Complex sentence: It has been demonstrated that the human brain has more acid sialic than the brain of the other mammal (2--4 times more), in fact the neuron membrane has 20 times more sialic acid than other cellular membranes.
Split into simpler sentences: It has been demonstrated that the human brain has more sialic acid than the brains of other mammals (2--4 times more). Neural membranes have 20 times more sialic acid than other cellular membranes.