Fueling Smarter: How Different Intake Strategies Affect Glucose During Endurance Workouts

As an endurance athlete and the founder of Project Blue Health Optimization, I’m always looking for ways to help people train smarter, recover better, and protect their long-term health. One of the most powerful tools for that is understanding how different fueling strategies affect blood glucose in real time.

This month, I have run a couple studies looking at blood glucose levels in relationship to different factors. For this specific study, I ran a self-experiment controlled over six different nights to do exactly that. The setup was simple: the exact same 1.5-hour indoor cycling workout (TrainerRoad “Glassy -2” — tempo intervals, TSS 72, Zone 3 effort), the same indoor exercise bicycle (Wahoo Kickr Bike Pro), a similar pre-workout dinner (balanced whole foods), same workout room conditions, same fan setup, and the same sipping protocol (equal-volume sips every 5 minutes for the first hour, then water only). The only variable was the first-hour of fueling.

I used a Dexcom Stelo CGM to capture continuous glucose data and recorded subjective energy and effort as well as the speed it took glucose returned to baseline. The six conditions were:

1. Water only

2. Skratch Labs Hydration Mix (30 g simple carbs — dextrose + cane sugar)

3. Soluble fiber 30 minutes pre workout + Skratch Hydration Mix (30 g simple carbs)

4. Skratch Labs Super High-Carb (cluster dextrin, 28.5 g carbs)

5. Repeat of Day 4 (cluster dextrin)

6. Ketone-IQ (exogenous ketone ester) 45 minutes pre + water only

Key Glucose Metrics

Day	Fueling Strategy	Avg Glucose (mg/dL)	Peak	Lowest	Variability (SD)	Return to Baseline
1	Water only	106.4	116	96	5.8	31 min
2	Simple carbs (Skratch)	110.3	128	95	11.1	26 min
3	Fiber + Simple carbs	103.2	124	70	15.5	41 min
4	Cluster dextrin	112.4	137	93	12.1	26 min
5	Repeat Cluster dextrin	111.1	130	94	10.4	32 min
6	Ketone-IQ + Water	100.9	115	86	10.0	~12–15 min

Discussion

The results were illuminating and aligned closely with the recent Noakes et al. review in Endocrine Reviews(2026). That paper analyzed over 160 studies and challenged the traditional view that muscle glycogen depletion is the primary cause of fatigue in endurance exercise. Instead, it argues that exercise-induced hypoglycemia (EIH) — a drop in blood glucose — is the central driver, as falling glucose signals the brain to protect itself by limiting effort. Carbohydrate ingestion works primarily by preventing or reversing EIH, not by refilling muscle glycogen.

• Ketone-IQ + Water (Day 6) produced the most stable glucose profile: lowest average, minimal variability, and fastest return to baseline. Energy felt “normal” and steady with no highs or crashes. This supports the idea that exogenous ketones provide an alternative fuel for the brain, reducing glucose dependence and lowering EIH risk, even without any carbs during exercise.

• Cluster dextrin (Days 4 & 5) was repeated for day 5 because the results from day 4 were so surprising. The complex carbs generated the highest peaks at this moderate dose (28.5 g/h). It did not produce dramatically flatter curves than simple sugars, which was consistent with my earlier Tour de Palm Springs observations.

• Simple carbs (Day 2) did not cause the extreme highs and lows that were expected. The average blood glucose fell between water and complex carbs.

• Fiber + simple carbs (Day 3) blunted early rises but led to the deepest low (70 mg/dL) and slowest recovery, suggesting delayed absorption was too pronounced for this workout.

• Water only (Day 1) was surprisingly stable without any glucose drops, indicating strong baseline metabolic flexibility, though late-ride fatigue was more noticeable.

Why This Matters for Endurance Athletes — and Prediabetes/Diabetes Risk

Many endurance athletes believe they are “metabolically protected” because they train hard and stay lean. However, repeated high-intensity efforts and high-carbohydrate fueling can lead to frequent glucose spikes and insulin surges, which over time contribute to insulin resistance and prediabetes risk, even in fit individuals.

The Noakes review highlights that maintaining stable blood glucose is critical not just for performance, but to avoid chronic metabolic stress. My data shows that personalized fueling can minimize unnecessary spikes and crashes, potentially reducing long-term inflammation and metabolic strain.

For athletes with prediabetes, diabetes, or family history, CGM-guided fueling becomes even more important. Strategies like exogenous ketones or lower-carb options (when appropriate) can help protect glucose stability without sacrificing training quality.

Practical Recommendations

• For stable glucose and clean energy in 60–90 minute Zone 3 sessions, exogenous ketones (Ketone-IQ) + water is highly effective.

• Moderate carbs (30 g/hr) reliably prevent major EIH while remaining gut-tolerable.

• Cluster dextrin needs to be looked at more closely. Our studies have not yielded more stable glucose curves when compared to simple sugars.

• Fiber preloading needs careful timing. I would only recommend for pre-diabetics or diabetics really watching their sugar intake.

• Post-workout meals should balance carbs with protein and fat to avoid reactive hypoglycemia.

Athletes should use CGM during training to discover their own optimal strategy rather than following generic advice.

Limitations and Future Directions

This was a single-subject study with moderate duration/intensity. Future work could include:

• Larger groups (n=4–8)

• Higher carb doses (60–90 g/h)

• Longer workouts (2.5–4+ hours)

• Combined strategies (e.g., low-carb + ketones)

• Other brands products

This experiment provides practical, real-world insight into how fueling affects glucose stability and perceived effort. Personalized metabolic testing is key to optimizing performance and protecting long-term health.

Full PDF report available here...

Fueling Smarter.pdf

Download PDF • 799KB

What’s your go-to fueling for tempo rides? Have you ever used CGM? Share in the comments!

Stay optimized!