Women in their early 40s with the highest intake of vitamin D and calcium from food sources may have a lower than average risk of starting menopause before age 45, a recent study suggests.

Taking vitamin D or calcium in supplement form had no benefit in the large study of U.S. nurses, the study team writes in American Journal of Clinical Nutrition, and there may be other substances in dairy foods that also contribute to their apparent protective effect.

“Early menopause can have substantial health impacts for women. It increases their risk of cardiovascular disease and early cognitive decline and osteoporosis,” lead author Alexandra Purdue-Smithe told Reuters Health.

In addition, as women are delaying having kids into their later reproductive years, having early menopause can have a substantial impact on their ability to conceive as they wish, which can have psychological and financial consequences, said Purdue-Smithe, an epidemiologist with the School of Public Health and Health Sciences at the University of Massachusetts Amherst.

“Given that (early menopause) affects roughly 10 percent of women in the U.S. and other Western populations, it felt like a worthwhile problem to start investigating and seeing if there are any potentially modifiable risk factors for it,” she said.

Menopause, when a woman stops menstruating and her levels of hormones like estrogen decline, typically happens between the ages of 45 and 55. Menopause before age 45 is considered “early.”

Vitamin D may be involved in some of the hormonal mechanisms of early menopause, but little is known about how dietary vitamin D and calcium affect the risk, Purdue-Smithe and her colleagues write.

They analyzed data from the Nurses’ Health Study II, a long-term study of more than 100,000 U.S. registered nurses who were 25 to 42 years old in 1989 when they began answering health questionnaires every two years.

The questionnaires were designed to assess the nurses’ lifestyles, behaviors and overall health. Questions about diet were asked five times over 20 years. Researchers followed the participants until 2011, by which time 2,041 women experienced early menopause.

“The women who consumed the most vitamin D from food sources had a 17 percent lower risk of having early menopause as compared to women who consumed the least,” Purdue-Smithe said. The researchers found this association only with dairy sources of vitamin D, like milk, not with non-dairy sources like oily fish.

Women who consumed the most calcium from food sources were also about 13 percent less likely to experience early menopause compared to women who consumed the least calcium, and once again, only dairy foods seemed to provide a benefit.

“Our next direction is to look at actual individual dairy foods and see if there’s something else going on with dairy itself,” Purdue-Smithe said.

The study team also found that taking high doses of calcium in supplement form was associated with a higher risk of early menopause. But the researchers speculate that these women might have been diagnosed with osteoporosis or other conditions that are also risk factors for early menopause.

“Most of what is known about the relationship between calcium and Vitamin D and women’s issues is related to bone health,” said Sandra Arevalo, a dietitian and director of the Nutrition Services and Community Outreach for Community Pediatrics at Montefiore Medical Center in New York.

Lack of Vitamin D and calcium in a woman’s diet, mainly as age progresses, increases her risk of low bone mineral density, osteoporosis and bone fractures, said Arevalo, who wasn’t involved in the study.

The top 10 food sources of calcium are low-fat yogurt, low-fat cheese, sardines, calcium-fortified soy milk, calcium-fortified orange juice, salmon, calcium fortified ready-to-eat cereal, turnips, kale and bok choi, she noted in an email.

The top 10 sources of Vitamin D are cod liver oil, swordfish, salmon, tuna fish, vitamin D fortified orange juice, low-fat vitamin D-fortified milk, yogurt, fortified margarine, sardines and liver, Arevalo said.