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The Role of Insulin-Like Growth Factor I in Age-Related Changes in Calcium Homeostasis in Men¹

Bone Metabolism Group (D.F., R.E.), University of Sheffield, Sheffield S5 7AU; and Department of Medicine (E.B.M.), University of Manchester, M13 9PL Manchester, United Kingdom

Address correspondence and requests for reprints to: Prof. Richard Eastell, Bone Metabolism Group, Clinical Science Center, Northern General Hospital, Herries Road, Sheffield S5 7AU, United Kindgom. E-mail: r.eastell{at}sheffield.ac.uk

The aim of this study was to evaluate hormonal influences on age-related changes in calcium homeostasis in men.

We recruited 178 healthy men, ages 20–79 (about 30 per decade). We measured serum calcium, phosphate, urinary calcium, and creatinine clearance. Dietary calcium intake and use of fish oils were determined by questionnaire. Fractional calcium absorption was estimated using the stable strontium technique in a subgroup of 60 men. PTH, 1,25-dihydroxyvitamin D [1,25(OH)₂D], 25-hydroxyvitamin D (25OHD), serum insulin-like growth factor I (IGF-I), and testosterone were measured in all men.

There was no change in serum calcium with age. There were decreases in serum phosphate, urinary calcium, and creatinine clearance with age (P < 0.02). Dietary calcium was unchanged. Strontium absorption decreased (P < 0.01), and PTH increased (P < 0.001) with age. The data for 1,25OH₂D were biphasic, reaching a peak at age 55 yr (P = 0.003). There was a linear increase in 25OHD with age (P = 0.009) that persisted after correcting for seasonal variation and was positively associated with fish oil use, therefore, the age-related changes in 25OHD were masked by self medication. There were log-linear decreases in IGF-I and testosterone with age (P < 0.0001).

Strontium absorption was not related to 25OHD or 1,25(OH)₂D, but was positively correlated with IGF-I. 1,25(OH)₂D correlated negatively with serum phosphate and calcium, but not PTH or creatinine clearance. IGF-I was positively associated with creatinine clearance, serum calcium, and phosphate and negatively associated with PTH (P < 0.001).

In this cross-sectional study of otherwise healthy, normally aging men, age-related decreases in IGF-I seem to have a greater impact on mineral absorption than does vitamin D status.

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