Familial fructokinase deficiency causes no symptoms because:

Fructose metabolism Fructokinase is the first enzyme for fructose metabolism. It conves fructose to fructose-1-P. In Fructokinase deficiency, Hexokinase (a non specific enzyme) can phosphorylate fructose as well as other sugars but it has high km (low affinity) for fructose. Glucose is the true substrate for this enzyme. Fructose 6-phosphate - the end product of Hexokinase reaction can enter glycolytic pathway to be utilized fuher, so it does not accumulate to produce toxic effects. EXTRA EDGE- Fructokinase deficiency causes Essential Fructosuria, while Aldolase B deficiency causes Hereditary Fructose Intolerance