If women were fed soy-based formula milk when they were babies, they are at increased risk of suffering from severe pain during their menstrual bleeds, according to the latest study to investigate the link.
In a study published today (Friday) in Human Reproduction, one of the world's leading reproductive medicine journals, researchers looked at data from 1,553 women aged between 23 and 35 who were enrolled in the Study of Environment, Lifestyle & Fibroids in the USA between 2010 and 2012. A total of 198 (13%) reported ever being fed soy formula milk.
They found that women who had ever been fed soy formula as babies were 40% more likely to have used hormonal contraception at some point to alleviate menstrual pain compared to women who had not been fed soy formula as babies; between the ages of 18 and 22 years they were 50% more likely to have experienced moderate or severe menstrual discomfort or pain with most of their periods.
Dr Kristen Upson, a postdoctoral fellow in the Epidemiology Branch at the US National Institute of Environmental Health Sciences (NIEHS) within the National Institutes of Health (NIH), North Carolina, USA, said: "Menstrual pain is the most common menstrual complaint and can substantially affect the quality of women's lives. Exposure to oestrogens during infant development, such as the phytoestrogens in soy formula, may affect reproductive health in adulthood.
"Previous research in young adults who participated in feeding studies as infants reported an increased risk of greater menstrual pain severity in adulthood with soy formula feeding. Evidence from subsequent animal studies support the disruptive effects of the soy formula phytoestrogen, genistein, on reproductive system development that persist into adulthood. Given these results, we were interested in further evaluating the association between infant soy formula feeding and menstrual pain in a cohort of young women. We decided to investigate this in a group of African-American women, since most of the participants were white in the previous study. The consistency of our results to that of the prior study suggest that our findings may be applicable to women in general."
When enrolled in the study, the participants were asked about whether they were fed soy formula as babies, how long for and whether this started within the first two months of birth. They were encouraged to ask their mothers to help them complete the questionnaire and 89% did receive their mothers' assistance. The participants were also interviewed on the telephone and asked about whether they had ever taken any prescription or over-the-counter medication, including hormonal contraception, to treat or prevent menstrual cramps, pelvic pain or discomfort, the age at which the medication was first used, and whether they were still using it. They were asked about the use of birth control pills, hormonal implants, patches, rings or shots, and intrauterine devices, the age they started using them, and the reasons for use. The researchers also asked participants about the frequency of moderate or severe menstrual discomfort or pain between the ages of 18 and 22.
Co-author, Dr Donna Baird, who is a senior investigator at the NIEHS/NIH and leads the Women's Health Group in the Epidemiology Branch at NIEHS, said: "Many studies of menstrual pain exclude women on hormonal contraception, but this can exclude those most bothered by menstrual pain since they may be using the medication to alleviate pain. Our questions about a women's prior experience with menstrual pain allowed us to include all the women in the study."
As the study is observational it is not able to show that soy formula causes the menstrual pain in adulthood, only that it is linked to it. Dr Baird said: "Although a growing body of research studies collectively lend support to the potential reproductive health consequences of early-life exposure to soy formula, further research is warranted before recommendations can be provided to parents and carers of young infants."
The researchers did not know the reasons for soy formula feeding, but it appears to be common in the USA. Dr Upson said: "A study from 2016 found that 12% of formula-fed babies had been fed soy formula in the past day."
She concluded: "Menstrual pain is the most common menstrual complaint and can have a substantial impact on the quality of life, affecting school performance, work productivity, and relationships. It may be affected by the disruption of reproductive system development earlier in life. Infancy is a unique time, not only for reproductive system development, but also for nutrition. In the early months after birth, an infant's nutrition primarily consists of breast milk and/or formula milk. This can lead to substantial exposure to the components in formula, including the phytoestrogens in soy formula. A greater understanding of hormonal exposures during periods of development that affect reproductive health in adulthood is needed to inform future prevention efforts and improve women's health."
Limitations of the study include the fact that women had to recall information on menstrual pain experienced in previous years, and the fact that the researchers relied on women reporting whether they were fed soy formula as infants, although the participation of the women's mothers is likely to have limited any reporting errors.