Your vitamin D is most likely low because of some combination of limited sun exposure, not enough vitamin D in your diet, or a body characteristic that makes it harder to produce or absorb the vitamin. A blood level below 20 ng/mL (50 nmol/L) is generally considered inadequate, and levels below 12 ng/mL (30 nmol/L) represent true deficiency. The good news is that once you understand which factors apply to you, most causes are straightforward to address.
How Your Body Makes Vitamin D
Vitamin D works differently from most vitamins. Rather than relying primarily on food, your body manufactures it when UVB radiation from the sun hits a cholesterol compound called 7-dehydrocholesterol in your skin. That compound converts into a precursor form of vitamin D, which then travels through your bloodstream to the liver and kidneys for two rounds of chemical activation before it becomes the hormone your body actually uses, called calcitriol.
This process depends on a surprisingly specific type of sunlight. Only UVB rays in the 295 to 315 nanometer wavelength range trigger the reaction. That means anything blocking those particular rays, from clouds to window glass to sunscreen, reduces your production. It also means the angle of the sun matters enormously, which is why where you live and what time of year it is have such a large impact on your levels.
You May Not Be Getting Enough UVB Light
The single most common reason for low vitamin D is insufficient UVB exposure, and geography plays a bigger role than most people realize. Researchers have documented a phenomenon called “vitamin D winter,” a stretch of months when the sun sits too low in the sky for UVB rays to reach the ground in meaningful amounts. In Boston (42°N latitude), skin exposed to sunlight from November through February produces no vitamin D at all. In Edmonton, Canada (52°N), that dead zone stretches from October through March. If you live anywhere at a similar latitude or farther north, you simply cannot make vitamin D from sunlight for a large part of the year, no matter how long you spend outside.
Even in sunnier months or lower latitudes, modern life works against you. Working indoors during peak daylight hours, wearing clothing that covers most of your skin, and applying sunscreen all reduce UVB exposure dramatically. This is the most fixable cause of low vitamin D for most people, but it’s also the one that conflicts most directly with skin cancer prevention, which is why supplementation often makes more practical sense than chasing sun exposure.
Your Diet Probably Isn’t Covering the Gap
Very few foods naturally contain meaningful amounts of vitamin D. Fatty fish like salmon, mackerel, and sardines are the best natural sources. Egg yolks, beef liver, and certain mushrooms exposed to UV light contain smaller amounts. In the U.S., milk, some orange juices, and many cereals are fortified with vitamin D, but the amounts added are modest.
The reality is that diet alone rarely provides enough vitamin D to keep blood levels in the adequate range, especially during months when sun exposure can’t pick up the slack. Most adults need at least 600 to 800 IU per day from all sources combined, and many researchers argue that’s a conservative estimate. If you eat little fish, avoid dairy, or follow a vegan diet, your dietary intake may be close to zero without supplementation.
Higher Body Weight Traps Vitamin D
Carrying excess body fat is one of the strongest predictors of low vitamin D, and the mechanism is straightforward: vitamin D is fat-soluble, so it gets absorbed and stored in adipose (fat) tissue. In people with obesity, this creates a trap. The vitamin gets pulled into fat cells but isn’t efficiently released back into the bloodstream because fat tissue in obesity becomes resistant to the normal hormonal signals that trigger fat breakdown.
Research published in the Journal of Clinical Endocrinology & Metabolism found that people with obesity needed roughly 1.8 times the supplemental dose to raise their blood levels by the same amount as people at a normal weight. So if you have a higher BMI, low vitamin D isn’t a sign that something is wrong with your health habits per se. Your body is just sequestering more of it, and you likely need a higher intake to compensate.
Skin Tone Has a Smaller Effect Than You May Have Heard
Melanin, the pigment that gives skin its color, does absorb some UVB radiation before it can trigger vitamin D production. For years, this was cited as a major reason for low vitamin D levels in people with darker skin. But more recent research comparing the lightest skin types to the darkest found that the inhibitory effect of melanin is relatively small, roughly a factor of 1.3 to 1.4. That means darker skin reduces vitamin D synthesis by about 30 to 40 percent compared to very fair skin, not the dramatic difference often claimed.
That said, even a modest reduction compounds over time, especially in combination with living at a high latitude, spending time indoors, or other risk factors. It’s one piece of the puzzle, not the whole picture.
Aging Reduces Your Skin’s Production Capacity
As you get older, your skin contains less of the cholesterol precursor that UVB converts into vitamin D. The production happens primarily in the outermost layers of the skin, and these layers thin with age. An older adult exposed to the same amount of sunlight as a younger person will produce significantly less vitamin D from that exposure. This is one reason vitamin D deficiency becomes increasingly common past age 65, and why recommended intake increases for older adults.
Gut Conditions That Block Absorption
Because vitamin D is fat-soluble, your intestines need to be absorbing fat properly for you to take in vitamin D from food or supplements. Several digestive conditions impair this process. Celiac disease, Crohn’s disease, ulcerative colitis, and cystic fibrosis can all damage or inflame the intestinal lining enough to reduce fat absorption. If you have one of these conditions and your vitamin D is low despite supplementation, malabsorption is a likely explanation. Gastric bypass surgery can also reduce the intestinal surface area available for absorption.
Liver or Kidney Problems
Remember that vitamin D needs two chemical conversions before it becomes active. The first happens in the liver, the second in the kidneys. If either organ isn’t functioning well, the process stalls. Chronic kidney disease is particularly notorious for disrupting vitamin D metabolism, because the kidneys are responsible for the final activation step. As kidney function declines, the body produces less of the active hormone form, which can trigger a cascade of problems with calcium and bone health. Liver disease can impair the first conversion step, though this typically requires significant liver damage.
Medications That Interfere
Certain drugs speed up the breakdown of vitamin D in your body. Antiseizure medications like carbamazepine, phenobarbital, and phenytoin are well-documented offenders. They rev up liver enzymes that metabolize vitamin D faster than normal, effectively draining your supply. Glucocorticoids (steroid anti-inflammatory drugs taken long-term) and some weight-loss medications that block fat absorption can also lower levels. If you started a new medication and your vitamin D dropped, the timing may not be a coincidence.
What Your Blood Test Numbers Mean
Vitamin D status is measured through a blood test for 25-hydroxyvitamin D, reported in either ng/mL or nmol/L. Here’s how to read your results:
- Below 12 ng/mL (30 nmol/L): Deficiency. This level is associated with bone-softening diseases in both children and adults.
- 12 to 19 ng/mL (30 to 49 nmol/L): Inadequate for bone and overall health in most people.
- 20 ng/mL (50 nmol/L) or above: Generally adequate for most people.
- Above 50 ng/mL (125 nmol/L): Potentially too high, with risk of adverse effects especially above 60 ng/mL.
These thresholds come from the National Academies of Sciences, Engineering, and Medicine. Some practitioners use higher cutoffs, but 20 ng/mL is the benchmark most widely accepted as sufficient.
Putting It All Together
Low vitamin D rarely has a single cause. It’s usually a stack of contributing factors. You live at a northern latitude, you work indoors, you’re over 50, you carry extra weight, and you don’t eat much fish. Each factor shaves off some of your supply, and together they push your levels below the adequate range. The most practical fix for most people is a vitamin D3 supplement, particularly during the colder months when sun-driven production shuts down entirely. The dose that makes sense for you depends on how many of these risk factors apply and how low your levels are, which is why a blood test is worth getting before and after you start supplementing.