Two athletes eat identical breakfasts. One shows a smooth 6.5 mmol/L postprandial peak returning to baseline in 90 minutes. The other spikes to 10.2 mmol/L and crashes into reactive hypoglycaemia at 45 minutes. Neither is diabetic. Both are elite level.
This is the insight that continuous glucose monitoring (CGM) is delivering to sports nutrition — and it is fundamentally changing how precision athletes design their fuelling protocols.
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The landmark study establishing the concept of personalised postprandial glucose response (PPGR) in healthy individuals was Zeevi and colleagues (2015), published in Cell. Following 800 non-diabetic participants with CGM over one week, they demonstrated that identical foods produced dramatically different glycaemic responses across individuals — with microbiome composition emerging as one of the strongest predictive factors (R=0.77 in their machine learning model).
For athletes, the implications are significant. Glycaemic variability — measured as the coefficient of variation (CV%) of interstitial glucose across the day — correlates with inflammatory markers, sleep quality, and perceived energy. Athletes with high CV% show less stable energy, disrupted appetite regulation, and in some research, impaired cortisol morning awakening response.
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Current CGM devices available to non-diabetic athletes include the FreeStyle Libre 2 and Dexcom G7. Worn as a disposable sensor on the upper arm or abdomen, they measure interstitial glucose every 1–5 minutes and transmit data to a smartphone. The incremental area under the curve (iAUC) — the area above baseline that a food creates — is the most meaningful metric for comparing responses between meals.
What athletes consistently discover when using CGM for the first time:
1. White rice before easy runs is fine; the same rice the night before hard intervals causes a reactive crash. The insulin sensitivity context of exercise timing changes everything. 2. Food sequencing matters. Shukla and colleagues (2017) demonstrated that eating protein and fats before carbohydrates at the same meal reduces iAUC by 37–46% — a strategy that requires no change in total intake, only ordering. 3. Individual trigger foods — oats spike some people dramatically while leaving others flat. Sports nutrition advice based on population-average glycaemic index values simply does not account for this interindividual variation.
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Practical CGM protocols for athletes:
- Wear for 14 days to capture training day vs rest day responses across different food exposures
- Log every meal and training session to correlate glucose behaviour with performance and fatigue
- Target post-meal peak below 7.8 mmol/L and return to baseline within 2 hours for meals not directly adjacent to exercise
- Pre-workout (within 60 min of high-intensity session): a moderate-GI carbohydrate that produces a rising but not spiking glucose curve entering the session is optimal — avoid starting a session in reactive hypoglycaemia
- Postprandial hyperglycaemia from large carbohydrate boluses followed by pronounced insulin-driven drops can be mitigated by splitting carbohydrate intake into two smaller portions 30 minutes apart
The criticism of CGM in non-diabetic athletes is valid: the absolute glucose values are less clinically important than in diabetic management, and some athletes risk developing food anxiety around glucose spikes that are physiologically normal and transient. CGM should be used as a learning tool for 1–4 weeks, not as a permanent monitoring dependency.
What CGM reveals — which no food diary can — is the real-time interaction between an athlete's meal composition, timing, microbiome state, sleep quality, and training load. That information can then inform a more stable, consistently structured meal plan.
For athletes designing their daily macro structure around training schedules, the meal generator at winsport.uk/tools/nutrition/macro-meal-generator builds personalised meal plans that account for energy timing, training day requirements, and macronutrient distribution — the foundations that CGM data consistently confirms matter most.
Have you used CGM as a non-diabetic athlete? What was the most surprising food response you discovered?