Race Day Carb Fueling: The Science of Not Bonking

At kilometre 32 of a marathon, a predictable physiological event occurs in under-fuelled runners: muscle glycogen levels drop below the threshold required to sustain race pace, the brain perceives a systemic energy crisis, and pace drops precipitously. Runners call this "hitting the wall." Physiologists call it glycogen depletion. Both groups agree that it is unpleasant and entirely avoidable.

Carbohydrate fueling is not a comfort strategy. It is a performance intervention grounded in clear metabolic science.

Why Carbohydrates Matter

Skeletal muscle has two primary fuel sources during running: glycogen (stored glucose) and fat. The ratio in which they are used is primarily determined by exercise intensity:

The problem: total glycogen stores in the liver and muscle are limited to roughly 400–500 g of carbohydrate in an average trained runner — enough to sustain roughly 90 minutes of moderate-intensity effort before stores approach critical depletion.

Any race longer than 90 minutes requires exogenous carbohydrate intake to maintain pace. This is not optional.

Carbohydrate Oxidation Rates

The gut can absorb and oxidise carbohydrates at a rate limited by intestinal transporters. Two transporters are relevant:

• SGLT1 transports glucose: maximum rate ~60 g/hour
• GLUT5 transports fructose: maximum rate ~30 g/hour

Using glucose alone, the effective ceiling is approximately $60$ g/hour. Using a combination of glucose and fructose (in roughly 2:1 ratio), oxidation rates of 90 g/hour are achievable, because the two transporters work in parallel without competing.

$\dot{m}_{\text{CHO}} = \dot{m}_{\text{glucose}} + \dot{m}_{\text{fructose}} \leq 90 \text{ g/hr}$

Most single-source gels (maltodextrin only) cap effective intake at 60 g/hour. Purpose-formulated dual-source products approach the 90 g/hour ceiling with appropriate gut training.

Race Distance Fueling Targets

Pre-Race Carb Loading

Muscle glycogen can be elevated above baseline by ~20% through a 48-hour high-carbohydrate protocol before a marathon. This provides approximately 80–100 extra grams of stored glycogen — equivalent to roughly 15–20 additional minutes of reserve at race pace.

The target pre-race carbohydrate intake is:

$\text{CHO (g)} = 10 \text{ to } 12 \text{ g} \times \text{body weight (kg)}$

For a 70 kg runner, this equates to 700–840 g of carbohydrate over 48 hours — approximately 350–420 g/day, or roughly double the average diet. This requires planning: think rice, pasta, bread, oat-based meals, and low-fat sources.

Day-before-race meal timing:

• Dinner (18:00–19:00): Large carbohydrate-dominant meal
• Breakfast race morning (3–4 hours pre-gun): 1–4 g/kg of carbohydrate, low fibre, familiar foods

Gut Training

The intestinal adaptations that allow $60$–$90$ g/hour absorption are trainable. Runners who consume carbohydrates during training increase intestinal SGLT1 expression over weeks. Runners who race having never taken gels in training often experience GI distress at race concentrations.

The rule: practice your race fueling strategy in long training runs at race effort. At minimum, use your intended race nutrition product twice before race day.

The "nothing new on race day" cliché is not superstition. It is physiology.

Common Mistakes

Starting too late: Waiting until you feel tired to take carbohydrates is waiting until wall-onset. Glycogen depletion is already advanced. Start fueling by 40–45 minutes into a marathon, regardless of how you feel.

Under-dosing: A single gel every 10K at marathon pace often delivers only 30–40 g/hour. Below 45 g/hour, you will likely deplete glycogen before finishing a 3.5-hour marathon.

Over-concentrating: Taking gels without sufficient water increases gut osmolality, slowing absorption and increasing GI distress risk. Always take gels with 100–150 mL of water, not with sports drink (which adds sodium to an already concentrated bolus).

Gut surprises: Coffee, high-fat foods, and high-fibre foods within 3 hours of race start increase GI stress during the race. The pre-race breakfast should be identical across your last three or four long runs before race day.

Practical Session Plan

For a 4-hour marathon runner consuming 70–80 g/hour from km 8:

This delivers approximately 148 g total across 29 km of fueled running — roughly 76 g/hour. Combined with pre-race glycogen stores, this is sufficient to maintain race pace through 42.2 km for most runners.

The wall is not inevitable. It is a planning failure.