The Journal of Clinical Endocrinology and Metabolism February 1999, Vol 84, No 2, 471
Aleman A, Verhaar HJ, De Haan EH, De Vries WR, Samson MM, Drent ML, Van der Veen EA, Koppeschaar HP

ABSTRACT
The GH/insulin-like growth factor-I (GH/IGF-I) axis is known to be involved in ageing of physiological functions. Recent studies indicate that the GH/IGF-I axis may be associated with cognitive functioning. The aim of the present study was to determine whether the age-related decline in circulating levels of IGF-I, as an index of anabolic status, is associated with cognitive functions that are known to decline with ageing, but not with cognitive functions not sensitive to ageing. Twenty-five healthy older men with well-preserved functional ability participated in the study. We also administered neuropsychological tests of general knowledge, vocabulary, basic visual perception, reading ability, visuocontructive ability, perceptual-motor-speed, mental tracking, and verbal long-term memory.
Performance on the last four tests decline with ageing, whereas the first four of these tests have been shown not to be sensitive to cognitive ageing. Mean age of the subjects was 69.1 +/- 3.4 (SD) yr. (range 65-76 yr), their mean body mass was 27.0 +/- 2.4 kg/m2, and their mean IGF-I level was 122 ng/mL (range: 50-220).
We found IGF-I levels to be significantly associated with the performances (controlled for education) on the Digit Symbol Substitution test (r=0.52, P=0.009) and the Concept Shifting Task (r=-0.55, P=0.005), which measure perceptual-motor and mental processing speed. Subjects with higher IGF-I levels performed better on these tests, performance on which is known to decline with ageing.

In conclusion, the results of this study support the hypothesis that circulating IGF-I may play a role in the age-related reduction of certain cognitive functions, specifically speed of information processing.