Part 9 (1/2)

according to an NIH report ent.i.tled The Pima Indians: Pathfinders for Health, ”people became more overweight.” ”If the Pima Indians could return to some of their traditions,” explained one NIH authority, ”including a high degree of physical activity and a diet with less fat and more starch, we might be able to reduce the rate, and surely the severity, of unhealthy weight in most of the population.”

The problem with this version of the Pima history is that obesity and overweight had been evident a century ago, when the relevant nutrition transition was from relative abundance to extreme poverty. From November 1901 to June 1902, the Harvard anthropologist Frank Russel lived on the Pima reservation south of Phoenix studying the tribe and its culture. Many of the older Pima, Russel noted in a report of the Bureau of American Ethnology, ”exhibit a degree of obesity that is in striking contrast with the 'tal and sinewy' Indian conventionalized in popular thought.”

Obesity among the Pima is not a new phenomenon, as demonstrated by this photo of ”Fat Louisa” taken in 1901 or 1902 by the Harvard anthropologist Frank Russell.

Russel 's a.s.sessment of the Pima's relative corpulence was then confirmed by the anthropologist and physician Ale Hrdli ka, who visited the Pima reservation in 1902 and 1905. ”Especial y wel -nourished individuals, females and also males, occur in every tribe and at al ages,” Hrdli ka reported, ”but real obesity is found almost exclusively among the Indians on reservations.”

For perhaps two mil ennia, the Pima had lived as both hunter-gatherers and agriculturalists. Game was abundant in the region, as were fish and clams in the Gila River. When the Jesuit missionary Eusebio Kino arrived among the Pima in 1787, the tribe was already raising corn and beans on fields irrigated with Gila River water. In the decades that fol owed, they took to raising cattle, poultry, wheat, melons, and figs. They also ate mesquite beans, the fruit of the saguaro cactus, and a mush of what Russel later cal ed ”unidentified worms.” In 1846, when a U.S. Army battalion pa.s.sed through Pima lands, the battalion's surgeon John Griffin described the Pima as ”sprightly” and in ”fine health.” He also noted that the Pima had ”the greatest abundance of food, and take care of it wel , as we saw many of their storehouses ful of pumpkins, melons, corn &c.”

Life began to change dramatical y the fol owing year, when a wagon route was opened to California ”by way of Tucson and the Pima vil ages.” This became the southernmost overland route for the California gold rush that began in 1849; tens of thousands of travelers pa.s.sed through the Pima vil ages on the way west over the next decade. They relied on the Pima for food and supplies.

With the arrival of Anglo-American and Mexican settlers in the late 1860s, the prosperity of the Pima came to an end, replaced by what the tribe referred to as ”the years of famine.” Over the next quarter-century, these newcomers hunted the local game almost to extinction, and the Gila River water, on which the Pima depended for fis.h.i.+ng and irrigating their own fields, was ”entirely absorbed by the Anglo settlements upstream.” By the mid-1890s, the Pima were relying on government rations to avoid starvation, and this was stil the situation when Hrdli ka and Russel arrived in the early 1900s.

Both Hrdli ka and Russel struggled with the dilemma of poverty coincident with obesity. Russel knew that the life of these Indians was arduous; sedentary behavior could not be a cause of obesity in the Pima. Instead, he proposed that a dietary factor was responsible. ”Certain articles of their diet appear to be markedly flesh producing,” Russel wrote. Hrdli ka suggested that ”the role played by food in the production of obesity among the Indians is apparently indirect.” He suggested that life on the reservation might be relatively sedentary and this could play a role-”the change from their past active life to the present state of not a little indolence”-but he did not appear particularly confident about it. After al , he wrote, obesity was quite rare among the Pueblo, ”who have been of sedentary habits since ancient times.” And obesity among the Pima was found ”largely but not exclusively” in the women, and the women of the tribe worked considerably harder than the men, spending their days harvesting the crops, grinding corn, wheat, and mesquite beans and carrying whatever burdens were not carried by pack animals.

Hrdli ka also noted that by 1905 the Pima diet already included ”everything obtainable that enters into the dietary of the white man,” which raises the possibility that this might have been responsible for the obesity. At the half-dozen trading posts that opened on the Pima reservation after 1850, the Indians took to buying ”sugar, coffee and canned goods to replace traditional foodstuffs lost ever since whites had settled in their territories.”

Neither Hrdli ka nor Russel suggested that the U.S. government rations might be the cause of obesity. But if the Pima diet on government rations was anything like that of tribes reduced to similar situations at the time on which data exist-including the Sioux on the Standing Rock Reservation in the Dakotas-then almost 50 percent of their calories came from sugar and flour.

Obesity in a.s.sociation with ”widespread poverty” was doc.u.mented again on the Pima reservation in the early 1950s by Bertram Kraus, a University of Arizona anthropologist working with the Bureau of Indian Affairs. According to Kraus, more than 50 percent of the children on the Pima reservation could legitimately be described as obese by their eleventh birthday. The local Anglos, Kraus wrote, got leaner as they got older (at the time, at least); this was not the case with the Pima. Kraus lamented the absence of dietary data to a.s.sess the nutritional state of the tribe, but this situation was remedied a few years later by Frank Hesse, a physician at the Public Health Service Indian Hospital on the Gila River Reservation. Hesse noted that the Pima diet of the mid-1950s was remarkably consistent from family to family and consisted of ”mainly beans, tortil as, chili peppers and coffee, while oatmeal and eggs are occasional y eaten for breakfast. Meat and vegetables are eaten only once or twice a week.” Hesse neglected to a.s.sess sugar consumption, but he did note that ”a large amount of soft drinks of al types is consumed between meals.” Hesse then concluded that 24 percent of the calories consumed by the Pima (the soft drinks not included) were from fat, which is certainly low by modern standards.*69 Over the next twenty years, the prevalence of obesity and diabetes among the Pima continued to rise, now coincident with a change in the foods distributed by government agencies and sold in the reservation trading posts. By the late 1950s, according to the Indian Health Service in Tucson, ”large quant.i.ties of refined flour, sugar, and canned fruits high in sugar” were being distributed widely on the reservations, courtesy of a surplus commodity food program run by the U.S. Department of Agriculture. When mechanization of the local agriculture industry brought a cash economy to the Pima, the local stores and trading posts ”started to carry high caloric pre-packed sweets, such as carbonated beverages (i.e., 'soda pop'), candy, potato chips, and cakes.” ”Soda pop is used in immense amounts,” as one 1962 study described it.

In April 1973, when the evils of dietary fat were stil widely considered hypothetical, the NIH epidemiologist Peter Bennett appeared before George McGovern's Senate Select Committee on Nutrition and Human Needs to discuss diabetes and obesity on the Pima reservation. The simplest explanation for why half of al adult Pima were diabetic, said Bennett, was the amount of sugar consumed, which represented 20 percent of the calories in the Pima diet. ”The only question that I would have,” Bennett had said, ”is whether we can implicate sugar specifical y or whether the important factor is not calories in general, which in fact turns out to be real y excessive amounts of carbohydrates.” Bennett's opinion was consistent with that of Henry Dobyns of the D'Arcy McNickle Center for the History of the American Indian, who is considered the foremost authority on Pima history. In 1989, Dobyns described obesity and diabetes in the tribe as being ”to some extent a result of inadequate nutrition” and added that this inadequate nutrition had come about because ”many of the poorer individuals subsist on a diet of potatoes, bread, and other starchy foods. Their traditional diet is beyond their reach, for they cannot catch fish in a dry riverbed and they cannot afford to buy much meat or many fresh fruits and vegetables.”

Studies of the Sioux of the South Dakota Crow Creek Reservation in the 1920s, Arizona Apaches in the late 1950s, North Carolina Cherokees in the early 1960s, and Oklahoma tribes in the 1970s al reported levels of obesity comparable to that in the United States today, but in populations living in extreme poverty. ”Men are very fat, women are even fatter,” as the University of Oklahoma epidemiologist Kel y West said of the local tribes of the 1970s.

”Typical y, their lifetime maximum weight has been 185 percent of standard.”

The early study of the Sioux, by two investigators from the University of Chicago, is particularly interesting, because it was one of the few published studies of diet, health, and living conditions in such a population, and it appeared the same year that the U.S. Department of the Interior released the results of a lengthy investigation of Native American living conditions. ”An overwhelming majority of the Indians are poor, even extremely poor,” the Interior Department reported, ”living on lands from which a trained and experienced white man could scarcely wrest a reasonable living.” The University of Chicago report said most of the Sioux lived in one-or two-room shacks; 40 percent of the children lived in homes without toilet facilities; water had to be hauled from the river. Little milk was consumed, although canned milk was included in the government rations. b.u.t.ter, green vegetables, and eggs were almost never eaten. No fruit was consumed.*70 Twenty-five to forty pounds of beef were issued per person as government rations each month, but this was ”not an indication of the amount consumed by each person,” the report noted, ”for the families who receive rations are not left alone to eat them. Issue day is visiting day for the families not on the ration rol , and often the visit lasts until the friends' or relatives' rations of meat are gone. The ration family, therefore, may be compel ed to live on bread and coffee for the remainder of the month.”

The staple of the Sioux diet on the reservation was ”grease bread,” fried in fat and made from white flour, supplemented by oatmeal, potatoes, and beans, some squash and canned tomatoes, black coffee, canned milk, and sugar. ”Almost two-thirds of the families, including 138 children, were receiving distinctly inadequate diets,” the report concluded. Fifteen families, with thirty-two children among them, ”were living chiefly on bread and coffee.”

Nonetheless, 40 percent of the adult women, over 25 percent of the men, and 10 percent of the children ”would be termed distinctly fat,” the University of Chicago investigators reported, whereas 20 percent of the women, 25 percent of the men, and a slightly greater percentage of the children were ”extremely thin.”

By the 1970s, when studies of obesity in populations began in earnest, the general att.i.tude was that obesity was simply a fact of life in developed nations. ”Even a brief visit to Czechoslovakia,” reported a Prague epidemiologist at the first International Conference on Obesity, in 1974, ”would reveal that obesity is extremely common and that, as in other industrial countries, it is probably the most widespread form of malnutrition.”

The observation that this was also true in poor populations in nonindustrialized countries, that obesity frequently coexists side-by-side with malnutrition and undernutrition, shows up with surprising consistency. In a 1959 study of African Americans living in Charleston, South Carolina, nearly 30 percent of the adult women and 20 percent of the adult men were obese although living on family incomes of from $9 to $53 a week. In Chile in the early 1960s, a study of factory workers, most of whom were engaged in ”heavy labor,” revealed that 30 percent were obese and 10 percent suffered from ”undernourishment.” Nearly half the women over forty-five were obese. In Trinidad, a team of nutritionists from the United States reported in 1966 that one-third of the women older than twenty-five were obese, and they achieved this condition eating fewer than two thousand calories a day-an amount lower than the United Nations' Food and Agriculture Organization recommendation to avoid malnutrition. Only 21 percent of the calories in the diet came from fat, compared with 65 percent from carbohydrates.

In Jamaica, high rates of obesity, again among adult women in particular, were first reported in the early 1960s by a British Medical Research Council diabetes survey. By 1973, according to Rolf Richards of the University of the West Indies, Kingston, 10 percent of al Jamaican men and nearly two-thirds of the women were obese in a society in which ”malnutrition in infancy and early childhood remains one of the most important disorders contributing to infant and childhood mortality.”

Similar observations were made in the South Pacific and throughout Africa. In Rarotonga in the South Pacific, for instance, in the mid-1960s, on a diet of only 25 percent fat, over 40 percent of the women were obese and 25 percent were ”grossly obese.” Among Zulus living in Durban, South Africa, according to a 1960 report, 40 percent of adult females were obese. Women in their forties averaged 175 pounds. In a population of urban Bantu ”pensioners,” the mean weight of women over the age of sixty was reported in the mid-1960s to be 165 pounds. ”Although dietary habits vary widely amongst the African countries, tribes and vil ages,” wrote B. K. Adadevoh from Nigeria's University of Ibadan in 1974, ”it is general y established that the African diet is rich in carbohydrates. Caloric intake for most is low and protein fal s short of the recommended al owance.”

It seems fair to a.s.sume that the lives of market women in West Africa in the 1960s or poor Jamaicans of the same era were nontoxic by any of the definitions that are commonly a.s.sociated with the current obesity epidemic. The Sioux of the mid-1920s, or the Pima of the 1900s or 1950s, living on reservations and relying on government rations to survive, clearly lived in a state of poverty that most of us today would find almost unimaginable.

Obesity in Africa is not a.s.sociated with prosperity. These photos from Nigeria, of market women and an obese eleven-year-old, date to the early 1970s.

So why were they fat? ”It is difficult to explain the high frequency of obesity seen in a relatively impecunious society such as exists in the West Indies, when compared to the standard of living enjoyed in the more developed countries,” Rolf Richards wrote about Jamaica in the 1970s. ”Malnutrition and subnutrition are common disorders in the first two years of life in these areas, and account for almost 25 per cent of al admissions to pediatric wards in Jamaica. Subnutrition continues in early childhood to the early teens. Obesity begins to manifest itself in the female population from the 25th year of life and reaches enormous proportions from 30 onwards.”

The question of what causes obesity in these impoverished populations has typical y been ignored by obesity researchers, other than to suggest that there is something unique about given groups of people that exacerbates the problem of obesity. The a.s.sumption, as The New Yorker writer Malcolm Gladwel wrote about the Pima in 1998, is that they are ”different only in degree, not in kind.”

The idea of specific populations predisposed to obesity is encapsulated in a notion now known as the thrifty gene-technical y, the thrifty-genotype hypothesis-that is now commonly invoked to explain the existence of the obesity epidemic and why we might all gain weight easily during periods of prosperity but have such difficulty losing it. The idea, initial y proposed in 1962 by the University of Michigan geneticist James Neel, is that we are programmed by our genes to survive in the paleolithic hunter-gatherer era that encompa.s.sed the two mil ion years of human evolution before the adoption of agriculture-a mode of life stil lived by many isolated populations before extensive contact with Western societies. ”Such genes would be advantageous under the conditions of unpredictably alternating feast and famine that characterized the traditional human lifestyle,” explained the UCLA anthropologist Jared Diamond in 2003, ”but they would lead to obesity and diabetes in the modern world when the same individuals stop exercising, begin foraging for food only in supermarkets and consume three high-calorie meals day in, and day out.” In other words, the human body evolved to be what Kel y Brownel has cal ed an ”exquisitely efficient calorie conservation machine.” And so, by this hypothesis, we suck up calories when they are abundant and store them as fat until they are cal ed upon in a time of need. ”Your genes match nicely with a scarce food supply,” Brownel explains, ”but not with modern living.” Such populations as the Pima and the descendants of African tribes, according to this logic, were until very recently stil trapped in this cycle of feast and famine and scarce food in general, and so their thrifty genes have yet to evolve to deal with times of continual plenty. The NIH researchers who study the Pima, as Gladwel reported, ”are trying to find these genes, on the theory that they may be the same genes that contribute to obesity in the rest of us.”

For the first few decades of its existence, this notion that we have evolved ”thrifty mechanisms to defend energy stores during times of privation” was invariably referred to as a hypothesis. That qualification is now often dropped, but the thrifty gene remains only a hypothesis, and one that rests on many a.s.sumptions that seem unjustifiable.

James Neel initial y proposed the idea of a ”thrifty genotype rendered detrimental by progress” to explain why diabetes was so prevalent in Western societies and yet apparently absent in primitive tribes, including the Yanomamo of the Brazilian rain forest, who were then the subject of Neel's research.

Neel was addressing the diseases of civilization and the kind of observations that led Peter Cleave to propose his saccharine-disease hypothesis. (Neel was unaware of Cleave's work at the time.) The enigma of Type 2 diabetes, Neel observed, is that it bestows significant evolutionary disadvantages upon anyone who has it. Diabetic women are more likely to die in childbirth and more likely to have stil births than healthy women; their children are more likely to be diabetic than those of healthy women. This implies that any genes that might predispose someone to become diabetic would evolve out of the population quickly, but this did not seem to have happened. One way to reconcile these observations is to imagine a scenario in which having a genetic predisposition to become diabetic is advantageous in some circ.u.mstances. (In a similar way, having the gene for sickle-cel anemia, normal y a disadvantage, provides protection against malaria, a major advantage in malarial areas, as Neel himself reported.) Since diabetic mothers are known to give birth to heavier children, Neel speculated that these diabetic genes bestowed an exceptional ability to use food efficiently, and thus an exceptional ability to convert calories into fat. Those with such thrifty genes, Neel explained, ”might have, during a period of starvation, an extra pound of adipose reserve” that would keep them alive when those who failed to fatten easily would die of starvation. So it would be beneficial to have such genes in the event of famine or prolonged food deprivation, which Neel now a.s.sumed must have been the case throughout our evolutionary history. Those same genes would lead to obesity and diabetes in an environment in which food was plentiful.

”If the considerable frequency of the disease is of relatively long duration in the history of our species,” Neel had asked to begin his discussion, ”how can this be accounted for in the face of the obvious and strong genetic selection against the condition? If, on the other hand, this frequency is a relatively recent phenomenon, what changes in the environment are responsible for the increase?”

The thrifty gene could be the answer only if diabetes was of long duration in the species-and there is no evidence of that. The disease seems to appear only after populations have access to sugar and other refined carbohydrates. In the Pima, diabetes appeared to be ”a relatively recent phenomenon,” as Neel himself later noted. When Russel and Hrdli ka discussed the health of the Pima in the early 1900s, they made no mention of diabetes, even while noting the presence of such ”rare” diseases as lupus, epilepsy, and elephantiasis.*71 As late as 1940, when El iott Joslin reviewed the medical records of the hospitals and physicians in Arizona, he concluded that the prevalence of diabetes was no higher among the Pima and other local tribes than anywhere else in the United States. Only in the 1950s, in studies from the Bureau of Indian Affairs, was there compel ing reason to believe that diabetes had become common. When Neel tested adolescent Yanomamo for the condition known as glucose intolerance, which might indicate a predisposition to diabetes, he found none, so had no reason to believe that diabetes existed before such isolated populations began eating Western foods. The same was true of an isolated tribe of Pima, discovered living in the Sierra Madre Mountains of northern Mexico. ”The high frequency of [Type 2 diabetes] in reservation Amerindians,” Neel later explained, ”must predominantly reflect lifestyle changes.”

By 1982, Neel had come to side with Peter Cleave in believing that the most likely explanation for the high rates of obesity and diabetes in populations like the Pima that had only recently become Westernized was their opportunity to ”overindulge in high sugar content foods.”

This left open the question of what biological factors or genes might determine who got obese and diabetic and who didn't in the presence of such foods, but it eliminated any reason to suggest that thrifty genes had ever bestowed some evolutionary advantage. ”The data on which that (rather soft) hypothesis was based has now largely col apsed,” Neel observed. He now suggested that either a tendency for the pancreas to oversecrete insulin and so cause hyperinsulinemia, or a tendency toward insulin resistance, which in turn would result in hyperinsulinemia, was the problem, which is consistent with the carbohydrate hypothesis of chronic disease. Both of these, Neel suggested, would be triggered by the ”composition of the diet, and more specifical y the use of highly refined carbohydrates.”

It wasn't until the late 1970s, just a few years before Neel himself publicly rejected his hypothesis, that obesity researchers began invoking thrifty genes as the reason why putting on weight seems so much easier than losing it. Jules Hirsch of Rockefel er University was among the first to do so, and his logic is noteworthy, because his primary goal was to establish that humans, like every other species of animal, had apparently evolved a homeostatic system to regulate weight, and one that would do so successful y against fluctuations in food availability. We eat during the day, and yet have to supply nutrients to our cel s al night long, while we sleep, for example, so we must have evolved a fuel storage system that takes this into account. ”To me, it would be most unthinkable if we did not have a complex, integrated system to a.s.sure that a fraction of what we eat is put aside and stored,” Hirsch wrote in 1977. To explain why these components might cause obesity so often in modern societies, he a.s.sumed as fact something that Neel had never considered more than speculation. ”The biggest segment of man's history is covered by times when food was scarce and was acquired in unpredictable amounts and by dint of tremendous caloric expenditure,” Hirsch suggested. ”The long history of food scarcity and its persistence in much of the world could not have gone unnoticed by such an adaptive organism as man. h.o.a.rding and caloric miserliness are built into our fabric.”

This was one of the first public statements of the notion that would evolve into the kind of unconditional proclamation made by Kel y Brownel a quarter century later, that the human body is an ”exquisitely efficient calorie conservation machine.” But it depended now on an a.s.sumption about human evolution that was contradicted by the anthropologic evidence itself-that human history was dominated by what Jared Diamond had cal ed the ”conditions of unpredictably alternating feast and famine that characterized the traditional human lifestyle.” Reasonable as this may seem, we have no evidence that food was ever any harder to come by for humans than for any other organisms on the planet, at least not until our ancestors began radical y reshaping their environment ten thousand years ago, with the invention of agriculture.

Both the anthropological remains and the eyewitness testimony of early European explorers suggest that much of the planet, prior to the last century or two, was a ”paradise for hunting,” in the words of the Emory University anthropologist Melvin Konner and his col aborators, with a diversity of game, both large and smal , ”present in almost unimaginable numbers.”*72 Though famines have certainly been doc.u.mented among hunter-gatherer populations more recently, there's little reason to believe that this happened prior to the industrial revolution. Those isolated populations that managed to survive as hunter-gatherers wel into the twentieth century, as the anthropologist Mark Nathan Cohen has written, were ”conspicuously wel -nourished in qualitative terms and at least adequately nourished in quant.i.tative terms.”

Hunter-gatherers lived in equilibrium with their environment just as every other species does. The oft-cited example is the !Kung Bushmen of the semi-arid Kalahari desert, who were studied by Richard Lee of the University of Toronto and a team of anthropologists in the mid-1960s. Their observations, Lee noted, were made during ”the third year of one of the most severe droughts in South Africa's history.” The United Nations had inst.i.tuted a famine-relief program for the local agriculturalists and pastoralists, and yet the Bushmen stil survived easily on ”some relatively abundant high-quality foods,” and they did not ”have to walk very far or work very hard to get them.” The !Kung women would gather enough food in one day to feed their families for the next three, Lee and his col eagues reported; they would spend the remaining time resting, visiting, or entertaining visitors from other camps.

The prevailing opinion among anthropologists, not to be confused with that of nutritionists and public-health authorities, is that hunting and gathering al ow for such a varied and extensive diet, including not just roots and berries but large and smal game, insects, scavenged meat (often eaten at ”levels of decay that would horrify a European”), and even occasional y other humans, that the likelihood of the simultaneous failure of al nutritional resources is vanis.h.i.+ngly smal . When hunting failed, these populations could stil rely on foraging of plant food and insects, and when gathering failed ”during long-continued drought,” as the missionary explorer David Livingstone noted of a South African tribe in the mid-nineteenth century, they could relocate to the local water holes, where ”very great numbers of the large game” also congregated by necessity. This resiliency of hunting and gathering is now thought to explain why it survived for two mil ion years before giving way to agriculture. In those areas where human remains span the transition from hunter-gatherer societies to farmers, anthropologists have reported that both nutrition and health declined, rather than improved, with the adoption of agriculture. (It was this observation that led Jared Diamond to describe agriculture as ”the worst mistake in the history of the human race.”) Although famines were both common and severe in Europe until the nineteenth century, this would suggest that those with European ancestry should be the most likely to have thrifty genes, and the most susceptible to obesity and diabetes in our modern toxic environments. Rather, among Europeans there is ”a uniquely low occurrence of Type 2 diabetes,” as Diamond puts it, more evidence that the thrifty-gene hypothesis is incorrect.

Species adapt to their environment over successive generations. Those that don't, die off. When food is abundant, species multiply; they don't get obese and diabetic.

When earlier generations of obesity researchers discussed the storage of fat in humans and animals, they a.s.sumed that avoiding excessive fat is as important to the survival of any species as avoiding starvation. Since the average 150-pound man with a body fat percentage of only 10 percent is stil carrying enough fat calories to survive one month or more of total starvation, it seems superfluous to carry around more if it might have negative consequences. ”Survival of the species must have depended many times both on the ability to store adequate yet not excessive amounts of energy in the form of fat [my italics], and on the ability of being able to mobilize these stores always at a sufficient rate to meet the body's needs,” observed George Cahil and Albert Renold, considered two of the leading authorities on the regulation of fat metabolism, in 1965. The total amount of fat stored, they suggested, ”should be kept sufficiently large to al ow for periods of fasting to which a given species in a given environment is customarily exposed, yet sufficiently smal to preserve maximum mobility.”

The thrifty-gene hypothesis, on the other hand, implies that we (at least some of us) are evolutionarily adapted to survive extreme periods of famine, but a.s.signs to humans the unique concession of having evolved in an environment in which excess fat acc.u.mulation would not be a burden or lead to untimely death-by inhibiting our ability to escape from predators or enemies, for instance, or our ability to hunt or perhaps even gather. It presupposes that we remain lean, or at least some of us do, only as long as we remain hungry or simply lack sufficient food to indulge our evolutionary drive to get fat-an explanation for leanness that the British metabolism researchers Nancy Rothwel and Michael Stock described in 1981 as ”facile and unlikely,” a kind way of putting it. The ”major objection” to the thrifty-genotype hypothesis, noted Rothwel and Stock, ”must be based on the observation that most wild animals are in fact very lean” and that this leanness persists ”even when adequate food is supplied,” just as we've seen in hunter-gatherers. If the thrifty-gene hypothesis were true of any species, it would suggest that al we had to do was put them in a cage with plentiful food available and they would fatten up and become diabetic, and this is simply not the case.

Proponents of the thrifty-gene hypothesis, however, wil invoke a single laboratory model-the Israeli sand rat-to support the notion that at least some wild animals wil get fat and diabetic if caged with sufficient food. ”When this animal is removed from the spa.r.s.e diet of its natural environment and given an abundant, high-calorie diet,” wrote Australian diabetologist Paul Zimmet in a 2001 article in Nature, ”it develops al of the components of the metabolic syndrome, including diabetes and obesity.”

But the sand-rat experiments themselves, carried out in the early 1960s at Duke University by the comparative physiologist Knut Schmidt-Nielsen, suggested that the abundance of food was not the relevant factor. Schmidt-Nielsen was trying to establish what aspect of the laboratory diet might be responsible for the obesity and diabetes that appeared in his sand rats. He had taken two groups of rats freshly trapped in Egypt and raised one on Purina Laboratory Chow-”49.4% digestible carbohydrates, 23.4% protein and 3.8% fat”-supplemented with ”fresh mixed vegetables,” and the other on the fresh vegetables alone. Both had access to as much food as they desired, but only the chow-eating rats got diabetic and obese. This suggested that something about Purina Chow was the determining factor. Perhaps the rats liked it better than vegetables, and so they ate more, although that, too, could be a physiological effect related to the nutrient composition of the chow. It might have been the density of calories in the rat chow, which have less water content than vegetables and so more calories per gram. It was also possible that the cause of the diabetes and obesity in these rats, as Schmidt-Nielsen suggested, was ”a carbohydrate intake that is greater than that occuring in the natural diet.”*73 Depending on the researchers' preconceptions, the Israeli sand rats could have been considered an animal model of the carbohydrate hypothesis, rather than the thrifty-gene hypothesis. Monkeys in captivity, by the way, wil also get obese and diabetic on high-carbohydrate chow diets. One of the first reports of this phenomenon was in 1965, by John Brobeck of Yale, whose rhesus monkeys got fat and mildly diabetic on Purina Monkey Chow-15 percent protein, 6 percent fat, and 59 percent digestible carbohydrates. According to Barbara Hansen, who studies diabetes and obesity and runs a primate-research laboratory at the University of Maryland, perhaps 60 percent of middle-aged monkeys in captivity are obese by monkey standards. ”This is on the kind of diet recommended by the American Heart a.s.sociation,” she says, ”high-fiber, low-fat, no-cholesterol chow.”

The world is ful of species that do fatten regularly, always to serve a purpose-long-distance migrations, reproduction, or survival during periods when food is either unavailable or too risky to procure. Hibernators seem to be an obvious choice to shed light on the a.s.sumptions underlying the thrifty-gene hypothesis. These animals acc.u.mulate enormous fat deposits in response to an environment that offers up periods of feast-spring, summer, and fal -and famine in the winter. Yet this acc.u.mulation goes unaccompanied by the chronic il s, such as diabetes, that appear in obese humans. Hibernating ground squirrels, for instance, wil double their weight and body fat in a few weeks of late summer. Dissecting such squirrels at their peak weight is akin to ”opening a can of Crisco oil,” as the University of California biologist Irving Zucker, a pioneer of this research, has described it, ”enormous gobs of fat, al over the place.”