Abstract Objectives To explore postprandial responses to isocaloric meals after ∼2 weeks on an ad libitum Low Carbohydrate (LC) diet vs. a Low Fat (LF) diet. Methods 16 healthy volunteers without diabetes were admitted to the NIH Clinical Center and randomized to consume a LC or LF diet for 2 weeks immediately followed by 2 weeks of the alternate diet. The LC diet was composed of ∼75% fat, ∼10% carbohydrate, and ∼15% protein; the LF diet was ∼75% carbohydrate, ∼10% fat, and ∼15% protein. Daily meals and snacks were matched for presented calories and participants were instructed to consume as much or as little as desired. On day 13 of each diet after an 8 hour fast, participants consumed a liquid meal containing 30% of energy requirements with a macronutrient composition corresponding to the prevailing diet. Blood was drawn at 0, 10, 20, 30, 60, 90, 120, 180, 240, 300, 360 minutes post meal consumption. Plasma concentrations of glucose, lactate, insulin, c-peptide, free fatty acids, and triglycerides were measured. Results 7 females and 9 males with an age of (mean ± SE) 28.7 ±1.7 y and BMI of 27.5 ± 1.5 kg/m2 completed the study. During the LC diet, baseline levels of triglycerides and lactate were significantly lower (−33.5 ± 9.1 mg/dl; P = 0.003, −0.18 ± 0.05 mM; P = 0.002, respectively) and glucose, insulin, and c-peptide also tended to be lower (−3.7 ± 2.0 mg/dl; P = 0.09, −2.2 ± 1.2 µU/ml; P = 0.08, −0.35 ± 0.17 ng/ml; P = 0.06, respectively) whereas free fatty acids were significantly higher (0.28 ± 0.06 mM; P = 0.0005) compared to the LF diet. Average postprandial levels of glucose, lactate, insulin, and c-peptide were significantly lower following the LC meal (−11 ± 3 mg/dl; P = 0.003, −0.88 ± 0.06 mM; P < 0.0001, −35 ± 9 µU/ml; P = 0.002, −2.5 ± 0.3 ng/ml; P < 0.0001, respectively) whereas free fatty acids and triglycerides were significantly higher (0.52 ± 0.03 mM; P < 0.0001, 32 ± 12 mg/dl; P = 0.03, respectively) compared to the LF meal. Conclusions Following a ∼2 week adaptation period to ad libitum LC vs. LF diets, isocaloric meals resulted in substantial differences in postprandial glucose, lactate, insulin, c-peptide, free fatty acids, and triglycerides. Funding Sources Intramural Research Program of the NIDDK.