SSE#64-Volume 10 (1997), Number 1
Table of Contents of all Sports Science Exchange Articles
- 1. The glycemic index (GI) of a food represents the magnitude of the increase in blood glucose that occurs after ingestion of the food.
- 2. GI tends to be lower for foods that have a high fructose content, exhibit high amylose/amylopectin ratios, are present in relatively large particles, are minimally processed, and are ingested along with fat and protein.
- 3. Consumption of lower GI foods 30-60 min prior to an endurance exercise bout tends to promote the following effects during exercise:
- Minimizes the hypoglycemia that occurs at the start of exercise.
- Increases the concentration of fatty acids in the blood.
- Increases fat oxidation and reduces reliance on carbohydrate fuel.
- The GI of a food consumed during exercise is probably less important than at other times because the insulin response to carbohydrate ingestion is suppressed during exercise.
- Consumption of high GI foods soon after exercise will probably optimally promote the restoration of muscle glycogen.
- Although manipulation of the GI of ingested foods may alter exercise metabolism, the effect of the GI on exercise performance is controversial and requires additional research.
Fuel Utilization During Exercise
The goals of dietary intervention for the athlete are to fill carbohydrate (glycogen) stores in the muscles and liver and to make both carbohydrate and fat readily available in the blood for use by the muscles. Carbohydrate fuel can support higher intensity exercise than can fat and is stored in more limited amounts in the body. The metabolic challenge is to maintain carbohydrate supply to the muscles but to somehow slow its depletion by relying optimally on fat as a fuel. Insulin plays a key role in fuel partitioning because insulin tends to increase the metabolism of carbohydrate and reduce fat use. An interesting question is whether or not certain foods can provide sufficient carbohydrate, affect insulin minimally, and also encourage fat use for energy.
Many studies have investigated the ergogenic value of consuming carbohydrate before, during, or after an exercise bout. There is overwhelming evidence that carbohydrate consumption before and/or during prolonged exercise can enhance endurance performance. Thus, a typical recommendation for the daily diet of athletes is to increase carbohydrate intake to at least 60% of the energy in the food ingested or to ingest at least 7 g of carbohydrate per kilogram of body weight. There are also recommendations about the amount and frequency of carbohydrate consumption during exercise (e.g., Walberg – Rankin, 1995), but these recommendations typically do not include any comment on the specific type of carbohydrate that should be consumed. The remainder of this review will summarize the evidence that consuming different types of carbohydrate causes different effects on exercise metabolism and, possibly, performance.
TYPES OF CARBOHYDRATE FOODS
Biochemically, most carbohydrate foods can be classified as mono-, di-, or polysaccharides. Simple and Complex Carbohydrates and the Glycemic Index
Carbohydrate foods are often classified as “simple” or ” complex” carbohydrates-mono- and disaccharides are grouped as “simple” and polysaccharides as “complex.” Although one might guess that simple molecules would be absorbed more rapidly than larger ones, this assumption is not always correct; digestion and absorption do not occur at the same rates for all carbohydrates within a biochemical grouping.
A newer system of carbohydrate classification is the “glycemic index” (GI). The term has been used for some time in clinical nutrition, particularly as it pertains to diabetes, but has only recently been used in the healthy, active population. This term refers to the relative degree to which the concentration of glucose in the blood rises after consumption of a food, i.e., the so-called “glycemic response.” Testing of the GI requires ingestion of 50 g of carbohydrate from a variety of foods, and measuring the blood glucose response over 2 h. After the blood glucose concentration over the two hours is graphically represented-with glucose concentration on the vertical axis and time on the horizontal axis-the area under the blood glucose curve is measured for each food and compared to consumption of 50 g of glucose as the reference. The glycemic index is given as a percentage, i.e., the percentage of the area under the blood glucose curve for the test food compared to that fo r glucose. Accordingly, a GI of 70 indicates that consuming 50 g of the food in question provokes an increase of blood glucose 70% as great as that for ingesting 50 g of pure glucose.
Factors that influence the glycemic index of a food include the biochemical structure of the carbohydrate, the absorption process, the size of the food particle, the degree of thermal processing, the contents and timing of the previous meal, and the co-ingestion of fat, fiber, or protein (Guezennec, 1995).
Mechanical or thermal processing of food that breaks the food into smaller particles or makes it more susceptible to the actions of the digestive enzymes increases the glycemic index of the food. For example, making flour from wheat will increase the glycemic index relative to ingesting wheat kernels. Finally, because ingestion of fat and protein tends to slow stomach emptying, absorption of carbohydrates and elevations in blood glucose usually occur more gradually if the carbohydrates are consumed along with fats and proteins.
Tables listing the glycemic index of foods have been developed mainly for use with diabetic persons (Foster-Powell & Brand Miller, 1995), but because blood glucose appears to be so critical to athletic performance these tables may also be useful for athletes.
THE INFLUENCE OF GLYCEMIC INDEX ON EXERCISE METABOLISM
Feedings Prior to Exercise
Food consumed prior to exercise should supply carbohydrate that can elevate or maintain blood glucose without dramatically increasing insulin secretion. This would theoretically optimize the availabilities of both glucose and fatty acids for use by the muscles. One concern about feeding carbohydrate prior to exercise is that a rapid increase in blood glucose- and thus insulin- might cause hypoglycemia at the start of the activity. A second effect of hyperinsulinemia prior to exercise is a reduction in lipolysis. Both of these conditions may increase reliance on muscle glycogen during the exercise.
The evidence suggests that consuming higher-GI foods 30-60 min before exercise causes more of a decrease in blood glucose upon the initiation of exercise and increases reliance on carbohydrate as a fuel during the exercise. These facts tend to identify lower-GI foods as promoting a preferable metabolic response prior to exercise. However, there is conflicting evidence on whether or not these metabolic differences have any effect on endurance performance.
Much research has focused on provision of food, particularly carbohydrate-rich items, during exercise to slow the depletion of body carbohydrate and thus delay the onset of fatigue. The concerns about carbohydrate feedings increasing insulin and thus depressing fatty acid availability are obviated when the carbohydrate is fed during exercise because the exercise-induced elevation in epinephrine depresses the release of insulin from the pancreas. After Exercise
A goal of feeding after exercise is to elevate glucose as soon as possible to provide substrate for glycogen synthesis; as reviewed by Robergs (1991), glycogen synthesis can occur more rapidly if carbohydrate is consumed quickly and in adequate amounts after exercise.
If no food is consumed after exercise, a low GI meal ingested prior to exercise may be warranted because it is likely to cause higher blood glucose and insulin concentrations after exercise than a high GI meal. However, glycogen synthesis will be faster if high GI meals are consumed as soon as tolerated after exercise. The increased blood glucose-and especially insulin-after exercise appear to be critical for resynthesizing muscle glycogen.
OTHER HEALTH ISSUES RELATED TO GLYCEMIC INDEX
There are several general health implications for high versus low GI diets. Much of the early research regarding the effects of GI used diabetic subjects because most of the complications of diabetes are related to excessive blood glucose levels; a lower GI diet moderates blood glucose in these individuals.
Because blood glucose has been implicated in appetite control, it has been suggested that a lower GI diet may increase satiety and make it easier to control food intake and body weight. Holt et al. (1992) tested the effects of six test meals on serum and glucose and insulin, and hunger. They found a direct relationship between GI and hunger during the 3 h after the meal, i.e., the high GI meals caused a greater feeling of hunger than did the low GI meals
Finally, total and low-density-lipoprotein cholesterol may decrease on a lower GI diet. Synthesis of cholesterol in the liver is sensitive to insulin concentrations, which tend to be higher with a high GI diet (Jenkins 1987; Kiens and Richter 1996). For example, Jenkins et al. (1987) reported a 15% drop in cholesterol of normal subjects after 2 wk on a low GI diet.
It is valuable to consume carbohydrate before, during, and after prolonged endurance exercise to provide fuel during exercise and substrate for glycogen synthesis following exercise. It is possible that carbohydrate foods with different GI may alter exercise metabolism and further affect performance. The research concerning GI and performance in athletes is limited, and recommendations concerning carbohydrate choices are still tentative. In addition, it is important to note that only a limited number of foods have been tested for their GI.
Consuming low versus high GI foods in the hour before exercise may moderate the decline in blood glucose that occurs at the beginning of exercise, reduce reliance on carbohydrate as a fuel, and increase lipid use during exercise. However, there is insufficient evidence to claim that these metabolic changes translate to reduced muscle glycogen depletion and improved endurance performance. Although fructose has a relatively low GI, it should be used in small Amounts and in combination with other carbohydrate sources because it often causes gastrointestinal distress. Other foods with a low GI that may be consumed before exercise include most fruits, pasta, rice, and possibly legumes if they are tolerated. The glycemic indices of commercial sports drinks have not been published, but drinks high in glucose would presumably have the highest GI, whereas those with more fructose or sucrose would tend to have a lower GI. It is important to note that the glycemic index of a food is not easily predictable. Multiple foods are generally consumed together; each food can impact the glycemic response of the other. In addition, the metabolic state of the person will influence glycemic index of a food. For example, a person with low glycogen stores will likely have less of an increase in blood glucose following food consumption than when initial glycogen stores are high.
The GI of foods consumed during exercise is probably not critical because the insulin response is muted during exercise. Thus, there will be less influence of GI on metabolic responses to exercise.
The best evidence for ingesting high GI foods is for post-exercise recovery of muscle glycogen. Several studies have shown an improved glycogen synthesis over at least the first hours of recovery when GI is high. High-GI foods include most breads, potatoes, and high-glucose sports drinks. If the recovery time is 20 h or longer, the GI of the carbohydrates ingested is probably less important than the quantity of carbohydrate consumed.
The possibility that a chronic diet of high-GI foods promotes higher insulin sensitivity and greater storage of muscle glycogen and triglycerides is intriguing for athletes, but this possibility need to be confirmed by studies using subjects who consume high-carbohydrate diets. Much more research needs to be done on the relationship between GI and general health, but because a low-GI diet seems likely to cause lower blood cholesterol and improved appetite control, a low-GI diet on an everyday basis is probably a good choice for athletes and non-athletes alike.
GLYCEMIC INDEXES OF COMMON FOODS
(Scale of 1 to 100)
- waffle – 76
- doughnut – 76
- bagel – 72
- wheat bread, white – 70
- bread, whole wheat – 69
- cornmeal – 68
- bran muffin – 60
- rice, white – 56
- rice, instant – 91
- rice, brown – 55
- bulgur – 48
- spaghetti, white – 41
- whole wheat – 37
- wheat kernels – 41
- barley – 25
- Rice Krispies – 82
- Grape Nuts Flakes – 80
- corn Flakes – 77
- Cheerios – 74
- shredded wheat – 69
- Grape Nuts 67
- Life – 66
- oatmeal – 61
- All Bran – 42
- watermelon – 72
- pineapple – 66
- raisins – 64
- banana – 53
- grapes – 52
- orange – 43
- pear – 36
- apple – 36
- potatoes, baked – 83
- potatoes, instant – 83
- potatoes, mashed – 73
- carrots – 71
- sweet potatoes – 54
- en peas – 48
- baked beans – 48
- chick peas – 33
- butter beans – 31
- lentils – 29
- kidney beans – 27
- soy beans – 18
- ice cream – 61
- yogurt, sweetened – 33
- milk, full fat – 27
- lk, skim – 32
- rice cakes – 82
- jelly beans – 80
- graham crackers – 74
- corn chips – 73
- life savers – 70
- angel food cake – 67
- wheat crackers – 67
- popcorn – 55
- oatmeal cookies – 55
- potato chips – 54
- chocolate – 49
- banana cake – 47
- peanuts – 14
- honey – 73
- sucrose – 65
- lactose – 46
- fructose – 23
- soft drinks – 68
- orange juice – 57
- apple juice – 41
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