Low vitamin D usually comes down to one or more of a handful of common causes: not enough sun exposure, living at a high latitude, having darker skin, carrying excess body weight, or having a digestive condition that blocks absorption. A blood level below 20 ng/mL is generally considered inadequate for bone and overall health, and below 12 ng/mL qualifies as outright deficiency. Most people with low levels have a combination of factors working against them rather than a single clear-cut reason.
How Your Body Makes Vitamin D
Vitamin D isn’t like most vitamins. Your body manufactures most of what it needs when UVB rays from the sun hit your skin. A compound already present in your epidermis gets converted into a precursor of vitamin D, which then enters your bloodstream and travels to your liver, where it’s transformed into the form doctors measure on blood tests. From there it goes to your kidneys for a final conversion into the active hormone your cells actually use.
This multi-step process means several things can go wrong along the way. You need adequate sun exposure, functioning skin production, a healthy liver, healthy kidneys, and enough magnesium (which regulates the enzymes involved in each conversion step). A problem at any point in the chain can leave you deficient even if the other steps are working fine.
You’re Not Getting Enough Sun
This is the most common reason. Modern life keeps people indoors. If you work in an office, commute in a car, and spend evenings at home, your skin may rarely see direct sunlight during the hours when UVB rays are strong enough to trigger vitamin D production. Sunscreen, while important for skin cancer prevention, also blocks UVB. Window glass filters out UVB entirely, so sitting in a sunny office doesn’t count.
Clothing matters too. The more skin you cover, the less surface area is available for production. People who wear long sleeves, head coverings, or full-length clothing for religious or cultural reasons are at higher risk of deficiency for this reason alone.
Where You Live Makes a Big Difference
The angle of the sun determines whether UVB rays are strong enough to reach your skin. During winter months at higher latitudes, the sun sits so low in the sky that virtually no vitamin D production occurs, regardless of how long you spend outside. This is sometimes called “vitamin D winter.”
How far north this affects you depends partly on your skin tone. Research modeling UV exposure found that in midwinter, fair-skinned people can’t produce sufficient vitamin D above roughly 47°N latitude. For people with medium skin tones, that threshold drops to about 44°N. For those with the darkest skin, it drops further to around 39°N. To put that in context, 39°N runs through Washington, D.C., Kansas City, and Sacramento. If you live north of those cities, winter sun alone won’t maintain your vitamin D levels regardless of how much time you spend outdoors.
Cloud cover, air pollution, and even altitude compound the problem. As a rough guide, when the UV index drops below about 2, even a fair-skinned person would need over an hour of midday exposure to produce a meaningful amount of vitamin D.
Darker Skin Needs More Sun Exposure
Melanin, the pigment that gives skin its color, acts as a natural sunscreen. It absorbs UVB radiation before it can reach the deeper skin cells where vitamin D production happens. This was an advantage for populations living near the equator, where intense sun is constant. But for people with darker skin living at higher latitudes, it creates a mismatch between their skin’s UV filtering and the available sunlight.
Studies comparing the most fair-skinned and darkest skin types found a melanin inhibition factor of roughly 1.3 to 1.4, meaning darker skin needs proportionally more UV exposure to produce the same amount of vitamin D. Combined with living in a northern city, this can make deficiency very difficult to avoid through sun alone.
Body Weight and Fat Storage
Vitamin D is fat-soluble, and your body stores it in fat tissue. In people with higher body fat, vitamin D gets pulled into fat cells and essentially locked away. Animal studies show that within 24 hours of ingesting vitamin D, a significant portion ends up in fat tissue, and it releases back into the bloodstream extremely slowly. After six weeks, roughly 80% of vitamin D was still trapped in fat stores.
This means that if you carry excess weight, a larger share of the vitamin D you make or consume gets sequestered before your body can use it. Obesity is consistently associated with lower blood levels of vitamin D, and people with higher BMIs often need more supplementation to reach adequate levels.
Digestive Conditions That Block Absorption
Since vitamin D from food and supplements is absorbed in the small intestine, any condition that disrupts that process can cause deficiency. Crohn’s disease, ulcerative colitis, and celiac disease all interfere with nutrient absorption and are well-known risk factors. Gastric bypass surgery, which physically reroutes food past parts of the small intestine, makes it harder to absorb enough vitamin D from what you eat.
If you have one of these conditions and your vitamin D is low, the deficiency is likely tied directly to malabsorption. Higher doses or specific supplement forms may be needed to compensate, since standard oral doses won’t be fully absorbed.
Medications That Deplete Vitamin D
Certain medications speed up how quickly your body breaks down vitamin D or interfere with its absorption. Anti-seizure drugs are among the most well-documented offenders. They can reduce absorption and increase excretion of several vitamins, including vitamin D. Long-term use of corticosteroids and some cholesterol-lowering drugs can also lower levels.
If you’ve been on any of these medications for months or years and your vitamin D is low, the medication may be a contributing factor worth discussing with your prescriber.
Kidney or Liver Problems
Because vitamin D must pass through both the liver and kidneys to become the active hormone your body uses, impaired function in either organ can leave you deficient even if your sun exposure and diet are adequate. Chronic kidney disease is especially impactful because the kidneys handle the final activation step. People with significant liver or kidney disease often have low vitamin D despite otherwise doing everything right.
Age
As you get older, your skin produces less vitamin D from the same amount of sun exposure. The concentration of the precursor compound in your skin declines with age, so an hour of midday sun at 70 produces meaningfully less vitamin D than the same hour at 25. Older adults also tend to spend more time indoors and may have reduced kidney function, compounding the problem.
What Your Blood Levels Mean
Vitamin D status is measured through a blood test for 25-hydroxyvitamin D. The NIH classifies results this way:
- Below 12 ng/mL (30 nmol/L): Deficiency. At this level, adults risk bone softening (osteomalacia), and children risk rickets.
- 12 to 19 ng/mL (30 to 49 nmol/L): Inadequate for bone and overall health in most people.
- 20 ng/mL or above (50 nmol/L): Generally adequate for healthy individuals.
- Above 50 ng/mL (125 nmol/L): Potentially too high, with possible adverse effects above 60 ng/mL.
These thresholds aren’t perfect. Optimal levels likely vary by age, ethnicity, and what health outcome you’re looking at. But if your result came back below 20, your body isn’t getting what it needs.
Why Diet Alone Rarely Fixes It
Very few foods naturally contain significant vitamin D. Fatty fish like salmon and mackerel are the best natural sources, but even a generous serving provides only a fraction of what your skin can make in minutes of strong sun exposure. Egg yolks contain small amounts. Fortified milk, orange juice, and cereals help but typically add only 100 to 150 IU per serving, which isn’t enough to correct a true deficiency.
This is why vitamin D is sometimes called the “sunshine vitamin.” Your body was designed to get most of it from UVB exposure, not food. When sun exposure drops, whether from lifestyle, geography, skin tone, or season, the gap between what you need and what you get through diet alone can be substantial. For many people, supplementation becomes the most practical way to bring levels back up, especially during winter months or if you have additional risk factors stacking against you.