Why Carb Loading Matters for Performance
Carbohydrates are your body's main fuel for high-intensity and prolonged exercise. Stored as glycogen in muscles and liver, they power efforts that last from a few minutes to several hours. When glycogen runs low, performance drops: you feel heavy, pace slips, and mental focus fades. As a fitness trainer working with runners and endurance athletes, I see how a structured carb-loading phase before a key event can increase glycogen stores and delay fatigue. This guide covers what the evidence says about timing, amounts, and how to apply it in practice.
What the Research Shows
Classic studies from the 1960s and 1970s established that depleting glycogen with hard training and then eating a high-carb diet for several days could nearly double muscle glycogen. Later research showed that a simpler approach — reducing training (taper) and increasing carbs for 24–48 hours — works well for most people without a strict depletion phase. Depletion can add fatigue and illness risk, so it is rarely recommended now. The goal is to start the event with full glycogen stores; taper plus high-carb intake achieves that for many athletes.
Who Benefits From Carb Loading
Carb loading is most relevant for events lasting roughly 90 minutes or more at a moderate to high intensity — half marathons, marathons, long triathlons, cycling sportives, and similar. For shorter or lower-intensity efforts, normal daily carbs are usually enough. If your event is under an hour or involves many short bursts with rest, focus on race-day nutrition and pacing rather than a full carb-load protocol.
Timing: The 24–48 Hour Window
In the final 24–48 hours before the event, shift the majority of your calories to carbohydrates while keeping protein and fat moderate. Aim for about 7–12 g of carbs per kilogram of body weight per day in that window. For a 70 kg athlete, that is roughly 500–840 g of carbs per day — a significant increase from a typical diet. Spread intake across meals and snacks so you are not overly full at any one time. Choose familiar, easily digested foods: rice, pasta, bread, oats, potatoes, bananas, and similar. Avoid high fibre or fatty meals the evening before if you have a sensitive gut.
Taper and Training Load
Carb loading works best when combined with a taper. In the last 2–7 days before the event, reduce training volume and intensity so that glycogen is not being burned at a high rate. This allows the extra carbs to fill stores instead of being used for daily training. A short, sharp session 2–3 days out is fine for some athletes to maintain readiness; avoid long or very intense work in the final 48 hours.
Race Morning and Pre-Event Meal
On the morning of the event, eat a carb-focused breakfast 2–4 hours before the start so you have time to digest. Again, choose foods you know sit well: toast, oatmeal, banana, or a simple sports nutrition product. Some athletes add a small top-up snack 30–60 minutes before the gun. During the event, continue to take in carbs (gels, drinks, bars) according to your plan — typically 30–60 g per hour for events over 90 minutes, and up to 90 g per hour if you have trained your gut for it.
Practical Meal Examples
In the 24–48 hours before a marathon or long race, a 70 kg athlete might aim for: breakfast — oatmeal with banana and honey; snack — rice cakes or fruit; lunch — pasta with light sauce and bread; snack — smoothie or sports drink; dinner — rice, chicken, and vegetables; evening snack — toast and jam. Adjust portion sizes to hit your carb target without excessive fullness. Hydrate well but do not overdrink; normal thirst and pale urine are good guides.
Mistakes to Avoid
Do not try a full depletion phase unless you have done it before under guidance; it can leave you tired and ill. Do not overeat to the point of discomfort; spreading intake is better than a few huge meals. Do not introduce new or exotic foods in the carb-load phase; stick to familiar options. Avoid very high fibre or gas-producing foods if you are prone to gut issues. Finally, do not neglect sleep and rest — the taper and carb load work together with recovery.
Supplements and Sports Nutrition
Carb-loading can be done with ordinary food. If you struggle to eat enough volume, liquid carbs (sports drinks, smoothies, or carb supplements) can help. Electrolyte drinks are useful in the days before and during the event to support hydration and sodium. There is no need for exotic products; focus on hitting your carb target with foods and drinks you tolerate well.
Individual Variation
Some athletes feel flat or heavy after a big carb load; others feel fuelled and ready. Body weight may increase by 1–2 kg due to water bound with glycogen — that is normal and not fat gain. If you have tried carb loading and felt worse, experiment with a moderate increase (e.g. 5–7 g/kg) or a shorter window (24 hours). Practice in training or in a B-race before your main goal event.
Summary
- Carb loading aims to maximize muscle and liver glycogen before events of about 90+ minutes.
- Combine taper with high-carb intake (7–12 g/kg/day) in the final 24–48 hours; no need for depletion.
- Eat familiar, easily digested foods; spread intake across meals and snacks.
- Race morning: carb breakfast 2–4 hours before; top-up if needed; then race nutrition per plan.
- Avoid depletion, overeating, and new foods; prioritize sleep and rest during taper.
