Are aldehydes toxic to humans? If so, what makes glucose less toxic?
Glucose is a particular kind of aldehyde.
Aldehydes are toxic to living organism because they react with amines in proteins, giving Schiff's bases and cross-linking proteins.
However, glucose and in general aldose sugars, which are polyhydroxy aldehydes, exist in unstable free aldehyde form only for short times, the equilibrium is shifted towards the hemiacetalic (alpha or beta) cyclic form in which they cannot anymore react with amines.