A large secondary analysis challenges the popular idea that individuals respond uniquely to specific foods. Researchers found that when you account for a person's normal day-to-day variability in blood sugar, the glycemic index reliably predicts how anyone will respond to a carbohydrate-rich food. Data from 382 healthy adults showed that prediction errors stayed within each person's own test-retest range about 90% of the time. The model needed no person-specific parameters — just the food's average glycemic index and the individual's baseline glucose variability. Practically, a glycemic index difference of at least 15 units is needed to produce a reliably different blood sugar response in a given person. These findings push back on claims from personalized nutrition companies that standard GI rankings are meaningless.
