Carbohydrates and training: how to fuel your workouts

Carbohydrates are frequently misunderstood in fitness discussions. They are often blamed for weight gain or treated as an optional extra for athletes. In reality, they are the most efficient energy source your body has for physical activity.

Understanding how carbohydrates work allows you to plan your meals around your training schedule. This ensures you have the energy to complete difficult sessions and recover properly afterwards.

How your body stores and uses energy

When you eat foods containing carbohydrates, your digestive system breaks them down into glucose. This glucose enters your bloodstream to be used for immediate energy. Your body tightly regulates the amount of glucose in your blood to keep your systems functioning smoothly. Any glucose you do not need right away is stored for later use.

Your body stores this reserve energy as glycogen. You have two main storage sites: your liver and your skeletal muscles.

Liver glycogen acts as a general reserve. Your brain relies heavily on glucose, and the liver releases it into the bloodstream to keep your blood sugar levels stable throughout the day and night.

Muscle glycogen acts as a local fuel source. The glycogen stored in your leg muscles will be used to power your legs during a run or a heavy squat. It cannot be transferred to your arms or back. Because muscle glycogen is locked in place, you must ensure your working muscles are adequately fuelled before a demanding training programme begins.

Your total storage capacity is limited. During long, hard efforts these stores can run low, often somewhere beyond the first hour and a half, although this varies a lot with the person, the pace and what they ate beforehand. Once they run low, your ability to maintain a high level of effort drops sharply.

Why training intensity matters

Your body uses a mixture of fat and carbohydrates for energy at all times. The ratio of this mixture changes depending on how hard you are working.

During rest or low-intensity activities like walking, your body relies primarily on fat. Fat is an abundant fuel source, but converting it into usable energy is a slow process.

As you increase your pace or lift heavier weights, your muscles need energy much faster than fat metabolism can provide. At this point, your body shifts to burning carbohydrates. Carbohydrates can be broken down rapidly, even when your oxygen intake is limited during heavy breathing.

If you perform high-intensity intervals, heavy weightlifting or fast running without adequate glycogen, your performance will suffer. You will likely feel sluggish, weak or unusually fatigued. You cannot force your body to burn fat at the speed required for maximum effort.

Understanding digestion speeds

Not all carbohydrate sources affect your body in the same way. The speed at which they digest determines when they are most useful for training.

Foods rich in fibre, such as oats, brown rice, beans and wholemeal bread, digest slowly. The fibre slows down the release of glucose into your bloodstream. This provides a steady, prolonged supply of energy without causing a sudden peak in blood sugar. These are ideal for your regular daily meals and for eating several hours before a workout.

Foods with very little fibre, such as white bread, plain pasta, white rice and fruit juice, digest quickly. They cause a rapid rise in blood sugar, delivering energy to your muscles almost immediately. While these are less useful for keeping you full during the day, they are highly practical shortly before or during a long training session.

Sports gels and drinks are simply concentrated, low-fibre carbohydrate. You do not need them for everyday training, and ordinary foods such as bananas, white bread with jam or sweets can do a similar job.

Timing your intake for training

You do not need a complex fuelling strategy for every workout. Your requirements depend entirely on the duration and intensity of your session.

Sessions under 60 minutes

If you are training for less than an hour, you rarely need to eat carbohydrates immediately beforehand. Your existing muscle glycogen stores are sufficient for a 45-minute weightlifting session, a short swimming workout or a 30-minute run.

You can simply eat a normal meal containing slow-digesting carbohydrates two to three hours before you start. If you train first thing in the morning, you can rely on the glycogen stored from your dinner the night before. There is no need to wake up early just to eat.

Sessions between one and two hours

When a workout stretches past the 60-minute mark, your glycogen stores will begin to deplete noticeably. Starting the session with full stores becomes much more critical for maintaining your performance.

Aim to eat a substantial meal rich in carbohydrates about three hours before training. If your schedule does not allow this, a smaller snack of fast-digesting carbohydrates 30 to 60 minutes before the session is a practical alternative. A banana, a slice of toast with jam or a handful of dried fruit will provide accessible energy without sitting heavily in your stomach.

Sessions over two hours

For long endurance events like marathon training or extended cycling, you will exhaust your stored glycogen before the session ends. You must consume carbohydrates while you are moving to maintain your pace.

General guidelines suggest taking in small amounts of easily digestible carbohydrates every 30 to 45 minutes during the activity. This is where low-fibre options such as a banana, sweets or a sports drink are convenient. Practise eating during your training to see what your stomach tolerates best. Avoid waiting until you feel completely exhausted to eat, as it takes time for the fuel to reach your muscles.

Dispelling common myths

Nutrition advice is often complicated by rigid rules that lack context. Understanding the facts helps you make better choices for your training programme.

Myth: Carbohydrates cause automatic weight gain

No single nutrient causes weight gain on its own. Weight management depends on your overall energy balance. If you consume more total energy than you expend, your body will store the excess. Carbohydrates hold water when stored as glycogen, which can cause temporary fluctuations on the scale, but this is not the same as gaining body fat. They are simply fuel for your activity.

Myth: You must consume sugar immediately after training

A common myth suggests you must consume fast-digesting carbohydrates within a strict 30-minute window after finishing a workout to recover. For most people training once a day or a few times a week, this is unnecessary. Your body will refill its glycogen stores gradually over the next 24 hours from your regular meals. Rapid refuelling is only necessary if you are training the same muscle groups again within eight to twelve hours.

Myth: Fasted training burns more total body fat

Training on an empty stomach does increase the proportion of fat you burn during that specific session. However, it does not appear to lead to meaningfully greater fat loss over time compared with training after eating. Fasted training often results in a lower intensity workout because you lack accessible energy. By eating beforehand, you can train harder, lift heavier and sustain your effort longer, which is generally more beneficial for long-term fitness and muscle retention.

Practical steps for daily planning

Matching your nutrition to your activity level requires a practical approach rather than rigid counting. You can adjust your intake based on what you have planned for the day, listening to how your body responds over time.

• Assess your daily activity: On days when you have a heavy training session, increase the portion of carbohydrates on your plate. On rest days, you can reduce this portion and increase your intake of vegetables and protein.
• Favour whole foods: For your main meals, choose options that retain their natural fibre. Potatoes, oats, quinoa and lentils provide sustained energy and support healthy digestion.
• Keep pre-workout snacks simple: If you need energy right before training, choose something bland and low in fat and fibre. Fat slows down digestion, which can leave food sitting in your stomach while you exercise.
• Listen to your performance: If you consistently feel weak during the final third of your workouts, or if your recovery takes unusually long, you may not be eating enough carbohydrates to support your training volume.

Adjusting your intake takes time. Start by making small changes to your pre-training meals and monitor how your energy levels respond during your workouts.

When to get it checked

Speak to a doctor or physiotherapist if you experience severe dizziness, fainting, chest pain or extreme breathlessness during exercise. You should also seek medical advice if you feel consistently exhausted despite resting, or if you experience sharp, worsening joint pain when training.

If you have a diagnosed medical condition, particularly diabetes or any disorder affecting blood sugar management, you must consult your doctor before changing your carbohydrate intake or starting a new training programme. Pregnant women and individuals taking prescription medication for blood pressure or metabolic conditions should also seek professional guidance to ensure their nutrition and exercise habits remain safe.