e. more processed and artificial foods) have been available recently, which has affected both height and weight on a population level.
Remember, though, that the sharp increase in “overweight and obesity” far outweighs the actual increase in weight, because of the rewriting of BMI standards in 1998. The main genuine increase in weight, the increase not explained by accounting methods, has been at the highest BMI levels — that is, the very fat have gotten fatter, while the population average has shifted upwards a bit but hasn’t changed drastically. This is consistent with the idea that your genetic predisposition sets your base level and your tendency to gain weight. Most modern people eat modern food, but those who are predisposed to start fat and who have parsimonious metabolisms are going to get fatter off the same diet.
So yes, certainly there’s an environmental component; certainly some people who are predisposed to be larger than average would be less larger than average if they were able to avoid, as you succinctly put it, “fast food, overprocessing, and synthetic ingredients” (which, remember, thin people consume too, because they are very difficult to avoid, and even more difficult at lower socioeconomic levels). If we were able to look beyond obesity hysteria and increase people of all sizes’ access to nutritious unprocessed food and active leisure activities, we might end up with a population that’s a bit less fat, in addition to being a lot more healthy. (I know which one I think is more important; official voices tend to disagree.) But that’s a far cry from saying that everyone can potentially be equally skinny, don’t you think?
Thanks, Kathryn, for the opportunity to clarify — and for writing an email that purports not to be a flame or troll that actually wasn’t a flame or troll.