Poor fertility and miscarriages may be a result of too little vitamin D

Impaired fertility and involuntary childlessness are common in the Nordic countries and there can be a number of reasons for these serious problems. However, according to a large Finnish study that is published in Nutrients, vitamin D deficiency, which is a widespread problem, may increase women’s risk of fertility problems and cause them to have a miscarriage.

Earlier studies have linked women’s problems with infertility and involuntary childlessness to low vitamin D levels in the body, but the studies have been small and shown conflicting results. In the new Finnish study, the researchers gathered data from a large Finnish population study (Northern Finland Birth Cohort) that included more than 3,000 women aged 31 years. The participants filled in questionnaires with information about family relations, lifestyle, health, sex life, and fertility. Their levels of vitamin D in the blood were also measured and compared with the following groups:

• Women who had not been examined for infertility
• Women who had been examined for infertility
• Women who had been treated for infertility
• Women who tried to conceive for 12 months before having success with it

The scientists adjusted their results for confounding factors and found that inadequate levels of vitamin D in the blood (below 30 nmol/L) was far more common among women with a history of impaired fertility and childlessness, compared with the reference group. At the same time, the reference group often had vitamin D levels that were higher than 75 nmol/L. Put differently, women who were most fertile generally tended to have blood levels of vitamin D that were substantially higher than the lower threshold value, which is 50 nmol/L.
The study also revealed that it generally took longer for women with low blood levels of vitamin D to become pregnant, and they also had an increased risk of miscarriage. The scientists conclude that vitamin D deficiency is related to a lower chance of becoming pregnant and lower fertility in general.

Vitamin D’s role in fertility

The form of vitamin D that we synthesize from sun exposure is passive. It is first converted into calcidiol (25(OH)D) or 25-hydroxyvitamin D – which is the form of vitamin D that is measured in the blood. Afterwards, 25(OH)D is converted into the active form that is called calcitriol (1,25 dihydroxyvitamin D3), which is considered a steroid hormone. Most cells in the body have receptors for this form of vitamin D, including the pituitary gland, the uterus, the ovaries, and the cervix that are important for the hormone balance and for fertility.
Vitamin D affects the enzymes 17β-hydroxysteroid dehydrogenase and aromatase that help regulate steroid hormones, including estrogen and testosterone. Moreover, vitamin D is important for the immune defense and the tolerance mechanisms of the immune defense, which make sure that the fetus is developed normally and not rejected by the body. The authors also address various lifestyle factors such as overweight, smoking, and alcohol that can affect fertility and also the body’s ability utilization of vitamin D.

Why do so many of us lack vitamin D?

At northern latitudes, we are only able to synthesize vitamin D in our skin when the sun sits sufficiently high in the sky. During the winter season, the sun is not strong enough to enable vitamin D synthesis, according to the authors. Many women also avoid getting too much sun exposure or use too much sun factor cream because they are afraid of skin cancer. While it is important to avoid getting sunburned, the sun does have health benefits. Furthermore, overweight people and type 2 diabetics have difficulty with synthesizing and activating vitamin D, which means they have an increased need for the nutrient.

• Health authorities recommend vitamin D supplementation for everyone during the winter period
• People who don’t get enough sun during the summer or have dark skin should also consider taking a supplement in the summertime.
• Pregnant and breastfeeding women are advised to supplement with vitamin D all year round
• It is essential for everyone to strive to have optimal blood levels of vitamin D at all times. The lower threshold value for vitamin D is 50 nmol/L
• According to the Finnish study, levels higher than 75 nmol/L are required for healthy fertility

Reference:

Johanna Lumme et al. Vitamin D Status in Women with a History of Infertility and Decreased Fecundability: A Population-Based Study. Nutrients. 2023

Brigham and Women´s Hospital. Vitamin D benefits and metabolism may depend on body weight. ScienceDaily January 15, 2023

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