A female infant appeared normal at bih but developed signs of liver disease and muscular weakness at 3 months. She had periods of hypoglycemia, paicularly on awakening. Examination revealed hepatomegaly. Laboratory analyses following fasting revealed ketoacidosis, blood pH 7.25, and elevations in both alanine transaminase (ALT) and aspaate transaminase (AST). Administration of glucagon following a carbohydrate meal elicited a normal rise in blood glucose, but glucose levels did not rise when glucagon was administered following an overnight fast. Liver biopsy revealed an increase in the glycogen content. To prevent the frequent episodes of hypoglycemia, which of the following dietary supplements would be most appropriate for this patient?