Dairy calves are exposed to isoflavones during the developmentally most sensitive period of their life.

Isoflavones represent a class of phytoestrogens present in plants. In dairy cows, dietary isoflavones have been shown to negatively affect reproductive performance. To the best of our knowledge, no studies have yet been conducted to determine if calves are pre- or neonatally confronted with isoflavones and their metabolites. In the present study, we hypothesize that isoflavones are passed on from the dam to the offspring in utero. Twenty-three pregnant Holstein Friesian dams and their calves, originating from three commercial dairy farms in Belgium, were included. Heparin blood samples were collected during the first, second, and third trimester of gestation from all pregnant dams. Heparin blood and hair samples were obtained from the offspring within 24 h after parturition. Colostrum samples were collected from a subset of eight dams to determine the concentration of isoflavones and their metabolites. During the first and second trimester of gestation, the dams were fed either a youngstock (nulliparous dams) or a lactation (multiparous dams) diet. During the third trimester, both groups received a similar dry cow diet. Genistein and daidzein levels were unaffected by diet type, while their metabolite [equol, dihydrodaidzein (DHD), and o-desmethylangolensin (ODMA)] concentrations were significantly higher in the lactation group. Furthermore, metabolite concentrations decreased significantly during gestation. Isoflavones and their metabolites were detected in all colostrum samples. No correlation could be found between levels in colostrum and blood of pregnant dams or calves. Peripheral levels of isoflavones and their metabolites were significantly lower in newborn calves in comparison to their dams. Genistein and daidzein concentrations were found to be significantly higher in the calves' hair versus blood samples, suggesting prenatal exposure to isoflavones for an extended period of time. In contrast, no isoflavone metabolites were detected in the calves' hair samples. This is the first study to demonstrate that dairy calves are exposed to isoflavones during the developmentally most sensitive period of their lives. Results obtained pave the way for more extensive research to examine which effects isoflavones might have on developing organ systems like the reproductive system.