The effect of diets with varying crude protein-to-precooked cornstarch (P:S) ratios on growth performance, feed efficiency, carbohydrase activity, and blood chemistry of juvenile Lutjanus guttatus was evaluated. Four isoenergetic (19.0 kJ g-1) diets containing different P:S ratios (46:18, 38:30, 34:35, and 31:40) were formulated to feed fish for ten weeks in triplicate. Each diet was tested in triplicate in groups of 35 fish per tank (average body weight 19.4 ± 0.3 g) to apparent satiation three times a day. The weight gain, specific growth rate, and feed conversion ratio obtained with P38S30 and P34S35 were not significantly different vs. P46S18. Survival does not present significant differences among treatments. Biological indices (condition factor, hepatosomatic index, viscerosomatic index, and intraperitoneal fat rate) showed no significant differences among treatments. Body protein content decreased significantly with increasing precooked cornstarch concentration, while body lipid levels increased. Blood chemistry values were within those of healthy juveniles, except for glucose values. An increase in pancreatic α-amylase and intestinal α-glucosidase blood chemistry values were within those of healthy juveniles, except for glucose values. An increase in pancreatic α-amylase and intestinal α-glucosidase levels concomitant with increasing precooked cornstarch concentration showed the capacity of juvenile L. gutattus to digest high precooked cornstarch levels. Growth performance, biological indices, and hematological parameters showed that L. guttatus could digest and metabolize up to 35% of precooked cornstarch through increased plasma glucose levels and the α-amylase and α-glucosidase activities. Without negative effect on growth performance.