Continuous glucose monitors (CGMs) — the small skin-adhered sensors that transmit real-time blood glucose readings to a phone app — have moved in the last 5 years from "diabetes-only medical device" to "consumer health tool." U.S. brands like Levels, Veri, Lingo, and Abbott's FreeStyle Libre 3 are now widely available without prescription, and the cost has dropped to roughly €70-120 per month.
For metabolically-curious adults over 40, a 2-4 week CGM trial can be one of the most informative interventions available. It's also one of the most easily-misused.
What a CGM actually measures
The sensor measures glucose concentration in the interstitial fluid (the fluid between cells, just beneath the skin) rather than directly in the blood. The two are highly correlated but not identical — interstitial readings lag blood glucose by about 5-10 minutes, and absolute calibration can drift.
This means CGM readings are excellent for trends and patterns but less reliable for single-point absolute values, particularly during rapid changes (like the steep rise after a high-carb meal). For most consumer applications, the trends are what matter.
What you'll learn in your first 2 weeks
Most metabolically-curious adults discover several useful things:
1. Your individual glucose response to specific foods
Glucose responses to identical meals vary substantially between individuals. Some people show large excursions to oats; others to white rice; others to bananas. The patterns are reasonably stable over time within an individual but very different across individuals — which means generic dietary advice ("avoid X") is less useful than personal data.
2. The cumulative effect of meal sequencing
The same meal eaten in different orders produces different glucose responses. Eating protein and vegetables before carbohydrates flattens the carbohydrate-induced spike substantially in most people. Eating a salad first before a starchy meal genuinely matters.
3. The post-exercise insulin sensitivity window
For 1-2 hours after intense exercise, your muscle glucose uptake is dramatically elevated. The same carbohydrates that produce a 50 mg/dL spike at 2pm produce a 20 mg/dL bump at 5pm if you've trained at 4pm. This is one of the more reliable patterns CGMs reveal.
4. The hidden glucose excursions you weren't aware of
Most people are surprised at how much certain "healthy" foods spike their glucose. Smoothies. Granola. Sweet potatoes. Dried fruit. Even some yoghurts. None of these are necessarily problematic, but the magnitude of the response is often surprising.
5. The impact of sleep and stress
One bad night of sleep visibly elevates next-day glucose responses across the board. A high-stress meeting can produce a 20-30 mg/dL glucose elevation without any food at all. The mind-body link is concrete and visible.
How to read a CGM well
The metrics that matter:
- Time in range — typically 70-180 mg/dL is the diabetic-care target; for non-diabetics, 70-140 mg/dL is a reasonable healthy target. Aim for 90%+ time in range.
- Average glucose — the rough proxy for HbA1c. An average of 100 mg/dL corresponds roughly to a 5.0% HbA1c.
- Glucose variability — how much your glucose swings up and down. Lower variability is better.
- Postprandial peaks — how high glucose goes after meals. Below 140 mg/dL is healthy; consistently above 180 mg/dL is a flag.
How to use a CGM badly
The pitfalls:
- Becoming food-anxious. CGMs can make people obsessive about every glucose excursion, including normal-and-healthy ones. Glucose is supposed to rise after meals; the question is how high and how long.
- Treating the data as more accurate than it is. Single-point readings are noisy. Trends matter; individual numbers don't.
- Eliminating foods based on single readings. Try a food 3-4 times in different contexts before drawing conclusions.
- Assuming non-diabetic targets equal diabetic targets. They don't. Healthy non-diabetics see post-meal glucose excursions to 140-160 mg/dL routinely; that's normal.
How long to wear one
For most metabolically-curious adults, a 2-4 week trial captures enough of your individual patterns to be useful. After that, the marginal information from continued monitoring drops sharply.
Some adults benefit from periodic re-checks — a 2-week trial every 6-12 months, particularly if you've made significant lifestyle changes and want to see the effect.
For people taking Grenov, a CGM trial in week 1 of starting and again in week 8-12 is one of the most informative ways to see whether the formula is doing what you want. Berberine's effects on postprandial glucose excursions tend to be visible within 2-3 weeks of starting and consolidate over the 12-week course. The CGM data is far more granular than HbA1c can be.
The honest summary
CGMs are a genuinely useful tool for metabolically-curious adults — informative, increasingly affordable, and personalising of dietary advice in ways generic guidelines can't be. They're also easy to misuse, particularly through over-anxiety about normal physiological responses.
Used well, they're one of the best self-quantification tools available. Used poorly, they're a fast track to food anxiety.
Pick the wear-window. Read the trends. Don't obsess.