My previous post talked about how it is possible for differences between groups to have one cause, and differences within those groups to have a different cause–that disproving changing genes as a cause of the ‘obesity epidemic’ does not prove that who gets fat and who doesn’t in our environment is due to, say, what your parents fed you as a child more than your genes. (There’s also a pretty good explanation of heritability and BMI here.) But is there positive evidence that it is not only possible, but that is indeed what’s happening–that genes play a fairly large role in who gets fat and who doesn’t in an “obesity epidemic”? Yes.
There’s a reason I’m being very careful to say “caused by changing genes” and not “caused by genes” as a great deal of popular writing does: whether and how much we gain weight given a change in environment is influenced by our genes, at both an individual and macro level. Just as the increase in height in response to more/better nutrition was part of our genetic potential, the population-wide increase in weight was possible because of our genetic potential. Both a general human tendency towards “thrifty genes”–genes that were influenced by a history of food shortages to store lots of fat in case of future food shortages–and differential “thriftiness” in individuals can lead to people putting on weight in good times in anticipation of bad times. And a tendency to put on more weight than before in good times after experiencing lean times–or a diet, since the intellectual part of our brains isn’t really talking to the homeostasis-keep-us-from-starving part of our brains–is also part of our genetic potential.
We can probably rule out “what your parents fed you as a child” as a reason why some people get fat and some don’t: adopted children’s BMIs are influenced a great deal by their biological parents’, with no significant influence from their adoptive parents’ BMIs. A study by Dr. Albert Stunkard of adult twins, some of whom had been adopted, found that the twins’ BMIs were very similar, and just as close together whether they were raised together or apart.
A deliberately-engineered change in environment can also show just how strong the genetic influence on weight is. A study by Dr. Claude Bouchard of 12 pairs of identical male twins found that within each pair, the two twins were highly similar to each other in amount and location of weight gain when Bouchard deliberately overfed the men by an extra 1000 calories a day–but there was a great deal of variation between different twin pairs. The pair gaining the most added about 29 lbs, while the pair gaining the least added about 9.5 lbs. (Most of the twin pairs returned to their original weight when taken out of the artificial-overfeeding environment.)
If it were just these three studies, then maybe things wouldn’t be so clear-cut. These were three of the ones that I remembered reading about in the past and/or that Traci Mann refers to in Secrets From The Eating Lab. But the title of this editorial from 2008 perhaps sums it up best: Obesity–still highly heritable after all these years.
(None of these measures of genetic effects on BMI are based on finding specific genes that influence BMI. Adoptee and twin studies are based on how much related or genetically identical individuals resemble each other, not on finding particular genes.)
Do any of these studies mean that significantly changing one’s BMI is completely impossible for everyone? No, but they do mean that for most people, the reason they weigh less or more than the average person in their environment is not because of some great effort or virtue on their part (or lack thereof), or because their parents gave them the exact right foods in the exact right ways, but because of their genes. And, sure, because of some of the new trendy research topics like gut bacteria, too. Some people will, through extraordinary effort, life circumstances, etc. be able to defeat their gene/environment combo, but most people who think they are thin through hard work would probably only be slightly fatter without their efforts. In other words, most thin people bragging about how they are thin through their own effort are the proverbial “born on third, think they hit a triple”. There was an editorial that I’ve never been able to find again, but it had a metaphor that really struck me: people would ask the writer, Isn’t it obvious that if you took two people and gave one free access to a constantly stocked fridge and gave the other carefully rationed meals, one would be fatter? The writer’s response: sure, but the difference in environment between people’s homes in obesity-epidemic countries is typically more like giving one person one constantly-stocked fridge and the other two constantly-stocked fridges.