Infant feeding with soy formula milk: effects on puberty progression, reproductive function and testicular cell numbers in marmoset monkeys in adulthood

Background This marmoset study addresses concerns about feeding human male infants with soy formula milk (SFM). Methods From age 4 to 5 days, seven male co-twin sets were fed standard formula milk (SMA) or SFM for 5-6 weeks; blood samples were subsequently collected at 10-week intervals. Testes from co-twins killed at 120-138 weeks were fixed for cell counts. Results SFM- and SMA-fed twins showed normal weight gain; puberty started and progressed normally, based on blood testosterone measurements. Body weight, organ weights (prostate, seminal vesicles, pituitary, thymus and spleen) and penis length were comparable in co-twins. All SMA- and 6/7 SFM-fed males were fertile. Unexpectedly, testis weight (P = 0.041), Sertoli (P = 0.025) and Leydig cell (P = 0.026) numbers per testis were consistently increased in SFM-fed co-twins; the increase in Leydig cell numbers was most marked in males with consistently low-normal testosterone levels. Seminiferous epithelium volume per tubule showed a less consistent, non-significant increase in SFM-fed males; raised germ cell numbers per testis, probably due to increased Sertoli cells, conceivably resulted in larger testes. Average lumen size, although greater in SFM-fed group, was inconsistent between co-twins and the difference was not significant. Conclusions Infant feeding with SFM has no gross adverse reproductive effects in male marmosets, though it alters testis size and cell composition, and there is consistent, if indirect, evidence for possible 'compensated Leydig cell failure'. Similar and perhaps larger changes likely occur in adult men who were fed SFM as infants.