Good Calories, Bad Calories

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, will 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 Association,” she says, “high-fiber, low-fat, no-cholesterol chow.”

The world is full 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 assumptions underlying the thrifty-gene hypothesis. These animals accumulate enormous fat deposits in response to an environment that offers up periods of feast—spring, summer, and fall—and famine in the winter. Yet this accumulation goes unaccompanied by the chronic ills, such as diabetes, that appear in obese humans. Hibernating ground squirrels, for instance, will 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, all over the place.”

Investigators who study hibernators, like Nicholas Mrosovsky, a University of Toronto zoologist, point out that weight gain, maintenance, and loss in these animals, and so perhaps in all species, is genetically pre-programmed and particularly resilient to variations in food availability. This program is characterized by its ability to adjust readily to changing circumstances and the unpredictability of the environment. Ground squirrels will gain weight through the summer at the same rate whether they’re in the wild or in the laboratory. They will lose it at the same rate during the winter whether they are kept awake in a warm laboratory or are in full hibernation, eating not a bite, and surviving solely off their fat supplies. “It is very hard to prevent them gaining and losing weight” on schedule, explains Mrosovsky, who did much of the original research in this area. When researchers surgically remove a sizable portion of fat from experimental animals—a procedure known as a lipectomy—the animals will restore the lost fat so that within months of the surgery they will be just as fat as they would have been without the surgery.^*74

Even the type of fat found in animals and humans is regulated in a way that accommodates differing internal and external environments. The fat in our limbs, for instance, is less saturated than the fat around our organs, and so is less likely to stiffen in cold weather. We will also change the fatty-acid composition of our subcutaneous fat with temperature—the colder it gets, the more unsaturated the fats. This same phenomenon, independent of the type of fat consumed, has been observed in pigs, rats, and hibernators. Another example of the evolutionary specificity of fat deposits can be seen in those desert animals that do not store fat subcutaneously, as humans and most animals do, apparently because it would inhibit heat loss and cooling. So there are fat-rumped and fat-tailed sheep, and fat-tailed marsupial mice, all desert-dwellers that carry their fat almost exclusively in the so-named locations.

The storage of fat, it seems clear, like all evolutionary adaptations, tends to be exquisitely well suited to the environment—both internal and external—in a way that maximizes benefits while minimizing risks. This is why most investigators who considered these issues in the 1970s and 1980s assumed that a tendency to gain any excessive weight during periods of abundance would be the kind of obvious liability that evolution would work to select out of the species rather than select in. The thrifty-gene hypothesis does not hold up. But without a thrifty gene, rendered detrimental by the abundance of food in modern societies and the absence of physical labor needed to procure it, how do we explain why gaining weight in modern societies still seems so much easier than losing it?

Chapter Fifteen

HUNGER

Khrushchev, too, looks like the kind of man his physicians must continually try to diet, and historians will some day correlate these sporadic deprivations, to which he submits “for his own good,” with his public tantrums. If there is to be a world cataclysm, it will probably be set off by skim milk, Melba toast, and mineral oil on the salad.

A.J. LIEBLING, The Earl of Louisiana, 1961

IN OCTOBER 1917, FRANCIS BENEDICT, director of the Carnegie Institution of Washington’s Nutrition Laboratory (located, as it happens, in Boston), put twelve young men on diets of roughly fourteen hundred to twenty-one hundred calories a day with the intention of lowering their body weights by 10 percent in a month. Their diets would then be adjusted as necessary to maintain their reduced weights for another two months, while Benedict and his colleagues meticulously recorded their psychological and physiological responses. A second squad of twelve men was studied as a comparison and then they were put on similar calorie- restricted diets. The results were published a year later in a seven-hundred-page report entitled Human Vitality and Efficiency Under Prolonged Restricted Diet.

Benedict hoped to establish whether humans could adjust to this lower nutritional level and thrive. His subjects lost the expected weight, but they complained constantly of hunger—“a continuous gnawing sensation in the stomach,” as described by the Carnegie report—and of being cold to the extent that several found it “almost impossible to keep warm, even with an excessive amount of clothing.” They also experienced a 30-percent decrease in metabolism. Indeed, Benedict’s subjects reduced their energy expenditure so dramatically that if they consumed more than twenty-one hundred calories a day—a third to a half less than they had been eating prior to the experiment—they would begin to regain the weight they had lost. The men also experienced significant decreases in blood pressure and pulse rate; they suffered from anemia, the inability to concentrate, and marked weakness during physical activity. They also experienced “a decrease in sexual interest and expression, which, according to some of the men, reached the point of obliteration.” That these phenomena were caused by the diet itself rather than the subsequent weight loss was demonstrated by the experience of the second squad of men, who manifested, according to the Carnegie report, “the whole picture…with striking clearness” after only a few days of dieting.