Ancient genes, modern meals: poking holes in the thrifty gene hypothesis.
The assumption is that America has a weight problem because human metabolism runs on ancient genes that are ill equipped for contemporary eating habits. In this line of thinking, a diet true to the hunter-gatherers we once were--heavy on protein, light on carbs--will make us skinny again. While the fad has attracted skepticism from those who don't buy the idea whole hog, there's still plenty of acceptance for one common premise about the evolution of obesity: Our bodies want to stockpile fat.
For most of human history, the theory goes, hunter-gatherers ate heartily when they managed to slay a fleeing mastodon. Otherwise, prehistoric life meant prolonged stretches of near starvation, surviving only on inner reserves of adipose. Today, modern humans mostly hunt and gather at the drive-thru, but our Pleistocene genes haven't stopped fretting over the coming famine.
The idea that evolution favored calorie-hoarding genes has long shaped popular and scientific thinking. Called the "thrifty gene" hypothesis, it has arguably been the dominant theory for evolutionary origins of obesity, and by extension diabetes. (Insulin resistance and diabetes so commonly accompany obesity that doctors have coined the term "diabesity.") However, it's not that difficult to find scientists who call the rise of the thrifty gene theory a feat of enthusiasm over evidence. Greg Gibson, director of the Center for Integrative Genomics at Georgia Tech in Atlanta, calls the data "somewhere between scant and nonexistent--a great example of crowd mentality in science."
Support for the thrifty gene theory maybe eroding in scientific circles, even while it's still going strong on the Internet. One analysis published in February in the American Journal of Human Genetics found no consistent association between 65 variations in possible thrifty genes and survival. Another study published in January in Biology Letters concluded that prehistoric hunter-gatherers actually ate more often, not less, than later societies that grew their own food. In describing the results, anthropologists from the University of Roehampton and University of Cambridge wrote that their finding challenges popular assumptions about the evolution of diet and today's epidemic of obesity and diabetes.
It's not that obesity has nothing to do with genetics, Gibson says. Of the 21,000 or so genes that make up the human genome, he estimates that perhaps hundreds influence body weight. Where the genetics get controversial, he says, is with the assumption that the overriding influence is a throwback to starvation--leaving humans at the mercy of genes that encourage overeating and the rapid accumulation of fat.
Instead, thrifty genes, if they exist, are just part of a complex genetic picture that contributes to the obesity epidemic, says Hertzel Gerstein, director of endocrinology and metabolism at McMaster University in Hamilton, Canada. The interaction between any one person's predisposition and the calorie-dense Western smorgasbord is still not well understood. "People are looking for an explanation," he says. "The thrifty gene hypothesis might be a piece of an explanation. However, if you accept it too uncritically, you close your mind and thinking to possibly better explanations."
And there are plenty of lesser-known theories that address the origins of obesity, including those rooted in the complex nature of evolution, the migration patterns of early humans and changes in gene function that don't involve mutations. Some explanations even stand out for their creativity, such as one from a team of researchers in India who propose that obesity is a consequence of declining human aggression.
Survival of the fattest
For all its recent fame, the thrifty gene hypothesis isn't new. Geneticist James Neel of the University of Michigan Medical School proposed it in 1962 in the American Journal of Human Genetics. "It must be remembered," he wrote, "that during the first 99 percent or more of man's life on Earth, while he existed as a hunter and gatherer, it was often feast or famine." The human who gorged and then held an extra pound or two in reserve when food was scarce was better able to survive. Thus, he concluded, the development of insulin resistance (a propensity for diabetes) conferred some physiological advantage that continues to exert itself.
In the five decades since its debut, the theory has "gone off in all sorts of directions," says Andrew Prentice, who studies international nutrition at the London School of Hygiene & Tropical Medicine. Prentice supports the general concept Neel proposed--that the genetic influences on body weight are the product of natural selection from lean times--but not in the way people commonly interpret it. For one, he doesn't think the advantage of fatness had much to do with mortality.
His research, particularly focused on women in Gambia, in West Africa, suggests that food shortage affects fertility, and that women with the highest body weight have greater reproductive success. (Alternatively, women who become dangerously thin cease to ovulate.) Once human societies became agricultural, they went through periods of both blight and plenty--much like the population Prentice studies today. He's found that plumpness is an advantage not because thinner members of a population are more likely to die, but because they are less likely to bear children and pass their genes to the next generation.
One of the most vocal critics of the thrifty gene hypothesis is John Speakman, who heads the Energetics Research Group at the University of Aberdeen in Scotland. If being fat offered a benefit over the course of human evolution, Speakman reasons, then even more of the population would be obese, in the same way humans have universally developed large brains and upright postures. With the thrifty gene idea and its different incarnations, "the common thread is that some time in our evolutionary history it was advantageous to be fat," he says. If this were true, and genes for obesity conferred a survival benefit, "the real problem is to explain why so few people get fat," he says. "Even in America, 60 to 70 percent of people are not obese. How come so many of us didn't inherit thrifty genes?"
Speakman became disenchanted with the thrifty gene hypothesis about a decade ago, deciding that it was based on simplistic assumptions about evolutionary dynamics. He set out to test it. Among other studies, in 2013 in Disease Models & Mechanisms, he published a mathematical analysis gauging how 32 known gene variations associated with body mass index would help people survive near starvation--and found that they made only a tiny difference.
The basic problem with the thrifty gene idea, he says, is that "it's based on a naive view of how evolution works." Natural selection, though powerful, isn't the sole architect of human DNA. Nature favors genes that help survival, but genes that are simply along for the ride will also be handed down through generations. In early human history, Speakman says, being overweight was a disadvantage. When a species is subject to predators, the slower, fatter members who are prone to overheating are the unlucky ones who will be picked off first. As humans developed fire and weapons, and grew less vulnerable to being hunted, he argues, heavier individuals survived--not because being fat was good, but because upper limits on body weight didn't matter as much any more.
He calls this idea the "drifty gene" hypothesis, a name he first proposed in 2008 in the International Journal of Obesity to reflect that obesity might not have been actively selected for, just passively allowed to float into the human genome. "If you have a mutation that happens but doesn't create a selective advantage, its influence is not going to be strong, but it's not going to be actively removed either," Speakman says.
Nifty, not thrifty
Speakman's isn't the only alternative to the idea that obesity is a holdover from ancient times of starvation. Elizabeth Genne-Bacon, a graduate student in genetics at Yale University, recently compiled evidence for the thrifty gene hypothesis along with other theories in a paper in the Yale Journal of Biology and Medicine.
"I see textbooks with [the thrifty gene hypothesis] all the time," she says. "It's elegant and it makes sense. It's easy to understand." However, she found the support to be surprisingly thin, writing that, "Obesity researchers are often not aware that there is, in fact, limited evidence to support the thrifty gene hypothesis."
For example, if thrifty genes were influencing metabolism, populations that most experienced famine should be particularly prone to diabetes. She points out that Europe has a lengthy history of war, disease and frequent food shortages, but European descendants have a lower incidence of diabetes than the indigenous people of the Americas and Pacific Islands who may have had less food turmoil in their distant past.
In her article, she lists other theories that might explain the evolution of obesity and diabetes:
The thrifty phenotype theory: This one exists in several forms. In a version proposed in 1992, researchers from University of Cambridge and Southampton General Hospital said the "thrifty phenotype" offers a survival advantage related primarily to obesity. Poor nutrition in the womb encourages the development of diabetes when food is abundant in adulthood even if it doesn't cause direct changes to DNA, according to the theory. The reason, as they later explained in 2001 in the British Medical Bulletin, is that undernourishment in early development produces changes that include both reduced insulin secretion and insulin resistance--hallmarks of diabetes.
Genetically unknown foods: This theory rests on the observation that obesity and diabetes occur rarely in native populations, such as those who live on the Pacific Island of Nauru or the Pima Indians of Arizona--until they develop a taste for the Western diet. First published in the late 1990s by Riccardo Baschetti, a physician in Padua, Italy, the theory proposes that humans evolved to eat foods with the natural sweetness of fruit (not Froot Loops), leaving bodies that are not able to cope with sugar and fat at every checkout stand.
Aggression control: A full male is a docile male, researchers from University of Pune, India and colleagues wrote in 2010 in Medical Hypotheses. As humans relied less on fighting to survive, a propensity for obesity emerged. (When lethargy won't get you killed, overindulgence isn't so bad.)
Climate adaptations: One of the latest hypotheses appeared in the journal Endocrinology in May. A team of researchers from the United Kingdom noted that diabetes and obesity are unevenly distributed among populations throughout the world; the highest risk for both diseases occurs among people adapted for warmer temperatures (such as native populations in the tropics or Central and South America). As humans migrated out of Africa beginning around 70,000 years ago, they settled in varied climates. Survival in colder parts of the world amplified genes that help preserve body temperature--a higher metabolic rate that keeps the body warm would confer some resistance to obesity. Genes adapted for warmer climates would lower metabolic rate, burn calories at a slower pace and make the body more inclined to accumulate fat.
Evolution through thick and thin
None of the theories could be right, or several could, says Jeffrey Friedman, the Rockefeller University geneticist best known for discovering leptin, the hormone that helps regulate appetite. When he announced the finding in 1994, many researchers thought Friedman had found a link to a long-sought thrifty gene because of leptin's direct relationship to appetite. Friedman himself went looking into the origins of obesity by conducting genetic studies on the island of Kosrae in Micronesia, where the native population was reduced to only 300 in the late 1880s, following a typhoon and an influx of communicable diseases from the West. Such a small, confined population creates a genetic bottleneck that makes isolating specific genes easier. As the island's eating patterns fell under Western influence, diabetes and obesity emerged as serious health threats.
So far, though, he hasn't found the answers he would like, including insight into evolutionary influences on diabetes. One of the largest studies of the Kosrae, published by Friedman and colleagues in PLoS Genetics in 2009, found that common gene variants associated with cholesterol and insulin response could not explain why some people became diabetic while others did not.
Despite the gaps in knowledge, it's a mistake to think that any one theory would explain an evolutionary tendency to become obese, Friedman says. He points out that humans in different parts of the world experienced different evolutionary pressures, so what's true in one population might not apply to the next.
"People think about evolution of this sort taking place over millennia or eons. Evolution can be evident in a single generation," he says. "Populations walk around with a spectrum of alleles, or genetic variants, and the frequency of one variant versus another can be changed dramatically even in a single generation."
Just as being overweight might have affected survival, so would being too thin. "You could draw the lines wherever you want, but the point is if you were infinitely fat or infinitely thin it wouldn't be good," he says. "And there's a biological system that's evolved that allows you balance in a particular environment."
The genetics of obesity has many influences, not just starvation or abundance or cold climates or warm, says obesity geneticist Claude Bouchard at Pennington Biomedical Research Center in Baton Rouge. Human DNA is a genetic soup that reflects any number of prehistoric dinner challenges. "Evolution must have been zigzagging," he says. "That's why it's hard to put all the pieces together."
Caption: Holding on to calories As time progressed, the threat of famine among early humans decreased, according to the thrifty gene hypothesis. But genes that once helped humans survive starvation remain and bodies are programmed to stockpile fat, even in times of increasing abundance, SOURCE: J. R. SPEAKMAN/ANNU. REV. NUTR. 2013
Caption: Menu of options Alternative theories to the thrifty gene hypothesis abound. So far, no single hypothesis on the genetic origins of obesity has gained universal acceptance. SOURCES: E. GENNE-BACON/YALE J. BIOL. MED. 2014; D. SELLAYAH ET AL/ENDOCRINOLOGY 2014
Caption: Warm climates, slow metabolism A new theory for the evolution of obesity proposes that as early humans migrated out of Africa and around the globe (dotted lines) and their bodies adapted to survive in different climates, those in warm climes ended up with a lower metabolic rate to help remain cool. That left them more vulnerable to metabolic syndrome, the cluster of symptoms that include high blood sugar and excess fat.
* John Speakman. "Evolutionary perspectives on the obesity epidemic: adaptive, maladaptive, and neutral viewpoints." Annual Review of Nutrition July 2013.
Why are some modern humans hefty? Theory Authors Description Thrifty gene James Neel, Human history was marked by feast or hypothesis U.S., 1962 famine. Humans who had fat reserves-- who were exceptionally efficient at storing fat--were more likely to survive. Drifty gene John Speakman, A counter to the thrifty gene theory. hypothesis U.K., 2008 Fatness was not a survival advantage. It just stopped being a disadvantage when humans no longer had to run from predators, so obesity drifted into the population. The thrifty Several A handful of hypotheses that revolve phenotype authors around the idea that poor nutrition in the womb encourages the development of diabetes when food is abundant in adulthood. Genetically Riccardo Obesity and diabetes occur when unknown Baschetti, populations are introduced to new foods foods Italy, 1998 they have not adapted to. Aggression Prajakta As humans relied less on fighting and control Belsare et al, aggression to survive, a propensity for India, 2010 obesity emerged. Over-indulgence becomes less of a problem when being docile is no longer a life-or-death calamity. Climate Dyan Sellayah Survival in cold parts of the world adaptations et al, U.K., favored genes that help preserve body 2014 temperature--a higher metabolic rate meant lower obesity and diabetes. In hot spots, lower metabolism meant the opposite.
Please note: Illustration(s) are not available due to copyright restrictions.
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|Date:||Sep 20, 2014|
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