A randomized crossover trial found that continuous glucose monitors may systematically overestimate glycemia compared with capillary blood sampling, with variations depending on the test food and individual differences.
In the study, published in The American Journal of Clinical Nutrition, investigators assessed glucose responses in 15 healthy adults across seven postprandial conditions.The trial included nine female and six male participants aged 18 to 65 years, with a BMI of 18 to 30 kg/m². Each participant completed seven laboratory visits, consuming carbohydrate challenges from different food sources, including glucose solution, whole fruit, blended fruit, and commercially available smoothies. Capillary and continuous glucose monitors (CGM) glucose readings were taken every 15 minutes for 120 minutes postingestion.
CGM-estimated fasting and postprandial glucose concentrations were 0.9 ± 0.6 and 0.9 ± 0.5 mmol/L higher compared with capillary estimates, respectively (both, P < .001). Further, the investigators found that CGMs overestimated the time spent above 7.8 mmol/L by approximately fourfold compared with capillary sampling, with baseline adjustment reducing but not eliminating this overestimation to about twofold (P < .01).
Differences in CGM vs capillary fasting glucose concentrations varied by participant (P = .001). The glycemic index (GI) derived from CGM readings was also inconsistent with capillary measurements. For instance, the GI for a commercial fruit smoothie was reported as 69 (95% confidence interval [CI] = 48–99) via CGM, whereas capillary sampling placed it at 53 (95% CI = 40–69, P = .05).
CGMs also ranked glycemic responses to test foods differently from capillary sampling. The incremental area under the curve order for capillary samples was 50 g glucose (CONTROL), commercial smoothie ingested over 30 ± 4min (SLOW), blended fruits (BLEND), commercial fruit smoothie (PRODUCT), equivalent carbohydrate as whole fruits (WHOLE), commercial smoothie with 5g inulin (FIBER), and commercial smoothie providing 30g carbohydrate (DOSE); while the CGM-ranked order was CONTROL, SLOW, PRODUCT, FIBER, BLEND, DOSE, and WHOLE.
The study emphasized that CGMs systematically overestimated glycemia as a result of differences in interstitial and blood glucose measurements. Capillary sampling remains the preferred method for accurate glycemic assessment.
Full disclosures are detailed in the study.
