The tricky thing about vitamin D deficiency is that most people with low levels don’t feel dramatically sick. The symptoms are common enough to be mistaken for stress, aging, or poor sleep. A blood level below 12 ng/mL is considered deficient by the National Academies of Sciences, while levels between 12 and 20 ng/mL are classified as inadequate. The only way to confirm where you stand is a simple blood test, but several signs and risk factors can tell you whether it’s worth checking.
Symptoms That Often Show Up First
Vitamin D deficiency doesn’t produce one telltale symptom. Instead, it tends to create a cluster of vague problems that build gradually. The most commonly reported signs include persistent fatigue that doesn’t improve with rest, aching bones (especially in the lower back, hips, and legs), and general muscle weakness or soreness. You might notice you’re getting sick more often, since vitamin D plays a direct role in immune function.
Mood changes are another frequent signal. Low vitamin D has been linked to increased sleepiness, cognitive fog, and depressive symptoms. Some people describe feeling mentally sluggish or having trouble concentrating. These overlap heavily with other conditions, which is part of why deficiency goes unrecognized for so long. If you’re dealing with several of these issues at once, particularly during winter months, low vitamin D is a reasonable suspect.
Less Obvious Warning Signs
Beyond the common symptoms, deficiency can affect your body in subtler ways. Slow wound healing, hair thinning, and frequent muscle twitches or cramps can all point to inadequate vitamin D. Physical performance tends to decline as well. Research has connected low levels with reduced muscle strength, decreased bone density, and a higher risk of fractures, even in people who wouldn’t consider themselves frail.
Changes in smell sensitivity and increased daytime sleepiness have also been documented in people with low levels, though these are less widely recognized. None of these signs alone confirm deficiency, but a pattern of several together, especially alongside risk factors, is a strong prompt to get tested.
Who Is Most at Risk
Certain groups are significantly more likely to be deficient, regardless of symptoms. Knowing whether you fall into one of these categories is often the most practical way to gauge your risk.
- People with darker skin. Higher melanin content reduces the skin’s ability to produce vitamin D from sunlight. This makes deficiency substantially more common in people with darker complexions.
- People over 65. Your skin becomes less efficient at synthesizing vitamin D as you age, and older adults tend to spend less time outdoors.
- People with obesity. A BMI over 30 is associated with lower vitamin D levels because fat cells sequester the vitamin, keeping it from circulating in the blood. Higher supplement doses are often needed to correct this.
- People who spend little time outdoors. If you’re homebound, work indoors during daylight hours, or consistently cover most of your skin with clothing, your body has limited opportunity to make vitamin D.
- People living at higher latitudes. If you live above roughly 40 degrees latitude (think New York, Madrid, Beijing, or anywhere farther from the equator), there are several months each year when the sun sits too low in the sky for your skin to produce meaningful vitamin D. When the UV index drops below 2, vitamin D synthesis is essentially zero, creating a “vitamin D winter.”
If two or three of these apply to you, the odds of having at least inadequate levels are high, even if you feel mostly fine.
How the Blood Test Works
The standard test measures a form called 25-hydroxyvitamin D, sometimes written as 25(OH)D, in your blood. Your liver converts vitamin D into this form before the rest of your body can use it, so it’s the most accurate snapshot of your overall vitamin D status. It’s a routine blood draw with no special preparation needed.
The National Academies set these thresholds:
- Deficient: below 12 ng/mL (30 nmol/L)
- Inadequate: 12 to 19 ng/mL (30 to 49 nmol/L)
- Sufficient: 20 ng/mL or above (50 nmol/L or above)
You may see different cutoffs elsewhere. The Endocrine Society previously used 30 ng/mL as the sufficiency threshold, but their 2024 guidelines moved away from endorsing specific target numbers. They also now recommend against routine screening in healthy adults, including those with darker skin or obesity, noting a lack of clinical trial evidence that testing and targeting a specific level prevents disease. That doesn’t mean testing is useless. It means your doctor will typically order it when you have symptoms, a relevant medical condition, or a clear reason to check rather than as a blanket screening tool.
What Happens if Deficiency Goes Untreated
Short-term, low vitamin D makes you feel lousy. Long-term, it causes real structural damage. In children, severe deficiency causes rickets, a condition where bones become soft and bend. In adults, it accelerates bone thinning (osteopenia and osteoporosis) and significantly increases fracture risk. Chronic deficiency has also been associated with higher rates of autoimmune diseases, high blood pressure, certain cancers, and increased susceptibility to infections. These associations don’t prove vitamin D deficiency directly causes all of these conditions, but the pattern is consistent and well-documented across large populations.
How Much Sunlight You Actually Need
Your skin produces vitamin D when exposed to UVB rays from the sun, but the amount of time required varies enormously depending on your skin tone, location, and the time of year. For people with lighter skin near the equator, as little as 3 to 15 minutes of midday sun with about 35% of skin exposed (think shorts and a t-shirt) can maintain adequate levels. People with very dark skin need considerably longer exposure. At 30 degrees latitude or higher, even daily noon sun exposure of under 15 minutes may not be enough for darker skin types.
The key window is around midday, when UVB intensity peaks. Morning and late afternoon sun, while pleasant, provides far less vitamin D per minute. And during winter months at higher latitudes, the sun never climbs high enough for any practical vitamin D production, no matter how long you stay outside. This is why supplementation matters most in winter and for people who can’t get regular midday sun.
Correcting Low Levels
If your blood test confirms deficiency, the typical approach is a loading dose to bring levels up quickly, followed by a smaller maintenance dose to keep them there. For severe deficiency in adults, doctors commonly prescribe around 50,000 IU per week for six weeks, or equivalent regimens spread over a longer period. After that, a daily maintenance dose of 800 to 2,000 IU is standard.
Most people notice improvements in energy and mood within a few weeks of starting supplementation, though bone-related benefits take longer to develop. Your doctor will usually recheck your blood levels after the loading phase to make sure you’ve reached an adequate range. It’s worth knowing that more isn’t necessarily better: blood levels above 50 ng/mL (125 nmol/L) can be associated with adverse effects, and levels above 60 ng/mL carry clearer risk.
Food sources contribute some vitamin D but rarely enough on their own. Fatty fish like salmon and mackerel, fortified milk and orange juice, egg yolks, and mushrooms exposed to UV light all contain vitamin D, but the amounts per serving are modest compared to what supplements or sunlight can provide. Think of dietary sources as a helpful baseline rather than a complete solution.