Acts on acetyl-D-mannosamine and glycolyl-D-mannosamine. an N-acyl-D-mannosamine + NAD(+) = an N-acyl-D-mannosaminolactone + H(+) + NADH NAM-DH may be applicable to the quantitative determination of sialic acid in serum. Belongs to the short-chain dehydrogenases/reductases (SDR) family.