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How Are Carbs Absorbed?


The majority of triathletes rely on carbs for energy, but how are carbohydrates absorbed and why should you care about different types?

When it comes to keeping your energy stores topped up for triathlon training or racing, you’ll find the vast majority of sports nutrition is concerned with getting carbohydrates into your system. These are delivered through a wide array of different nutrition products such as energy drinks and gels.

Depending on the different carb ingredients included in your sports nutrition, your body will react in different ways. Some carbs are absorbed fast, giving a brief sugar hit while others are designed to keep you going for longer.

Simple and Complex Carbs

Carbohydrate sources are usually categorised into either simple or complex carbs. The simplest are monosaccharides, which can’t be broken down any further so are quickly absorbed. These include dextrose and glucose.

Disaccharides such as cane sugar are broken down into two distinct carbohydrate sources, so take a little longer to process. Meanwhile polysaccharides contain several molecules to break down.

Energy Drink Sugar
Sugary carbs are the main constituent of most energy drinks. (Photo: Triathlon Vibe)

The longer the carbohydrate chain, the more digestion that’s needed to break it down into energy your muscles can use. This means simple carbs can quickly deliver fuel while more complex carbs take longer to hit the bloodstream and can provide a more sustained flow of energy.

Complex carbs also require more energy for the breaking-down process. This can place additional stress on the stomach and intestines as blood is being diverted to your working muscles.

Carbohydrate Absorption

Once the carbs are broken down into their simplest form, they are absorbed into the bloodstream through the wall of the small intestine. This causes insulin to be released from the pancreas, which induces the body’s cells to absorb the sugar.

The amount of insulin being released depends on the speed at which sugar hits the bloodstream – the more quickly it’s absorbed the more insulin that’s released. So, most short-chain carbs cause a higher, more short-lived spike in blood sugar levels than most long-chain carbs, which tend give more balanced sugar levels.

Glycaemic Index

It’s not always the case that insulin response decreases as carb complexity increases.

The potential sugar spike of a fuel source can be established by looking at its glycaemic index (GI), a simple scale rated from 0 to 110+ with higher numbers meaning a bigger peak in blood sugar levels.

Athlete stomach
The glycemic index of carbs affects insulin response and absorption. (Photo: Jannes Jacobs – Unsplash)

Maltodextrin, a very popular polysaccharide used in many sports drinks, causes a massive insulin spike. That’s because of its high glycaemic index of 110, compared to table sugar’s rating of 65.

Glycogen and Fat Storage

If you’re exercising hard, the body provides energy to working muscles for immediate use. Or, if the body’s not in direct need of the energy, it will try to store it for later use as glycogen in the muscles. However, once those stores are filled, any additional sugar is stored as fat.

The latter result is important to note for triathletes, especially those in the pursuit of increased wellbeing through healthy weight loss or improved body composition.

As the body can store roughly 90 minutes’ worth of glycogen in the muscles and liver without additional energy intake, you’ll need to carefully consider whether you really need to pump extra sugar into your system. It’s usually best saved for those super high-intensity workouts that quickly use up muscle glycogen or longer sessions that go beyond this timeframe.

Of course, this doesn’t mean you shouldn’t replace the fluid and salt that’s also lost while exercising during shorter and sweatier sessions to help avoid dehydration.

Triathlon Vibe
Triathlon Vibe
Triathlon Vibe is the home of triathlon training advice for beginner to expert triathletes. From sprint to Ironman, we share how to swim, bike and run stronger and faster.


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