Observations were made over 6 hours for pre - and post-working memory ( WM ) , glycaemic and insulin responses and biomarkers of Alzheimer 's disease ( AD ) ( 0 , 2 , 4 and 6 hours ) : amyloid precursor protein ( APP ) , - secretase subunits presenilin-1 ( PS1 ) , presenilin-2 ( PS2 ) , and glycogen synthase kinase ( GSK-3 ) .