Vitamin D deficiency is remarkably common after 40 — not because something goes wrong, but because the ways we naturally got it (regular sun exposure, mostly) collide with modern life: indoor work, sunscreen, less time outside, and a natural decline in the skin's ability to produce it with age. Vitamin D affects far more than bones, including immune function and mood. A blood test is the only reliable way to know your level, and food plus supplementation (if needed) are the practical fix.
Vitamin D has a branding problem. Most people file it under "bone health" and stop thinking about it — but low levels are so widespread that some research puts a large share of American adults, women over 40 included, below optimal range.
It's less an exotic health issue and more a predictable side effect of how most people actually live now.
Why is vitamin D deficiency so common?
Vitamin D is unusual among nutrients because your body can manufacture it — but only when skin is exposed to enough direct sunlight, at the right angle, for long enough. That equation has quietly broken down for most people: indoor jobs, sunscreen (which is genuinely the right skincare choice, so this isn't an argument against it), higher latitudes, and simply spending less unstructured time outdoors than previous generations all reduce production.
On top of that, skin becomes less efficient at synthesizing vitamin D from sunlight with age, so the same amount of sun exposure produces less vitamin D at 45 than it did at 25. Few foods naturally contain much vitamin D either, which means diet alone rarely closes the gap without fortified foods or supplements.
What does vitamin D actually affect?
Bone health: vitamin D is essential for calcium absorption, directly tying into the bone density picture covered in our bone density guide — without enough of it, the body can't use dietary calcium effectively no matter how much you consume.
Immune function: vitamin D receptors are present throughout the immune system, and research links low levels with a higher likelihood of frequent infections, though it's not a guaranteed cause-and-effect.
Mood: some research associates low vitamin D with low mood and seasonal patterns of it, adding a thread to the broader picture in our anxiety guide — though vitamin D is one factor among many, not a stand-alone explanation.
Muscle function: adequate vitamin D supports muscle strength, which matters for the same reasons covered in our gut health guide's discussion of whole-body nutrient absorption.
Where does vitamin D actually come from?
- Sunlight exposure on bare skin (unreliable and inconsistent for most lifestyles)
- Fatty fish (salmon, mackerel, sardines)
- Egg yolks
- Fortified foods (milk, some plant milks, some cereals)
- Supplements (D3 is the form most commonly recommended)
Because so few whole foods are naturally rich in vitamin D, fortified foods and supplements do a lot of the practical work for most people, especially after 40.
How much vitamin D do you actually need?
General guidelines suggest 600-800 IU daily for most adults, but that's a population-wide starting point, not a personalized target. Because deficiency is so common and the "right" dose depends on your actual blood level, a simple test — then adjusting intake based on the result — is a more precise approach than guessing from a supplement label.
When should you talk to a doctor about vitamin D?
Worth a conversation at your next physical, or sooner if you have persistent fatigue, bone or muscle aches, or frequent infections without an obvious explanation. It's a simple, inexpensive blood test, and knowing your actual number beats guessing either way.
Frequently asked questions
How do I know if I'm vitamin D deficient?
Fatigue, bone or muscle aches, and low mood are common signs, though they overlap with many other causes. A blood test (25-hydroxyvitamin D) is the only reliable way to know.
Can I get enough vitamin D from sunlight alone?
Possible but unreliable — it depends on season, latitude, skin tone, sunscreen use, and time spent outside. Food and supplements matter as reliable backups.
How much vitamin D should women over 40 take?
General guidelines suggest 600-800 IU daily, but the right amount depends on your actual blood level — testing first is more reliable than guessing.
Can you take too much vitamin D?
Yes, from supplements taken in excess over time — it can build up and affect calcium levels. Uncommon at standard doses, but worth periodic checking rather than self-prescribing high doses.
Sources
- National Institutes of Health, Office of Dietary Supplements — vitamin D fact sheet. ods.od.nih.gov
- Harvard T.H. Chan School of Public Health — vitamin D. hsph.harvard.edu
Chris S. — Founder & Writer
Chris is 46 and living the same chapter she writes about. She started The Well Chapter to turn solid sources into calm, practical guidance for women navigating the changes that come after 40 — without hype, fear, or miracle promises.