After the flood, God promised Noah that “seedtime and harvest, and cold and heat, and summer and winter, and day and night shall not cease” (Genesis 8:22). The seasons are part of His design, and so is the way our bodies change with them.
One of the biggest of those changes comes in November, and almost nobody feels it arrive. The sun sits lower in the sky, and your skin stops making much vitamin D.
Why the sun stops helping in November
Your skin makes vitamin D when a particular band of sunlight, called UVB, reaches it. In summer that light comes in at a steep angle and plenty of it gets through. In winter the sun is lower, the light passes through more of the atmosphere on its way down, and most of the UVB is filtered out before it reaches you.
Researchers in Boston measured this in 1988. Skin exposed to the Boston sun made no vitamin D at all from November through February. In Edmonton, Canada, the stretch ran from October through March.
Dr. Michael Holick, who led that work, puts the line at about 33 degrees latitude. Above it, skin makes very little vitamin D through most of the winter. That line runs close to Atlanta and Los Angeles, so most of the country lives north of it.
Why it matters more after 40
Two more things lower the amount your body makes.
Skin makes less vitamin D as we age. Dr. Holick’s research puts a 70-year-old at about a quarter of the vitamin D-making capacity of a young adult.
Winter also keeps most of us indoors, and window glass blocks UVB completely. Sitting by a sunny window warms you without making any vitamin D.
For women in and past menopause, bone is where low vitamin D shows up most. Vitamin D is what lets the body take in calcium from food. When there isn’t enough of it, the body draws calcium out of bone to keep the level in the blood steady. Vitamin D also supports muscle strength and the immune system, which matters in the months when colds go around.
What to do through the winter
Get outside at midday when you can. Daylight sets your sleep and wake rhythm even when the sun is too low to make vitamin D, and a walk in it does you good. From March on, midday sun on your face and arms starts making vitamin D again.
Eat mushrooms that have seen the sun. Mushrooms make vitamin D when they are exposed to ultraviolet light, much the way skin does. Look in the produce section for packages labeled “UV-treated” or “high in vitamin D.” In spring and summer you can do it at home: slice fresh mushrooms and set them gill side up in the midday sun for an hour before you refrigerate or dry them. Mushrooms are a good winter food. For most people they aren’t enough by themselves to hold a healthy level until spring.
Feed the nutrients vitamin D works with. Your body needs magnesium to turn vitamin D into the form it uses. Pumpkin seeds, almonds, black beans and dark leafy greens are all rich in it. Those same greens supply vitamin K, which works with vitamin D on bone.
Use a supplement for the winter months. North of that 33-degree line, a vitamin D3 supplement is the practical way for most people to hold their level until spring. We carry Vitamin D3 & K2, which pairs D3 with vitamin K2 to help direct the calcium vitamin D brings in toward your bones. Vitamin D is fat-soluble, so take it with a meal that includes some avocado, nuts or seeds.
Know your number. The blood test is called 25-hydroxyvitamin D. Many integrative practitioners like to see it between 50 and 80 ng/mL. A test in late winter shows you your low point, and one in late summer shows your high. How much you take depends on where you start, and a nutritionally minded health care professional can help you settle on an amount.
A season to plan for
“Truly the light is sweet, and a pleasant thing it is for the eyes to behold the sun” (Ecclesiastes 11:7). The shorter days are part of how God made the year, and a few simple habits keep your vitamin D where it should be until spring.
Start this week. Put a midday walk on your calendar, add mushrooms to your grocery list, and if you haven’t had your vitamin D checked, ask for the test.
Frequently asked questions
Can I get enough vitamin D from food on a plant-based diet?
Mushrooms exposed to ultraviolet light are the main plant food that carries it, and some fortified plant milks add a little. Most people get the rest from summer sun and a winter supplement.
Does sitting in a sunny window count?
No. Window glass blocks the UVB light your skin needs to make vitamin D.
Should I keep taking vitamin D in the summer?
Many people lower or stop their dose in summer if they get regular midday sun on bare skin. A test in late summer tells you whether you held your level.
Can you take too much vitamin D?
Yes. Vitamin D is stored in the body, and very high doses taken for a long time can build up. Testing once or twice a year keeps you in a good range.
Why is K2 paired with D3?
Vitamin D raises how much calcium you absorb. Vitamin K2 helps move that calcium into bone, where you want it.
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Sources
- Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. Journal of Clinical Endocrinology and Metabolism, 1988;67(2):373-378.
- Holick MF. Vitamin D deficiency. New England Journal of Medicine, 2007;357(3):266-281.
- Holick MF. Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. American Journal of Clinical Nutrition, 2004;80(6 Suppl):1678S-1688S.
- Uwitonze AM, Razzaque MS. Role of magnesium in vitamin D activation and function. Journal of the American Osteopathic Association, 2018;118(3):181-189.
- Cardwell G, Bornman JF, James AP, Black LJ. A review of mushrooms as a potential source of dietary vitamin D. Nutrients, 2018;10(10):1498.
Educational content. Not medical advice. Talk with a nutritionally minded health care professional about your own health.
We offer these teachings freely to support clarity, stewardship, and responsible care. If this article was helpful, you may explore additional resources here:
MyHDiet.com — plant-based nutrition, enzymes, detox foundations
WipeOutToxins.com — toxin-free living and prevention
CleanseYourBlood.com — circulation and blood-flow support
HealthRetreat.com — Hallelujah Diet Health Retreat
AMPMForHealth.com — everyday wellness rhythms and curated tools
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