Studies of ileal digestibility show that we use cooked starch very efficiently. The percentage of cooked starch that has been digested by the time it reaches the end of the ileum is at least 95 percent in oats, wheat, potatoes, plantains, bananas, cornflakes, white bread, and the typical European or American diet (a mixture of starchy foods, dairy products, and meat). A few foods have lower digestibility: starch in home-cooked kidney beans and flaked barley has an ileal digestibility of only around 84 percent.
Comparable measurements of the ileal digestibility of raw starch are much lower. Ileal digestibility is 71 percent for wheat starch, 51 percent for potatoes, and a measly 48 percent for raw starch in plantains and cooking bananas. The differences conform to test-tube studies of a wide range of items showing that raw starch is poorly digested, often only half as well as cooked starch. Starch granules eaten raw frequently pass through the ileum whole and enter the colon unchanged from when they were eaten. This “resistant starch” is vivid testimony to the deficits of a raw starch diet, explaining why we like our starch cooked and contributing to the weight loss that raw-foodists experience.
The principal way cooking achieves its increased digestibility is by gelatinization. Starch inside plant cells comes as dense little packages of stored glucose called granules. The granules are less than a tenth of a millimeter (four-thousandths of an inch) long, too small to be seen with the naked eye or to be damaged by the milling of flour, and they are so stable that in a dry environment they can persist for tens of thousands of years. However, as starch granules are warmed up in the presence of water they start to swell—at around 58
o
C (136
o
F) in the case of wheat starch, a well-studied and representative example. The granules swell because hydrogen bonds in the glucose polymers weaken when they are exposed to heat, and this causes the tight crystalline structure to loosen. By 90
o
C (194
o
F), still below boiling, the granules are disrupted into fragments. At this point the glucose chains are unprotected, and gelatinize. Starch does not necessarily stay gelatinized after being cooked. In day-old bread the starch reverts and becomes resistant. This might help explain why we like to toast bread after it has lost its initial freshness.
Gelatinization happens whenever starch is cooked, whether in the baking of bread, the gelling of pie fillings, the production of pasta, the fabrication of starch-based snack foods, the thickening of sauces, or, we can surmise, the tossing of a wild root onto a fire. As long as water is present, even from the dampness of a fresh plant, the more that starch is cooked, the more it is gelatinized. The more starch is gelatinized, the more easily enzymes can reach it, and therefore the more completely it is digested. Thus cooked starch yields more energy than raw.
This effect is detected easily in blood measurements. Within thirty minutes of a person eating a test meal of pure glucose, the concentration of glucose in his or her blood rises dramatically, before returning to base levels in just over an hour. The effect of eating cornstarch is almost identical as long as it is cooked. But following a meal of raw cornstarch, the value of blood glucose remains persistently low, peaking at less than a third of the value for cooked cornstarch.
The effects of cooking are captured by comparing the glycemic index of cooked and raw foods. Glycemic index (GI) is a widely used nutritional measure of a food’s effect on blood sugar levels. High-GI foods, such as pure sugar, white bread, and potatoes, are good sources of energy after exercise, but for most people they are poor foods because they easily lead to excessive weight gain. In addition, the calories they offer tend to be “empty,” being low in protein, essential fatty acids, vitamins, and minerals. Low-GI foods, such as whole-grain bread, high-fiber cereals, and vegetables, reduce weight gain, improve diabetes control, and lower cholesterol. Cooking consistently increases the glycemic index of starchy foods.
Animal protein has been almost as important as starch in diets throughout our evolution, and it remains a strongly preferred food today. Yet the effects of cooking on the energy derived from eating meat have never been formally investigated, particularly the effects due to meat’s complex structure. Even the effects on proteins are a matter of debate. Until recently some scientists, such as David Jenkins, saw cooking as reducing protein digestibility. Others claim cooking protein is beneficial or has no effect. Recent studies of the digestion of eggs are starting to resolve the argument, showing for the first time that cooked protein is digested much more completely than raw protein.
In contrast to the new finding, in the past raw eggs have often been claimed to be an ideal source of calories, for reasons that sound logical. “An egg should never be cooked,” wrote raw-foodists Molly and Eugene Christian in 1904. “In its natural state it is easily dissolved and readily taken up by all the organs of digestion, but the cooked egg must be brought back to liquid form before it can be digested, which puts extra and unnecessary labor upon those over-worked organs.” This kind of argument persuaded generations of bodybuilders. The first muscleman with wide popular appeal was Steve Reeves, Hollywood’s movie Hercules of the 1950s, who famously ate raw eggs every day for breakfast. Celebrated strongmen like Charles Atlas and Arnold Schwarzenegger touted their merits too—as Mr. Universe, Schwarzenegger swallowed his eggs mixed with thick cream. Raw egg-eating by muscular athletes has even entered popular culture. In 1976 Sylvester Stallone’s boxing hero Rocky Balboa swallowed them as part of his training regimen in the movie
Rocky
. Thirty years later, in
Rocky Balboa
, he was still downing raw eggs. The quantity eaten by these legendary figures could be daunting: “Iron Guru” Vince Gironda, a popular teacher of bodybuilders, recommended up to thirty-six raw eggs a day.
Raw eggs would seem to provide an excellent food supply not only because their protein needs no chewing but also because their chemical composition is ideal. The amino acids of chicken eggs come in about forty proteins in almost exactly the proportions humans require. The match gives eggs a higher biological value—a measure of the rate at which the protein in food supports growth—than the protein of any other known food, even milk, meat, or soybeans. Raw eggs have other natural advantages. Their shells make them safer from bacterial contamination than cuts of meat. When aborigines on the beaches of Australia’s tropical north coast are thirsty, they look for turtle nests and readily drink raw egg whites. Eggs are the only unprocessed animal food that can safely be stored at room temperature for several weeks.
But even though eggs appear to be both high-quality and relatively safe when eaten raw, hunter-gatherers prefer to cook them. Unlike Australians, the Yahgan hunter-gatherers of Tierra del Fuego “would never eat half-cooked, much less raw eggs.” The Yahgan bored holes in eggshells to prevent them from bursting, buried the eggs on the edge of the fire, and turned them until they were quite hard inside. When not drinking eggs to slake their thirst, Australian aborigines would take similar pains, throwing emu eggs in the air to scramble them while still intact. They would then put them into hot sand or ashes and turn them regularly to cook them evenly, taking about twenty minutes. Such care suggests that the hunter-gatherers knew better than the musclemen.
In the late 1990s a Belgian team of gastroenterologists tested the effects of cooking for the first time, using a new research tool that allowed the investigators to follow the fate of egg proteins after they had been swallowed. The researchers fed hens a diet rich in stable isotopes of carbon, nitrogen, and hydrogen. The labeled atoms found their way into the eggs, allowing the experimenters to monitor the fate of protein molecules when the eggs were eaten. To determine how much of an egg meal was digested and absorbed in the body, they adopted the same method that had been used for studies of starch digestibility: they collected the food remains from the end of people’s small intestine, the ileum. Any protein that was undigested by the time it reached the ileum was metabolically useless to the person who ate it, because in the large intestine bacteria and protozoa digest the food proteins entirely for their own benefit.
At first the experimenters worked only with ileostomy patients, but later they were able to check their results with healthy subjects as well. The ileostomy patients and healthy volunteers each ate about four raw or cooked eggs, containing a total of 25 grams (0.9 ounces) of protein. Results were similar for the two groups. When the eggs were cooked, the proportion of protein digested averaged 91 percent to 94 percent. This high figure was much as expected given that egg protein is known to be an excellent food. However, in the ileostomy patients, digestibility of raw eggs was measured at a meager 51 percent. It was a little higher, 65 percent, in the healthy volunteers whose protein digestion was estimated by the appearance of stable isotopes in the breath. The results showed that 35 percent to 49 percent of the ingested protein was leaving the small intestine undigested. Cooking increased the protein value of eggs by around 40 percent.
The Belgian scientists considered the reason for this dramatic effect on nutritional value and concluded that the major factor was denaturation of the food proteins, induced by heat. Denaturation occurs when the internal bonds of a protein weaken, causing the molecule to open up. As a result, the protein molecule loses its original three-dimensional structure and therefore its natural biological function. The gastroenterologists noted that heat predictably denatures proteins, and that denatured proteins are more digestible because their open structure exposes them to the action of digestive enzymes.
Even before the Belgian egg study, there were indications that cooking can be responsible for enough denaturation to strongly influence digestibility. In 1987 researchers chose to study a beef protein, bovine serum albumin (BSA), selected because it is a typical food protein. In cooked samples, digestion by the enzyme trypsin increased four times compared to that of uncooked samples. The researchers concluded that the simple process of denaturation by heat (causing the protein molecule to unfold and lose its solubility in water) explained its greatly increased susceptibility to digestion.
Heat is only one of several factors that promote denaturation. Three others are acidity, sodium chloride, and drying, all of which humans use in different ways.
Acid is vital in the ordinary process of digestion. Our empty stomachs are highly acidic thanks to the secretions of a billion acid-producing cells that line the stomach wall and secrete one to two liters of hydrochloric acid a day. Food entering the stomach buffers the acidity and causes a more neutral pH, but the stomach cells respond rapidly and secrete enough acid to return the stomach to its original intense low pH, less than 2. This intense acidity has at least three functions: it kills bacteria that enter with the food, activates the digestive enzyme pepsin, and denatures proteins. Denaturation looks particularly important.
Marinades, pickles, and lemon juice are acidic, so if applied for sufficient time they can contribute to the denaturing of proteins in meat, poultry, and fish. It is no surprise that we like seviche, raw fish marinated in a citrus juice mixture, traditionally for a few hours. Hunter-gatherers have likewise been reported mixing acidic fruits with stored meats. The Tlingit of Alaska stuffed goat meat with blueberries and stored salmon spawn mashed with cooked huckleberries. Many other North American groups made pemmican by mixing dried and pounded meat with various kinds of berries, and Australian aborigines mixed wild plums with the pounded bones and meat of kangaroo. While pleasing flavors and improved storage might be enough to account for such mixtures, increased digestibility could also contribute to explaining the broad use of these acidic preparations. Animal protein that has been salted and dried, such as fish, is likewise denatured and thereby made more digestible. Increased digestibility from denaturation also helps account for our enjoyment of dried meats such as jerky or salted fish.
Although gelatinization and denaturation are largely chemical effects, cooking also has physical effects on the energy food provides. Research on the topic began with a misfortune almost two hundred years ago. On June 6, 1822, twenty-eight-year-old Alexis St. Martin was accidentally shot from a distance of about a meter (three feet) inside a store of the American Fur Company at Fort Mackinac, Michigan. William Beaumont, a young, war-hardened surgeon, was nearby and arrived within twenty-five minutes to find a bloody scene that he described eleven years later: “A large portion of the side was blown off, the ribs fractured, and openings made into the cavities of the chest and abdomen, through which protruded portions of the lung and stomach, much lacerated and burnt, exhibiting altogether an appalling and hopeless case. The diaphragm was lacerated and perforation made directly into the cavity of the stomach, through which food was escaping at the time your memorialist was called to his relief.”
Beaumont took St. Martin to his own home. To everyone’s surprise, St. Martin survived, and Beaumont continued to house and care for him after he stabilized. In a few months the patient resumed a vigorous life, and he became so strong that he eventually even paddled his family in an open canoe from Mississippi to Montreal. Although the fist-sized wound mostly filled in, it never completely closed. For the rest of St. Martin’s life, the inner workings of his stomach were visible from the outside.
The ambitious Beaumont realized that he had an extraordinary study opportunity. He began on August 1, 1825. “At 12 o’clock, M., I introduced through the perforation, into the stomach, the following articles of diet, suspended by a silk string, and fastened at proper distances, so as to pass in without pain—viz.:—a piece of highly seasoned
a la mode
beef; a piece of
raw, salted, fat pork
; a piece of
raw, salted, lean beef
; a piece of
boiled, salted beef
; a piece of
stale bread
; and a bunch of
raw, sliced cabbage
; each piece weighing about two drachms; the lad [St. Martin] continuing his usual employment around the house.”