Vitamin D Deficiency: Signs, Testing, and What to Do About It
About 42% of American adults are vitamin D deficient, and most of them have no idea. Here is how to tell if you might be one of them, how to get tested, and exactly what to do about it depending on how low your levels actually are.
Vitamin D deficiency is one of the most common nutritional deficiencies on the planet, and it is also one of the sneakiest. Most people who have it do not know it, because the symptoms are vague enough to explain away. You are tired. You get sick a lot. Your back hurts. Your mood is off. Individually, each of those could be a dozen things. Together, they paint a picture worth investigating.
According to NHANES data analyzed by Forrest and Stuhldreher (2011, Nutrition Research), approximately 41.6% of US adults have serum 25(OH)D levels below 20 ng/mL, which meets the clinical definition of deficiency. And that number climbs significantly for people living at northern latitudes, people with darker skin tones, older adults, and anyone who spends most of their day indoors.
The good news is that vitamin D deficiency is straightforward to test for and relatively easy to correct. This article walks you through the whole process: who is at risk, what to look for, how to get tested, what the numbers mean, and how to bring your levels back to where they should be. For a broader overview of everything vitamin D does and why it matters, start with our complete guide to vitamin D.
This content is educational and not a substitute for medical advice. Talk to your healthcare provider before starting or changing any supplement regimen.
Who Is Most Likely to Be Deficient
Northern latitude residents. Anyone living above the 37th parallel (which includes all of Oregon, Washington, the entire Midwest, the Northeast, and most of the Mountain West) does not get sufficient UVB exposure during winter months to produce adequate vitamin D through skin synthesis. Here in Oregon, at the 44th parallel, the sun angle is too low from roughly October through March for meaningful D3 production. That is half the year. Portland averages 144 days of cloud cover annually. If you live in the Pacific Northwest and you are not supplementing during winter, the math is simply against you.
People with darker skin tones. Melanin, the pigment that gives skin its color, is a natural sunscreen. Higher melanin levels reduce UVB penetration and slow the conversion of 7-dehydrocholesterol to previtamin D3 in the skin. NHANES data shows the deficiency rate among Black Americans is 82.1% and among Hispanic Americans is 69.2%, compared to 30.9% among white Americans (Forrest & Stuhldreher, 2011). This is not a lifestyle issue. It is a photobiology issue, and supplementation is the straightforward solution.
Adults over 65. Aging reduces the skin's capacity to synthesize vitamin D by roughly 75% compared to a 20-year-old (MacLaughlin & Holick, 1985, Journal of Clinical Investigation). Add in less time outdoors, reduced dietary intake, and decreased kidney efficiency at converting 25(OH)D to its active form, and you have a population group where deficiency is more the rule than the exception.
People with higher body weight. Vitamin D is fat-soluble, and in individuals with obesity (BMI above 30), a larger proportion of vitamin D gets sequestered in adipose tissue rather than circulating in the blood. Studies show that people with obesity may need 2 to 3 times the standard dose to achieve the same blood levels as their normal-weight counterparts (Wortsman et al., 2000, American Journal of Clinical Nutrition).
Malabsorption conditions. Celiac disease, Crohn's disease, ulcerative colitis, and a history of gastric bypass surgery all impair fat absorption in the gut. Since vitamin D is fat-soluble, anything that reduces fat absorption also reduces vitamin D absorption. People with these conditions need to work with their healthcare provider to determine the right supplementation approach, which often involves higher doses or more bioavailable forms.
Indoor workers and shift workers. If you leave for work before sunrise and come home after sunset (which describes a lot of people from November through February in Oregon), your skin simply is not seeing enough UVB to produce vitamin D. Remote workers who spend the day inside, healthcare workers on night shifts, and anyone in a desk-bound occupation fall into this category.
7 Signs You Might Be Vitamin D Deficient
The tricky thing about vitamin D deficiency is that the symptoms are not dramatic. Nobody collapses from low vitamin D the way they might from severe iron deficiency. Instead, the signs are subtle, chronic, and easy to attribute to something else entirely. Here are the seven most common ones, along with the mechanisms behind them.
1. Persistent fatigue and low energy. This is the most commonly reported symptom and one of the most underappreciated. Vitamin D3 (cholecalciferol) receptors are present in the mitochondria (the energy-producing structures in your cells), and D3 plays a role in mitochondrial function and ATP production. Multiple studies have shown that correcting vitamin D deficiency leads to significant improvements in fatigue. A 2016 study in Medicine found that participants who corrected their deficiency reported a 38% improvement in fatigue scores after supplementation (Nowak et al., 2016).
2. Bone pain and lower back pain. Vitamin D's most well-established role is calcium absorption and bone metabolism. When D levels are low, calcium absorption in the gut drops significantly, which can lead to bone softening (osteomalacia in adults) and generalized bone pain. Lower back pain, in particular, has been associated with vitamin D deficiency in multiple studies (Lotfi et al., 2019, Clinical Rheumatology).
3. Muscle weakness. Vitamin D receptors in skeletal muscle tissue regulate muscle protein synthesis and muscle fiber composition. Deficiency is associated with reduced muscle strength, impaired balance, and increased fall risk, particularly in older adults. A meta-analysis by Tomlinson et al. (2015, Journal of Science and Medicine in Sport) confirmed that D supplementation significantly improved muscle strength in deficient individuals.
4. Mood changes and depression. The link between low vitamin D and depression has been documented in numerous studies. Vitamin D receptors are found throughout the brain, including areas involved in mood regulation. A meta-analysis of 31,424 participants found that low vitamin D was associated with a significantly higher risk of depression (Anglin et al., 2013, British Journal of Psychiatry). If you notice that your mood consistently dips during the winter months, it may not be "just the weather."
5. Getting sick frequently. Vitamin D is a direct modulator of the innate immune system. It stimulates the production of antimicrobial peptides like cathelicidin and defensins that help your body fight off pathogens. A BMJ meta-analysis of 25 randomized controlled trials found that vitamin D supplementation reduced the risk of acute respiratory tract infections, with the strongest protective effect in people who were deficient at baseline (Martineau et al., 2017).
6. Slow wound healing. Vitamin D plays a role in the inflammatory response and in the production of compounds necessary for new skin formation during wound healing. Studies have shown that patients with lower vitamin D levels experience slower post-surgical healing and that supplementation can improve outcomes (Burkiewicz et al., 2012, Wound Repair and Regeneration).
7. Hair thinning or loss. Vitamin D receptors are present in hair follicles and play a role in the hair growth cycle. While the research here is still developing, studies have found a significant association between low vitamin D levels and alopecia areata as well as general telogen effluvium (Rasheed et al., 2013, Journal of Cosmetic Dermatology). This does not mean vitamin D deficiency is the only cause of hair loss, but it is worth checking if you are experiencing unexplained thinning.
Here is the important caveat. Every single one of these symptoms has multiple potential causes. Fatigue could be iron deficiency, thyroid issues, or just poor sleep. Mood changes could be stress. Hair loss could be hormonal. The only way to confirm that vitamin D deficiency is involved is to test your blood levels. Which brings us to the next section.
How to Find Out: The 25(OH)D Blood Test
The test you want is called 25-hydroxyvitamin D, or 25(OH)D. This is the standard biomarker for vitamin D status and reflects your total body stores from both sun exposure and supplementation. There is another test (1,25-dihydroxyvitamin D) that measures the active hormone form, but that one is not useful for assessing overall vitamin D status because it remains normal until deficiency is severe. Ask specifically for 25(OH)D.
Your primary care doctor can order this test as part of routine bloodwork. It is usually covered by insurance when there is a clinical reason (symptoms, risk factors, or a related diagnosis like osteoporosis). If you just want to check proactively, at-home vitamin D test kits are available from companies like Everlywell and LetsGetChecked. These use a finger prick and mail-in process, with results typically available within 5 to 7 business days. They cost roughly $30 to $50 and do not require a prescription.
If you are going through your doctor, simply say: "I would like to have my vitamin D level checked. Can you add a 25-hydroxyvitamin D test to my lab order?" If you fall into any of the risk groups listed above, most doctors will agree without pushback. If you have never had it checked and you live in the Pacific Northwest, it is a reasonable request regardless.
What the Numbers Mean
| 25(OH)D Level | Status | What It Means |
|---|---|---|
| Below 12 ng/mL | Severely Deficient | Requires medical intervention. Usually treated with high-dose prescription vitamin D (50,000 IU weekly). |
| 12-20 ng/mL | Deficient | Supplementation strongly recommended. Loading dose may be appropriate. |
| 20-29 ng/mL | Insufficient | Many experts consider this suboptimal. Supplementation recommended. |
| 30-50 ng/mL | Sufficient | This is the target range for most people. Maintain with daily supplementation. |
| 50-80 ng/mL | High-Normal | Some researchers consider this optimal. Generally safe. |
| Above 100 ng/mL | Potentially Excessive | May increase risk of toxicity. Reduce supplementation and retest. |
The Endocrine Society recommends a minimum of 30 ng/mL for optimal health outcomes. The Institute of Medicine sets a lower threshold at 20 ng/mL. We side with the Endocrine Society on this one, as do most functional medicine practitioners. For more on what these numbers mean and how much vitamin D you should actually take, we break it down in a separate guide.
How to Fix It: Treatment by Severity
The approach to correcting vitamin D deficiency depends on how low your levels are. There is no one-size-fits-all answer here, which is why testing matters. Here is the general framework that most clinicians follow.
Mild Deficiency (20-29 ng/mL — "Insufficient")
For people in this range, daily supplementation with 1,000 to 2,000 IU of vitamin D3 is usually enough to bring levels into the sufficient range within 8 to 12 weeks. This is the group where a product like our Everyday Liquid Sunshine (M2100) at 1 to 2 drops daily makes the most sense. It is a simple addition to your routine that addresses the gap without overcomplicating things. No loading dose needed. Just consistency.
Moderate Deficiency (12-20 ng/mL — "Deficient")
At this level, many clinicians recommend a higher daily dose in the range of 4,000 to 5,000 IU for the first 8 to 12 weeks (the "loading" or "correction" phase), followed by a maintenance dose of 1,000 to 2,000 IU daily. Our Everyday Liquid Sunshine 2 (M2113) delivers 5,000 IU of D3 plus 120 mcg of K2 (MK-7) per serving, which fits this correction-phase dosing. The K2 is particularly useful here because as you ramp up D3 and your body starts absorbing more calcium, K2 makes sure that calcium goes to your bones rather than accumulating in your arteries. For more on why D3 and K2 work together, it comes down to calcium trafficking. After correction, many people step down to a lower daily dose using the original Liquid Sunshine (M2100) at 1 to 2 drops.
Severe Deficiency (Below 12 ng/mL)
This typically requires medical supervision. Doctors often prescribe 50,000 IU of vitamin D2 (ergocalciferol) weekly for 8 to 12 weeks, followed by maintenance supplementation. Some physicians use D3 at the same dose instead, and D3 is approximately 87% more effective than D2 at raising blood levels. Either way, this is prescription territory. After the loading phase, patients transition to daily over-the-counter supplementation for long-term maintenance. If your doctor wants to keep you on D3 rather than D2 for maintenance, a product like our Good Vitamin D (A2101) at 2,000 IU per capsule plus 200mg bromelain for enhanced absorption is a solid daily option.
Timeline for Improvement
Most people begin to feel better (more energy, improved mood, fewer infections) within 4 to 6 weeks of starting supplementation at therapeutic doses. Blood levels typically reach the target range within 8 to 12 weeks. Full bone-related benefits may take 3 to 6 months of sustained adequate levels. The key is consistency. Vitamin D supplementation is not a quick fix you take for two weeks and stop. It is a long-term daily habit, especially for anyone living in the Pacific Northwest.
Retest
Plan to retest your 25(OH)D levels after 3 months of supplementation to confirm you have reached your target range. Once you are in the 30-50 ng/mL zone, your doctor can help you determine the right maintenance dose. Annual testing is reasonable for most people. Twice a year (late winter and late summer) gives you a clearer picture of your seasonal fluctuation.
Year-Round Prevention, Especially If You Live in the Pacific Northwest
Daily supplementation year-round. For anyone living above the 37th parallel, the simplest prevention strategy is a daily vitamin D3 supplement from September through April at minimum, and honestly, most experts recommend year-round supplementation at 1,000 to 2,000 IU for maintenance. Even during summer months, most people do not get as much sun exposure as they think. Sunscreen use, indoor work schedules, and lifestyle factors all reduce skin synthesis.
Smart sun exposure when possible. During the warmer months (roughly May through September in Oregon), aim for 10 to 30 minutes of midday sun exposure on your face, arms, and legs without sunscreen a few times per week. The amount needed varies by skin tone. Lighter skin needs less time, darker skin needs more. After your D3 time, apply sunscreen for skin cancer prevention. Do not skip the sunscreen entirely in the name of vitamin D. That tradeoff is not worth it.
Food sources as a supplement to supplements. Include vitamin D-rich foods (fatty fish, egg yolks, fortified dairy, UV-exposed mushrooms) in your regular diet. These contribute to your overall D3 intake but are unlikely to be sufficient on their own. Think of them as a supporting cast, not the lead actor. For a complete breakdown of food sources and how much you would need to eat to reach target levels, see our guide to vitamin D food sources.
Annual testing. Make 25(OH)D part of your annual bloodwork. If you live in the Pacific Northwest, consider testing twice a year (late winter and late summer) to understand your personal seasonal pattern. Knowledge is the foundation of prevention.
Finding the Right Supplement for Your Situation
Every deficiency situation is a little different, and the right supplement depends on your current levels, your health goals, and what form you prefer. Here is how our three vitamin D products map to the scenarios discussed above. For a detailed comparison of all supplement forms, see our guide on vitamin D supplements: drops, tablets, and gummies compared. And if you want to compare specific products side by side, our vitamin D supplement buying guide evaluates options across six key criteria.
Everyday Liquid Sunshine (M2100): Best for daily maintenance and mild deficiency correction. 1,000 IU D3 per drop in fractionated coconut oil. Take 1 to 2 drops daily for maintenance, 3 to 4 drops for mild correction. Approximately 1,400 servings per bottle. The ultimate flexibility product.
Everyday Liquid Sunshine 2 D3+K2 (M2113): Best for moderate deficiency correction and higher-dose maintenance. 5,000 IU D3 + 120 mcg K2 (MK-7) per dropper. The D3+K2 combination is especially valuable during the correction phase when calcium metabolism is ramping up.
Good Vitamin D (A2101): Best for people who prefer capsules and want enhanced absorption. 2,000 IU D3 + 200mg bromelain per capsule. Bromelain is a natural enzyme from pineapple that supports nutrient absorption. Solid choice for daily maintenance after correcting a deficiency.
†These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.
Sources & References
- Forrest KY, Stuhldreher WL. "Prevalence and correlates of vitamin D deficiency in US adults." Nutrition Research. 2011;31(1):48-54. PMID: 21310306
- MacLaughlin J, Holick MF. "Aging decreases the capacity of human skin to produce vitamin D3." Journal of Clinical Investigation. 1985;76(4):1536-1538. PMID: 2997282
- Wortsman J, Matsuoka LY, Chen TC, et al. "Decreased bioavailability of vitamin D in obesity." American Journal of Clinical Nutrition. 2000;72(3):690-693. PMID: 10966885
- Nowak A, Boesch L, Andres E, et al. "Effect of vitamin D3 on self-perceived fatigue." Medicine. 2016;95(52):e5353. PMID: 28033244
- Lotfi A, Abdel-Nasser AM, Hamdy A, et al. "Hypovitaminosis D in female patients with chronic low back pain." Clinical Rheumatology. 2019;26(11):1895-1901.
- Tomlinson PB, Joseph C, Angioi M. "Effects of vitamin D supplementation on upper and lower body muscle strength levels in healthy individuals." Journal of Science and Medicine in Sport. 2015;18(5):575-580. PMID: 25156880
- Anglin RE, Samaan Z, Walter SD, McDonald SD. "Vitamin D deficiency and depression in adults: systematic review and meta-analysis." British Journal of Psychiatry. 2013;202:100-107. PMID: 23377209
- Martineau AR, Jolliffe DA, Hooper RL, et al. "Vitamin D supplementation to prevent acute respiratory tract infections." BMJ. 2017;356:i6583. PMID: 28202713
- Burkiewicz CJ, Guadagnin FA, Skare TL, et al. "Vitamin D and skin repair." Wound Repair and Regeneration. 2012;20(6):822-828.
- Rasheed H, Mahgoub D, Hegazy R, et al. "Serum ferritin and vitamin D in female hair loss." Journal of Cosmetic Dermatology. 2013;12(1):36-41.
- Heaney RP, Recker RR, Grote J, et al. "Vitamin D3 is more potent than vitamin D2 in humans." Journal of Clinical Endocrinology & Metabolism. 2011;96(3):E447-E452. PMID: 21177785
- Bischoff-Ferrari HA, Willett WC, Wong JB, et al. "Prevention of nonvertebral fractures with oral vitamin D and dose dependency." Archives of Internal Medicine. 2009;169(6):551-561. PMID: 19307517
- Manson JE, Cook NR, Lee IM, et al. "Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease." New England Journal of Medicine. 2019;380:33-44. PMID: 30415629
Bottom line
Vitamin D deficiency is absurdly common, especially if you live at a northern latitude, have darker skin, are over 65, or spend most of your day indoors. The symptoms are sneaky enough that most people write them off as normal life stuff. Get a 25(OH)D blood test. It is a simple, inexpensive test that gives you a clear answer. If you are deficient, the correction process is well-understood and typically takes 8 to 12 weeks with the right supplementation. If you are not deficient, a daily maintenance dose of 1,000 to 2,000 IU of D3 is a smart preventive strategy for most adults. Either way, test first. Guess later.
FAQ
How long does it take to correct a vitamin D deficiency?
It depends on the severity of your deficiency and your dose. For mild deficiency (20-29 ng/mL), most people reach sufficient levels within 8 to 12 weeks on 1,000 to 2,000 IU daily. For moderate deficiency (12-20 ng/mL), plan on 8 to 12 weeks at 4,000 to 5,000 IU daily. Severe deficiency (below 12 ng/mL) often requires prescription-strength doses of 50,000 IU weekly for 8 to 12 weeks. Symptoms like fatigue and mood improvements often show up within 4 to 6 weeks, before blood levels fully normalize.
Can I just get more sun instead of taking a supplement?
In theory, yes. In practice, it depends entirely on where you live, what time of year it is, and your skin tone. If you live above the 37th parallel (which includes Oregon, Washington, and most of the northern US), UVB rays are too weak from October through March to produce meaningful vitamin D in your skin, no matter how long you stand outside. During summer months, 10 to 30 minutes of midday sun on exposed skin a few times per week can contribute, but most people still do not produce enough. Supplementation is the reliable solution, especially for anyone in the Pacific Northwest.
What is the best vitamin D supplement for deficiency?
For correcting a deficiency, look for a supplement using vitamin D3 (cholecalciferol), not D2, as D3 is approximately 87% more effective at raising blood levels. Liquid forms offer the best absorption since D3 is already dissolved in a carrier oil. If your deficiency is moderate to severe, a D3+K2 combination is smart because the K2 helps direct the increased calcium absorption to your bones.
Should I take vitamin D with food?
Yes, ideally with a meal that contains some dietary fat. Vitamin D is fat-soluble, meaning it needs fat present in your digestive tract for optimal absorption. Studies show that taking vitamin D with a fat-containing meal can increase absorption by up to 50% compared to taking it on an empty stomach. If you use a liquid supplement that already contains a carrier oil (like fractionated coconut oil), the fat is built right in, so timing with meals is less critical but still a good habit.
Can vitamin D deficiency cause weight gain?
The relationship between vitamin D and weight is complicated and likely bidirectional. Low vitamin D levels are strongly correlated with obesity, but it is not clear that deficiency causes weight gain. What is more likely is that excess adipose tissue sequesters vitamin D (pulling it out of circulation), which makes people with higher body weight more prone to deficiency. There is some evidence that correcting deficiency may support weight management efforts, but vitamin D supplementation alone is not a weight loss strategy. The practical takeaway: if you have a higher body weight, you likely need a higher dose of vitamin D to maintain adequate blood levels.
Is vitamin D deficiency dangerous?
Prolonged, severe vitamin D deficiency can lead to serious health consequences. In children, it causes rickets (skeletal deformities). In adults, it causes osteomalacia (bone softening) and significantly increases the risk of osteoporosis and fractures. Beyond bone health, research has linked chronic deficiency to increased susceptibility to infections, higher risk of autoimmune conditions, mood disorders, and cardiovascular risk factors. The good news is that deficiency is entirely correctable with proper supplementation and monitoring. It is not a condition you have to live with.
How often should I get my vitamin D levels tested?
If you have never been tested, start there. After that, if you are actively correcting a deficiency, retest after 3 months of supplementation to confirm you have reached your target range (30-50 ng/mL). Once your levels are stable, annual testing is reasonable for most people. If you live in the Pacific Northwest, testing twice a year (once in late February or March when levels are lowest, and once in August or September when levels peak) gives you a clear picture of your personal seasonal variation and helps you dial in the right year-round supplementation strategy.