They have been called “the ultimate wearable.”

It’s a fashiony boast for a medical device, but then again it’s from a promotional video for FreeStyle Libre by Abbott Laboratories, one of the best-selling continuous glucose monitors in the country. Similar units include Medtronic’s Guardian Connect, the Dexcom G6 and the Senseonics product Eversense.

The devices offer diabetic patients a way to see their blood glucose rising in real time, data transmitted from a hair-sized sensor just below the skin to an app on their phone. Where finger-stick glucose meters used by millions of Americans allow a snapshot of blood sugar levels at a moment in time, CGM’s, which are not covered by most forms of insurance and can cost anywhere from a few hundred to a few thousand dollars, depict the metric as a fluid process.

Just ate a banana? Wearers of CGM’s can check their phone and see if their body is happy with the news or becoming concerned. How about a bowl of grapes? You might be surprised with the result. Similar to the popularity of heart rate monitors in the 1990s, CGM’s offer a window into the bodily workload, except it’s for the exertion placed on the body’s system for clearing glucose from the blood.

“Even people who are well, who don’t have insulin resistance are having these daytime surges when we put CGM’s on them,” said obesity clinician Sarah Hallberg in a talk last year about her dietary research. “So ... this is a problem with everyone.” In a recent paper published in PLOS Biology, researchers from Stanford reported that 15% of 57 healthy individuals studied wearing CGM’s hit pre-diabetic glucose levels after meals.

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Elevated blood sugar is dangerous to vital organs over time. In healthy persons, the arrival of glucose will trigger the pancreas to release insulin, an emergency hormone for moving excess sugar into fat cells. For those with Type 2 diabetes, the system is failing.

“I first started using a CGM after I reversed my diabetes, and am using it now to see how I tolerate individual foods,” says John Somsky, 50, a computer programmer and former diabetic patient from Hugo, Minn.

Somsky reversed his diabetes and lost 150 pounds on a low carbohydrate, high fat diet with intermittent fasting. He says the news from his CGM can be surprising.

“I was eating steak at a restaurant, and it had some spices on it, and it bumped my blood glucose up to 135 or 140. I found there was sugar in their spice mix," he said.

Somsky learned if he eats close to bed, his blood sugar will take half of the night to recover from a meal. He learned that, “if I just eat carrots, it will raise my blood sugar more than if eat carrots dipped in ranch.”

Specialists say this is both a limitation and advantage of the devices. Foods affect blood sugar differently in combination with other foods.

In the meantime, phones are beginning to answer questions about the world inside as well as outside of us, and the CGM market is expected to reach $1.3 billion in the next five years.