The glycemic index for the athlete

The glycemic index (GI) appeared as a tool for people with diabetes, but quickly migrated into fitness: "slow carbs are good, fast ones are bad". For an athlete this scheme is too simplified. Depending on the moment of the training day, a "fast" carbohydrate can be the best choice. The editorial team explains how GI works, where its limits are and how to apply it sensibly.

What the glycemic index is

The glycemic index was proposed by Jenkins and colleagues in 1981. It shows how strongly a product raises the level of glucose in the blood over the two hours after consuming a serving containing 50 g of digestible carbohydrates, compared with a reference — pure glucose, whose GI is taken as 100.

Products are conventionally divided into three groups: low GI (55 and below), medium (56–69) and high (70 and above). Products with a high GI are digested quickly and give a sharp glucose peak, while those with a low GI are digested slowly and raise glucose smoothly.

GI is affected by many factors: the type of starch, the content of fibre, fats and protein, the degree of processing of the product, the method of preparation. For example, overcooked pasta has a higher GI than pasta cooked "al dente", and a ripe banana higher than a green one.

International GI tables, in particular the edition of Atkinson and colleagues (2008), have gathered values for thousands of products. However, even for a single product the data from different countries can differ substantially due to variety, processing and testing methodology, so the GI value is a guide, not an exact constant.

Glycemic load and the limitations of GI

The main drawback of GI is that it does not take into account the amount of carbohydrates in a real serving. Watermelon has a high GI, but in an ordinary serving there are few carbohydrates, so its real effect on glucose is moderate. To account for this, glycemic load (GL) was devised: GI × grams of carbohydrates in the serving ÷ 100.

The second limitation — GI is measured for an individual product eaten on an empty stomach. In real life we eat mixed dishes: rice with chicken and vegetables, bread with butter and cheese. Protein, fat and fibre slow absorption, and the GI of a dish differs from the GI of the individual ingredients.

The third limitation — individual differences. The glucose response to the same food differs between people depending on insulin sensitivity, the microbiome, the time of day and previous physical exertion. In trained people, muscles absorb glucose especially actively after a workout.

So GI is useful for an athlete as a general characteristic of the "speed" of carbohydrates, but it should not be turned into a rigid system. The amount of carbohydrates and their timing usually matter more than the exact GI value.

GI before training

In the 1990s the idea was popular that products with a low GI before training are better: they give more stable glucose and do not cause a "reactive" drop in sugar at the start of exertion. The study by Thomas and colleagues (1991) showed that lentils before prolonged work on a cycle ergometer prolonged the time to exhaustion compared with potato.

However, subsequent work did not confirm a stable advantage of a low GI, especially if the athlete consumes carbohydrates during exertion as well. The review by Burke and colleagues (2011) concluded that in that case the GI of the pre-workout food has little effect on the result.

A short-term decrease in glucose at the start of a workout after high-GI food really does happen, but in most people it does not worsen performance and has no symptoms. People who feel weakness or dizziness should move the food closer to the start or choose options with a lower GI.

• 3–4 hours before the start — a full meal with carbohydrates, a moderate amount of protein and a small amount of fat and fibre.
• 1–2 hours before — a lighter snack: a banana, oatmeal, toast with honey.
• Before a competition — only tried and tested products that the stomach tolerates well.

More important than GI in practice is the comfort of the gastrointestinal tract. Legumes with a low GI are rich in fibre and can cause bloating during running, so before intense sessions they are often avoided, despite the "correct" GI.

During exertion and for recovery

During prolonged work of more than 60–90 minutes, the goal is to quickly deliver glucose to the muscles. Here high-GI products — gels, sports drinks, jelly sweets, white bread with honey — are the best choice. According to the recommendations of Burke and colleagues (2011), for exertion lasting 1–2.5 hours 30–60 g of carbohydrates per hour are advisable, and for longer ones — up to 90 g/h provided glucose and fructose are combined.

After a prolonged or exhausting workout, muscles restore glycogen fastest in the first hours. The study by Burke, Collier and Hargreaves (1993) showed that a diet with high-GI products over 24 hours of recovery provided a greater store of muscle glycogen than a diet with a low GI at the same amount of carbohydrates.

For rapid recovery, when the next workout or competition stage will take place in a few hours, roughly 1.0–1.2 g of carbohydrates per kilogram of body weight every hour during the first 4 hours is recommended. It is exactly then that a high GI provides a real advantage.

But if there is a day or more until the next workout, the significance of GI for glycogen recovery decreases: the main thing is the total amount of carbohydrates per day. In this case you can choose products that are better suited for general health.

GI in the athlete's everyday diet

Outside the training "windows", a diet dominated by products with a low and medium GI — whole grains, legumes, vegetables, fruit — provides more fibre, trace elements and better satiety. For health this is advantageous regardless of sporting goals.

For athletes who control their weight or are at a stage of reducing fat mass, products with a lower GI can help control appetite better. At the same time, there is no evidence that a low GI in itself accelerates fat burning at the same calorie intake.

For endurance sports with a large training volume, an excessive enthusiasm for "slow" carbohydrates can be a problem: a large volume of fibre makes it harder to consume the needed amount of carbohydrates and causes discomfort in the stomach. For such athletes, part of the carbohydrates in the form of refined products is quite appropriate.

For people with diabetes who train, GI and GL are important in sport too, but the nutrition plan and glucose control must be agreed with an endocrinologist, since physical exertion changes the need for glucose-lowering medications.

Editorial conclusions

The glycemic index is a useful but limited tool. It describes the speed of carbohydrate absorption, but does not take into account the serving size, mixed dishes and individual differences.

For an athlete there are no "bad" and "good" carbohydrates: during prolonged exertion and for rapid recovery, high-GI products are appropriate, while in the everyday diet — low and medium GI ones.

The amount of carbohydrates and their timing are mostly more important than the exact GI value, and tolerance by the stomach before the start is more important than any tables.

The editorial team also recommends familiarising yourself with our materials about carbohydrate loading, about sports gels and isotonic drinks and about nutrition in the heat.

References

1. Jenkins DJ, Wolever TM, Taylor RH, et al. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr. 1981;34(3):362–366.
2. Atkinson FS, Foster-Powell K, Brand-Miller JC. International tables of glycemic index and glycemic load values: 2008. Diabetes Care. 2008;31(12):2281–2283.
3. Burke LM, Collier GR, Hargreaves M. Glycemic index — a new tool in sport nutrition? Int J Sport Nutr. 1998;8(4):401–415.
4. Burke LM, Collier GR, Hargreaves M. Muscle glycogen storage after prolonged exercise: effect of the glycemic index of carbohydrate feedings. J Appl Physiol. 1993;75(2):1019–1023.
5. Thomas DE, Brotherhood JR, Brand JC. Carbohydrate feeding before exercise: effect of glycemic index. Int J Sports Med. 1991;12(2):180–186.
6. Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci. 2011;29 Suppl 1:S17–S27.
7. Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine joint position statement. Nutrition and athletic performance. Med Sci Sports Exerc. 2016;48(3):543–568.