Fuel 101: Carbohydrates

Fuel 101: Carbohydrates

Your Engine. Not Your Enemy.

Carbohydrates have spent the last decade being blamed for everything. Weight gain, inflammation, energy crashes, the lot. Low-carb diets have gone from fringe to mainstream, and the message that has filtered through to a lot of athletes is that carbs are something to be managed carefully, limited strategically, and ideally kept in check.

For endurance athletes, this is genuinely dangerous advice. Carbohydrates are not a dietary vice to be rationed. They are the primary fuel source for any exercise above a gentle stroll. If you want to train hard, race well and recover properly, you need to understand what carbs do and how to use them.

How carbohydrates work

When you eat carbohydrates, your body breaks them down into glucose. Some of that glucose goes straight into your bloodstream to fuel what you're doing right now. The rest gets stored as glycogen, primarily in your muscles and liver, ready to be called upon when you need it.

Your glycogen stores are finite. Depending on body size and training status, most people can store somewhere between 400 and 600 grams of glycogen, equating to roughly 1,600 to 2,400 calories of carbohydrate energy. That sounds like plenty. During a hard session, you can burn through it surprisingly quickly.

Once glycogen runs low, intensity drops. Your body can still produce energy from fat, but not at the rate required for moderate to hard training. This is the bonk, the wall, the sudden and very unpleasant realisation that you've run out of fuel. It is entirely avoidable.

The difference between types

Not all carbohydrates behave the same way, and the timing of what you eat matters.

Simple sugars are made up of one or two sugar molecules and digest quickly. Fruit, sports drinks, gels, white bread. These are ideal before and during training because they reach the bloodstream fast. They are not ideal in large amounts outside of exercise because the rapid blood sugar rise and subsequent drop can leave you feeling worse than before.

Complex carbohydrates are longer chains of sugar molecules that take more time to break down. Oats, rice, sweet potato, legumes, whole grains. These release energy more gradually, which makes them better suited to meals a few hours before training and to general daily eating. They also tend to come with fibre, vitamins and minerals that simple sugars don't.

Fibre is technically a carbohydrate but your body doesn't digest it. It slows digestion, feeds the healthy bacteria in your gut and keeps you feeling full. Most athletes don't need to worry about fibre timing around training itself. Getting enough across the day through vegetables, legumes and wholegrains is what matters.

The 30/60/90 framework

During exercise that lasts longer than about 60 minutes, you need to be taking carbohydrates in. How much depends on the duration and intensity of the session.

• Up to about 90 minutes: aim for 30 to 45 grams of carbohydrate per hour.

• 90 minutes to three hours: aim for 45 to 60 grams per hour.

• Beyond three hours: up to 90 grams per hour is achievable with the right combination of glucose and fructose sources, because they use different transport pathways in the gut.

These numbers feel large to a lot of athletes at first. The gut needs training just like legs do. Start at the lower end and gradually increase over weeks of consistent practice. Gastrointestinal issues during races are almost always a result of trying intake levels in competition that haven't been practised in training.

Why low-carb fails endurance athletes

Fat oxidation is a real thing. Your body does burn fat as fuel, particularly at low intensities. Some athletes have trained their fat metabolism to be quite efficient. But fat cannot be converted to energy quickly enough to sustain efforts above roughly 60 to 70 percent of maximum heart rate.

For anyone doing intervals, tempo runs, hard group rides, races or really any session that involves sustained effort, glycogen is what makes it possible. Running low on it mid-session is not a fitness problem. It is a fuelling problem.

Low-carb approaches may have a place for very specific athletes in very specific contexts. For most endurance athletes training regularly, restricting carbohydrates means restricting performance.

Carbs and your training week

Not every day requires the same amount of carbohydrate. A rest day or easy recovery session doesn't demand the same glycogen stores as a long ride or a track session. Eating more carbs on hard days and slightly fewer on easy days, sometimes called carb periodisation, lets you fuel the sessions that matter without eating more than you need overall.

The principle is simple: match your carbohydrate intake to the energy demands of the day. Fuel the work that needs fuelling.

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