In 1990, Dr. Claude Bouchard and colleagues published a simple but fascinating study demonstrating the importance of genetics in body fatness (1). They took advantage of one of the most useful tools in human genetics: identical twins. This is what happens when a single fertilized egg generates two embryos in utero and two genetically identical humans are born from the same womb. By comparing identical twins to other people who are not genetically identical (e.g., non-identical twins), we can quantify the impact of genes vs. environment on individual characteristics (2).
Dr. Bouchard and colleagues recruited 12 pairs of male identical twins, determined how many calories each person needed to eat to maintain weight, then overfed each person by 1,000 kilocalories per day for 84 days. Although every participant gained weight and fat, "individual changes in body composition and topography of fat deposition varied considerably." Weight gain ranged from 9 pounds (4.3 kg) to 29 pounds (13 kg), despite the fact that all subjects were eating the same calorie excess! This is a really striking result if you think about it. Even after minimizing variability in half of the energy balance equation (energy in), there remained large differences in the amount of weight and fat people gained. Some were able to effortlessly "burn off" most of the excess calories.
However, weight gain within pairs of twins was much more similar than weight gain between pairs of twins. In fact, even the distribution of the gain was far more similar within pairs than between them. If one twin gained 15 lbs around the midsection, the other usually gained about the same amount of weight, and in the same place.
This suggests that a person's genetic makeup determines how well his body naturally 'resists' fat gain during overeating, and also where his body tends to store fat. Some people are genetically predisposed to gaining fat, and others are genetically predisposed to gaining fat in unhealthy places.
Dr. John de Castro and colleagues have taken this research even further. In a large and ambitious study (resulting in a number of papers; here are a few: 3, 4, 5, 6), his group recorded the dietary habits of identical and non-identical twins to quantify the genetic contribution to a range of eating-related variables. Here's a little snippet from one abstract that drives home the main point (3):
Significant heritabilities were also found for the amount of food energy ingested daily, as well as its macronutrient, alcohol, and water content. Linear structural modeling analysis showed that 65% of the variance in energy intake was attributable to heredity. In addition, the pattern of intake, meal frequency, size, and meal macronutrient, alcohol, and water composition, were also found to have strong heritabilities.
In other words, nearly every major aspect of food behavior is strongly impacted by genes*. Other papers from the same study showed that genes have a strong influence on the amount of food it takes to feel full, how likely it is that the sensation of fullness will terminate a meal, the perception of palatability, how much influence palatability has on calorie intake, cognitive dietary restraint, and almost every other measurable characteristic.
This is just a small taste of the literature that demonstrates convincingly that genetics plays a major role in obesity. Twin studies (and similar designs) overall have found that genes explain about 70 percent of the differences in body fatness between people (8). This suggests that when it comes to obesity, it's important to pick the right parents.
What Can We Learn From Obesity Genetics?
Genetics has a lot to teach us about obesity. First, as described above, genetic makeup is a major influence on body fatness and virtually all of its determinants. Your genes determine, to a large extent, how likely you are to become obese, or how hard you'll have to work to remain lean if you're genetically predisposed to obesity. This is one of the reasons why it's unfair to judge obese people for their fatness. Of course behavior does matter, but a) most obese people were dealt a more challenging hand than most lean people, and b) behavior is also strongly heritable, so even that isn't freely chosen.
Beyond the moral/philosophical implications of this research, there's also a strong scientific reason to care about the genetics of obesity: it points to the biological processes that determine body fatness, and it can do so in a relatively unbiased fashion. Genetics is an important tool for understanding how body fatness is regulated, including in humans. I'll discuss this in the next post. This brings us to the third reason for caring about obesity genetics: if we know what biological processes are involved, we can attempt to target them to prevent or reverse obesity, and perhaps individualize treatments. This could involve diet, lifestyle, surgery, and/or drug therapy.
* This statement will sound funny to geneticists because everything we do is strongly impacted by genes. Genes are required for having a body, a brain, and expressing behaviors at all. I've simplified the language for a general audience, but really what we're talking about is the extent to which genetic variability within our species can account for variability in individual characteristics within our species. A more accurate way of saying the sentence I've asterisked would be "individual variability in nearly every major aspect of food behavior is strongly accounted for by genetic variability."