There is a reason the same patients who felt fine in August come in during February wondering what happened. Between those two months, one of the few nutrients your body manufactures rather than eats stops being manufactured. It is a predictable, measurable, and correctable annual event, and most people never test for it.
Your skin makes vitamin D when UVB radiation reaches it. The catch is that UVB has to pass through the atmosphere at a steep enough angle, and in winter it does not.
Springfield sits near 37 degrees north. Above roughly 35 degrees, the sun's winter angle is low enough that very little usable UVB reaches the ground for several months. From late October through February, you can stand outside in Missouri all day and produce almost none.
Several other things stack on top:
Stored summer vitamin D does carry over, but it depletes over weeks to a couple of months. By midwinter, most people are running on whatever they eat or supplement.
Calling it a vitamin undersells it; it functions more like a hormone, with receptors throughout the body:
The National Library of Medicine's vitamin D overview covers the basics of synthesis, intake, and the bone diseases that frank deficiency causes.
Deficiency is quiet, which is the problem. When symptoms appear, they are easy to attribute to something else:
Every one of those has other possible causes, which is exactly why a blood test settles it faster than guessing.
Worth testing sooner rather than later if you:
The right test is 25-hydroxyvitamin D, written as 25(OH)D. That is the storage form, and it reflects your actual status.
A few things worth understanding about the numbers. Levels below 20 ng/mL are generally treated as deficient and levels of 20 to 30 as insufficient. Many functional medicine practitioners, including our team, prefer patients land comfortably above 30, and often in the 40s to 50s, because that range is where immune and musculoskeletal function appear to do best. There is genuine debate about optimal targets above that, and anyone claiming certainty is overstating the evidence.
The useful part is the trend. A level checked in October and again in February tells you how steeply you drop and how much supplementation you actually need, which is more informative than a single reading ever is.
Vitamin D is also one of several nutrients worth looking at together, which is why we often fold it into broader micronutrient testing rather than running it alone.
Supplementing is straightforward, with a few details that matter:
Vitamin D can be overdone. It is fat-soluble, it accumulates, and genuine toxicity causes dangerously high calcium levels. Every case we have seen came from someone self-prescribing very high doses for months without testing. This is a nutrient to correct deliberately, with a follow-up test, rather than a nutrient to take aggressively and hope.
Fatigue and low mood in winter are not always a vitamin D problem, even when levels are low. Thyroid dysfunction, iron deficiency, B12 deficiency, sleep apnea, and seasonal changes in light exposure all produce overlapping symptoms. We cover that overlap in our article on seasonal mood and sleep changes.
Correcting vitamin D while ignoring an untreated thyroid condition will disappoint everyone involved. The point of testing broadly is to find out which problem you actually have.
417 Integrative Medicine runs comprehensive nutrient testing for adults and children at 1335 E Republic Rd, Suite D, Springfield, MO 65804, serving Springfield, Nixa, Ozark, Republic, Battlefield, Willard, and the surrounding communities.
Our adult functional medicine program interprets results against optimal ranges rather than only the thresholds that flag disease, doses to your individual result, and retests to confirm the correction held. October is the right month to establish your baseline, before the seasonal drop rather than after it.
Ready to find out where your levels actually sit? Schedule your consultation at 417 Integrative Medicine, or call (417) 363 3900.
