Fast Carbs vs. Slow Carbs for Cyclists: When to Eat What

THE MYTH OF "SIMPLE VS. COMPLEX"

Many cyclists confuse simple with fast and complex with slow, but don’t be fooled! They’re not the same.

The confusion comes from a mix-up between chemistry (how a molecule is shaped) and biology (how fast your body actually processes it).

For decades, we’ve been told that complex carbs are slow and good while simple carbs are fast and bad. But when you look at how the human digestive system functions, that rule completely breaks down.

In fact, as sports nutrition expert Asker Jeukendrup explains in this article, not all complex carbs are slow and not all carbs behave exactly as their “simple” or “complex” label suggests.

Whether a carb is simple or complex comes down to chemistry; it's the molecular structure of the carbohydrate.

Simple carbs have one or two sugar molecules (glucose, fructose, table sugar, for example). Complex carbs are made up of long, winding chains of sugar molecules bonded together, like potatoes, rice and oats.

But simple versus complex doesn’t always have to do with how fast they digest, it’s just the shape of their molecules.

What actually matters on a ride is the biology aspect – how fast or slow a carb digests and raises your blood sugar, which is measured by the Glycemic Index.

We can look at a few examples that show how simple doesn’t always mean fast and complex doesn’t always mean slow.

For example, maltodextrin is a favourite ingredient in energy gels and sports drinks. It has a long chain of glucose molecules linked together, so technically, it is a complex carb. But the bonds holding it together are so weak that the enzymes in your saliva break those chains easily, allowing it to pass through the stomach rapidly. It has a complex structure, but it digests quickly and its GI is higher than table sugar.

Fructose is the sugar found naturally in fruit. Chemically, it is a simple carbohydrate consisting of just a single sugar molecule. You would think it would spike your blood sugar immediately, but it doesn't. The fibre in fruit slows it down.

Fructose is processed differently than glucose and must first be metabolized by the liver before contributing significantly to energy supply.

When you consume it, it has to wait in line to pass through your gut, travel down the portal vein to your liver, and wait for your liver to convert it into glucose. These extra steps act like a bottleneck. So while its chemistry has a simple structure, an apple has a slow delivery on results.

When you are looking for fuel to avoid bonking on a hard trainer session or an outdoor climb, stop looking at whether a food is considered simple or complex.

Instead, look at its digestibility. You want foods that clear your stomach quickly and require minimal processing by your liver so the energy goes straight to your cycling muscles.

In other words:

BEST FAST-DIGESTING CARBS FOR CYCLING (AND WHEN TO USE THEM)

On Sundays, my friends and I hit the road for a long, chatty Zone 2 cruise. I feel like I can roll along for hours carefree with almost no food at all, other than a banana or an oatmeal bar before I leave the house. My body is happily burning fat for fuel, and my energy feels limitless.

But Saturday morning? I roll up to the local group ride, which happens to be a pack of big, powerful guys. The second the pace snaps, my entire reality shifts. Because I’m a smaller rider, just staying in their draft and matching their raw speed means my engine is immediately forced into the redline. They might be cruising, but I’m working. And maybe crying on the inside.

I can’t afford the luxury of digesting a slow carb, and I certainly can't burn fat fast enough to keep up. If I’m not sucking down fast-acting energy gels or downing Gummy Nerds like nobody's business, I’m going to bonk and get dropped.

Why? Your body’s capacity to store carbohydrates (as glycogen) is limited to about 90-120 minutes of moderate cycling. When the pace quickens, you need fast carbs to survive.

Most rides lasting longer than 60-90 minutes benefit from carbohydrate intake, particularly when intensity is moderate to high. Aiming for 60 to 90+ grams of fast carbs per hour keeps your blood glucose high and spares your liver and muscle glycogen.

Some 30-60 minutes before your ride, eating a quick snack like a banana or a slice of white toast with honey will top off your tank without leaving a heavy mass sitting in your stomach.

After your ride, you’ll want to top off your stores pretty quickly. Those first 30 minutes after a grueling ride are affectionately known as the "glycogen window."

Fast carbs combined with protein trigger a rapid insulin response, rushing nutrients into depleted muscles to kickstart recovery.

BEST SLOW-DIGESTING CARBS BEFORE LONG RIDES

Slow carbs are the foundation of your training nutrition. You need them; just maybe not while you're riding your bike.

Slow carbs are an important part of your daily meals. Breakfast, lunch, and dinner on non-consecutive training days should focus on slow carbs to:

• Keep your insulin stable
• Prevent energy crashes
• And provide essential micronutrients

Slow carbs are also good a few hours before a ride. For example, a bowl of oatmeal or sweet potato a few hours before rolling out allows your body plenty of time to digest the food and get it into your muscles without causing a pre-race insulin spike.

Use slow carbs on your off-bike and recovery days, too. When your energy expenditure is low, slow carbs provide satiety and steady energy without causing unnecessary blood sugar spikes.

WHY "SLOW CARBS DURING EXERCISE" IS MOSTLY A MYTH – WHAT THE RESEARCH SAYS

Every few years, a new trend emerges suggesting that cyclists should eat slow-release carbs (like specialized starches or fat-adapted diets) during exercise to maintain stable energy.

The sports science community overwhelmingly disagrees.

When you are riding hard, blood flow is diverted away from your stomach and sent to your hardworking quads and hamstrings. Your GI tract is compromised.

Trying to digest fibre, fats, or complex slow carbs during a hard block of intervals is a fast track to bloating, cramping, stomach ache and nausea. The carbs won’t digest fast enough to hit your blood stream when you need it.

In this BikeRadar article by Jack Evans, performance nutritionist Will Girling explains that hitting the right mix of calories, carbs, protein and fat to support your daily training will help you to improve more quickly.

The right carbs at the right times will have a huge impact on whether your ride is successful or not.

INDOOR TRAINING NEEDS FAST FUEL, TOO

Don't make the mistake of thinking indoor cycling requires less fuelling.

In fact, because indoor trainers provide a constant resistance with no coasting or descents, your calorie burn can be even higher. Indoor rides are a great way to practice race nutrition and work on increasing your carb intake per hour.

High intensity intervals are fast-carb territory. Those savage VO2 max sessions or punchy ROUVY races need fast carbs to keep going.

Your body can’t burn fat fast enough to sustain these efforts; feed it gels or sports drinks (or Nerds Gummy Clusters).

On the other hand, if you are planning a steady, two-hour cruise through a scenic ROUVY route? This is where your slow-carb pre-loading shines.

Eat a clean, slow-carb meal two hours prior, and you'll easily coast through the first hour before needing to reach for your on-the-bike fast carbs.

DON'T FORGET THE INDOOR CLIMATE FACTOR

Heat and intensity don't disappear just because you're at home.

Because you lack the natural cooling wind of outdoor riding, you sweat more indoors. Drinking a sports drink loaded with fast carbs and electrolytes solves your fuelling and hydration needs simultaneously.

COMMON CARB MISTAKES THAT COST YOU WATTS

Making a mistake on your carb intake can slow you down.

For one thing, eating fibre during a race can cause major GI distress, which will make riding miserable.

But being afraid of sugar is another mistake that can slow you down. Although sugar off the bike can lead to crashes; sugar on the bike saves your race because it delivers fast fuel to your hardworking muscles.

Remember that under-fuelling short, intense rides will also hurt your results. You might think, "It's only an hour on ROUVY, I don't need carbs," but an hour of high-intensity sweet-spot intervals will completely drain your immediate glucose stores, killing your power in the final 15 minutes.

WRAP-UP: MATCH THE CARB TO THE CADENCE

At the end of the day, there is no such thing as a "good" or "bad" carbohydrate – there’s only the right tool for the ride.

Leave the slow carbs, whole grains, and veggies for your recovery days and your pre-ride breakfast prep.

But when the pace rises, the intervals start, or you are trying to hold onto a fast wheel, don't be afraid of the sugar.

Whether you’re packing your top-tube bag with Nerds Gummy Clusters for Saturday's group ride or mixing a high-carb drink for a grueling virtual block on ROUVY, remember the golden rule:

See you on the leaderboard – just make sure your pockets are full!

RELATED READING

• ROUVY, “Endurance Nutrition For Cyclists And Triathletes: What To Eat & When”
• ROUVY, “The Best Energy Gels For Cycling: What To Use, When, And Why It Matters”
• Harvard T.H. Chan School of Public Health, "The Nutrition Source: Carbohydrates And Blood Sugar"
• International Society of Sports Nutrition, “Position Stand: Nutrient Timing”
• ROUVY, “Low-Carb And Cycling: Does It Really Work For Riders?”