In the last several years, there has been an increasing amount of scientific research
demonstrating the importance of vitamin D in reproductive health. For example,
men with vitamin D deficiency have poorer fertility. A large Danish study found that vitamin
D supplementation resulted in more live births. I talked about that study in this previous episode.
In today’s episode of InfertilityTV,
I am going to discuss vitamin D levels and their impact on the menstrual cycle.
What is Vitamin D? Vitamin D is an oil-soluble vitamin that has several important functions in
the body. It helps to absorb dietary calcium and phosphorus from the intestines. Vitamin D
keeps the calcium and phosphate levels in the blood normal, thereby promoting bone health.
There are two main ways in which you get vitamin D in your body. Most of the
vitamin D in humans is made in the skin when exposed to sunlight. The amount of sunlight
needed to make adequate amounts of vitamin D3 varies, depending upon the person's age,
skin color, sun exposure, and underlying medical problems. The production of vitamin D3 from the
skin decreases with age. In addition, people who have darker skin need more sun exposure
to produce adequate amounts of vitamin D3, especially during the winter months.
Another important source of vitamin D is foods like sardines, salmon, eggs and liver. However,
in humans, less than 20% of vitamin D is obtained from the diet. In the United States,
commercially fortified cow's milk is the largest source of dietary vitamin D.
The length of a person's cycle can be shorter or longer. To learn about the cycle,
let’s use a hypothetical example. Let’s say that counting from the first day of
the period until the first day of the next period, is 28 days. In this case,
we would expect ovulation to occur around day 14. The time from the beginning of
her period until ovulation is called the follicular phase. People who ovulate early
have a short follicular phase. People who ovulate late have a long follicular phase
As long as they have intercourse around the time of ovulation, people who have a long cycle have
the same chance for pregnancy as people with a short cycle. However, people with a longer cycle
have a few less opportunities to get pregnant over time because it takes longer for them to ovulate.
We also know that in some cases, people with longer cycles are not ovulating. In fact,
the longer the cycle, the more likely it is that there is no ovulation.
A few years ago, scientists published the results of the Time to Conceive study. In this study,
people who were intending to become pregnant were recruited if they did not have any known problems
with fertility or ovulation problems. They had blood drawn at the beginning of the study and
kept diaries of their attempts at pregnancy and were provided with home ovulation tests.
Menstrual cycle length ranged from 19 to 68 days. They called cycles long if
they lasted 35 days or more. They called them short, if they were 25 days or less.
This study found that people with low levels of vitamin D were more likely to
have longer cycles and more likely to have a longer follicular phase. In other words,
people with low vitamin D levels took longer to ovulate. This was most likely to be seen in
people with a vitamin D level less than 20 but still had an effect at levels between 20 and 30.
Vitamin D levels were not associated with the length of the luteal phase.
Our Infertility TV Bottom line is this: If the length of your cycles is longer,
more than 35 days, it is possible that your vitamin D levels are low. Vitamin D levels
are easy to check with a blood test and easy to fix with vitamin D supplementation. This
study did not look at whether supplementation with vitamin D will shorten the length of the
cycle. This remains to be seen. However, given the mounting evidence regarding vitamin D and
fertility and general health, making sure your vitamin D levels are normal is a reasonable goal.