Abstract

The effects of a low-protein high-carbohydrate (LPHC) diet (8% protein 65% to 72% carbohydrate) were compared to those of regular rabbit chow (14% to 16% protein 57% to 64% carbohydrate) on sex hormone-binding globulin (SHBG) capacity in 12 male rabbits. The six rabbits who were fed the LPHC diet for 8 weeks showed a significant increase in their mean SHBG capacity (mean ± SE: from 70 ± 16 nmol/L to 332 ± 45 nmol/L, P < .01) whereas the six rabbits fed the standard diet showed a slight decrease (from 106 ± 22 nmol/L to 76 ± 20 nmol/L, NS). These changes in SHBG capacity were mirrored by a decrease in percent-free T (from 2.64 ± 0.26% to 1.64 ± 0.16%, P < .01) in the LPHC diet group and no change in percent-free T in the regular diet group (from 2.36 ± 0.21% to 2.19 ± 0.10%). The changes in SHBG capacity and percent-free T were not associated with significant changes in testosterone (T), free T, estradiol (E 2), thyroxine, triiodothyronine, thyroxine-binding globulin, or cortisol binding globulin levels. In a subsequent experiment, testosterone cyclopentyl proprionate (TCP) was administered to six male rabbits while on regular rabbit chow and again after 6 weeks of the LPHC diet. TCP administration did not cause any significant change in the SHBG capacity, but the LPHC resulted again in a significant ( P < .05) increase in SHBG capacity from 80 ± 18 nmol/L to 198 ± 22 nmol/L. There was an inverse correlation between SHBG capacity and percent-free T but no significant correlation between SHBG and T, SHBG and free T concentration, percent free T and T, or between weight and SHBG. The administration of a LPHC diet results in a significant increase in SHBG capacity and a decrease in precent free T. While changes in circulating T levels can effect the SHBG capacity, the increase noted after diet is not due to changes in T levels. This increase does not appear mediated by changes in circulating E 2 or thyroid hormones and the exact mechanism remains uncertain.

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