What is already known about this subject • Although placebo effects have been shown on subjective continuous variables such as pain, placebo effects on objective continuous variables remain uncertain.• The present, pilot, follow‐up investigation represents the first to assess a placebo effect on the objective continuous measurement of acute postprandial plasma glucose. What this study adds • Placebo effects may be operating on postprandial plasma glucose outcomes.• Cornstarch sources of placebo may decrease the plasma glucose response to a 75‐g oral glucose tolerance test, rendering them as positive controls when assessing postprandial outcomes.• Other carbohydrate sources used as placebos in research may show similar effects.AimsPlacebo effects in clinical trials remain uncertain. To investigate a placebo effect on acute postprandial plasma glucose, we conducted a follow‐up investigation on a previous study.MethodsThe effect of placebo (9 g encapsulated cornstarch +500 ml water, taken at −40 min) on the plasma glucose response to a 75‐g oral glucose tolerance test (OGTT) was assessed in a previous study in 12 healthy subjects (gender, five male, seven female; age 27 ± 6 years; body mass index 24 ± 3.4 kg m−2). This was compared with the effect of a water control (500 ml water taken alone at −40 min) on the same outcome in the same subjects in a follow‐up study.ResultsCornstarch placebo decreased plasma glucose area under the curve during the 75‐g OGTT by 28% [Δ (95% confidence interval) −63.3 min mmol−1 l−1 (−218.33, 91.66), P < 0.02] compared with the water control (P < 0.05).ConclusionsPostprandial plasma glucose outcomes may be vulnerable to placebo effects.