Effects of dietary phytoestrogens on plasma testosterone and triiodothyronine (T3) levels in male goat kids

Abstract

Background: Exposure to xenoestrogens in humans and animals has gained increasing attention due to the effects of these compounds on reproduction. The present study was undertaken to investigate the influence of low-dose dietary phytoestrogen exposure, i.e. a mixture of genistein, daidzein, biochanin A and formononetin, on the establishment of testosterone production during puberty in male goat kids.

Methods: Goat kids at the age of 3 months received either a standard diet or a diet supplemented with phytoestrogens (3-4 mg/kg/day) for approximately 3 months. Plasma testosterone and total and free triiodothyronine (T3) concentrations were determined weekly. Testicular levels of testosterone and cAMP were measured at the end of the experiment. Repeated measurement analysis of variance using the MIXED procedure on the generated averages, according to the Statistical Analysis System program package (Release 6.12, 1996, SAS Institute Inc., Cary, NC, USA) was carried out.

Results: No significant difference in plasma testosterone concentration between the groups was detected during the first 7 weeks. However, at the age of 5 months (i.e. October 1, week 8) phytoestrogen-treated animals showed significantly higher testosterone concentrations than control animals (37.5 nmol/l vs 19.1 nmol/l). This elevation was preceded by a rise in plasma total T3 that occurred on September 17 (week 6). A slightly higher concentration of free T3 was detected in the phytoestrogen group at the same time point, but it was not until October 8 and 15 (week 9 and 10) that a significant difference was found between the groups. At the termination of the experiment, testicular cAMP levels were significantly lower in goats fed a phytoestrogen-supplemented diet. Phytoestrogen-fed animals also had lower plasma and testicular testosterone concentrations, but these differences were not statistically significant.

Conclusion: Our findings suggest that phytoestrogens can stimulate testosterone synthesis during puberty in male goats by increasing the secretion of T3; a hormone known to stimulate Leydig cell steroidogenesis. It is possible that feedback signalling underlies the tendency towards decreased steroid production at the end of the experiment.

Figure 1

Effects of phytoestrogens on the plasma testosterone concentrations in male goat kids. Goat kids at the age of 3 months received either a standard diet (controls) or a diet supplemented with phytoestrogens (3 - 4 mg/kg/day) for a period of ~3 months (August 19 to November 7). At week 8 of the experiment (i.e. October 1), phytoestrogen-exposed animals (closed circles, solid line) had significantly (* P < 0.05) higher testosterone concentrations than controls (open circles, dashed line).

Figure 2

Effects of phytoestrogens on plasma total triiodothyronine (T₃) concentrations in male goat kids. Goat kids at the age of 3 months received either a standard diet (controls) or a diet supplemented with phytoestrogens (3 - 4 mg/kg/day) for a period of ~3 months (August 19 to November 7). At week 6 of the experiment (i.e. September 17), phytoestrogen-exposed animals (closed circles, solid line) had significantly (* P < 0.05) higher total T₃ concentrations than controls (open circles, dashed line).

Figure 3

Effects of phytoestrogens on the plasma free triiodothyronine (T₃) concentrations in male goat kids. Goat kids at the age of 3 months received either a standard diet (controls) or a diet supplemented with phytoestrogens (3 - 4 mg/kg/day) for a period of ~3 months (August 19 to November 7). At week 9 and 10 of the experiment (i.e. October 8 and 15), phytoestrogen-exposed animals (closed circles, solid line) had significantly (* P < 0.05) higher free T₃ concentrations than controls (open circles, dashed line).

Figure 4

Testicular testosterone concentrations in goat kids at the end of the experiment. Goat kids at the age of 3 months received either a standard diet (controls) or a diet supplemented with phytoestrogens (3 - 4 mg/kg/day) for a period of ~3 months (August 19 to November 7). Data are expressed as mean ± S.E.M.

Figure 5

Testicular cAMP concentrations in goat kids at the end of the experiment. Goat kids at the age of 3 months received either a standard diet (controls) or a diet supplemented with phytoestrogens (3 - 4 mg/kg/day) for a period of ~3 months (August 19 to November 7). Data are expressed as mean ± S.E.M. * P < 0.05.