CGM Devices May Overestimate Blood Sugar After Meals

What Was Studied and Why It Matters

Continuous glucose monitors (CGMs) have become one of the most popular tools for tracking blood sugar — especially after meals. But a 2025 study published in The American Journal of Clinical Nutrition raises an important question: are CGMs actually giving us accurate readings when we eat different types of foods?

Researchers at a UK laboratory recruited 15 healthy adults and had them consume seven different carbohydrate challenges on separate days — including straight glucose, whole fruits, blended fruit, and commercial fruit smoothies. Every 15 minutes for two hours, blood sugar was measured two ways: by the CGM sensor worn on the body, and by a traditional fingerstick (capillary) blood test, which is considered the gold standard for accuracy.

What the Study Found

The results were eye-opening. CGMs consistently read higher than fingerstick tests — by about 0.9 mmol/L (roughly 16 mg/dL) both before and after meals. That gap may sound small, but its real-world impact was significant:

• CGMs overestimated the amount of time participants spent above a high blood sugar threshold by approximately four times compared to fingerstick testing.
• Even after researchers tried to mathematically correct for the baseline difference, CGMs still overestimated high blood sugar readings by about two times.
• The glycemic index (a measure of how quickly a food raises blood sugar) was rated higher by CGM than by fingerstick for the same smoothie — 69 versus 53 — a difference that could meaningfully change food choices.
• The degree of inaccuracy varied from person to person, meaning there's no single correction factor that works for everyone.

What This Means for Real Patients

If you rely on a CGM to decide what foods to eat, adjust insulin doses, or judge whether your blood sugar is in a safe range after meals, this study suggests you should use that data carefully. Your CGM is an excellent tool for spotting trends — like whether your blood sugar is rising or falling — but it may not give you a precise number, particularly after eating carbohydrate-rich foods like smoothies or fruit.

For critical decisions — such as confirming a high blood sugar reading before adjusting medication — a fingerstick test using a traditional glucose meter remains the most reliable option. Whether you use a CGM, a glucometer, or both, having the right supplies on hand matters. MDS Diabetes carries a full range of glucose testing supplies, including fingerstick meters and testing strips, so you're never left without an accurate backup option.

Practical Takeaways

• Don't rely solely on CGM readings to judge how a specific food affects your blood sugar.
• Use fingerstick tests to confirm unusually high post-meal readings before making medication changes.
• Talk to your diabetes care team about how to interpret CGM data alongside traditional testing.
• Understand that CGM accuracy can vary — not just by food type, but also by individual body chemistry.

Bottom Line

CGMs are valuable tools, but this research confirms they can overestimate blood sugar levels — especially after meals — and the error isn't the same for every person or every food. Fingerstick testing remains the gold standard for accurate readings when precision matters most. Use your CGM for trends, but keep traditional testing supplies close for the moments that count.