The glycemic index


The glycemic index

The “glycemic index” (GI) is a term used to describe the blood glucose response to a particular food. Specifically, it is the area under the curve of the blood glucose vs. time plot when one consumes 50 g available carbohydrates from a specific food compared with pure glucose [1]. The concept of GI is used more colloquially, however, to simply refer to how quickly sugar reaches the blood stream after food consumption. A food with a high GI will lead to a rapid, and often high, rise in blood sugar while a food with a low GI will lead to a slower, more sustained blood glucose concentration. Because blood glucose stimulates insulin secretion, foods with a high GI tend to stimulate a rise in blood insulin levels as well.

Using the GI of carbohydrates is a nutritionally-advanced technique to regulate blood glucose and insulin concentrations to optimize athletic performance. Athletes tend to consume carbohydrates before exercise to maintain blood glucose concentrations and fuel exercise. After exercise, athletes tend to consume carbohydrates for two purposes: 1) to restore glycogen stores, and 2) to spike insulin concentrations and further promote entry of amino acids into muscle cells, thereby promoting accretion of lean body mass. It is generally believed that GI can be leveraged to deliver the desired amount of glucose to muscle to maximally activate either locomotive action or muscle protein synthesis. However, unfortunately, the scientific literature does not support these techniques as strongly as one might expect. This article summarizes the studies that investigate how the GI of carbohydrates affects blood glucose levels during exercise, exercise performance, and post-exercise anabolic processes.
Low GI carbohydrates may be your best bet for a pre-workout meal

Consumption of a high GI food will result in quicker availability of carbohydrates in the blood for utilization during exercise. After blood glucose reaches its peak, however, blood glucose is likely to quickly fall back to baseline. It has been shown in some (e.g., [2], [3]) but not all (e.g., [4], [5]) studies that carbohydrate oxidation is greater during exercise after a high GI pre-workout meal. Accordingly, fat oxidation is greater during exercise after a low GI pre-workout meal. With that said, there is no consensus as to whether a high- or low GI pre-workout meal will lead to better performance. In fact, a recent (preliminary) meta-analysis concluded that the GI of a pre-exercise meal did not significantly affect performance at a subsequent time trial or time-to-exhaustion exercise session [6].

When looking across all studies, if a difference in performance was noted, which was not always the case ([5], [7]–[9]), participants performed better when they consumed the low GI pre-workout meal compared to the high GI meal ([4], [10]–[13]).

The variability in results highlights the fact that many factors play into how GI affects performance. For example, if carbohydrates can be consumed during exercise, a low GI pre-workout meal may be best, whereas if not, a high GI pre-workout meal may be best [6]. Also, the duration of the exercise is important; longer endurance events will likely benefit from a low GI pre-workout meal [3], whereas it may not be as important for shorter events. Future research will be necessary to tease out the effects of the intensity of the exercise, the nature of the exercise (i.e., aerobic vs. anaerobic), differences in individuals such as habitual diet and training status, and other factors.
A high GI carbohydrate will accelerate restoration of glycogen stores

In the case where restoration of glycogen levels needs to be accomplished quickly, as in a double-header or when one has two workouts in a day, high GI carbohydrates (with protein) are typically recommended after the first workout to accelerate the glycogen synthesis process [14], [15]. However, evidence regarding performance effects after a glycogen-depleting workout and then a post-workout meal is equivocal. Some studies (e.g., [16]) show that a high GI post-workout meal will result in more complete glycogen synthesis and an improved workout, but others (e.g., [17]) have reported that a low GI post-workout diet will increase fat oxidation which, in turn, can improve subsequent endurance performance. Brown et al. [18] confirmed that glycogen restoration is faster with high GI carbohydrates and fat oxidation is greater with low GI carbohydrates, and subsequently demonstrated no difference in athletic performance.

In the case of a single workout per day, there is no particular urgency in restoration of glycogen stores. Consumption of an adequate amount of carbohydrates, regardless of timing and, presumably, GI, will allow glycogen to be restored before the next workout [15], [19], with restoration of muscle glycogen occurring by about 24 h post-workout under these conditions. Additionally, in line with what was mentioned earlier, habitual consumption of low GI carbohydrates promotes fat oxidation and prevents high fluctuations in blood sugar, which may overall be a healthier choice.
The use of GI with post-workout protein is largely understudied in regard to maximizing muscle protein synthesis

It is generally believed that a post-workout insulin spike is ideal for maximizing anabolic processes. Because high GI foods can do that, they are recommended post-workout. However, despite these wide-spread claims, there is little scientific support for either the necessity of the carbohydrate-induced insulin spike or the requirement of high GI foods for maximal rates of muscle protein synthesis.

In a 2013 review, Aragon & Schoenfeld noted that the principle role of insulin in the post-workout window is to prevent muscle protein breakdown. Protein itself can stimulate insulin secretion, and the degree to which it can stimulate insulin secretion (to 15-30 mU/L, or 3-4 times greater than fasting levels) may be sufficient to prevent muscle protein breakdown (reviews: [15], [20]). Indeed, Kreider et al. [21] performed a study in which participants consumed a post-workout meal composed of whey protein plus either sucrose, honey, or maltodextrin. They concluded that, in combination with whey protein, any of these carbohydrates was sufficient to stimulate protein synthesis. However, these three carbohydrate sources all have a relatively high glycemic load. Unfortunately, there are no studies that directly compare the chronic effects of post-workout high- and low GI carbohydrates on anabolic processes.
High and low GI foods

So, what foods have high or low GIs? In general, highly processed foods that are mostly carbohydrates have high GIs. These include sports drinks like Gatorade®, cornflakes, instant rice, instant mashed potatoes, baked potatoes, crackers, and pretzels. Less processed, carbohydrate-based foods tend to have lower GIs such as whole grain bread, lentils, and apples. Foods with carbohydrates that also have protein and/or fat also tend to have lower GIs, such as milk and dark chocolate. There are many sources available with more complete lists of foods and their corresponding GIs. A table with over 2,480 individual food items is available here [22]:

Atkinson, F. S., Foster-Powell, K., & Brand-Miller, J. (2008). International Tables of Glycemic Index and Glycemic Load Values: 2008. Diabetes Care, 31(12), 2218–2220.
Conclusions

The GI can be leveraged to quickly restore glycogen after a workout; however, there is equivocal evidence that the GI of carbohydrates is important in regard to the extent of glycogen restoration after a workout and subsequent performance, spiking insulin for maximum muscle protein synthesis after a workout, and maximizing performance before a workout. Importantly, this does not mean that GI is not important, it means that GI is not always important. Athletic events vary greatly in their physical demands (e.g., intensity, duration, requirements for strength, agility, cognition) and athletes’ bodies also vary greatly (e.g., training status, habitual diet). There are many athletes who likely benefit from consuming low GI carbohydrates before a long run or high GI carbohydrates after a lifting session. In the coming years, the literature will likely resolve some of its discrepancies with further research that distinguishes between some of the key variables listed above. In the meantime, the best way to determine if you are sensitive to glycemic load is to conduct some self-experimentation in the off-season to find which nutritional strategies will help you reach your performance potential.
References


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