The Glycemic Index & Carbohydrate Fueling

by | Aug 26, 2021 | 0 comments


In this blog, Dr. Bucci discusses how the Glycemic Index can help inform a comprehensive carbohydrate fueling strategy – and the pitfalls that explain why it’s only one input and not the complete answer for maximizing efficient energy production.

Glycemic Index (GI)

The Glycemic Index (GI) is a characteristic of carbs in different foodstuffs that affects choosing what carbs to use during long-term exercise. Since glucose is by far the main sugar in the bloodstream, GI tracks the change in blood glucose levels for 4-6 hours after eating on an empty stomach, and it’s measured by administering 100 grams (about a quarter pound) dry weight of a specific carb source. The area under the curve for the rise in blood sugar from 100 grams of glucose is defined as a GI = 100.

Usually 100 grams of a single source – pure sugars/starches, bananas, spaghetti, and so on – is used, and then compared to the rise in blood glucose. A GI of 100 means the tested source raises blood sugar the same as an equal amount of glucose. A GI of more than 100 means that foodstuff will raise blood sugar more than glucose itself, and conversely, a GI under 100 means that foodstuff does not raise blood sugar as much as glucose.

Over the years, single nutrients/foods, meals, packaged foods and nutrient/food combinations have been catalogued with a GI number so one can get a rough idea of how much and how quickly a substance will increase blood glucose. The GI was developed specifically for diabetics so they could make better food choices to keep their blood sugar from going up too much, but its application clearly extends into any research or discipline focused on blood sugar levels – like, say, endurance athletics.

GI Pitfalls

Testing Variations Depend on the Setting – Quality Counts
Despite the appeal of its simplicity, GI alone is not a very helpful or realistic number for practical usage. In order for the GI number to mean something, there are variables in determining GI that need to be known, some of which are usually standardized, but which leave room for faulty conclusions and assumptions. How much of the carb source was actually consumed? And with how much water/liquid? Was the test done after establishing a baseline for the subject with a placebo? Or was the GI number simply compared to a textbook average GI of 100 from a table to save time, effort, and expense? How long did the testing period last? Sometimes a single blood glucose level test at 2 hours after eating (called a 2hrPP or postprandial) is used as a quick and dirty GI, since 2 hours is the usual time most foodstuffs produce a peak blood level.

It’s easy to imagine how the added stressor of endurance exercise can complicate all of the above in pretty essential ways.

Stomach Emptying
The GI number is also dependent on starting with an empty stomach, because how fast and evenly a substance is emptied from the stomach is a major determinant of GI. Generally, fats, fibers and proteins slow stomach emptying when mixed with carbs, resulting in a lower GI. The amount of water present is also a consideration.

This creates clear issues in applying GI learning to real life situations and, especially, to endurance athletics, where any consumable’s GI will be impacted by myriad variables that in turn impact stomach emptying. More food and drink means slower stomach emptying, so GI calculated under controlled clinical conditions can’t be relied on for an accurate assessment when the chips are down.

GI Does Not Equal Carb or Calorie Content – Quantity Counts
Though GI only looks at blood glucose, whatever else is in the substance tested also determines how blood glucose increases, regardless of how much “carbohydrate” it has. In a vacuum, giving 50g of glucose yields a GI of 50; when those 50g are delivered as part of a larger quantity of foodstuff that also introduces variables like those discussed above, then those 50g of glucose may not translate to an exact GI of 50. If you just consider the GI number as a measurement of glucose in the food, you could be deceived by a low GI foodstuff which may actually contribute more total carb calories through other sugars and different uptake speeds.

A substance can also be slowly converted to glucose, longer than the glucose measurement time period, again giving a false impression of glucose delivery. Pure fructose has a GI of ~25, but it is a pure sugar carb calorie. For decades, fructose’s low GI tricked the medical community into thinking that you can eat a lot of fructose without raising blood sugar too much (and justifying high-fructose corn syrup intakes too).

What was missed is that fructose has the same calories as glucose, so eating more fructose may keep blood sugar low but easily leads to getting more calories than eating glucose itself would have. Also, fructose is quickly converted to triglycerides, which was a hidden risk for gaining body fat weight and cardiovascular ill health, explaining why even “controlled” diabetics still suffered long-term complications of too much sugar intake.

GI Is Helpful for Endurance Athletics
Despite all of these complications and variables, GI does provide guidance on how to get the most and fastest blood glucose increase from specific substances, helping you maximize blood sugar levels during long-term exercise. In general, higher-GI sugars and foods are more desirable for elite athletes needing to maintain blood sugar levels (fuel!) during exercise.

Of course, not all sources are created equal. As we’ve discussed here and elsewhere, some sugar sources (read: fructose) can be traps, leading to net-zero energy production and impeding the kinds of sugars that more easily produce energy.

Glycemic Index of Some Sugars and Foods

Food or Sugar Glycemic Index (GI)*
Corn syrup (Glucose syrup) 115
Maltose 105
Maltodextrins (Glucose polymers) 90-110
Glucose (Dextrose) 100
White rice 87-94
Rice crackers 87
Instant Mashed Potatoes 87
Gatorade® 78
Watermelon 76
White bread (wheat flour) 75
Cornflakes 75-85
Sucrose (table sugar) 65
Popcorn 65
Soft drinks, Sodas 60
Honey  55-61
Banana (varies with ripeness) 55
Orange juice 50
Lactose 46
Fructose 15

* Keep in mind that there is some variance for each value, usually ~5-10%, along with natural variation of carb content in foods. Also, 100 grams is a big amount of glucose seldom taken by itself – that’s 400 Calories!


Types of Sugars

In terms of immediate use within the body, glucose is the most important carbohydrate since it fits into the energy-producing pathways that generate ATP – which is cellular energy. The body eventually breaks down all sugars and carbohydrate sources into glucose, which is the primary form in which sugar enters cells to be used for energy. That’s why it’s the basis for the Glycemic Index.

Glucose is a fast carb, and even during times of exercise at or above threshold, glucose can be easily digested. It’s directly absorbed by the small intestine and directly transported to the cells to be metabolized with highest percentage of conversion to metabolic energy (ATP). Glucose can also be stored as glycogen (chains of glucose) within muscles and the liver, and can also be converted to fats for long-term energy storage if consumed in excess of use.

The only problem with glucose as exercise fuel is that, eventually, your cell’s glucose receivers max out, and any additional glucose can’t be absorbed into muscles, spiking blood sugar levels.

Glucose Polymers
As noted above, anything besides glucose needs to be converted to glucose to become fuel, but there are alternative glucose sources that can streamline the uptake process. Maltose (glucose-glucose disaccharide; GI 105), for example, delivers easily unpackable glucose to cells, and it does it even a little faster than pure glucose alone.

Glucose polymers are another efficient delivery option. AKA dextrins or starches, glucose polymers are simply short, straight (or unbranched) chains of glucose molecules, often called maltodextrins. Even larger chains get glucose into the bloodstream as fast or often faster than pure glucose itself, because amylase, the enzyme that converts starches to glucose in the gut, is in abundance.

The glycogen in which we store energy is itself a large, highly branched-chain glucose polymer in muscles and the liver, designed to be broken down into single glucose molecules relatively quickly by specific enzymes that can attack a spherical glycogen molecule simultaneously in 3D. (Your gut does not have these enzymes, so eating glycogen produces a low GI, making it a poor consumable source of blood glucose.)

These properties of glucose polymers mean that, in EFS-PRO, the CCD (Cyclic Cluster Dextrin, a highly branched chain polymer) is better suited for fast stomach emptying and fast enzymatic conversion to glucose, giving EFS-PRO advantages over other carb sources for delivering glucose to working muscles.

Fructose has a relatively low GI of 20±5 and is a simple sugar (monosaccharide) like glucose and galactose. Fructose is 75% sweeter than glucose and is generally found in honey and fruits in addition to its many uses as a food-sweetening additive.

It is absorbed more slowly into the bloodstream than straight glucose and sucrose, and therefore has a less erratic effect on blood sugar levels at rest. Diabetics or those that are very sensitive to changes in blood sugar find fructose to be advantageous. But, as a result of its slow absorption, beverages that contain fructose can cause gastric upset and slow gastric emptying. Research suggests that fructose is more tolerable when combined with sucrose and glucose.

Avoid beverages that list “high fructose corn syrup” as primary ingredients as they will slow fluid uptake and not provide optimal sugars to support exercise energy requirements. As a pre-exercise meal, or between workouts, fructose is an excellent source of carbohydrates; however, we omit it from EFS, EFS-PRO, and Liquid Shot because it’s a drag on ATP production during training.

Galactose is a simple sugar that has recently shown up in sports drinks. Lactose is the primary sugar in dairy products and is composed of one molecule of glucose and one of galactose. Because of its galactose content, it is more slowly absorbed into the bloodstream than pure glucose, and it’s therefore more blood sugar-friendly.

The GI of galactose could not be found on any of the official GI lists*, though at least one popular sports drink does claim that galactose is absorbed quickly, like glucose, without a subsequent increase in insulin release; however, this absorption is limited to getting into intestinal cells. Enzymes are still needed to convert galactose to glucose, either in gut cells or other tissues, which slows the delivery of glucose to generate ATP.

Also, lactose intolerance makes galactose undesirable for long-term endurance events in many persons, and it often will not manifest until a large amount has been ingested, with adverse gastrointestinal consequences.

With a GI of 68±5, our final sugar, sucrose, sits between glucose and fructose on the Glycemic Index, which makes sense as it’s composed of one molecule of glucose and one molecule of fructose. This is simply table sugar. White or brown, powdered or granulated – the everyday kitchen staple. Chemically, it’s a disaccharide comprising one molecule of glucose and one of fructose. It is usually made from refining extracts of sugar beets or sugar cane.

Sucrose is the barometer for sweetness: glucose has a lower sweetness than sucrose, but fructose is 1.5 times sweeter. Again, given that it incorporates both, sucrose’s sweetness profile isn’t surprising.

Taste is critical for a sports drink to be utilized correctly – if you do not drink it, it cannot help performance; however, adding more sucrose or fructose to give a sweeter taste to encourage consumption comes at the cost of slower glucose delivery. A happy medium of combining glucose, sucrose, and glucose polymers with higher GI than glucose can be adjusted to achieve a similar GI to glucose alone while improving taste.

This approach can also actually increase glucose delivery through the mechanism of Multiple Transportable Carbohydrates, or MTCs, which we take advantage of in EFS, EFS-PRO, and Liquid Shot.

Multiple Transportable Carbohydrates (MTCs)

For our purposes here, we’ll simply summarize the concept of MTCs. (For a deep dive, click here.) Under the right conditions, blending other sugars with glucose can add to the total glucose delivered to working muscles when glucose consumption is maximal and cellular glucose transport receptors (such as GLUT4) are saturated. For the best results, the disaccharide sucrose stars in the role of “other sugars.”

The process essentially uses the two sugars in sucrose (one glucose and one fructose) to produce the energy equivalent of a single glucose, letting you raise the fuel tank’s ceiling even if your body’s ability to process pure glucose is already maxed out. So the energy is more expensive, costing two sugars instead of one, but that’s because the cheaper energy production options are already exhausted. (This is, of course, a gross oversimplification of the process. Click the link above for a more comprehensive exploration.)

Goldilocks Syndrome for Glucose Delivery

As the above demonstrates, maximizing energy production isn’t as simple as providing high-GI carb sources. Instead, a combination of high-GI glucose polymers (specific maltodextrins and/or CCD), plain old glucose, and deceptively low-GI sucrose can maximize glucose delivery to working muscles by three avenues: encouraging consumption through taste, maximize stomach emptying, and saturating all available glucose entry points in the gut and at the cells.

Luke Bucci PhD

Luke Bucci PhD

Chief Scientific Officer

Dr. Bucci is the industry leader in sports nutrition. He’s also an accomplished author and lecturer and holds multiple patents and patent applications on clinical laboratory testing methods and nutritional supplements.
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