The Glycemic Index & Carbohydrate Fueling

Types of Sugars

Glucose
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
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
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.

Sucrose
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.


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