Insulin-like growth factor-I and cognitive function in healthy older men

The GH/insulin-like growth factor-I (GH/IGF-I) axis is known to be involved in aging of physiological functions. Recent studies indicate that the GH/I GF-I axis may be associated with cognitive functioning. The aim of the pres ent study was to determine whether the age-elated decline in circulating le vels of IGF-I, as an index of anabolic status, is associated with cognitive functions that are known to decline with aging, but not with cognitive fun ctions not sensitive to aging. Twenty five healthy older men with well-preserved functional ability partic ipated in the study. We also administered neuropsychological tests of gener al knowledge, vocabulary, basic visual perception, reading ability, visuoco nstructive ability, perceptual-motor speed, mental tracking, and verbal lon g-term memory. Performance on the last four tests decline with aging, where as the first four of these tests have been shown not to be sensitive to cog nitive aging. Mean age of the subjects was 69.1 +/- 3.4 (sn) yr (range 65-7 6 yr), their mean body mass index was 27.0 +/- 2.4 kg/m(2), and their mean IGF-I level was 122 ng/mL (range: 50-220). We found IGF-I levels to be sign ificantly associated with the performances (controlled for education) on th e Digit Symbol Substitution test (r = 0.52, P = 0.009) and the Concept Shif ting Task (r = -0.55, P = 0.005), which measure perceptual motor and mental processing speed. Subjects with higher IGF-I levels performed better on th ese tests, performance on which is known to decline with aging. In conclusion, the results of this study support the hypothesis that circul ating IGF-I may play a role in the age-related reduction of certain cogniti ve functions, specifically speed of information processing.