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CARBOHYDRATES AND
PROTECTIVE FIBER
CONTROVERSY SURROUNDS THE humble carbohydrate. Is it good or bad? From the mid 1950s to the 1990s, they were the good guys, the heroes. Low in fat, they were supposed to be our salvation from the “epidemic” of heart disease. Then, the Atkins onslaught of the late 1990s recast them in the role of dietary villain. Many advocates avoided all carbohydrates—even vegetables and fruits. So, are carbohydrates good or bad?
Insulin and insulin resistance drive obesity. Refined carbohydrates, such as white sugar and white flour, cause the greatest increase in insulin levels. These foods are quite fattening, but that doesn’t necessarily mean that all carbohydrates are similarly bad. “Good” carbohydrates (whole fruits and vegetables) are substantially different from “bad” (sugar and flour). Broccoli will likely not make you fat, no matter how much you eat. But eating even modest amounts of sugar can certainly cause weight gain. Yet both are carbohydrates. How do we distinguish the two?
GLYCEMIC INDEX AND GLYCEMIC LOAD
DR. DAVID JENKINS of the University of Toronto began to tackle this problem in 1981 with the glycemic index. Foods were ranked according to their ability to raise glucose levels. Since dietary protein and fat did not raise blood glucose appreciably, they were excluded from the glycemic index, which measures only carbohydrate-containing foods. For those foods, glycemic index and insulin-stimulating effect are closely correlated.
The glycemic index uses identical 50-gram portions of carbohydrate. For example, you might take foods such as carrots, watermelon, apples, bread, pancakes, a candy bar and oatmeal, measure out a portion of each to contain 50 grams of carbohydrate, then measure the effect on blood glucose. Then you compare the foods against the reference standard—glucose—which is assigned a value of 100.
However, a standard serving of food may not contain 50 grams of carbohydrate. For example, watermelon has a very high glycemic index of 72, but contains only 5 percent carbohydrate by weight. Most of watermelon’s weight is water. You would need to eat 1 kilogram (2.2 pounds!) of watermelon to get 50 grams of carbohydrate—far in excess of what a person would eat at one sitting. A corn tortilla, though, has a glycemic index of 52. The tortilla is 48 percent carbohydrate by weight, so you would only have to eat 104 grams of the tortilla (close to what a person would reasonably eat during a meal) to get 50 grams of carbohydrate.
The glycemic load index attempts to correct this distortion by adjusting for serving size. Watermelon turns out to have a very low glycemic load of 5 while the corn tortilla still ranks high at 25. But whether you use the glycemic index or glycemic load, you’ll find there is a clear distinction between refined carbohydrates and unrefined traditional foods. Western refined foods have a very high glycemic index and glycemic load scores. Traditional whole foods have low glycemic load scores, despite containing similar amounts of carbohydrate—an essential distinguishing feature. (See Figure 16.11.)
Carbohydrates are not inherently fattening. Their toxicity lies in way they are processed.
Figure 16.1. Glycemic load values for some common foods.
Refining significantly increases the glycemic index by purifying and concentrating the carbohydrate. Removal of fat, fiber and protein means that the carbohydrate can be digested and absorbed very quickly. In the example of wheat, modern machine milling, which has almost completely replaced the traditional stone milling, pulverizes the wheat into the very fine white powder we know as flour. Cocaine users will know that very fine powders are absorbed into the bloodstream much faster than coarse grains—that’s what allows for higher “highs,” both for cocaine and for glucose. Refined wheat causes our glucose levels to spike. Insulin levels follow.
Second, refining encourages overconsumption. For example, making a glass of orange juice may require four or five oranges. It is very easy to drink a glass of juice, but eating five oranges is not so easy. By removing everything other than the carbohydrate, we tend to overconsume what is left. If we had to eat all the fiber and bulk associated with five oranges, we might think twice about it. The same applies to grains and vegetables.
The problem is one of balance. Our bodies have adapted to the balance of nutrients in natural food. By refining foods and only consuming a certain component, the balance is entirely destroyed. People have been eating unrefined carbohydrates for thousands of years without obesity or diabetes. What’s changed, and recently too, is that we now predominantly eat refined grains as our carbohydrate of choice.
WHEAT: THE WEST’S GRAIN OF CHOICE
WHEAT HAS LONG been a symbol of nutrition. Wheat, along with rice and corn, is one of the first domesticated foods in human history. Yet these days, what with gluten sensitivity and obesity, wheat does not have a friend to call its own. But how can wheat possibly be so bad? As discussed in chapter 9, wheat has been cultivated since ancient times.
But by the 1950s, Malthusian concerns of overpopulation and worldwide famine arose again. Norman Borlaug, who would later win the Nobel Peace Prize, began experimentating with higher-yield wheat varieties, and thus was born the dwarf-wheat variety.
Today, an estimated 99 percent of all wheat grown worldwide is dwarf or semi-dwarf varieties. But where Dr. Borlaug bred naturally occurring strains together, successors quickly turned to new technologies to enhance mutations. The new varieties of wheat were not tested for safety, but were merely assumed to be safe in this new atomic age.
It is clear that the dwarf wheat varieties of today are not the same as those fifty years ago. The Broadbalk Wheat Experiment2 documented the change in nutritional content over the last half century. Even as grain yields skyrocketed during the Green Revolution, the micronutrient content plummeted. Today’s wheat is simply not as nutritious as in previous generations. That surely cannot be good news.
Another clue to wheat’s changing character is the enormous increase in the prevalence of celiac disease, which is a reaction against gluten protein that damages the small intestine. Wheat is by far the predominant source of gluten in the Western diet, often by a factor of 100 or more. By comparing archived blood samples from Air Force men over a period of fifty years, researchers discovered that the prevalence of celiac disease appears to have quadrupled.3 Could this be a result of new wheat varieties? This question has not yet been satisfactorily answered, but the possibility is troubling.
Processing methods have changed significantly over the centuries. Wheat berries were traditionally ground by large millstones powered by animals or humans. The modern flourmill has replaced traditional stone grinding. The bran, middlings, germ and oils are efficiently and completely removed, leaving the pure white starch. Most of the vitamins, proteins, fiber and fats are removed along with the outer hull and bran. The flour is ground to such a fine dust that its absorption by the intestine is extremely rapid. The increased rate of glucose absorption amplifies the insulin effect. Whole wheat and whole grain flours retain some of the bran and germ, but suffer from the same problem of rapid absorption.
Starches are hundred of sugars all linked together. Most (75 percent) of the starch found in white flour is organized into branched chains called amylopectin; the remainder into amylose. There are several classes of amylopectin: A, B and C. Legumes are particularly rich in amylopectin C, which is very poorly digested. As the undigested carbohydrate moves through the colon, gut flora produces gas causing the familiar “tooting” of the bean eater. While beans and legumes are very high in carbohydrate, much of it is not absorbed.
Amylopectin B, found in bananas and potatoes, is intermediate in terms of absorption. The most easily digested is amylopectin A found in—you guessed it—wheat. Wheat is converted to glucose more efficiently than virtually any other starch.
However, despite all the concerns discussed in this chapter, observational studies consistently demonstrate that whole grains are protective against obesity and diabetes. Where is the disconnection? The answer, here is fiber.
THE BENEFITS OF FIBER
FIBER IS THE non-digestible part of food, usually of a carbohydrate. Common types of fiber include cellulose, hemicellulose, pectins, beta-glucans, fructans and gums.
Fiber is classified as soluble or insoluble based on whether it is dissolvable in water. Beans, oat bran, avocado and berries are good sources of soluble fiber. Whole grains, wheat germ, beans, flax seeds, leafy vegetables and nuts are good sources of insoluble fiber. Fiber can also classified as fermentable or non-fermentable. Normal bacteria residing in the large intestine have the ability to ferment certain undigested fiber into the short-chain fatty acids acetate, butyrate and propionate, which can be used as an energy source. They may also have other beneficial hormonal effects, including the decreased output of glucose from the liver.4 Generally, soluble fiber is more fermentable than insoluble.
Fiber has multiple purported mechanisms of health, but the importance of each is largely unknown. High-fiber foods require more chewing, which may help to reduce food intake. Horace Fletcher (1849–1919) believed strongly that chewing every bite of food 100 times would cure obesity and increase muscle strength. Doing so helped him lose 40 pounds (18 kilograms), and “Fletcherizing” became a popular weight-loss method in the early twentieth century. Fiber may decrease the palatability of food and thus reduce food intake.
Fiber bulks up foods and decreases its energy density. Soluble fiber absorbs water to form a gel, further increasing its volume. This effect helps fill the stomach, which increases satiety. (Stomach distention may signal a sensation of fullness or satiety through the vagus nerve.) Increased bulk may also mean that the stomach takes more time to empty. Therefore, after meals rich in fiber, blood glucose and insulin levels are slower to rise. In some studies, half the variance of the glucose response to starchy foods depended on their fiber content.5
In the large intestine, the increased stool bulk may lead to increased caloric excretion. On the flip side, fermentation in the colon may produce short-chain fatty acids.