Popular high-protein diets are helping some women shrink their waistlines and feel healthier. But new research reveals these diets might also reduce fertility, by compromising the health of embryos. These findings add to existing criticisms of high-protein diets, which some claim contribute to metabolic problems, mood disorders and gout.

Reproductive medicine researchers in Colorado found female mice that consumed too much protein are less likely to produce live offspring. A test group of mice ate a diet consisting of twenty-five percent protein, while the control group had a fourteen percent protein diet. One month later, about one-hundred-seventy embryos from the mice were transferred into the wombs of surrogate mouse mothers.

The results showed that maternal diet before conception affected embryo health. Only sixty-five percent of the embryos from the high-protein group developed into fetuses, while eighty-one percent in the control group did. Spontaneous abortion later in pregnancy was also more likely in the high-protein eaters— sixteen percent versus just one percent in the control group.

The culprit here is the accumulation of ammonium in the reproductive tract that results from the breakdown of food proteins. Ammonium has been shown to significantly interfere with embryo development in the petri dish environment. Now it appears to be dangerous to the developing embryo in utero.

Researchers say they don’t know if these findings apply to humans. But it’s possible a protein-packed diet full of meat and eggs could have long-term, unintended effects on women’s own eggs, and, eventually, on their childbearing abilities.