NOT JUST SIMPLE VERSUS COMPLEX
Carbohydrates were once divided into two categories: simple and complex. Simple carbohydrates like sugar were portrayed as the bad boys of nutrition, while complex carbohydrates like bread or rice were regarded as the golden children. That was a gross oversimplification. Not all simple carbohydrates are bad, and not all complex carbohydrates are good. Later in this chapter, I will describe two far more useful ways of categorizing carbohydrates: by their effect on blood sugar (the glycemic index) and by whether they come from refined or whole grains.
Simple carbohydrates are sugars. The simplest simple carbohydrates are glucose (sometimes called dextrose), fructose (also called fruit sugar), and galactose (a part of milk sugar). Table sugar is sucrose, a combination of one molecule of glucose and one of fructose. Milk contains lactose, which is made by joining one molecule of glucose with one of galactose. Simple carbohydrates provide us with energy and little else.
Complex carbohydrates are more . . . well, complex. In essence they are long chains of linked sugars. There are many types of complex carbohydrates in our food. The main one is starch, a long chain of glucose molecules. The human digestive system can rapidly break down starch and other complex carbohydrates into their component sugars. Some complex carbohydrates, like fiber, are quite indigestible and pass largely unchanged through the stomach and the intestines. Although fiber doesn’t contribute any energy or molecular building blocks to the body, it is still an important part of our diet. Because the term “complex carbohydrate” includes both starch and fiber, it is a useless and potentially misleading term.
WHY CARBOHYDRATES MATTER
In the average American diet, carbohydrates contribute about half of all calories. In one study from the National Health and Nutrition Examination Survey (NHANES), a staggering half of these “carbohydrate calories” came from just eight sources:3
• soft drinks, sodas, and fruit-flavored drinks
• cake, sweet rolls, doughnuts, and pastries
• pizza
• potato chips, corn chips, and popcorn
• rice
• bread, rolls, buns, English muffins, and bagels
• beer
• french fries and frozen potatoes.
Using data from NHANES, my colleagues and I calculated that about 80 percent of the carbohydrates in the U.S. diet come from sugar, refined starch, or potatoes. That means about 40 percent of all the calories we consume come from sugars, highly refined and easily digested grains, or potato starch, all of which are rapidly converted to blood sugar.
Fast Fact: Potatoes and Corn—Starches, Not Vegetables
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Although the USDA says that potatoes and corn are vegetables, your body treats them more like white rice and other rapidly digested grains than like vegetables (see "The Spud Is a Dud" on page 167). Eating other vegetables instead of potatoes and corn can help control your weight and your health. While corn is technically a whole grain, it has been so intensively bred for high starch content that it is no longer the same nutritious food that Tisquantum (Squanto) gave to the Pilgrims as they tried to eke out a living in North America.
When you eat a slice of bread, a potato, or some candy, your body breaks down the digestible carbohydrates it contains. Starch is converted to glucose, which is rapidly absorbed into the bloodstream and swiftly shuttled to the farthest reaches of the circulatory system. Because these simple sugar molecules are a primary fuel for most of the body’s tissues, complex mechanisms are in place to make sure that the level of glucose in the bloodstream doesn’t shoot too high or drift too low.
Figure 15. Response to Eating Carbohydrates. Easily digested carbohydrates make blood sugar and insulin rise faster and higher—and fall further—than slowly digested carbohydrates.
The rise in blood glucose is followed quickly by a parallel rise in insulin (see Figure 15). This hormone, produced by special cells in the pancreas, ushers glucose into muscle and other cells. As cells sponge up glucose, blood sugar levels fall, followed closely by falling insulin levels. Once your blood sugar nears its baseline, the liver begins releasing stored glucose to maintain a constant supply.
Corn Syrup Isn’t to Blame
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One of the many dramatic changes in the American diet over the past fifty years has been in how we satisfy our craving for sugar. Until the 1970s, we relied almost exclusively on sucrose (table sugar) from sugarcane and sugar beets, with a bit of honey, maple sugar, and molasses thrown in for variety. Today more than half of our sugar comes from corn, much of it in the form of high-fructose corn syrup. It’s found in everything from sugared sodas to ketchup and baby food.
Why the change? High-fructose corn syrup tastes a bit sweeter than sucrose. It’s easier to blend into beverages. And it costs a few pennies less per pound than sucrose.
High-fructose corn syrup has been cast as one of the villains behind the obesity epidemic for a reason: the jump in its use closely parallels the trajectory of obesity rates.
The body metabolizes fructose differently from glucose. Table sugar contains glucose and fructose in equal proportions (50 percent for both), since sucrose is made of one glucose molecule joined to one fructose molecule. High-fructose corn syrup is pure glucose mixed with pure fructose in almost equal proportions (55 percent fructose and 45 percent glucose). So table sugar and corn sweeteners have much the same physiological impact on blood sugar, insulin, and metabolism.
Some people claim that they’ve lost weight by cutting high-fructose corn syrup from their diets. That may be so, but it probably isn’t because they eliminated corn sweeteners. What likely happened is that by eliminating corn sweeteners they took in fewer added sugars and thus fewer calories.
So far, high-fructose corn sweetener doesn’t seem to be a greater dietary disaster than any other kind of added sugar. So swapping a soda made with high-fructose corn sweetener for one made with “natural sugar” won’t improve your health. What’s important is limiting your intake of all added sugars.
The World Health Organization recommends keeping added sugars under 10 percent of daily calories (roughly 12 teaspoons or 50 grams), and getting under 5 percent of calories from them “would provide additional health benefits.”4
To help us keep track of added sugars, the FDA is now requiring food companies to list added sugar on their “Nutrition Facts” labels. You may be surprised at how much added sugar some of your favorite foods contain.
After a snack or meal brimming with easily digested carbohydrates, blood sugar bolts upward. The resulting flood of insulin drives down the blood sugar level, sometimes too fast and too far. If there isn’t any more digestible carbohydrate in the stomach or intestines, your gut and brain start sending out hunger signals to make you grab for more food even as the liver starts releasing stored glucose. In contrast, slowly digested whole-grain carbohydrates smooth out this glucose-insulin roller coaster. Because it takes longer for the digestive system to break down whole grains into sugar molecules, blood sugar and insulin levels rise more slowly and peak at lower levels. A more drawn-out process also means it takes longer to get hungry again.
THE PROBLEM OF INSULIN RESISTANCE
In a growing number of people, the body’s tissues don’t respond to insulin as they should. Instead, they resist its “Open up for sugar” signal. This resistance to insulin keeps the amount of sugar in the bloodstream at high levels for longer periods and forces the pancreas to produce extra insulin in order to jam glucose into cells. Like an overworked, undermaintained pump, insulin-making cells in the pancreas may wear out and eventually stop producing enough insulin to keep blood sugar under control. Insulin resistance and faltering insulin production are early signs of type 2 diabetes, which was once called non-insulin-dependent diabetes and adult-onset diabetes.
Several things contribute to insulin resistance. Here are four key factors:
Obesity is at the top of the list. The further you get above a healthy
weight (see page 40), the more your body has trouble handling glucose.
Inactivity comes next. The less active you are, the lower the ratio of muscle to fat you have, even if your weight is perfectly fine. Fat cells don’t handle glucose as efficiently as muscle cells, especially regularly exercised muscle cells. The less muscle you have, the harder it is to clear glucose from the bloodstream. Working your muscles with daily physical activity improves their ability to remove glucose from the blood, even when you are resting or sleeping.
Dietary fats play a modest role in insulin resistance, with low intake of polyunsaturated fat and high intake of trans fats leading to greater resistance.
Genes also play a part. Insulin resistance is more common among Native Americans, Pacific Islanders, and people of Asian heritage than it is among those of European descent. But, like everyone else, people with a genetic predisposition to insulin resistance can beat the condition by staying lean, being physically active, and eating the right diet.
Insulin resistance isn’t just a blood sugar issue. It has also been linked with a variety of other problems, including high blood pressure, high levels of triglycerides, low protective HDL cholesterol, heart disease, and possibly some cancers.
HIGH-CARBOHYDRATE DIETS ARE ESPECIALLY BAD FOR PEOPLE WHO ARE OVERWEIGHT
People who are overweight fare worse on high-carbohydrate diets than do lean people. In the Nurses’ Health Study, for example, eating a lot of easily digested carbohydrates is most strongly connected to increased odds of having a heart attack among women who are overweight. What’s more, experiments in which volunteers were asked to follow high-carbohydrate, low-fat diets ended up with heart-unhealthy changes in levels of HDL and triglycerides, not to mention higher levels of blood sugar and insulin.5 These adverse changes are the most pronounced in overweight people.
Put more plainly, a low-fat, high-carbohydrate diet may be a terrible eating strategy for individuals who are overweight and not physically active. They fare better with a diet that includes fewer refined carbohydrates, more whole and intact grains, more healthy protein, and more good fats. Whether or not you are overweight, the switch from refined to whole grains will be good for you because of the increased intake of micronutrients.
THE GLYCEMIC INDEX: HOW CARBOHYDRATES AFFECT YOUR BODY SUGAR
Some carbohydrate-rich foods make blood sugar spike in a flash. Others yield their sugars more slowly, acting like those sustained-release cold capsules you may have seen advertised on television.
Not long ago, the rule of thumb was that sugars triggered rapid rises in blood sugar and insulin, while complex carbohydrates caused more delayed responses. But nutrition researcher David Jenkins and his colleagues at the University of Toronto overturned this conventional wisdom by systematically testing the impact of different foods on blood sugar levels (see “Measuring the Glycemic Index and Glycemic Load” on page 121). The carbohydrate ranking they developed, called the glycemic index (GI), counters the notion that all complex carbohydrates are good and all simple ones are bad.6 The higher a food’s glycemic index, the faster and stronger it affects blood sugar and insulin levels. As a reference point, pure glucose—the rapidly digested essence of blood sugar—is assigned a score of 100. On this scale, anything below 55 or so is considered a low-glycemic-index food.
Some of the glycemic index rankings are exactly what you might expect. An apple has a glycemic index of 38. A serving of old-fashioned (not instant) oatmeal has a glycemic index of 58. Jelly beans have a glycemic index of 78. Other rankings come as a surprise. Cornflakes, surely a complex carbohydrate, are in the 80s, while ice cream and a Snickers bar—which most people would assign to the simple carbohydrate camp—have lower glycemic index rankings than white bread, a classic complex carbohydrate. Perhaps surprisingly, whole-grain bread can have just as high a glycemic index value as white bread if the flour is finely ground. However, its higher content of fiber and other nutrients sets it apart as a healthier choice.
Low Glycemic Foods and Prevention of Diabetes: the Building Blocks of Evidence
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Over time, high blood sugar levels and a high demand for insulin will damage insulin-secreting cells in the pancreas and lead to type 2 diabetes. That’s good reason to suspect that diets with a high glycemic index will increase the risk of this disease. And there’s evidence for it: in large cohort studies, my research team and others have found that people consuming diets with a high glycemic index have a greater risk of developing type 2 diabetes. This was borne out in an updated analysis of data from the Nurses’ Health Study and Health Professionals Follow-Up Study.7 Among more than 175,000 women and men followed for up to twenty-four years, 15,027 developed type 2 diabetes. In all three cohorts, participants who consumed diets with the highest glycemic index were 33 percent more likely to have developed diabetes than those consuming diets with the lowest glycemic index.8
A large study using a drug, not diet, adds to what I think is conclusive evidence that there’s a cause-and-effect relationship between the glycemic index and the development of diabetes. The drug, called acarbose (brand name Precose), has long been used to treat diabetes. It specifically inhibits the body’s ability to chop up starch molecules into glucose molecules. In effect, it converts a high glycemic food into a low glycemic food without affecting its content of fiber or micronutrients. In a large randomized trial that compared acarbose with placebo, those taking the drug had a 25 percent lower risk of diabetes, as well as a similar reduction in risk for cardiovascular disease and high blood pressure.9
Foods with a high glycemic index offer a fast energy boost by quickly increasing blood sugar levels. (That’s one reason some people who use insulin to treat diabetes are urged to carry glucose tablets when they travel or exercise.) But such foods also promote equally swift drops in blood sugar that may trigger the early return of hunger. In contrast, the steadier, more sustained release of glucose from low-glycemic-index foods can stave off hunger for longer periods. There is also now strong evidence that eating foods lower on the glycemic index will help keep diabetes at bay (see “Whole Grains Protect Against Diabetes” on page 124).
Glycemic Index and Glycemic Load Values for Commonly Eaten Foods (Relative to Glucose)
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The glycemic index and glycemic load offer information about how a food affects blood sugar and insulin. The lower the glycemic index or glycemic load, the less the food affects blood sugar and insulin levels. A glycemic index below 55 and a glycemic load below 10 are considered low.
Foods
Serving Size
Glycemic index (%)
Carbohydrate (grams)
Glycemic Load*
Pancake
2 six-inch
83
56
46
Comflakes
1 cup
81
48
38
Total
1 cup
76
40
31
Grape-Nuts
1/2 cup
71
41
29
Coca-Cola
12 ounces
63
39
25
Cranberry juice
1 cup
68
36
24
White rice
5 ounces
64
36
23
Jelly beans
1 ounce
78
28
22
Snickers bar
1 bar (2 ounces)
68
32
22
Raisin Bran
1 cup
61
35
21
Pasta
1 cup
42
47
20
Shredded Wheat
2 biscuits
75
20
15<
br />
Potatoes, mashed
1 cup
74
20
15
Cheerios
1 cup
74
20
15
Oatmeal (rolled oats)
1 cup
58
22
13
Banana (ripe)
1 medium
51
25
13
Orange juice
1 cup
52
23
12
White bread
1 slice
70
14
10
Strawberry jam
1 tbsp
51
20
10
Pizza Hut Super Supreme Pizza
2 slices
36
24
9
Whole wheat bread
1 slice
71
13
9
English muffin
1 muffin
77
11
8
Ice cream
1/2 cup
61
13
8
All-Bran
1/2 cup
42
21
8
Sugar, table
1 tsp
68
10
7
Baked beans
1 cup
48
15
7
Apple
1 medium
38
15
6
Pumpernickel (dark rye bread)
Eat, Drink, and Be Healthy Page 15