Ask the Nutritionist: Vitamin D


“Tell me more about vitamin D. How and where do I get enough vitamin D for general wellness?”

Many people have heard of vitamin D and understand its relationship to the sun. I often hear people say “soaking up my vitamin D” when a nice, sunny day arrives. But is it really as simple as that? Of course not. Today I will clarify what vitamin D is, why you need to make sure you are getting enough, and how to make sure you are getting what you need.

Vitamin D 101

Vitamin D is a fat-soluble vitamin with hormone-like properties that is found naturally only in a few foods and can be synthesized in the skin from the sun’s UVB rays. It is commonly known for its facilitative role in bone health by increasing calcium absorption and for its role in improving seasonal affective disorder. However, many people do not know that it also is required for proper immune function, hormone health, cellular growth and development, and blood sugar and blood pressure regulation. That’s a lot!

Many studies have found an inverse relationship between vitamin D status and autoimmune disorders, diabetes, eczema, cancer, depression, and more. The current recommended dietary allowance (RDA) is set at 400IU per day
for infants, 600IU per day for children and adults, and 800IU for the elderly.

Source of Vitamin D

Vitamin D from the Sun

The sun doesn’t give you vitamin D3, it merely starts a process. Upon the skin’s exposure to the sun’s UVB rays, pre-vitamin D3 is converted to inactive vitamin D3 (cholecalciferol). After conversion in the skin, cholecalciferol is quickly transported to the liver and then to the kidneys to be metabolized to active vitamin D3, also known as calcitriol. Therefore, if the liver or kidneys are not functioning properly, vitamin D status can be impaired.

However, the darkness of your skin, the fat deposition underneath your skin, and the coverage on your skin (including sunscreen and clothing) all play a role in if and how much pre-vitamin D3 is converted to cholecalciferol. If you have darker skin tone, are elderly (less fat under skin), wear a protective clothing layer, or apply a sunscreen greater than 10 SPF, you will have reduced or no vitamin D3 conversion.

Furthermore, above the 40th parallel north (or below the 42nd parallel south), there is inadequate UVB radiation to support vitamin D synthesis from mid-October to mid-March. Bellingham is at the 48th parallel north.

Plus, the best time for good UVB exposure is between 10 am–2 pm. If you are working an indoor job from 9 am to 5 pm, are fully clothed, and wear sunscreen on your face, then you will not synthesize any vitamin D. If you think you will get some vitamin D when sitting in a sunny spot inside, think again. It doesn’t matter what time of year it is, or time of day, if you are sitting in front of a window, all UVB rays are blocked and you will not synthesize any vitamin D.

Nutritionist Tip: Get outside around noon for 15 minutes and expose your hands, arms, and face (without sunscreen) in the late spring, summer, and early fall months to synthesize about 1000IU. Then layer on the sunscreen or seek shade.

Vitamin D from Food

Vitamin D is only found naturally in very few foods including fatty fish like salmon and sardines (340IU per 3 ounces), eggs (40IU per egg), and liver (40IU per 3 ounces). Mushrooms, although advertised as a source of vitamin D, often do not provide much useable vitamin D unless the grower purposefully has exposed the mushrooms to UV light. One cup of sliced “unexposed” crimini mushrooms only provides 5IU, whereas the “exposed” provides around 400IU. Ask your grocer what kind they offer. However, there are other foods on the market that are fortified with vitamin D such as dairy and plant milks, orange juice, and some cereals, usually providing anywhere between 50-100IU per serving.

Nutritionist Tip: Enjoy fatty seafood like salmon, UV “exposed” mushrooms, and fortified milk or non-dairy milks multiple times per week for substantial food sources of vitamin D.

Vitamin D from Supplements

Vitamin D supplements can be found as vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Vitamin D2 is synthesized via UV irradiation of yeast, whereas D3 is synthesized via UV irradiation of lanolin. However, research studies have found that vitamin D2 may not be as effective in increasing active (calcitriol) vitamin D3 serum levels. Taking supplements is essential when adequate dietary intake and proper sun exposure are lacking.

How Much to Supplement?

The optimal intake of vitamin D to support general health and well-being remains controversial. Researchers have found a U-shaped curve regarding vitamin D status, indicating that both low and high vitamin D serum levels are correlated with disease development and progression. The Linus Pauling Institute recommends that generally healthy adults supplement with 2000IU of vitamin D3 daily. Some may need more or less depending on the factors previously discussed.

More is not always better! High-dose vitamin D supplementation that is not monitored can lead to abnormally high serum calcium concentrations, which can damage the kidneys and heart. Research suggests that daily intakes of less than 10,000IU per day in healthy individuals is very unlikely to result in toxicity.

Nonetheless, it is best to test not guess! Work with a health care practitioner to check your vitamin D status 1–2 times per year to get a feel for your unique needs. Research studies suggest that a serum vitamin D concentration between 40 ng/mL and 60 ng/mL is ideal.

Nutritionist Tip: Supplement with 1000–2000IU per day and check your vitamin D levels annually to ensure a serum vitamin D between 40–60ng/mL. Make sure to check your multivitamin, as they often already contain some vitamin D.

Resources for The Inspired Individual

Overall, low vitamin D status can impact your health in many ways. Unfortunately, testing vitamin D status is not as routine as it should be, especially here in the Pacific Northwest. Ensuring optimal vitamin D levels year-round can help keep you feeling your best. Be an advocate for yourself and request vitamin D labs at your annual doctor visit or see the resources below for additional helpful research, testing, guidelines, and applications.