You probably know the feeling. One day it is still summer, and the next the sun is setting before dinner.
Many of us just chalk the first weeks of fall up to being a bit tired.
Under the hood, something else may be going on. Once the days get short and the sun sits low in the sky, your skin can barely make any vitamin D from sunlight. Levels can slide fast from there.
According to the NIH Office of Dietary Supplements, blood levels of this vitamin below 20 ng/mL are considered inadequate for bone and overall health.
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Deficiency is already common year round. A review in Frontiers in Nutrition that pooled NHANES data from 2001 to 2018 found about one in four US adults had levels below 50 nmol/L, which is 20 ng/mL. The share rose in the winter months.
The most commonly missed sign
The symptom most people brush off is a low-grade tiredness that creeps in through October and November.
According to the Cleveland Clinic, fatigue, bone pain, muscle weakness and mood changes are among the main signs of a shortage.
That tiredness often gets written off as the usual seasonal slump. Autumn fatigue has many causes, and a low vitamin D level is only one of them. But it is the one that is easiest to check with a simple blood test.
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7 warning signs to watch for as fall sets in
The symptoms of a vitamin D shortage are often vague, which is why they are missed. The seven most commonly reported ones:
- Persistent fatigue that does not lift after a full night of sleep.
- Low mood or new depressive symptoms, especially from mid-October onward.
- Bone or muscle aches, often in the lower back, hips or legs.
- Frequent colds or minor infections, because vitamin D helps activate the immune cells that fight viruses and bacteria.
- Brain fog and trouble focusing on ordinary tasks.
- Hair thinning, especially a sudden pickup in shedding during the fall.
- Slow healing of cuts and scrapes.
Any single one of these can have many other causes, and none of them proves a vitamin D shortage on its own.
Two or three showing up together, in the weeks after the clocks go back, is the pattern worth paying attention to.
What the blood test numbers mean
If your doctor orders a 25-hydroxyvitamin D test, which measures how much vitamin D is actually in your blood, the result comes back in ng/mL. The NIH uses three main categories.
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Serum 25(OH)D | What it means |
|---|---|
Below 12 ng/mL | Deficient Also read: Your relationships may leave a measurable mark on a specific brain network |
12 to 19 ng/mL | Inadequate for bone and overall health |
20 ng/mL and above | Sufficient for most people Also read: Short on steps? Picking up the pace for 30 minutes may lower death risk |
An older cutoff popularized by the Endocrine Society in 2011 placed insufficiency between 20 and 29 ng/mL.
The Society stepped back from that specific target in its 2024 guideline for healthy adults, after finding the clinical trial evidence did not support a single sharp threshold.
What you can do as the sun gets weaker
The recommended daily intake set by the National Academies is 600 IU for most adults, rising to 800 IU after age 70.
Food can cover some of that. Fatty fish like salmon, trout and sardines, egg yolks, and fortified milk, yogurt or cereal all carry some vitamin D.
For adults aged 75 and older, the 2024 Endocrine Society guideline suggests a daily low-dose vitamin D supplement because of a possible drop in mortality risk.
For healthy adults under 75, the guideline does not recommend routine extra vitamin D beyond what food and a standard multivitamin provide.
A short walk outside around midday still helps, even on cold days, because any direct sunlight on bare skin counts.
North of roughly the latitude of San Francisco, however, the midwinter sun no longer carries enough UVB for the skin to make much vitamin D at all.
If you have unexplained fatigue, muscle aches or a persistent low mood that started in the last few weeks, ask your doctor whether a 25(OH)D blood test makes sense for you.
This article is made and published by Mie Hermansen, who may have used AI in the preparation.