Research: Daily Carb Intake Above 12g/kg Bodyweight Diminishes Race Performance

A peer-reviewed study published in the International Journal of Sport Nutrition and Exercise Metabolism has challenged the high-carb-loading gospel that has dominated endurance nutrition for the past 30 years.

What they foundResearchers at Loughborough University followed 48 trained cyclists for 16 weeks. The cyclists were divided into four groups: low-carb (3g/kg/day), moderate-carb (7g/kg/day), high-carb (12g/kg/day), and very-high-carb (15g/kg/day). All groups performed the same training load and were tested on a 40 km time trial every 4 weeks.

Results3g/kg/day: Significantly worse time trial performance, with progressive degradation over 16 weeks7g/kg/day: Performance stable, modest improvement over baseline12g/kg/day: Performance improved 4-6% over baseline15g/kg/day: Performance equivalent to 12g/kg, but with significantly more GI distress and reported gut discomfort

What it meansThe traditional high-carb-loading approach (Burke's 10-12g/kg/day recommendation) is now confirmed as the upper limit of useful intake. Going above 12g/kg doesn't improve performance and increases GI risk during high-intensity efforts.

Practical recommendationsEndurance training (Zone 2): 5-7g/kg/day is sufficientHigh-intensity training (intervals, VO2max): 7-10g/kg/dayRace day (events >90 min): 10-12g/kg/day for 2-3 days before, plus in-race fueling at 60-90g/hourDaily intake above 12g/kg: No performance benefit, increased GI risk

What it doesn't changeIn-race fueling (60-90g/hour for events >90 min) remains the same. The optimal in-race carb intake is well established by prior research. The new finding is specifically about daily training intake, not race-day nutrition.