The CGM Beginner Playbook.

What a CGM is, in 30 seconds.

A continuous glucose monitor is a coin-sized sensor you wear on the back of your upper arm. A short filament sits in interstitial fluid (the fluid between skin cells), measures glucose every 1–5 minutes, and streams the reading to your phone via Bluetooth or NFC. Sensors last 10–15 days. Glucose readings lag blood by ~5–15 minutes — interstitial fluid catches up to blood, so spike timing is slightly delayed.

Originally built for type 1 diabetes management, CGMs are now FDA-approved for over-the-counter sale to non-diabetic adults (Stelo, Lingo, Libre Rio). They are not a diagnostic tool for diabetes in this population — they are a behavioral feedback loop for understanding how your body responds to food, exercise, sleep, and stress.

Should you bother?

Honestly, most metabolically healthy adults learn what they need to know from two weeks of CGM wear and never use one again. The information is real, but the n=1 nature limits long-term ROI. Wear one if:

• You are pre-diabetic (HbA1c 5.7–6.4%) or have a family history of T2D. CGM data is genuinely actionable here.
• You are visibly skinny but suspect metabolic dysfunction (post-meal sleepiness, hunger 2 hours after a meal, central fat despite normal BMI). "Thin outside, fat inside" syndrome shows up clearly on a CGM.
• You are training endurance + tracking fueling. CGM during long runs / rides surfaces under-fueling and gel timing.
• You are recomping (cutting fat / gaining muscle) and want to understand insulin sensitivity — useful for timing carbs around training.
• You are curious for 2 weeks, will draw conclusions, and then stop. Diminishing returns set in fast.

Skip CGM if you have a history of disordered eating — the per-meal feedback loop can fuel orthorexic behavior. Same applies if you tend toward health anxiety; CGM noise (random spikes, sensor errors) can read as disease.

Choosing a device.

All three U.S. consumer CGMs use Abbott\'s FreeStyle Libre 3 sensor underneath, repackaged for the non-diabetic market. Differences come from the software, subscription model, and onboarding:

• Stelo (Dexcom) — uses the Dexcom G7 platform. 15-day sensor, no fasting glucose displayed (only trends and ranges). $99 / 2 sensors / 30 days. Best for people who already use Dexcom or want the cleanest app.
• Lingo (Abbott) — gamified "Lingo Count" score that translates glucose variability into a single daily number. 14-day sensor. ~$50 / sensor, monthly subscription. Best for first-time users who want a simple metric.
• Levels — software layer on top of FreeStyle / Dexcom hardware. Includes meal logging, food scoring, and habit-coaching features. ~$199/year subscription + sensor cost. Best for people who want detailed analysis and are willing to log meals.
• Nutrisense — similar software-layer to Levels but with nutritionist coaching included. Higher price, more guidance.

For a 2-week diagnostic wear: Stelo is the cheapest entry point. If you want ongoing tracking + meal analysis, Levels or Nutrisense earn their subscription. Lingo is the best middle ground for people who hate logging and want one summary score.

What to actually measure.

Forget the post-meal spike, mostly. Three numbers matter more:

• Fasting / waking glucose. Optimal: 70–90 mg/dL. Consistently >100 suggests insulin resistance regardless of HbA1c.
• Time in range (70–140 mg/dL). Optimal: >90% of the day. Healthy adults run >95%. Pre-diabetics often drop to 70–85%.
• Glucose variability (coefficient of variation, CV). Optimal: <15%. CV is more predictive of cardiovascular endpoints than average glucose alone. High variability = your insulin response is brittle.

The post-meal spike is interesting but its magnitude depends on what you ate, when you ate, how recently you exercised, your sleep last night, and where the sensor is in its 14-day cycle. A 30 mg/dL rise after a meal of pasta is fine. A 60 mg/dL rise after a meal of oatmeal and berries is also fine. Chase the trend, not the individual reading.

How to read post-meal spikes.

Three patterns are worth recognizing:

• Healthy spike. Glucose rises 20–40 mg/dL post-meal, peaks at 45–75 minutes, returns to baseline within 2 hours. Your insulin response is working.
• Brittle spike. Glucose shoots up >60 mg/dL, peaks fast, then crashes below baseline (reactive hypoglycemia). Suggests insulin overshoot — often seen with simple carb meals without protein/fat.
• Plateau spike. Glucose rises and stays elevated >140 mg/dL for 2+ hours. Suggests delayed insulin response, gastric emptying issues, or pre-diabetes.

The same person can produce all three patterns depending on context. After a poor night of sleep, even a healthy meal can produce a plateau. After heavy exercise, even a sugary meal can produce a healthy-looking spike because the muscle is glucose-hungry.

Levers that actually change the numbers.

Most people end CGM week 1 frustrated because they cut sugar and the numbers barely move. The biggest movers, in order:

• Walk 10 minutes after meals. Single most impactful intervention. Light movement post-prandial reduces the spike 20–30% via muscle glucose uptake. The Inzucchi studies on post-meal walks are striking.
• Order foods within a meal: fiber → protein/fat → carb. The Stanford "food order" trials show 30–40% lower post-meal spikes from the same calories simply by eating the salad before the pasta.
• Resistance training 2–3× per week. Muscle is the primary disposal site for blood glucose. More muscle = better glucose tolerance, full stop.
• Sleep 7+ hours. One night of 4-hour sleep produces post-meal glucose curves comparable to early prediabetes. Spiegel Lancet 1999.
• Vinegar or lemon juice with the meal. 1–2 tbsp apple cider vinegar in water before a high-carb meal lowers the spike ~20%. Acetic acid slows gastric emptying.
• Switch refined to whole carbs. White bread → sourdough or whole grain. Sugar-sweetened drinks → water. Modest but consistent.

Sensor accuracy and noise.

Modern CGMs are accurate to within ~10% of a finger-prick reading. They are noisier in the first 24 hours after insertion (warm-up artifact), during dehydration, and toward the end of sensor life (days 12–15). Compression artifacts (sleeping on the sensor) cause false lows.

If a reading seems wrong (very high or very low without symptoms), it usually is. Don\'t panic on a single point — look at the 30-minute trend instead.

A 2-week diagnostic protocol.

If you only want to wear one CGM in your life, here is the playbook:

• Week 1 — baseline. Eat normally. Do not change anything. Just log what you eat and when. Note your fasting glucose, time in range, and CV.
• Week 2 — intervention. Walk 10 min after every meal. Change food order (fiber/protein/fat before carbs). Compare same meals from week 1 to week 2 — same breakfast, same lunch, different post-meal context. The delta is your CGM-derived intervention effect.
• After. Pick the 2–3 interventions that produced the biggest effect on you specifically, and make them permanent.