Quick and Slow Forms of Carbohydrate

Just as carbohydrate is referred to as simple or complex and sugars or starches, it can also be categorized as quick or slow. The quick or slow refers to a complex system called the glycemic index (GI). The glycemic index is theoretically based on how 50 grams (200 calories) of carbohydrate (not counting fiber) in a food will affect blood sugar levels. For example, white bread is a carbohydrate high on the glycemic index and supposedly causes a rapid spike in blood sugar, while beans are considered low on the glycemic index and cause a more gradual increase in blood sugar levels. Table 6.1 provides the glycemic index and glycemic load (glycemic response to a standard serving of food) of popular sports foods. That is, a person might eat 200 calories of carbohydrate from pasta in a sitting,

Table 6.1 Glycemic Index and Glycemic Load of Popular Sports Foods

Food

Glycemic index

Glycemic load

Serving size

Coca-Cola

63

16

8 oz (240 ml)

Apple juice

40

12

8 oz (240 ml)

Gatorade

78

12

8 oz (240 ml)

Chocolate milk (1.5% fat with Nesquik)

41

5

8 oz (240 ml)

Rice cakes

78

17

1 oz (30 g)

Bagel, white Lenders

72

25

2.5 oz (75 g)

Wonder Bread

73

10

1 oz (30 g)

Spaghetti

58

28

6 oz (175 g)

Cheerios

74

15

1 oz (30 g)

Oatmeal, cooked

69

16

1 cup

Banana, underripe

42

10

4 oz (125 g)

Orange

42

5

4 oz (125 g)

Snickers bar

68

23

2 oz (60 g)

PowerBar, chocolate

56

24

2.3 oz (67 g)

Created from data in K. Foster-Powell, S.H. Holt and J.C. Brand-Miller, 2002, "International table of glycemic index and glycemic load values: 2002," American Journal of Clinical Nutrition 76(1):5-56.

Created from data in K. Foster-Powell, S.H. Holt and J.C. Brand-Miller, 2002, "International table of glycemic index and glycemic load values: 2002," American Journal of Clinical Nutrition 76(1):5-56.

but most people do not eat 200 calories of carbohydrate from rice cakes at one time. Hence, the actual glycemic load of a food differs from its glycemic index.

The glycemic index was initially developed to help people with diabetes better regulate their blood glucose. But people with diabetes generally eat foods in combinations (e.g., a sandwich with bread, turkey, and tomato), which alters the glycemic index of the meal (Franz 2003). Athletes, however, commonly eat foods solo (a banana, a bagel). Hence, exercise scientists became curious about the possibility that quick or slow forms of carbohydrate might affect exercise performance because they affect blood glucose in different ways. Could athletes use this ranking system to determine what to eat before, during, and after exercise?

In theory, low-glycemic index foods (apples, yogurt, lentils, beans) provide a slow release of glucose into the bloodstream, and high-glyce-mic index foods (sports drinks, jelly beans, bagels) quickly elevate blood sugar. Could low-GI foods help endurance athletes perform better by providing sustained energy during long bouts of exercise? Are high-GI foods best to consume immediately after exercise to rapidly refuel the muscles and, thereby, enhance subsequent performance?

Although this seems logical, I tell my athletes to disregard all the hype about the glycemic index and simply enjoy fruits, vegetables, and whole grains without fretting about their glycemic effect. Too many factors influence a food's glycemic index, including where the food was grown, the amount eaten, added fat, the way the food is prepared, and whether the food is eaten hot or cold. To make the glycemic index even less meaningful, each of us has a differing daily glycemic response that can vary approximately 43 percent on any given day (Vega-Lopez et al. 2007). Also, keep in mind that well-trained muscles can readily take up carbohydrate from the bloodstream. Hence, athletes secrete less insulin than do unfit people. This means most athletes don't get the blood sugar spikes seen in unfit people. Athletes also don't commonly get type 2 diabetes; exercise is an excellent way to manage blood sugar.

All things considered, you, as an athlete, have little need to obsess about a food's glycemic effect because you don't even know your personal response to the food. Plus, the sports nutrition research fails to clearly show performance benefits from these theories (Burke, Collier, and Hargreaves 1998). The research does indicate that the best way to enhance endurance is to consume carbohydrate before and during exercise—tried-and-true choices that taste good, settle well, and digest easily. You need not choke down low-glycemic index kidney beans thinking they will offer sustained energy, when they actually might only create digestive distress. To enjoy sustained energy, simply eat a tried-and-true preexercise meal or snack and then, after the first hour, consume about 200 to 250 calories of carbohydrate per hour of endurance exercise. (See chapters 9 and 10 for more information about fueling before and during exercise.)

For athletes who train hard or compete within 4 to 6 hours of the first session, choosing high-glycemic index recovery foods is a smart choice. High-GI foods provide glucose quickly and refuel depleted glycogen stores quicker than a lower glycemic index choice. Yet, 24-hour research suggests a low-GI diet might actually contribute to better performance the second day (109 versus 99 minutes of running to exhaustion) (Stevenson, Williams, and Biscoe 2005; Stevenson et al. 2005). The low-GI diet might facilitate better replacement of intramuscular fat stores (important for endurance) as well as enhance the use of fat for fuel, instead of the limited (and limiting) glycogen stores.

The bottom line: If you have to rapidly refuel from one bout of exhausting exercise to prepare for a second bout of exercise, eat enough easy-to-digest carbohydrate—at least 0.5 gram of carbohydrate per pound (1 g per kg) of body weight, or about 300 calories for a 150-pound (68 kg) person every two hours for four to six hours—and enjoy a balance of healthy fat and protein to take care of all the recovery needs, not just carbohydrate for glycogen. (See chapter 10 for more information about recovery.)

Insulin and Fat Storage

What about the popular notion that high-glycemic index foods are fattening because they create a rapid rise in blood sugar, stimulate the body to secrete more insulin, and thereby (supposedly) promote fat storage? Not so simple. Excess calories are fattening, not excess insulin. Insulin can stimulate the appetite, as well as fat deposition, and that's where high-GI carbohydrate gets a bad reputation.

We need more research to determine whether physically fit people will lose weight more easily with a diet based on low-GI foods. Indeed, even the research on low-GI foods and weight loss in overweight people is unclear. In one study of obese people (ages 18 to 35) with high insulin secretion, a low-GI diet (that lowered the insulin response) contributed to about 13 pounds (6 kg) of weight loss in 18 months—more than the 2.5 pounds (1 kg) lost by a comparison group who ate a higher-carbohydrate, lower-fat diet (Ebbeling et al. 2007). Yet, in another yearlong study of overweight adults (average age 35 years), a low-GI diet resulted in no differences in weight loss, hunger, or satiety compared with those who ate high-GI meals (Das et al. 2007). Stay tuned!

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