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Growth Hormone Deficiency and Physical Function

University of Toronto Toronto, Ontario, Canada M5G 1X5

To the editor:

We agree with Dr. Sartorio and his colleagues that the effects of rhGH treatment on muscle size, strength, and anaerobic capacity in adult GH-deficient (GHD) patients are limited. We did not see marked improvement in muscle strength despite an increase in mean fiber area (both type I and type II fibers) following rhGH treatment as compared with placebo or baseline (1). However, the significant improvement in aerobic function that we have described (1) suggests that the capacity of skeletal muscle to utilize oxygen for energy expenditure maybe enhanced and/or there is an enhanced ability to deliver oxygen in response to rhGH treatment. The improvement in cardiac function associated with rhGH treatment (2) is certainly compatible with the latter possibility. Nevertheless, it may well be that the interface of oxygen delivery and utilization is critical in GHD subjects and that examination of muscle function alone may be less revealing in the GH deficiency state.

Dr. Sartario’s studies comparing childhood-onset GHD adults to age-, sex-, and activity-matched control subjects suggest that the reduced muscle size and strength in GHD adults is a function of reduced body size. Although this explanation is plausible for short-statured adults who have been GHD since childhood, this does not seem to be the situation for GHD adults who have a normally developed body size. Moreover, while correction for reduced body size in adults with childhood-onset GHD may negate the physiological differences seen in absolute terms, the functional consequences cannot be so easily dismissed. The fact remains that to perform activities of daily living (such as walking, climbing stairs, rising from a chair, etc.) these GHD adults must move a larger body mass, carrying a heavier load of fat with less metabolic machinery available to perform the work than their non-GHD counterparts.

We agree with Dr. Sartario and his colleagues, who suggest that future studies need to consider the onset of GHD. Such differences may be a potential source contributing to the variable responses to GH treatment on muscle and functional performance in GHD adults.

References

1. Woodhouse LJ, Asa SL, Thomas SG, Ezzat S. 1999 Measures of submaximal performance evaluate and predict functional response to growth hormone (GH) treatment in GH-deficient adults. J Clin Endocrinol Metab. 84:4570–4577.[Abstract/Free Full Text]
2. Lombardi G, Colao A, Ferone D, et al. 1997 Effect of growth hormone on cardiac function. Horm Res. 48:38–42.

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