The Carb-Insulin Myth: Tuning Out the Noise in your CGM Data

Apr 2

For decades, the standard playbook for endurance performance was simple: Eat carbs. Lots of them. Carbs fuel muscle glycogen, and muscle glycogen fuels performance. But in recent years, a strong counter-narrative has emerged: the “Carb-Insulin Hypothesis.” This model suggests that all carbohydrates—from a banana to a bagel—spike insulin, and because insulin is a storage hormone, those carbs are locked away as fat, leading to weight gain, metabolic inflexibility, and performance decline.

Enter the Continuous Glucose Monitor (CGM). Originally designed for people with diabetes, these wearable sensors track glucose levels in real-time, 24/7. Suddenly, athletes could see the direct impact of their pre-workout oatmeal or mid-race gel.

The combination of the Carb-Insulin Hypothesis and real-time data has created a perfect storm of anxiety. Many active individuals now look at their CGM charts with dread, obsessing over every peak. But the truth is, a large portion of what you see on that graph is noise, especially if you are an athlete.

If you are using a CGM for performance optimization, it’s time to separate myth from physiology.

The Myth: All Spikes Are Bad

The core of the Carb-Insulin panic is the idea that any glucose "spike" (often defined simply as a rise above baseline) is damaging. The assumption is that a spike means excessive insulin, and excessive insulin means you are now storing fat and preventing fat adaptation.

The Reality for Athletes:

This model ignores any nuance and context. The way a sedentary, insulin-resistant person responds to 50g of carbs is completely different from how a metabolically healthy athlete responds to the same meal.

Spikes During Exercise are Functional — One of the most common sources of confusion among new CGM users is a massive spike during a hard workout, even when fasted.

Why it happens: When you perform high-intensity or explosive work (intervals, sprints, heavy lifting), your body perceives a massive need for immediate fuel. To meet this demand, you release cortisol and adrenaline. These hormones signal your liver to pour stored glucose into the bloodstream at a rapid rate. Your liver is doing its job! It’s mobilizing fuel to save your performance. This is a functional spike.

The Noise: You don’t need to do anything about a mid-workout spike. The circulating glucose is meant to be used.

Spikes Pre/Post-Workout are About Restoration

Eating carbohydrates before a workout ensures you have accessible fuel. Eating carbs afterward is essential for initiating muscle glycogen resynthesis—the most critical part of recovery for high-training-load athletes. This replenishment window is when your muscles are most receptive to glucose. Yes, your glucose will rise, but it is moving into the cells that need it.

What You Should Be Paying Attention To with Your CGM

If you aren't obsessing over every 140 mg/dL peak, what are you looking for? The true value of a CGM for an athlete is in long-term patterns, not acute snapshots.

1. Fueling Stratification by Intensity

Low Intensity (Zone 2/Fat Max) — During easy, long-distance sessions, your glucose curve should ideally be stable and flat (e.g., in the 80–110 mg/dL range). If your glucose is highly variable or frequently dips low during a low-intensity run, it may indicate you are metabolically inflexible and relying too heavily on glucose even when your body should be burning fat.
High Intensity (Threshold/VO2 Max) — As intensity increases, you should see those functional rises. A completely flat line during a grueling interval set might actually mean you are under-fueled and your liver can't keep up with demand.

2. Spotting the "Bonk" Before it Happens

One of the best uses of a CGM in competition is catching reactive hypoglycemia or a steady, unexplained slide in glucose during long efforts.

The Trend: If you fuel heavily (e.g., a massive gel) right before a race start, your glucose will spike, prompting a strong insulin release. When you then start racing hard, your muscles demand glucose simultaneously with the insulin, causing a rapid, severe crash (hypoglycemia) within the first 30 minutes. The CGM can teach you to time your pre-race fuel better.
The Slide: During an ultra or long triathlon, seeing your glucose steadily drop from 110 mg/dL to 90 to 80 to 70 mg/dL, despite continued fueling, is a clear sign that you are running on empty. You can reactively increase your carbohydrate intake before you feel the cognitive and physical failure of "hitting the wall."

3. Nighttime "Canaries in the Coal Mine"

Some of the most valuable data happens while you are asleep. Your overnight glucose patterns can be a powerful metric of recovery and systemic stress.

High Overnight Glucose: If you typically wake up with glucose significantly higher than your standard baseline (e.g., waking at 110 mg/dL when you usually wake at 85 mg/dL), this can be a strong signal of systemic stress. Possible causes include severe overtraining, insufficient caloric intake relative to training load (low energy availability), lack of sleep, or systemic inflammation.
Nocturnal Dips: Frequent, extreme dips into the 50s mg/dL during the night can sometimes mean your glycogen stores were too depleted to maintain stable blood sugar while fasting.

What Should You Tune Out?

To keep your sanity while wearing a CGM, you must learn to ignore every single number coming your way:

1. Compression Lows

If you wear your sensor on your arm, sleeping on that arm can compress the interstitial fluid around the sensor, causing it to register a "false low" (e.g., a 55 mg/dL reading at 3 AM with no other symptoms). If you wake up and the number instantly pops back up, it was a compression low. Ignore it.

2. High Readings that Stabilize Fast

A spike to 160 mg/dL after a meal containing carbohydrates is completely normal for a non-diabetic human. If your curve comes back to baseline within 2 to 3 hours, your body’s regulation system is working optimally. The problem is chronically elevated glucose or high variability (swinging from 60 to 180 all day long), not a single, transient spike.

3. Small Fluctuations (±10 mg/dL)

Interstitial glucose monitoring has a delay compared to blood glucose (around 5–15 minutes). Furthermore, sensors have a margin of error. Your chart will never be a perfect line. Small, jagged movements up and down 5–10 points throughout the day are normal physiological variance and sensor error. Do not chase them.

Focus on the Forest, Not the Trees

A CGM is a powerful biofeedback tool, but only if you use it within the context of athletic physiology. Do not let the Carb-Insulin Hypothesis scare you away from fueling your performance.

Use your CGM to understand how different fuels affect you at different intensities, to catch fueling mistakes during long efforts, and to monitor your systemic recovery overnight. Tune out the compression lows, the functional exercise spikes, and the minor variance.

The goal isn't a flat line. The goal is a fueled, powerful, and adaptable metabolic engine.