The Continuous Glucose Monitor Fad will Fade

 When Tom, a 55-year-old executive and beach-tennis enthusiast, got his annual blood work, he was rattled. He saw an A1c of 5.8%. Having just read a book on increasing healthspan, he thought: “That’s prediabetes,” imagining insulin shots and diet overhauls. Eager to take charge, he jumped on the wellness bandwagon, went to Amazon and bought a $100 continuous glucose monitor (plus extra supplies) to track his blood sugar in real time. He figured spikes after meals would show if his insulin was faltering, and he could watch his progress on an app. But weeks of fixating on the app left him confused. What was he really measuring, and what did it really mean?  

How A1c works and doesn’t work
A1c measures the percentage of hemoglobin coated with glucose, reflecting average blood sugar over two to three months. A reading of 5.7–6.4% suggests prediabetes; below 5.7% is normal, and 6.5% or higher indicates diabetes. It’s a snapshot, not a real-time insulin gauge. But A1c can be misleading. High readings can result from anemia (e.g., iron deficiency prolongs red blood cell life, increasing glucose attachment), kidney disease, or ethnicity—African Americans and Hispanics often have higher A1c without elevated glucose. Low readings can stem from blood loss, sickle cell disease, or frequent blood donations, which shorten red blood-cell lifespan. Some people have smaller blood cells (e.g., in thalassemia trait) or higher red blood cell turnover (in certain genetic variants), both within normal ranges, but these distort A1c by altering hemoglobin exposure to glucose. For Tom, mild anemia likely nudged his 5.8% higher, not insulin resistance. And it could have been temporary. 

The CGM fad
Tom’s pivot to a CGM reflects a biohacking craze, but these devices, built for diabetics, mislead non-diabetics. CGMs track interstitial glucose, showing post-meal spikes (e.g., 160 mg/dL from pasta) or stability (under 130 mg/dL). Big spikes might hint at insulin resistance, but normal glucose can hit 140–180 mg/dL after carbs, affected by stress or sleep. A week of data can’t diagnose insulin resistance, which is a slow-burn condition. Tom’s pizza spikes were normal.

Obsessing over CGM numbers, fueled by YouTube influencers, risks pointless diet tweaks or anxiety. Even Peter Attia, a guy I respect a lot, wears one. Why? He keeps saying he’s not a fan of overtesting. But he probably gets more podcast views when he shows people his glucose levels on his phone app. Peter is smart enough to debunk the fad and spread the message that they are only useful for people who are injecting insulin.

Peter gives one scenario where a CGM can benefit a non-diabetic patient: if a person is wearing a monitor for months and you have a glucose profile, then when giving a new drug or changing something, you could see if that has any effect on insulin resistance if the glucose numbers increase, and then if you reverse the change and the numbers go back down to baseline, you can assume it was whatever you changed. That’s kind of a billionaire biohacker toy, because if you’re measuring A1c every six months or so, that will generally tell you things are going in the wrong direction. But to have people wearing monitors just to get a baseline and then watch for changes seems over the top to me. Maybe it can help patients that were diabetic and are now under control, but I don’t think it’s practical to measure every little thing on normal people.

The OGTT solution
If Tom wanted to know about insulin resistance, which doesn’t change very quickly, he should’ve chosen the gold standard — an oral glucose tolerance test (OGTT). After a 75g glucose drink, blood glucose is measured at fasting, 1-hour, and 2-hour intervals. A 2-hour reading below 130 mg/dL is normal; 130–190 mg/dL flags prediabetes. At $50–$200, often paid by insurance, it beats CGMs ($50–$100/month) and provides better data. Dr. Rachel Rubin cautions, “Misinterpreting normal responses with trendy tools like CGMs can lead to unnecessary interventions.” Not only that, because other factors can influence glucose numbers, Tom has no idea what his actual insulin-resistance level is. 

Remarkably, Tom later read this blog and went to get an OGTT. It showed a 2-hour glucose of 125 mg/dL. Is that normal? To put this in context, I’ll put both ranges into a table to help you interpret the results: 

Tom’s value of 125 mg/dL is more accurate than his previous A1c number, so he should use that. It’s a good number, but it helps to understand what he should do and not do at this point. 

Most people talk about the “glycemic index” of a particular food (candy bars are higher than crackers), what really matters is the glycemic load. Tom should be aware that eating a small chocolate-chip cookie is different from eating two small cookies. People in his range should not worry about eating carbs, but they should try to limit how much sugar they get at one time. More glucose spikes will tend to raise his insulin resistance. A glucose monitor would show this, but common sense says it’s better not to put the second cookie in his mouth. 

Now that Tom knows he’s in the normal range, he doesn’t need another test unless he feels he’s been getting too much glucose. He can keep playing beach tennis, live a normal life, and not worry too much about measuring biomarkers. In general, a blood test once a year should show you anything to worry about and investigate. Symptoms will tell you even more. 

Skip the fad. Don’t over test. Get your A1c measured every six months and adjust your lifestyle to bring that number down to 5.5 or lower. If you don’t get a reliable number from an A1c test, get an OGTT. But the goal is not to measure. The goal is to change behavior. 

To learn more, read my Facts page. 

Disclaimer: Consult a healthcare provider before pursuing medical tests.

To learn more, read my Facts page.

Disclaimer: Consult a healthcare provider before pursuing medical tests.