Continuous Glucose Monitoring (CGM)
Real-time blood sugar dynamics on demand
Plain English
A continuous glucose monitor is a small sensor worn on the skin that measures blood sugar levels every few minutes around the clock. Unlike a single fasting blood glucose reading, a CGM shows the full picture: how high your glucose spikes after meals, how quickly it returns to baseline, and how stable it stays overnight. It turns glucose from a snapshot into a trend.
The Mechanism
Glucose enters the bloodstream from digested carbohydrates and is delivered to cells by insulin, which acts as the delivery signal. A CGM sensor sits just beneath the skin in interstitial fluid, the fluid surrounding cells, and measures glucose concentration there using a small enzymatic electrode. Interstitial readings lag behind actual blood glucose by approximately 5 to 15 minutes, which is why CGM values during a rapidly rising or falling phase differ slightly from a finger-stick test.
The data stream from a CGM reveals patterns that single readings cannot: postprandial spikes (the rise after eating), the glucose trough in the late afternoon, nocturnal stability, and the dawn phenomenon, the early-morning glucose rise driven by cortisol and growth hormone releasing before wake. These patterns vary considerably between individuals eating identical meals, a finding established by Zeevi et al. at the Weizmann Institute (2015) in a study of 800 participants.
CGM is approved for diabetes management, where it replaces multiple daily finger-sticks and enables real-time dosing decisions. In healthy adults without diabetes, it is increasingly used as a metabolic health tool, though continuous reference ranges for this population are still being established.
Why It Matters
One fasting glucose reading tells you almost nothing. A week of CGM data tells you everything.
For people with diabetes or prediabetes, CGM removes the guesswork from insulin and medication decisions and dramatically reduces the risk of dangerous hypoglycemic episodes. For metabolically healthy adults, CGM reveals how specific foods, sleep quality, stress, and exercise timing affect glucose stability in ways a standard annual blood panel never could. The practical signal: a postprandial spike above 160 mg/dL or nocturnal instability below 70 mg/dL warrants attention regardless of fasting glucose.
Common Misconception
Many people assume CGM is only for diabetics. In reality, published research shows significant postprandial glucose variability in metabolically healthy adults eating the same meals, and that variability predicts long-term metabolic health. CGM data can reveal early insulin resistance years before HbA1c or fasting glucose leave the normal range.
What a Healthy Range Looks Like
Hypoglycemic
Below 70 mg/dL
Dangerously low; requires immediate intervention in diabetics; in healthy adults often signals missed meals or overexertion
Optimal fasting
70–99 mg/dL
Standard fasting target; most healthy adults spend overnight here
Postprandial rise
100–140 mg/dL
Normal post-meal zone; a healthy system returns to baseline within 1–2 hours
Elevated
140–180 mg/dL
Prediabetic territory on a sustained basis; brief spikes are common after high-glycemic meals
Diabetic range
Above 180 mg/dL
Clinically elevated; consistent readings here indicate poor glycemic control
For metabolically healthy adults, continuous glucose should spend the vast majority of time between 70 and 140 mg/dL. Postprandial peaks above 160 mg/dL and overnight dips below 70 mg/dL both warrant investigation. Compare your trends to your own personal baseline, not population averages.
Signs It Is Disrupted
- Afternoon energy crashes 1–2 hours after carbohydrate-heavy meals
- Waking at 3–4am with a racing heart or hunger (nocturnal glucose instability)
- Intense cravings for sugar or refined carbs within 2 hours of eating
- Brain fog or difficulty concentrating in the hours after meals
- HbA1c creeping above 5.4 despite no obvious dietary changes
- Fasting glucose consistently in the upper-normal range (95–99 mg/dL)
How to Improve It
Which Devices Track It
Dexcom (G6, G7)
Clinical standard; FDA-approved for insulin dosing decisions in diabetics. 10-day sensor life. Reads every 5 minutes. Mean absolute relative difference (MARD) of 9–10% in healthy glucose ranges.
Abbott Libre (2, 3)
14-day sensor; Libre 3 streams in real-time via Bluetooth. Slightly wider accuracy range at glucose extremes. Most widely used CGM globally.
Stelo (Dexcom)
First FDA-cleared OTC CGM for non-diabetic adults. 15-day sensor. Designed for metabolic health monitoring in people without diabetes or insulin use.
Levels Health
Software layer built on Libre or Dexcom hardware; provides metabolic score, trend analysis, and meal tagging. Accuracy is entirely dependent on the underlying sensor.
3 Things to Remember
CGM turns glucose from a single-point snapshot into a continuous trend, revealing postprandial spikes, overnight stability, and meal-by-meal responses that a fasting test cannot detect.
Healthy adults show significant individual variation in glucose response to identical meals; a personal CGM week is more informative than any population-level glycemic index table.
Walking after meals, sequencing food (vegetables and protein before carbs), and prioritizing sleep are the three highest-leverage daily levers for flattening glucose variability.
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