Every January, patients come into our Naperville office describing the same cluster of complaints. They are tired in a way that sleep does not fix. Their muscles ache for no obvious reason. They have caught every cold that went through the office. Some of them have already searched vitamin D deficiency symptoms online and suspect what is going on. Many of them are right.
Illinois winters are hard on vitamin D levels, and not because people here eat differently or spend less time outside than anyone else in the northern half of the country. The problem is geometry. For a large stretch of the year, the sun over Naperville simply cannot do the job it does in June.
The Latitude Problem: Why Winter Sun Does Not Work Here
Your skin makes vitamin D when a specific band of ultraviolet light, UVB, hits it. UVB only reaches the ground in meaningful amounts when the sun is reasonably high in the sky. Researchers have long noted that at latitudes above roughly 37 degrees north, the winter sun sits too low for the skin to produce much vitamin D at all. Naperville sits near 42 degrees north.
In practical terms, that means from roughly October through April, you can stand outside at noon on a clear day in Naperville and your skin will produce little to no vitamin D. The light feels like sunlight. It will not sunburn you. But the UVB that drives vitamin D synthesis is mostly filtered out by the angle it has to travel through the atmosphere.
Add in the realities of a Midwest winter, short days, heavy coats, long hours indoors, and most people go five or six months drawing down the vitamin D they stored up over the summer. Some start the winter with a healthy reserve. Many do not. National survey data has repeatedly shown that low vitamin D status is common in the United States, and it is more common in northern states and during winter months. We will not pin a made-up percentage on it, but if you live in Illinois and have never tested your level, there is a genuine chance you are running low right now.
What Vitamin D Actually Does
Vitamin D gets credited with everything from curing depression to preventing cancer, and most of that is ahead of the evidence. Here is what is actually well established, and what is still being studied.
Bone and calcium
This is the core job. Vitamin D allows your gut to absorb calcium. Without enough of it, your body pulls calcium from bone to keep blood levels stable, which over time weakens the skeleton. Severe, prolonged deficiency causes rickets in children and osteomalacia, or soft bones, in adults. This part is not controversial. It is the reason vitamin D has a recommended daily intake at all.
Immune function
Immune cells carry vitamin D receptors, and low vitamin D status has been associated with more frequent respiratory infections in observational research. Supplementation trials have shown modest benefit in some groups, particularly people who started out deficient. It is fair to say vitamin D supports normal immune function. It is not fair to call it an immune shield, and we will not.
Muscle
Vitamin D plays a role in muscle function, and deficiency can show up as muscle weakness and aches. In older adults, correcting low vitamin D has been studied for reducing fall risk, with mixed but suggestive results.
Mood
This is the area where the honest answer is the least satisfying. Low vitamin D has been associated with low mood and seasonal mood changes in observational studies, but supplementation trials for depression have produced inconsistent results. Correcting a true deficiency is reasonable and sensible. Expecting a vitamin to resolve a mood disorder is not. If your mood tanks every winter, vitamin D status is one thing worth checking, alongside a real conversation with a clinician.
Vitamin D Deficiency Symptoms Worth Paying Attention To
Mild deficiency is often silent, which is part of what makes it easy to miss. When symptoms do show up, they tend to be vague and easy to blame on winter itself:
- Persistent fatigue that does not track with how much you sleep
- Muscle aches, cramps, or general weakness
- Bone pain or tenderness, often in the lower back, hips, or legs
- Getting sick more often than usual
- Low mood, particularly a pattern that worsens in the darker months
- Slow recovery from workouts or minor injuries
None of these symptoms proves a vitamin D problem on its own. All of them overlap with thyroid issues, anemia, hormone imbalances, poor sleep, and plain midwinter malaise. That overlap is exactly why testing matters, which we will get to shortly.
Who Runs Lowest
Winter at this latitude affects everyone, but a few groups are reliably more likely to be deficient:
- People with darker skin. Melanin reduces the skin’s ability to produce vitamin D from the same amount of sun, so higher-melanin skin needs considerably more exposure to make the same amount.
- Indoor workers. If you commute in the dark, work under fluorescent light, and drive home in the dark, your sun exposure from November to March can round to zero.
- Older adults. Skin becomes less efficient at producing vitamin D with age, and the kidneys become less efficient at converting it to its active form.
- People with digestive conditions. Celiac disease, Crohn’s disease, and a history of gastric bypass surgery can all impair absorption of vitamin D from food and pills.
- People carrying extra body fat. Vitamin D is fat soluble, and more of it gets sequestered in fat tissue rather than circulating where it is needed.
If you fall into more than one of these categories and you live through Illinois winters, you should assume nothing about your level until you have measured it.
Test First, Then Supplement
Here is advice we give even though it occasionally costs us a sale: do not start high-dose vitamin D without knowing your number. The blood test is called 25-hydroxyvitamin D, and it tells you where you actually stand. Some people who feel terrible in February have perfectly fine vitamin D and a thyroid or hormone issue instead. Some people taking a daily supplement are still low because they absorb it poorly. And while vitamin D toxicity from supplements is rare, it is real at very high sustained doses, so megadosing blindly is a bad plan.
Testing also catches the lookalikes. Fatigue, low mood, and poor recovery sit at the intersection of vitamin D, B12, thyroid function, and sex hormones. A proper workup sorts out which one is actually yours. We offer hormone testing at our Naperville office for exactly this reason, and a vitamin D level is a simple add-on to a broader panel when the symptom picture is murky. If you want to talk through your symptoms before committing to labs, a virtual consultation is an easy place to start.
Why Food Alone Rarely Fixes It
A common first instinct is to eat your way out of the problem. The math is discouraging. Few foods contain meaningful vitamin D naturally. Fatty fish like salmon and trout are the best sources, egg yolks and UV-exposed mushrooms contribute small amounts, and most of the rest comes from fortified foods like milk and some cereals. A serving of salmon is a genuinely good dose. A glass of fortified milk is a modest one. Hitting adequate intake every single day through food alone requires a diet most people will not sustain, which is why supplementation is the standard recommendation for people who test low, especially in winter.
Your Options: Daily D3 or a Monthly Injection
For most people, the fix is unglamorous: a consistent daily oral vitamin D3 supplement, dosed based on your blood level, with a recheck after a few months. D3 is inexpensive, well studied, and effective when taken reliably. If that describes you, take the pill and spend your money elsewhere.
The injection route exists for the people that routine does not serve. A vitamin D3 injection at our office is $40, takes under 15 minutes, and is given monthly. It makes sense for two groups in particular: people who know themselves well enough to admit the daily pill will not happen, and people with absorption problems, like the GI conditions mentioned above, for whom an intramuscular dose bypasses the gut entirely.
Many patients pair it with other vitamin injections in the same quick visit. The most common combination in winter is vitamin D with B12, since B12 injections address a different and frequently overlapping cause of the same fatigue. The two deficiencies feel similar from the inside, which is one more argument for testing rather than guessing.
Getting Through an Illinois Winter With Your Levels Intact
The sun will not be back in a useful way until spring, but running low from October to April is optional. If the symptoms above sound familiar, start with a level, not a megadose. SMooth Solutions Medspa is a physician-supervised practice, and we can test your vitamin D as part of a broader look at why you feel the way you do, then match the fix to the result, whether that is a simple daily D3 or a monthly injection you never have to remember.
Book a virtual consultation or call 630-381-8952. We are at 1867 Bay Scott Circle, Suite 111, Naperville, IL 60540, with a second location at SMooth Skin Studio in Oswego.