Growth hormone administration and exercise effects on muscle fiber type and diameter in moderately frail older people

OBJECTIVE: Reduced muscle mass and strength are characteristic findings of growth hormone deficiency (GHD) and aging. We evaluated measures of muscle strength, muscle fiber type, and cross sectional area in response to treatm ent with recombinant human growth hormone (rhGH) with or without a structur ed resistance exercise program in frail older subjects. DESIGN. Placebo-controlled, randomized, double blind trial. SETTING: Outpatient clinical research center at an urban university-affilia ted reaching hospital. PARTICIPANTS: Thirty-one consenting older subjects (mean age 71.3 +/- 4.5 y ears) recruited as a subset of a larger project evaluating rhGH and exercis e in older people, who underwent 62 quadricep-muscle biopsies. INTERVENTION: Random assignment to a 6-month course of one of four protocol s: rhGH administered subcutaneously daily at bedtime, rhGH and a structured resistance exercise program, structured resistance exercise with placebo i njections, or placebo injections only. MEASUREMENTS: Muscle biopsy specimen s were obtained from the vastus lateralis muscle. Isokinetic dynamometry st rength tests were used to monitor individual progress and to adjust the wei ghts used in the exercise program. Serum insulin-like growth factor-I (IGF- I) was measured and body composition was measured using a Hologic QDR 1000W dual X-ray densitometer. RESULTS: The administration of rhGH resulted in significant increase in cir culating IGF-I levels in the individuals receiving rhGH treatment. Muscle s trength increased significantly in both the rhGH/exercise (+55.6%, P =.0004 ) as well as the exercise alone (+47.8%, P =.0005) groups. There was a sign ificant increase in the proportion of type 2 fibers between baseline and si x months in the combined rhGH treated subjects versus those not receiving r hGH (P =.027). CONCLUSIONS: Our results are encouraging in that they suggest an effect of growth hormone on a specific aging-correlated deficit. IGF-I was increased by administrating rhGH and muscle strength was increased by exercise. The a dministration of rhGH to frail older individuals in this study resulted in significant changes in the proportions of fiber types. Whether changes in f iber cross-sectional area or absolute number occur with long-term growth ho rmone administration requires further study.