What Causes Vitamin D Deficiency?

Vitamin D deficiency is not evenly distributed. Some people are far more likely to end up low, and the reasons are specific and mostly known.

Roughly 70% of US adults sit below the level considered sufficient, based on a 2018 analysis of 26,010 adults in the British Journal of Nutrition. But that average hides the part worth knowing, which is who carries most of the risk.

Your level is the result of three inputs: what your skin makes from sunlight, what you eat, and what you supplement. Sunlight normally does most of the work. Deficiency happens when that pathway gets blocked and nothing replaces it.

WHO IS MOST AT RISK OF VITAMIN D DEFICIENCY?

Most people carry more than one of these. An office worker in Chicago who uses sunscreen has three.

DOES SKIN TONE AFFECT VITAMIN D?

Yes, and it is one of the larger effects. Melanin is effective at absorbing UVB, which is what makes it protective against sun damage. The same property means less UVB reaches the cells that convert the precursor molecule into vitamin D.

The practical result is that the same time outdoors produces less vitamin D. NHANES data found higher rates of insufficiency in US adults with darker skin. This is a difference in how much sun is needed, not a difference in how the vitamin works once it is in you.

WHY DOES LATITUDE MATTER SO MUCH?

Because UVB has to pass through the atmosphere at a steep enough angle to reach you at all.

A 1988 study by Webb, Kline and Holick measured vitamin D synthesis in winter sunlight and found that skin in Boston, at 42.2°N, produced no measurable previtamin D3 from November through February. In Edmonton, at 52°N, that window ran October through March. Holick has summarized it plainly: in Boston you cannot make vitamin D in your skin from November through March no matter how long you stay outside.

Most of the US population lives above 37°N. For a large share of the country, winter sun is not a source of vitamin D at any duration. We went through whether you can get enough from the sun in more detail.

DOES BODY WEIGHT AFFECT VITAMIN D LEVELS?

It does. Vitamin D is fat-soluble and distributes into fat tissue, which means more body fat leaves less circulating in the blood where a test measures it.

A 2000 study by Wortsman and Holick in the American Journal of Clinical Nutrition found that participants with obesity showed a 57% lower rise in circulating vitamin D after UVB exposure than lean participants, despite similar precursor levels in the skin. Researchers still debate whether the mechanism is sequestration in fat tissue or dilution across a larger body volume. The practical outcome is the same: the same input produces a lower reading.

IS VITAMIN D DEFICIENCY GENETIC?

Partly. Variation in the genes affecting vitamin D binding protein and the enzymes that metabolize it influences how individuals respond to the same intake. This is one reason two people can take an identical dose and land at different levels.

It is not a large enough factor to override the behavioral and environmental ones above, and there is no routine clinical test for it. It is a reason to check your own number rather than assume a standard dose lands you in the same place it lands someone else.

HOW DO YOU KNOW IF YOU ARE DEFICIENT?

A serum 25-hydroxyvitamin D test, written 25(OH)D. It is a standard panel any provider can order, and it is not usually part of a routine annual physical, so you generally have to ask.

Symptoms are not a reliable substitute. Many people who are low notice nothing at all, and the things people do notice are vague enough to have many other causes. If your result comes back low, we covered what the number actually means and what to do about it.

WHAT IF YOU ARE IN SEVERAL RISK GROUPS?

The factors stack. Darker skin plus a northern winter plus indoor work is not three small effects, it is three blockages on the same pathway.

The useful response is not to fix your latitude or your job. It is to stop depending on a pathway that is closed for you and supply the vitamin directly, consistently, in a form that absorbs. Vitamin D is fat-soluble, so it needs fat present to be taken up properly, which is why the carrier matters and why oil-based forms outperform dry tablets and gummies.

That is the reasoning behind D3X: liquid D3 and K2 in organic olive oil, so the fat that drives absorption arrives with every dose rather than depending on what you had for breakfast.* Three ingredients, no fillers, one dropper a day.

If you are in two or more of the groups above, the question is not really whether to supplement. It is whether you will still be doing it in three months.