How continuous BG monitors work

Continuous glucose monitors sit in the interstitial fluid under the skin and track glucose 24/7 for about 7-15 days. A transmitter sends the readings to a phone. Some systems can also link with an insulin pump. Readings can lag blood glucose by approximately 5-15 minutes, which matters when you try to act on data in real time.

Overall glucose metabolism

After you eat, carbohydrates break down to glucose, blood glucose rises, and the pancreas releases insulin so cells take up glucose and levels return to normal. Glucose naturally fluctuates throughout the day with food and exercise. Normal fasting levels often sit around 3.5-5.5 mmol/L (sometimes up to 6). With a higher-carb meal (e.g., ~100 g carbs), glucose may move from approximately 5 mmol/L to approximately 9 mmol/L, peaking 30-60 min after eating and returning to baseline within 2-3 hours. This pattern is normal. Protein can also contribute to glucose. When insulin resistance is present, the rise can last longer.

Positives

Reactive hypoglycaemia, especially pre-exercise

Some people who eat simple carbs 30-45 min before training see a spike then a dip that hits as exercise starts. A CGM can help confirm this if symptoms like dizziness or fatigue show up. You could then tweak timing or forms of carbohydrate.

Overnight “random hypos”

Wearables can reveal hypoglycaemic episodes during sleep. It only matters if symptoms occur, yet a CGM can help rule in or out low glucose as the reason someone wakes overnight instead of guessing.

Ruling out hypos in day-to-day life

Some people eat extra at certain times because they think they’re preventing a hypo. Tracking can show whether those dips are actually happening.

A very cautious positive: individual responses to foods

Glycaemic responses vary between people. A monitor might help you spot your own patterns. However, many things skew readings, such as illness, stress, and daily variability, so you need patience and care to avoid over-interpreting one-off spikes.

Negatives

Fear of carbs and an obsession with “stable” blood glucose levels

Seeing post-meal spikes can fuel carb fear in both athletes and non-athletes. Athletes may then avoid simple carbs or reduce carbs in ways that worsen their nutrition. Fast-acting options (e.g., juice) can look “bad” on a graph, yet in context, they are useful.

Cost

CGMs cost money to both wear and replace.

Misinterpreting your own data

Without context and guidance, it’s easy to misread trends and make unhelpful changes.

Data delay

Most devices run approximately 5-15 minutes behind blood glucose. That delay limits in-the-moment decisions during sport. The data works better for spotting trends and adjusting later.

Do we recommend it?

We rarely recommend CGMs for healthy people. We would consider them when there are clear reasons, such as suspected pre-exercise dips or overnight lows. If someone is level-headed, well-informed, and willing to view the numbers in context, using a CGM can be reasonable. For people with type 1 diabetes or those using an insulin pump, the calculus is obviously different. For general healthy users, especially when the device drives carb fear, we lean against routine use.