Wintertime months mean the sun sets sooner in many places. Unfortunately, this limits the hours we can spend outside and get sunshine. In addition, we are bundled up when it is cold, limiting skin exposure to the sun's rays.
Even if you live in sunny California, Texas, or Florida, you still might be deficient in this essential vitamin. Sunlight alone is not enough to supply our bodies with the amount of Vitamin D that it needs. Read more on why we need vitamin D, vitamin D through the seasons, recommended amounts to take, and more below.
Just because the weather is warm doesn't guarantee you are getting enough vitamin D.
First, why do we need vitamin D?
Vitamin D is an important vitamin for everyone at every age. It is a fat-soluble vitamin responsible for absorbing the mineral calcium into your intestines and bloodstream. Without enough vitamin D3, you are at higher risk for bone fractures and poor oral health. (5) Sufficient vitamin D plays a role in healthy blood pressure and cardiovascular health.
Infants need vitamin D to support their growing bones and the formation of healthy teeth. Vitamin D works best when combined with vitamin K2 to mineralize your baby's bones and teeth. Then, before teeth erupt, vitamin D keeps the baby's gums healthy.
A systematic review of controlled clinical trials, which included a large number of children, showed that sufficient vitamin D levels can protect against dental cavities. Vitamin D supplementation reduced the risk of developing dental cavities in about 47% of children. (6)
Vitamin D helps keep our immune system healthy and can reduce the risk of certain viruses. In addition, it keeps our brains and cognitive function sharp and decreases the risk of type 2 diabetes. It may also help improve symptoms of eczema.
Vitamin D in different seasons
Just because the weather is warm doesn't guarantee you are getting enough vitamin D. One cross-sectional study including men and women ages 10-70 showed that even in an area of high humidity, the seasonal variation of vitamin D in the subject's bloodstream was significantly lower in the winter months compared to the summer. (3) In the summer, people often avoid the midday sun and sit in the shade or cover more of their skin to avoid sunburn, which also means getting less vitamin D from sunshine.
The season in which someone was born can affect lifelong vitamin D status. A significant cross-sectional retrospective investigation showed that people born in the winter had significantly lower total vitamin D later on as adults compared to those born in the summer. It showed the risk of developing a vitamin D deficiency was 11% higher for those born in the winter. (7)
The UV index is a scale of ultraviolet radiation levels. The UV index is high in the summer, and our skin is exposed to more ultraviolet light. In the winter, UV exposure is much lower, so even though it may feel good to have the sun shining on our faces, we are not getting as much vitamin D because the angle of the sun's rays just doesn't hit the earth to deliver more UV light. Most of the United States gets little vitamin D from sun exposure from November through February. (9)
Other factors that limit our vitamin D sufficiency
Living in higher altitudes
The further you live from the equator, there is less UV light from the sun
Air pollution - especially for those living in densely populated areas
Reduced skin exposure because of clothing covering more of the skin or reduced time in the sun at midday for at least 20-30 minutes
Having a darker skin tone - darker skin pigmentation requires more prolonged sun exposure to absorb vitamin D (8)
Using sunscreen can limit the amount of UV light our skin can absorb
Being overweight - being obese is correlated with lower vitamin D levels and may affect the bioavailability of vitamin D
Breastfed infants whose mothers do not have sufficient vitamin D levels - breastmilk has vitamin D. Still, many mothers are deficient themself, and their breastmilk doesn't supply their baby with adequate vitamin D. Because of this, mothers either need to test their vitamin D serum levels and supplement themselves with the recommended 6400 IU/day or give their baby a vitamin D3 supplement.
How much vitamin D do we need? (1)
RDA - stands for Recommended Daily Allowance.
0-12 months - RDA is 400 IU/day unless the baby is exclusively breastfed and the mother is taking 6400 IU/day
1-70 years - 600 IU/day
During pregnancy and lactation - 600 IU/day
Over 70 years - 800 IU/day
What is the best form of vitamin D to take?
Vitamin D3 is more available to our body than vitamin D2. In addition, because of vitamin D3's longer half-life, it stays in the body longer and can raise levels better than D2.
In one study, infants were given either a supplement of vitamin D2 or D3 for three months. The group with vitamin D2 had an increase of 4ng compared to infants given vitamin D3, who had a rise of 9ng. This meant that 75% of infants in the D2 group had sufficient levels after supplementation compared to 96% of infants in the vitamin D3 group. (2)
Vitamin D in our diet
A healthy diet with foods high in vitamin D contributes to our overall vitamin D levels. Still, it is not a sufficient way for most people to maintain their vitamin D status.
Foods high in vitamin D
Shiitake, white, and portobello mushrooms exposed to at least 6 hours of UV light
What to look for in a vitamin D supplement?
It contains vitamin D3, not vitamin D2
High quality, organic and non-gmo
It also has vitamin K2 as MK-7. Vitamin D3 absorbs calcium, but it is vitamin K2 that then transports the calcium from the bloodstream to where it is needed in the body, like your bones and teeth. Without vitamin K2, too much calcium can build up in your arteries. (4)