Vitamin D is a fat-soluble vitamin that helps maintain bone health, as well as male and female reproductive health. Vitamin D regulates the levels of calcium and phosphorus, which supports calcium absorption and bone health – especially important during the preconception period, pregnancy and the postnatal period. It can be difficult to get enough Vitamin D from natural sources, including food and sun exposure, but it's not impossible. Let's dive in.
What are the good sources of Vitamin D?
According to the National Health and Nutrition Examination Survey, a vast majority of us in the United States do not get enough Vitamin D from food. In fact, a whopping 92% of men and 97% of women did not get enough Vitamin D from food, despite the fact that some food and beverages are fortified with Vitamin D in the United States.
Vitamin D synthesis in the skin
This is partially because a significant portion of the Vitamin D that our body uses is synthesized in our skin. Vitamin D synthesis occurs when we are outside, being exposed to the sun – specifically, to the UVB radiation. UVB doesn’t penetrate glass, so even if you spend a lot of time indoors by a sunlit window, that doesn’t help with your Vitamin D levels – your skin needs direct exposure to UVB outdoors.
How much Vitamin D is generated in your skin depends on many factors, including:
- Angle and strength of the sunlight, which varies based on where you live, the season or the time of day.
- Amount of UVB that reaches your skin, which depends on the cloud cover, whether you wear sunscreen or long sleeves, as well as the amount of melatonin you have.
- Your age, with older people being able to synthesize less Vitamin D than younger people.
Depending on these factors, you may not be getting enough Vitamin D from sun exposure. For example, people who live in northern regions often have low blood levels of Vitamin D in winter. Similarly, people who are diligent with sun protection (which is a good idea for skin cancer prevention!) may not get enough Vitamin D from endogenous synthesis.
Natural, whole-food food sources of Vitamin D
There aren’t many natural food sources of Vitamin D – another reason we don’t get enough Vitamin D from food sources alone. However, it is possible to improve your Vitamin D intake from natural, whole-food sources when you focus on these foods:
- Farmed rainbow trout has close to 650 IU of Vitamin D in a 3-oz serving. This is about 80% of the recommended daily value.
- Similarly, salmon has about 570 IU of Vitamin D, at about 70% of the daily recommendation.
- Other fatty fish like sardines and tuna also contain some Vitamin D, but at much lower levels of 40-45 IU per a 3-oz serving.
- White button mushrooms that are grown under UV light may be a surprising natural source of Vitamin D. They have about 350 IU, in a half-cup serving (raw). This is one of the few natural sources of Vitamin D.
National Institutes of Health Office of Dietary Supplements has a more comprehensive list here that shows many foods with no or negligible amount of Vitamin D.
Foods commonly fortified with Vitamin D
Because natural, whole-food sources of Vitamin D are few, some foods are fortified with Vitamin D in the United States. In fact, most of the Vitamin D in the typical American diet comes from Vitamin D-fortified foods:
- Almost all milk is fortified with Vitamin D in the United States. A one-cup serving of milk typically contains about 120 IU of Vitamin D. Caveat: Dairy products made from milk, such as cheese, ice cream and yogurt, typically don’t have added Vitamin D.
- Some orange juice and breakfast cereals have added Vitamin D.
- For those of us with milk allergy or lactose intolerance, plant-based milk products like almond milk, soy milk and oat milk, are often fortified with Vitamin D at a similar level found in cow’s milk (120 IU per cup).
Dietary supplements with Vitamin D
The need for Vitamin D and the difficulty of getting enough from natural food sources and sun exposure alone are well known. The same National Health and Nutrition Examination Survey showed that over a quarter of the US population (28%) take dietary supplements that contain Vitamin D. While those of us who don’t take a Vitamin D supplement had less than 200 IU/day of Vitamin D in our diet, people who take a Vitamin D supplement had more than 4 times that amount in their diet.
The takeaway on Vitamin D sources
Natural, whole foods provide more benefits than any specific nutrient in isolation, but supplements can be an integral part of your healthy eating habits, mainly by bridging the nutritional gap in your diet.
Vitamin D is the case in point. It’s an essential vitamin that helps maintain healthy bones and teeth. It can also be difficult to get a full recommended amount from food and sun exposure alone, depending on where you live, your skin color, your dietary practice and more.
In fact, even using a conservative definition of 30 ng/mL as a threshold of inadequate Vitamin D levels, 18% of US adults have been found to be at risk of Vitamin D inadequacy. (Here, inadequacy means lower-than-optimal levels, but not to the point of outright deficiency.) Over 17% of African Americans (who have more trouble absorbing the UBV to synthesize the vitamin) are outright deficient in Vitamin D, while about 8% of the general population ages 20-39 are deficient.
If you think you might not be getting enough Vitamin D from food sources and sun exposure, it may be a good idea to consider Vitamin D supplementation to help support bone and teeth health, reproductive health in both men and women, as well as prenatal health. Your fertility doctor or a general healthcare provider can give you specific recommendation, but please do reach out if you have any questions. We are with you.
