The Blood Sugar Buzz: Why It Matters

If you’ve ever felt sluggish after a big meal or had a sudden craving for something sweet, blame your blood sugar. Glycemic responses—the way your body reacts to different foods—play a huge role in energy levels, metabolism, and long-term health. Keep your blood sugar steady, and you’ll feel like a rockstar. Let it swing like a rollercoaster, and you’re in for energy crashes, mood swings, and potential long-term issues like insulin resistance.

This is where the glycemic index (GI) comes in. It ranks foods based on how much they spike your blood sugar, with glucose (pure sugar) scoring a perfect 100. In theory, the lower the GI, the better for steady energy and metabolic health.

But here’s the catch—tracking blood sugar isn’t as simple as it sounds.

Enter the CGM Hype Train

Continuous Glucose Monitors (CGMs) have gone mainstream. No longer just for diabetics, these little sensors are being marketed to health-conscious folks as the ultimate biohacking tool. Companies claim they’ll help you optimize your diet, avoid blood sugar spikes, and stay in the metabolic sweet spot. Some influencers even treat them like a magic weight-loss gadget.

Sounds great, right? Well, not so fast.

The Problem With CGMs (That No One’s Talking About)

CGMs don’t measure blood sugar directly. Instead, they track glucose levels in interstitial fluid (the stuff between your cells) and estimate what’s happening in your bloodstream. But this estimation isn’t always accurate.

A recent study found that CGMs tend to overestimate blood sugar spikes, especially with certain foods like smoothies. They also exaggerate how long people spend in so-called “danger zones” of high blood sugar. Translation? You might be stressing over blood sugar levels that aren’t even that bad.

Some key takeaways from the research:

  • CGMs tend to inflate fasting glucose levels in some people more than others.
  • Blood sugar response rankings can change depending on the food. A smoothie might look way worse than whole fruit, even if the real difference is small.
  • One-size-fits-all calibration won’t fix the problem. Individual variation means no universal correction factor can make CGMs perfectly reliable.

What This Means for You

If you’re using a CGM to tweak your diet, take the data with a grain of salt (or sugar in this case, ha). Just because your CGM tells you a certain food is “bad” doesn’t mean it’s actually causing massive blood sugar swings. It could just be the way the device interprets glucose movement in your body.

For truly accurate blood sugar tracking, capillary blood tests (aka finger pricks) remain the gold standard. They directly measure glucose in the bloodstream, making them far more reliable for things like glycemic index calculations.

The Bottom Line

CGMs are a cool tool for spotting trends, but they’re not the final word on glycemic responses. If you’re making major diet decisions based on CGM readings alone, you might be getting misled. The best approach? Use a mix of real-world data, how you feel after eating, and validated methods like capillary blood tests if you need precision.

So next time your CGM freaks out over a smoothie, don’t panic—it might just be bad at math. 😉

Hutchins KM, Betts JA, Thompson D, Hengist A, Gonzalez JT. Continuous glucose monitor overestimates glycemia, with the magnitude of bias varying by postprandial test and individual – A randomized crossover trial. Am J Clin Nutr. 2025 Feb 22:S0002-9165(25)00092-9. doi: 10.1016/j.ajcnut.2025.02.024. Epub ahead of print. PMID: 40021059.