Women fed soy-based formula milk as babies are more likely to suffer from severe period pains, according to US scientists.

They looked at data from more than 1500 women, nearly 200 of whom had been fed soy-based baby formula as infants. Women fed soy were 50 per cent more likely to have experienced moderate or severe menstrual discomfort or pain during most of their periods, and 40 per cent more likely to have used hormonal contraception to alleviate menstrual pain.

However, the study cannot show that soy formula actually causes menstrual pain, only that an association exists.

In a study published today in Human Reproduction, a leading reproductive medicine journal, researchers looked at data from 1553 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 per cent) reported ever being fed soy formula milk.

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They found that women who had ever been fed soy formula as babies were 40 per cent 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 per cent 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."

The researchers did not know the reasons for soy formula feeding, but it appeared to be common in the USA. Dr Upson said: "A study from 2016 found that 12 per cent 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 was likely to have limited any reporting errors.