The Fat Gene
What role does genetic heritability play in body composition?
In his book, The Agile Gene: How Nature Turns on Nurture, Matt Ridley explains the falsehood of the nature versus nurture debate:
"...It is genes that allow the human mind to learn, to remember, to imitate, to imprint, to absorb culture, and to express instincts. Genes are not puppet masters or blueprints. Nor are they just the carriers of heredity. They are active during life; they switch each other off and on; they respond to the environment. They may direct the construction of the body and brain in the womb, but then they set about dismantling and rebuilding what they have made almost at once - in response to experience. They are both cause and consequence of our actions. Somehow the adherents of the "nurture" side of the argument have scared themselves silly at the power and inevitability of genes and missed the greatest lesson of all: The genes are on our side."
Ridley discusses recent developments in genetics and our understanding of the role that genes play in our lives. As he alluded to in the above paragraph, genes aren't a fixed archaic blueprint. They are responsive to our environment and partially affect how we direct our experiences in those environments. It's a dance between nurture and nature and they both play off of each other.
Very little of Ridley's book discusses body composition. He much more commonly delves into how nature and nurture affect intelligence and personality, but there are remarkable similarities which he occasionally highlights.
Bodyweight Heritability: Differences aren't necessarily causes
"In this respect, personality is about as heritable as body weight. The correlation between two siblings in weight, according to one study, is 34 percent. The similarity between parents and children is a little lower, 24 percent. How much of this similarity is due to the fact that they live together and eat similar food, and how much to the fact that they share the same genes? Well, identical twins reared in the same families have a correlation of 80 percent while fraternal twins reared together have only 43 percent similarity, which suggests that genes matter rather more than shared eating habits. What about adoptees? The correlation between adoptees and their adoptive parents is only 4 percent, and that between unrelated siblings in the same family is just 1 percent. By contrast, identical twins reared apart are still 72 percent similar in weight.
Conclusion: Weight is largely due to genes, not eating habits, so throw away the diet advice and let rip with the ice cream? Of course not. The study says nothing about the causes of weight, it only reveals something about the causes of difference in weight within a particular family. Given the same access to food, some people will put on more weight than others. People are getting fatter in western societies, not because their genes are changing but because they are eating more and getting less exercise. But when everyone has similar access to food, the ones who put on weight fastest will be the ones with certain genes. So, variation in weight can be inherited, even while environment raises the average."
Stay with me here and try not to get hung up on the oversimplification of "eating more and getting less exercise."
At this point it's clear that body composition (I hate using the generic term "weight") is affected by genetic factors as well as environment. Not a huge surprise there. But what exactly are those genetic factors and how are they affecting us?
It's hard to say. Studies done on twins and siblings make it difficult to trace down root causes of genetic influence. They show us that genes do stuff, but not which ones, and quite often we're not even sure what exactly they're doing because we only see specific measurable effects.
"Heritability is usually highest for those features of human nature caused by many genes rather than by the action of single genes. And the more genes are involved, the more heritability is actually caused by the side effects of genes than the direct effect."
Early environment, like oxygen, only matters much when it's inadequate.
In many cases, environment becomes an issue only when it is significantly altered from the norm. A comparison is made to vitamin C. It has noticeable negative consequences if you don't have enough, but once your levels are adequate taking extra doesn't do much. This is part of the reason genes play such a significant role in many studies on siblings. The environment is already normalized.
"Recall the heritability of weight. In a western society, with ample access to food, those who put on weight faster will be the ones with genes that nudge them into eating more. But in a desolate part of the Sudan, say, or Burma, where extreme poverty is rife and famine just around the corner for many people, everybody is hungry and the fat people are probably the rich ones. Here variation in weight is caused by the environment, not the genes. In the jargon of the scientist, the effect of the environment is non-linear; at the extremes, it has drastic effects. But in the moderate middle, a small change in environment has a negligible effect."
Again, ignore the over-simplified quantitative focus on the eating part. Feel free to change "eating more" to "eating more processed artificial food made from corn syrup, flour and soybeans" if it helps.
Genetic Influences Increase with Age and Self-Directed Environmental Exposure
An interesting fact coming from this discussion is that genetic influence increases with age while shared environmental factors decrease. Adoptive siblings have predictably similar IQ's as children, but as they age the correlation disappears. Likewise, identical twins raised separately will have IQ's more comparable to their adopted families than to each other, but as they progress into adulthood they become increasingly more identical as similarity to their adopted family drops away. Ridley doesn't mention if this applies to body comp as well, but it seems likely.
Back to the fat genes. How is it that a complex variety of genes can influence body composition through their combined "side effects?" What are those side effects? Surprisingly, a major factor may be in how genes nudge us into different environments and experiences.
Just as much as genes may affect things like insulin sensitivity (and environment factors such as parental eating habits during gestation can affect those genes) they also influence behaviors.
Genes: Agents of Nurture
"The "environment" is not some real, inflexible thing, it is a unique set of influences actively chosen by the actor himself or herself. Having a certain set of genes predisposes a person to experience a certain environment. Having "athletic" genes makes you want to practice a sport; having "intellectual" genes makes you seek out intellectual activities. The genes are agents of nurture.
Appetite versus Aptitude
As a parallel, how do genes affect weight? Presumably through controlling appetite... Is it the gene or the ice cream that that causes fatness? Well, it is obviously both. The genes are causing the individual to go out and expose himself to an environmental factor, in this case ice cream. Surely it is bound to be the case with intelligence. The genes are likely to be affecting appetite more than aptitude. They do not make you intelligent, they make you enjoy learning. Because you enjoy it, you spend more time doing it and you grow to be more clever. Nature can only act via nurture. It can act only by nudging people to seek out the environmental influences that will satisfy their appetites. The environment acts as a multiplier of small genetic differences, pushing athletic children toward the sports that reward them and pushing bright children toward the books that reward them.
Reading this makes me think of my own childhood. As a kid, I loved reading and learning. My older brother brought his schoolwork home with him each day and taught it to me. As a result, I knew how to read before I began pre-school. I remember riding in the car, excitedly reading street signs to my parents, thrilled at cracking the mystery of what those combinations of letters revealed.
My parents bought me children's encyclopedias and I would actually sit in my room reading them like story books. Within a few years I was pulling my parents college books off the shelves and attempting (briefly and unsuccessfully) to teach myself French or Spanish.
It wasn't that I was forced into it; I sought the activity out because I found it inherently rewarding.
Likewise, I loathed organized sports but loved being outside and playing and being physically active. I eventually found my way into the weight room at school. There were no playbooks to memorize, costumes to wear or people with whistles yelling at me. I could structure whatever I did in the gym to suit my own needs and began learning by trial and (lots of) error.
My interests revolved around sports with no strict organization and free form expression, like snowboarding. I trained daily in the weight room, even after football practice. Most people thought that I did this because I wanted the strength to transfer into football or some other organized school sport. I encouraged this misperception because it increased the chances of my getting away with working out as often as I pleased (such as when I should have been in algebra class) but the truth was that I couldn't have cared less about school sports and was interested in lifting weights as an end unto itself.
Nature Turns On Nurture
As a kid, I was physically weak and remarkably skinny. I remember starting my working sets on the bench press with an empty bar because adding ten pounds to either side would have been a one rep max. As an eighth grader, I weighed less than everyone on my football team, including the seventh graders.
I've since spent six years in military special operations, can deadlift just over 500 pounds, have single digit bodyfat and the physical freedom to surf, climb, run, swim or do just about any physical activity I can come up with.
I didn't have "athletic genes" but I had genes that made me want to pursue learning about and practicing strength training and a pretty good variety of adventure sports. I was genetically prone to seek environments and experiences that led me to become athletic. Nature drove nurture, and nurture altered nature.
Since you're reading this and have made it to the end of an article a good deal longer than the typical internet attention span, we probably have something in common in this sense.
Regardless of the physical traits you've drawn in the genetic lottery, your inherent predispositions are leading you to seek information like what's contained in this blog and apply it to your world, creating your own self-directed environmental experiences. If you can find reward and meaning in this process, you have everything it takes to bring your body and physical life to the place you desire.