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 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 alike, and 800IU for the elderly.
SOURCES 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 into 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, the time of year and time of day also play an important factor. In latitudes above 42 degrees North (or below 42 degrees South), there is inadequate UVB radiation to support vitamin D synthesis from mid-October to mid-March. Keep in mind Bellingham is at 48 degrees North. Plus, the best time for good UVB exposure is between 10am – 2pm. Therefore, if you are working an indoor job from 9am to 5pm, are fully clothed, and wear sunscreen on your face, then you will not synthesize any vitamin D. And 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 will be 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 you can 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 3oz), eggs (40IU per egg), and liver (40IU per 3oz). 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 wellbeing 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 all the factors discussed previously. However, 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 1-2x per year to get a feel for what your unique needs are. 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 doctors visit or see the resources below for more helpful research, testing, guidelines, and applications.
Krause’s Food & The Nutrition Care Process, 14thEdition. Pages: 1071-1072.
Linus Pauling Institute. Vitamin D. https://lpi.oregonstate.edu/mic/vitamins/vitamin-D#RDA
GrassrootsHealth. Resources. https://grassrootshealth.net/documentation/
Like to read? Then get your evidence based nutrition information here! All posts written by Selva Wohlgemuth, MS, RDN Functional Nutritionist & Clinical Dietitian