Glossary: Carbohydrate periodisation

What carb periodisation is

Adjusting daily and per-workout carbohydrate intake to match the training session ahead and the adaptations you are targeting. The two extremes:

High carb for high intensity: top up glycogen before and during VO2, threshold, race-pace, or race-day work.
Low carb for low intensity: do some easy aerobic work in a lower-carb state to support fat oxidation adaptations.

Why "train low, race high"

A popular periodisation pattern. The idea:

Train low: some easy aerobic sessions done without pre-fueling and with limited in-session carbs trains the body's fat oxidation pathways. Slower in the moment, potentially useful for long-distance race fueling economy.
Race high: races are fully fueled. The trained fat adaptations remain useful, but you do not handicap your performance on race day.

What Grepiac's fueling planner assumes

The fueling planner defaults to a fully-fueled approach (race high, high-intensity days fully fueled). It does not auto-prescribe low-carb sessions; that is a coaching decision communicated through notes or the workout plan.

What carb periodisation is NOT

Not the same as keto. Periodisation alternates; ketogenic diets keep carbs low permanently.
Not a magic shortcut. Most adaptations from periodisation are modest and require months of consistent application.
Not for everyone. Athletes with disordered-eating history, women in certain cycle phases, and athletes approaching A races should typically NOT train low.

Practical rules of thumb

Easy aerobic (Grepiac Zone 1-2): can be done low-carb occasionally.
Sweet spot, threshold, VO2 (Zone 3b through 6): full fuel.
Race-pace specificity sessions: race fueling, not low-carb experiments.
Long sessions (3+ hours): always carry race-day carbs. Even on otherwise low-carb days.

How to communicate

If a coach prescribes a session as "train low":

Write a CoachNote explaining the intent and the limits.
Mark it visible to the athlete.
Include the fall-back plan if the athlete feels truly bad mid-session (eat, do not push through to harm).