Vitamin D Deficiency: The Most Common Lab Finding We See

Vitamin D is the most common deficiency I see on lab work, and it is not close. Depending on the season, roughly 4 in 10 American adults test insufficient, and many test outright deficient. Most of them feel fine, which is exactly the problem.

What vitamin D actually does

Vitamin D behaves more like a hormone than a vitamin. Receptors for it exist in nearly every tissue in your body. It regulates calcium absorption and bone health, which most people know. It also plays documented roles in immune function, muscle strength, mood, and hormone production. Low levels are associated with higher all cause mortality, more frequent respiratory infections, low testosterone in men, and worse outcomes across a long list of conditions.

Association is not causation, and vitamin D research is famously messy. But the combination of high prevalence, low cost to fix, and wide physiologic reach makes it one of the highest value labs we check.

Why so many people are low

Your skin makes vitamin D from sunlight, specifically UVB. Modern life works against that at every turn: we work indoors, we wear sunscreen when outside, and anyone north of roughly Atlanta gets too little UVB for synthesis for a good part of the year. Darker skin, older age, and higher body fat all further reduce production. Food sources like fatty fish and fortified dairy contribute far less than people assume.

Testing: the number to know

The blood test is 25-hydroxyvitamin D. Reference ranges vary by lab, but here is how I interpret it:

• Below 30 ng/mL: deficient. This needs correction.
• 30 to 40 ng/mL: insufficient. Most people pass a standard lab check here but sit below where the benefit likely lives.
• 60 to 80 ng/mL: the range I target for most members.
• Above 100 ng/mL: unnecessary, and toxicity becomes possible with sustained very high supplementation.

Fixing it

Sunlight is the natural fix, but for most people a daily supplement is the reliable one. In my stack I use vitamin D3 combined with K2, which supports directing calcium into bone rather than soft tissue. Typical corrective doses run 2,000 to 5,000 IU of D3 daily, taken with a meal that contains fat since it is fat soluble. The right dose is the one that gets your blood level into range, which is why we retest rather than guess.

The takeaway

Get your level tested, correct it if it is low, and retest to confirm the dose is right. Few things in preventive medicine are this cheap to find and this cheap to fix.

This article is educational and is not medical advice. Supplement doses should be guided by your own lab work and physician.