Carb Loading for Endurance Athletes

Carbohydrate loading — also called glycogen supercompensation — is a well-established pre-competition strategy used by endurance athletes to maximize liver and muscle glycogen stores beyond their normal resting levels. When executed correctly, it can meaningfully extend time to fatigue and improve performance in prolonged events.

Why Glycogen Is the Limiting Factor in Endurance Performance

During sustained moderate- to high-intensity exercise (65–80% VO2max), muscle glycogen is the dominant fuel source. A well-trained athlete can store approximately 400–600 g of glycogen (1,600–2,400 kcal), enough to support roughly 90–120 minutes of continuous hard effort. Once glycogen is depleted, the body can only produce energy at the rate that fat oxidation permits — much slower — causing the sharp performance decline known colloquially as "hitting the wall" or "bonking."

Carbohydrate loading aims to push glycogen stores above baseline, effectively extending the endurance ceiling.

Who Benefits from Carb Loading?

Carb loading is beneficial for events lasting 90 minutes or longer at moderate to high intensity:

Marathon and half-marathon running
Long-course triathlon (Olympic distance and above)
Road cycling events >90 minutes
Cross-country skiing, rowing, swimming (1500 m+)
Soccer, rugby, and other high-intensity team sports with extended playing time

For events shorter than 60–75 minutes, glycogen stores are rarely the limiting factor, and carb loading provides no measurable benefit. For strength and power sports, it is also not relevant.

The Classic Protocol vs. Modern Protocol

Classic 6-Day Protocol (Astrand, 1960s–1980s)

This original protocol involved a depletion phase (3 days of exhaustive training + low-carbohydrate diet) followed by a loading phase (3 days of rest + very high carbohydrate intake). While effective at supercompensating glycogen, the depletion phase was physically and psychologically taxing and carried injury risk.

Modern 3-Day Protocol (Current Standard)

Research by Sherman and colleagues demonstrated that trained athletes can achieve comparable glycogen supercompensation without the grueling depletion phase. The modern protocol is now standard:

Days 1–3 before competition: taper training volume significantly (reduce duration by 40–50%); maintain normal to slightly elevated carbohydrate intake (5–7 g/kg/day)
Days –3 to –1 (the 3 days before competition): increase carbohydrate intake to 8–12 g per kilogram of body weight per day; keep fat and protein moderate; reduce training to light activity only
Day before competition (–1): complete rest or very light activity; 8–10 g/kg of carbohydrates; focus on familiar, digestible foods
Competition morning: 1–4 g/kg of carbohydrates 1–4 hours before the start

What to Eat During the Loading Phase

Achieving 8–12 g/kg/day of carbohydrates requires deliberate food choices. For a 70 kg athlete, that is 560–840 g of carbohydrates per day — a large amount that warrants planning.

Staple foods: white rice, pasta, bread (white or sourdough), potatoes, oatmeal, corn, bagels
Liquid carbs: fruit juices, sports drinks, rice milk — useful when solid food volume becomes difficult
Sugary foods in moderation: sports gels, bananas, dates, honey can help top up intake
Minimize: high-fat foods (reduce gastric emptying), high-fiber foods (increase GI distress risk), novel foods, alcohol

Expected Physiological Changes

Successful carb loading produces several notable changes:

Muscle glycogen increases from ~100–150 mmol/kg dry weight (normal) to ~200–250 mmol/kg dry weight (supercompensated)
Body weight increases by 1–2 kg due to water retention — glycogen is stored with approximately 3 g of water per gram of glycogen. This is expected and not fat gain.
Muscle heaviness or tightness is common; this resolves quickly during exercise warm-up

Performance Benefits and Limitations

In controlled research studies, glycogen supercompensation has been shown to:

Improve time to exhaustion at submaximal intensity by 20–30%
Improve time-trial performance over 90+ minute events by 2–3%
Allow athletes to maintain higher intensities later in the race (critical for kick finishes)

The benefits are most pronounced in events where glycogen depletion is the actual performance bottleneck. In shorter, higher-intensity events or well-paced efforts where pace is the limiting factor, the improvement may be less noticeable in practice.

During-Race Carbohydrate Strategy

Even with maximal pre-loaded glycogen, exogenous carbohydrate intake during events >75–90 minutes further enhances performance:

30–60 min efforts: mouth rinse with carbohydrate solution (no need to swallow) activates reward pathways and improves performance
60–90 min efforts: 30–45 g of carbohydrates per hour
90–150 min efforts: 45–60 g of carbohydrates per hour
>2.5 hour efforts: 60–90 g per hour using multiple carbohydrate types (glucose + fructose in 2:1 ratio) to maximize intestinal absorption