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Science, Vol 305, Issue 5684, 637-639 , 30 July 2004
Peering Under the Hood of Africa's RunnersConstance Holden
Kenyans dominate endurance running, and West Africans excel as sprinters. With a physiological explanation in hand, researchers are now probing the genetics of this geographic mastery
Theories abound about what Kenya-born writer and runner John Manners calls "the greatest geographical concentration of achievement in the annals of sport." Is it the high altitude that fosters big lungs and efficient oxygen use? Is it their maize-based diet? Or the fact that many children run to school? A grueling training regimen, perhaps? Such questions have inspired a handful of researchers to try to define the Kenyan magic. Meanwhile, scientists are unraveling why athletes whose ancestors come from the other side of the continent--West Africa--have emerged as the world's fastest sprinters. Fuel economy A decade later, the scientists have ruled out most of the popular explanations for Kenyans' domination of running. Altitude is not the key to the riddle, they have found, because there's no difference between Kenyans and Scandinavians in their capacity to consume oxygen. And the Kenyan diet is on the low side for essential amino acids and some vitamins as well as fat, says Dirk Christensen of the Copenhagen center: "In spite of the diet, they perform at high level." The running-to-school hypothesis was demolished as well: Kenyan children aren't any more physically active than their Danish peers. Do Kenyans try harder? The researchers found that the Danes actually pushed themselves harder on a treadmill test, reaching higher maximum heart rates. An important clue is the ability of Kenyans to resist fatigue longer. Lactate, generated by tired, oxygen- deprived muscles, accumulates more slowly in their blood. Comparisons of lactate levels have suggested to Saltin's group that Kenyan runners squeeze about 10% more mileage from the same oxygen intake than Europeans can. Just as more aerodynamic cars get better gas mileage, the Kenyan build helps explain their fuel efficiency. A recent British TV documentary described the Kalenjin as possessing "birdlike legs, very long levers that are very, very thin [on which they] bounce and skip" along. Saltin's group has quantified this observation. Compared with Danes, the thinner calves of Kenyans have, on average, 400 grams less flesh in each lower leg. The farther a weight is from the center of gravity, the more energy it takes to move it. Fifty grams added to the ankle will increase oxygen consumption by 1%, Saltin's team calculates. For the Kenyans, that translates into an 8% energy savings to run a kilometer. "We have solved the main problem," declares Henrik Larsen of the Copenhagen center. "Kenyans are more efficient because it takes less energy to swing their limbs." Other scientists say the jury is still out on the Kenyan question. But "I think Saltin is probably the most correct that anyone is at the moment," says physiologist Kathryn Myburgh of the University of Stellenbosch in South Africa, who is exploring the role of Kenyans' training.
However, slim lower legs are not the whole story. Kenyan runners also have a higher concentration of an enzyme in skeletal muscle that spurs high lactate turnover and low lactate production. Saltin says that this results in an "extraordinarily high" capacity for fatty acid oxidation, which helps wring more energy out of the muscles' biochemical reactions. Because intense training alters the body's biochemistry, Saltin says that he can't say for sure whether the ezyme levels are due to genes or training. But he adds, "Ithink it's genetic." Research in South Africa jibes with the Copenhagen group's findings. A team led by exercise physiologist Adele Weston of the University of Sydney, Australia, compared black South Africans, whose running strengths are similar to those of Kenyans, with white runners. The two groups had similar VO2 max values--that is, when putting out maximum effort, they used up the same amount of oxygen per kilogram of body weight per minute. But the black runners were more efficient in their oxygen consumption, lasting on a treadmill at maximum speed for twice as long as the whites. As with the Kenyans, the black South African runners accumulated less lactate and had higher levels of key muscle enzymes. A little more
twitchy Various studies have shown that West African athletes have denser bones, less body fat, narrower hips, thicker thighs, longer legs, and lighter calves than whites. But the differences between East and West Africans are even more striking. The fabled Kenyan runners are small, thin, and tend to weigh between 50 and 60 kilograms, whereas West African athletes are taller and a good 30 kilograms heavier, says Timothy Noakes, a prominent exercise physiologist and researcher at the University of Cape Town.
The differences don't stop with body shape; there is also evidence of a difference in the types of muscle fibers that predominate. Scientists have divided skeletal muscles into two basic groups depending on their contractile speed: type I, or slow-twitch muscles, and type II, fast-twitch muscles. There are two kinds of the latter: type IIa, intermediate between fast and slow; and type IIb, which are superfast-twitch. Endurance runners tend to have mostly type I fibers, which have denser capillary networks and are packed with more mitochondria. Sprinters, on the other hand, have mostly type II fibers, which hold lots of sugar as well as enzymes that burn fuel in the absence of oxygen. In the 1980s, Claude Bouchard's team at Quebec's Laval University took needle biopsies from the thigh muscles of white French Canadian and black West African students. They found that the Africans averaged significantly more fast-twitch muscle fibers--67.5%--than the French Canadians, who averaged 59%. Endurance runners have up to 90% or more slow-twitch fibers, Saltin reports. Bouchard, now at Louisiana State University in Baton Rouge, says his team looked at two enzymes that are markers for oxidative metabolism and found higher activity of both in the West Africans, meaning they could generate more ATP, the energy currency of the cell, in the absence of oxygen. The study suggests that in West Africa there may be a larger pool of people "with elevated levels of what it takes to perform anaerobically at very high power output," says Bouchard. Although training can transform superfast-twitch type IIb fibers into the hybrid type IIa, it is unlikely to cause slow- and fast-twitch fibers to exchange identities. Myburgh says there is evidence that, with extremely intensive long-distance training, fast IIa fibers can change to slow type I fibers. So far, however, there is no evidence that slow-twitch fibers can be turned into fast-twitch ones. As an athlete puts on muscle mass through training, new fibers are not created, but existing fibers become bigger.
Running ACEs The differences in physique and muscle makeup that underlie the dominance of Kenyan endurance runners and West African sprinters doubtless have a strong genetic component. But researchers are only just getting off the starting mark in the search for genes that influence running performance. Bouchard's group, for example, is collecting DNA samples from 400 runners and other top endurance athletes from the United States and Europe, but he says they haven't spotted any running genes yet. There are a couple of intriguing possibilities, though. In 1999,
a team headed by Kathryn North of the Children's Hospital at
Westmead in Australia described two versions of a gene that affects
production of Alejandro Lucia Mulas of the European University in Madrid is taking DNA samples from Eritrean runners to explore another candidate: different versions of the gene for angiotensin-converting enzyme (ACE). Lucia says the less active version, or I allele, of this gene is associated with less muscle, less fluid retention, and more relaxed blood vessels--which would enhance oxygen uptake--and appears to be more prevalent in endurance runners. And in Scotland, sports physiologist Yannis Pitsiladis has launched a major onslaught on the Kenyans' secrets with the International Centre for East African Running Science. Headquartered at the University of Glasgow, the virtual center will bring together research on demography, diet, and socioeconomic factors as well as genes. Pitsiladis says he has spent the last 3 years in East Africa collecting DNA samples from their "living legends" and now has DNA from 404 Kenyan and 113 Ethiopian athletes. His team has found a higher prevalence of the I allele for the ACE enzyme in male marathoners compared with men from the general Ethiopian population. But Pitsiladis thinks his numbers may lack significance given the variability of the trait in African populations. "At the moment there is no evidence" that East Africans have a genetic advantage in running, he says. None of the data negate the importance of cultural habits and training. But as Entine quotes anthropologist and sports science expert Robert Malina, who is retired from Michigan State University, "Differences among athletes of elite caliber are so small that if you have an advantage that might be genetically based ... it might be very, very significant." Next month's Olympic games in Athens should demonstrate yet again that West African runners are built for speed and Kenyans built to endure.
Copyright © 2004 by The American Association for the Advancement of Science. All rights reserved. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Triumph of fast twitch. Carl
Lewis, a U.S. sprinter with West African roots, winning the
400-meter race at the 1992 Olympics in Barcelona.
Road Test. Masks monitored
Kenyans' oxygen use.
-actinin-3, a protein found only in fast-twitch muscles.
They found the less efficient version of the gene--which results in
poorer energy conversion--in 18% of the members of a group of
Caucasians. In 2003, North's group reported in the American
Journal of Human Genetics that only 6% of a group of sprinters
had the gene defect; 26% of endurance runners had it. The authors
surmise that
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