Vitamin D Deficiency: Symptoms, Causes, and How to Get Enough

Vitamin D is unlike any other nutrient. Your body can manufacture it when sunlight hits your skin, which is why researchers often call it the "sunshine vitamin" or, more accurately, a prohormone. Yet despite this built-in production system, vitamin D deficiency is staggeringly common. According to the National Institutes of Health Office of Dietary Supplements, roughly 35 percent of U.S. adults have insufficient vitamin D levels, and some estimates place outright deficiency at over 40 percent of the population.

The consequences are not trivial. Vitamin D plays critical roles in bone metabolism, immune defense, mood regulation, and muscle function. When your levels drop too low, the symptoms can be subtle at first, easily mistaken for general fatigue or stress, but they compound over time into serious health issues.

In this comprehensive guide, we will cover exactly what vitamin D does in your body, the warning signs of deficiency, what causes levels to drop, how much you actually need, the best food and supplement sources, and how to track your daily intake so you never fall short again. If you are interested in the broader picture of vitamins and minerals, our guide on macronutrients vs. micronutrients is a great starting point.

What Does Vitamin D Do in Your Body?

Vitamin D is a fat-soluble vitamin that functions more like a hormone once activated. After your skin synthesizes it from UVB rays, or after you absorb it from food or supplements, your liver converts it to 25-hydroxyvitamin D (calcidiol), the form measured in blood tests. Your kidneys then convert it to its active form, 1,25-dihydroxyvitamin D (calcitriol), which travels throughout your body and influences hundreds of genes.

Here are the major roles vitamin D plays:

Bone Health and Calcium Absorption

Vitamin D's most well-established function is regulating calcium and phosphorus absorption in the gut. Without adequate vitamin D, your body can only absorb about 10 to 15 percent of dietary calcium. With sufficient levels, that absorption rate jumps to 30 to 40 percent. This is why vitamin D and calcium are so closely linked, and why a deficiency in one often worsens the other. For a deeper look at calcium needs, see our article on how much calcium you actually need.

Immune System Function

Vitamin D receptors are found on nearly every immune cell in your body. Research published in the British Medical Journal found that vitamin D supplementation reduced the risk of acute respiratory infections by 12 percent overall, and by up to 70 percent in individuals who were severely deficient. The Cleveland Clinic notes that vitamin D helps activate T cells, the immune system's frontline defenders.

Mood and Mental Health

Vitamin D receptors are concentrated in areas of the brain involved in mood regulation, including the prefrontal cortex, hippocampus, and hypothalamus. A meta-analysis published in the British Journal of Psychiatry found that low vitamin D levels were associated with a significantly increased risk of depression. While supplementation alone is not a treatment for clinical depression, correcting a deficiency can be an important piece of the puzzle.

Muscle Function

Vitamin D influences muscle protein synthesis and neuromuscular function. The Mayo Clinic notes that deficiency is associated with muscle weakness, particularly in the proximal muscles of the thighs and upper arms, increased fall risk in older adults, and impaired physical performance.

Emerging Research: Cancer, Heart Disease, and Autoimmunity

Large-scale observational studies have linked low vitamin D levels to increased risks of colorectal cancer, cardiovascular disease, multiple sclerosis, and type 1 diabetes. The landmark VITAL trial, a randomized controlled trial of over 25,000 participants published in the New England Journal of Medicine, found that while vitamin D supplementation did not significantly reduce overall cancer incidence, it did reduce cancer mortality by 25 percent in analyses that excluded the first two years of follow-up. Research in this area is ongoing and promising.

Vitamin D Deficiency Symptoms: 7 Warning Signs

Vitamin D deficiency symptoms are notoriously nonspecific, meaning they overlap with dozens of other conditions. This is one reason deficiency goes undiagnosed for so long. If you are experiencing several of the following symptoms simultaneously, it is worth asking your doctor for a 25-hydroxyvitamin D blood test.

1. Frequent Illness or Infections

If you catch every cold that goes around the office or find yourself dealing with respiratory infections multiple times a year, low vitamin D could be a contributing factor. Vitamin D directly supports both the innate and adaptive immune systems. A study in the Archives of Internal Medicine found that individuals with vitamin D levels below 30 ng/mL were significantly more likely to report a recent upper respiratory tract infection, even after adjusting for variables like season, age, and body mass.

2. Fatigue and Tiredness

Chronic, unexplained fatigue is one of the most commonly reported symptoms of vitamin D deficiency, and one of the most frequently overlooked. A study published in the North American Journal of Medical Sciences found that 77.2 percent of patients who reported fatigue had low vitamin D levels. Many of these patients saw significant improvements in fatigue scores after supplementation brought their levels back to the sufficient range. If you feel persistently tired despite adequate sleep, vitamin D is worth investigating alongside other common culprits like iron deficiency.

3. Bone and Back Pain

Because vitamin D is essential for calcium absorption, a deficiency can lead to inadequate bone mineralization. In adults, this manifests as osteomalacia, a softening of the bones that causes deep, aching bone pain, particularly in the lower back, pelvis, hips, and legs. The NIH ODS notes that prolonged deficiency can contribute to osteoporosis, increasing fracture risk. Large observational studies have found a strong correlation between low vitamin D status and chronic lower back pain.

4. Depression and Mood Changes

Seasonal affective disorder (SAD) has long been linked to reduced sunlight exposure during winter months, and vitamin D is believed to be one of the key mediating factors. Beyond seasonal patterns, chronic low mood, irritability, and anxiety can all be associated with insufficient vitamin D. A Harvard Health review noted that several studies have found an association between low vitamin D levels and depression, particularly in older adults.

5. Slow Wound Healing

If cuts, scrapes, and surgical wounds seem to take unusually long to heal, vitamin D deficiency may be part of the reason. Vitamin D plays a role in controlling inflammation and fighting infection, both of which are critical to the wound healing process. It also stimulates the production of compounds essential for forming new skin. Research suggests that adequate vitamin D levels support every phase of wound healing, from initial inflammation to tissue remodeling.

6. Muscle Pain and Weakness

Diffuse muscle pain, weakness, and muscle cramps are classic signs of vitamin D deficiency that are frequently misdiagnosed as fibromyalgia or chronic fatigue syndrome. Vitamin D receptors are present in skeletal muscle tissue, and the active form of the vitamin influences muscle contraction and strength. A study in the journal Pain found that vitamin D deficiency was significantly more prevalent among patients with chronic widespread pain. If you are also experiencing muscle cramps, it is worth checking your magnesium levels as well, since these deficiencies often occur together.

7. Hair Loss

While some hair loss is normal, excessive shedding or thinning can be linked to nutrient deficiencies, including vitamin D. Vitamin D stimulates hair follicles, and receptors for the vitamin are found in the cells that form hair follicles (keratinocytes). Severe deficiency has been associated with alopecia areata, an autoimmune condition that causes patchy hair loss. Research published in the journal Skin Pharmacology and Physiology found that women with hair loss had significantly lower vitamin D levels than those without hair loss.

What Causes Vitamin D Deficiency?

Understanding the root causes helps you assess your own risk. Vitamin D deficiency is rarely about a single factor. It is usually a combination of the following:

• Limited sun exposure: People who work indoors, live in northern latitudes (above 37 degrees N), wear extensive clothing, or consistently use sunscreen are at higher risk. Between November and March in much of the U.S. and all of Canada and Northern Europe, the sun's angle is too low for skin to produce meaningful vitamin D.
• Darker skin pigmentation: Melanin reduces the skin's ability to produce vitamin D from sunlight. Studies from the NIH show that Black Americans have substantially lower average 25(OH)D levels than White Americans, with deficiency rates estimated at 82 percent versus 30 percent, respectively.
• Age: As you get older, your skin becomes less efficient at synthesizing vitamin D. An older adult's skin produces approximately 75 percent less vitamin D than a younger person's skin given the same sun exposure. Additionally, kidney function declines with age, reducing the conversion to the active form.
• Obesity: Vitamin D is fat-soluble, meaning it gets sequestered in body fat. People with a BMI over 30 often require two to three times more vitamin D to achieve the same blood levels as those at a healthy weight. The Endocrine Society specifically recommends higher supplementation doses for individuals with obesity.
• Malabsorption conditions: Celiac disease, Crohn's disease, ulcerative colitis, and cystic fibrosis can all impair the gut's ability to absorb fat-soluble vitamins, including vitamin D.
• Kidney and liver disease: Since both organs are involved in converting vitamin D to its active form, impaired function in either one can lead to deficiency even with adequate intake.
• Certain medications: Glucocorticoids, anticonvulsants, antiretroviral medications, and some cholesterol-lowering drugs can accelerate vitamin D metabolism and lower blood levels.
• Exclusively breastfed infants: Breast milk contains very little vitamin D, which is why the American Academy of Pediatrics recommends all breastfed infants receive 400 IU of supplemental vitamin D daily beginning shortly after birth.

How Much Vitamin D Do You Need?

The recommended daily intake of vitamin D varies by age, health status, and which expert body you consult. The most widely cited guidelines come from the Institute of Medicine (now the National Academy of Medicine) and the Endocrine Society. Here are the current Recommended Dietary Allowances (RDAs) from the NIH Office of Dietary Supplements:

Age Group	RDA (IU/day)	RDA (mcg/day)	Tolerable Upper Limit (IU/day)
Infants 0-12 months	400*	10*	1,000-1,500
Children 1-13 years	600	15	2,500-4,000
Teens 14-18 years	600	15	4,000
Adults 19-70 years	600	15	4,000
Adults 71+ years	800	20	4,000
Pregnant and lactating women	600	15	4,000

*Adequate Intake (AI) rather than RDA, since insufficient evidence exists to establish an RDA for infants.

It is worth noting that these RDAs are considered conservative by many experts. The Endocrine Society recommends that adults at risk of deficiency take 1,500 to 2,000 IU per day to maintain blood levels above 30 ng/mL, and some researchers argue that optimal levels may be even higher, in the range of 40 to 60 ng/mL.

IU vs. mcg: You will see vitamin D measured in both International Units (IU) and micrograms (mcg). The conversion is straightforward: 1 mcg of vitamin D equals 40 IU. So 600 IU is the same as 15 mcg, and 1,000 IU equals 25 mcg.

If you want to understand where vitamin D fits within the bigger picture of all the micronutrients your body needs, our article on how to track micronutrients walks you through the full process.

Best Food Sources of Vitamin D

Very few foods naturally contain significant amounts of vitamin D, which is part of the reason deficiency is so widespread. Most dietary vitamin D comes from fortified foods and fatty fish. Here are the top food sources according to the NIH ODS and the USDA FoodData Central database:

Food	Serving Size	Vitamin D (IU)	% Daily Value*
Cod liver oil	1 tablespoon	1,360	170%
Trout (rainbow), cooked	3 oz	645	81%
Salmon (sockeye), cooked	3 oz	570	71%
Mushrooms (UV-exposed)	1/2 cup	366	46%
Sardines, canned in oil	2 sardines	46	6%
Milk, fortified (whole, 2%, skim)	1 cup	120	15%
Soy milk, fortified	1 cup	100-120	13-15%
Orange juice, fortified	1 cup	100	13%
Cereal, fortified	1 serving	80	10%
Egg yolk	1 large	44	6%
Tuna, canned in water	3 oz	40	5%
Beef liver, cooked	3 oz	42	5%
Cheese, Swiss	1 oz	6	1%

*Daily Value (DV) is based on 800 IU (20 mcg) for adults and children aged 4 years and older.

A few things stand out from this table. Fatty fish dominates the list, which means if you do not eat fish regularly, meeting your vitamin D needs through food alone is extremely difficult. Fortified foods like milk, plant milks, and orange juice help bridge the gap, but even three cups of fortified milk only provide about 360 IU, well below what many experts consider optimal. This is where understanding nutrient density becomes valuable; it helps you choose foods that pack the biggest nutritional punch per calorie.

UV-exposed mushrooms are a noteworthy option for vegetarians and vegans. When mushrooms are exposed to ultraviolet light (either sunlight or commercial UV lamps), they produce vitamin D2 in significant amounts. Check the label, as regular store-bought mushrooms that have not been UV-treated contain very little vitamin D.

Vitamin D Supplements: D2 vs. D3 and Optimal Dosing

Given the difficulty of getting enough vitamin D from food and sun alone, supplements are a practical solution for many people. But not all vitamin D supplements are created equal.

Vitamin D2 (Ergocalciferol) vs. Vitamin D3 (Cholecalciferol)

• Vitamin D3 is the form your skin produces naturally and is found in animal-based foods. It is more effective at raising and maintaining blood levels of 25(OH)D. A meta-analysis published in the American Journal of Clinical Nutrition found that D3 was approximately 87 percent more potent than D2 in raising serum 25(OH)D concentrations.
• Vitamin D2 is derived from fungi and yeast. It is the form found in UV-exposed mushrooms and is the type typically used in prescription-strength supplements. While it can raise blood levels, it is less stable and breaks down faster in the body.

Bottom line: For most people, vitamin D3 is the better choice. Vegan D3 supplements derived from lichen are widely available for those who prefer plant-based options.

How to Take Vitamin D Supplements Effectively

• Take with fat: Vitamin D is fat-soluble. Taking it with a meal that contains fat improves absorption by up to 50 percent, according to research from the Cleveland Clinic.
• Consistency over timing: Whether you take it in the morning or evening does not matter much. What matters is taking it consistently. Daily dosing is generally preferred over large weekly or monthly boluses, as studies suggest daily supplementation maintains more stable blood levels.
• Get tested first: Before starting high-dose supplementation (above 2,000 IU per day), get a 25(OH)D blood test. This lets you and your doctor determine the right dose based on your current levels. A level below 20 ng/mL is considered deficient, 20-29 ng/mL is insufficient, and 30 ng/mL or above is generally considered sufficient.
• Do not exceed the upper limit without medical supervision: While vitamin D toxicity is rare, it can occur with sustained intake above 10,000 IU per day. Toxicity causes dangerously elevated blood calcium levels (hypercalcemia), leading to nausea, kidney stones, and in severe cases, kidney failure.

The Vitamin D and Calcium Connection

Vitamin D and calcium are deeply interdependent. Without adequate vitamin D, your body cannot absorb enough calcium from your diet, no matter how much calcium you consume. Conversely, having plenty of vitamin D but insufficient calcium means your body may pull calcium from your bones to maintain blood calcium levels, weakening your skeleton over time.

The Mayo Clinic recommends that adults aim for both adequate vitamin D (600-800 IU per day from the RDA, though many people need more) and adequate calcium (1,000-1,200 mg per day depending on age and sex). Taking these nutrients together, either from food or supplements, can improve absorption of both.

This interdependence extends to other micronutrients as well. Magnesium, for instance, is required for the enzymes that convert vitamin D to its active form. A magnesium deficiency can therefore impair vitamin D metabolism even if your vitamin D intake is adequate. This is one of many reasons a holistic approach to micronutrient tracking matters more than focusing on a single nutrient in isolation. Read more about how these minerals interact in our guides on calcium and magnesium deficiency.

How to Track Your Vitamin D Intake Daily

Knowing how much vitamin D you need and which foods contain it is only useful if you can actually monitor your intake over time. The challenge is that vitamin D content varies significantly between foods, fortification levels differ between brands, and sun exposure is difficult to quantify. This makes consistent tracking essential.

Here is a practical framework:

1. Log your food intake consistently. This does not have to be tedious. Modern tracking tools make it much faster than manual lookups. Acai lets you track vitamin D from food photos alongside 244 other micronutrients, so you get a complete picture of your daily intake without manually searching databases for every ingredient.
2. Include supplements in your tracking. If you take a vitamin D supplement, log it alongside your food. This gives you an accurate total daily intake number.
3. Review weekly trends, not just daily numbers. A single day under your vitamin D target is not a problem. A consistent pattern of falling short, week after week, is. Acai's weekly dashboard flags when you are consistently low on any micronutrient, including vitamin D, so you can adjust before a deficiency develops.
4. Get blood work done periodically. Tracking intake tells you what is going in. Blood tests tell you what your body is actually doing with it. Ask your doctor for a 25(OH)D test at least once a year, ideally at the end of winter when levels tend to be lowest.
5. Adjust for seasonal changes. If you live in a northern climate, your vitamin D intake strategy should differ between summer (when you get some from sun exposure) and winter (when you likely need more from food and supplements).

If you are not sure which tracking method is right for you, our comparison of the best micronutrient tracking apps can help you decide. And if you want to start with the basics, our guide on how to hit your macros covers the foundational habits that make any kind of nutrition tracking sustainable.

Frequently Asked Questions About Vitamin D

How long does it take to correct a vitamin D deficiency?

It depends on the severity of your deficiency and the dose you take. In general, with appropriate supplementation (typically 2,000-5,000 IU per day for moderate deficiency, or higher loading doses prescribed by a doctor for severe deficiency), most people see their blood levels reach the sufficient range within 8 to 12 weeks. The Endocrine Society suggests retesting blood levels after 3 months of supplementation to confirm your dose is adequate.

Can you get too much vitamin D?

Yes, but it is extremely unlikely from food and sun alone. Vitamin D toxicity almost always results from excessive supplementation, specifically sustained intake above 10,000 IU per day for extended periods. Symptoms of toxicity include nausea, vomiting, weakness, frequent urination, and dangerously high blood calcium levels that can lead to kidney damage. Stick to the tolerable upper limit of 4,000 IU per day unless directed otherwise by your doctor.

Can I get enough vitamin D from sunlight?

It depends on where you live, your skin tone, and your lifestyle. As a general guideline, 10 to 30 minutes of midday sun exposure on your face, arms, and legs (without sunscreen) two to three times per week can produce adequate vitamin D for many fair-skinned individuals during summer months. However, during winter at latitudes above 37 degrees N, the sun's UVB rays are too weak for vitamin D synthesis. Darker-skinned individuals may need three to six times more sun exposure to produce the same amount. For most people, relying on sun alone is not a reliable strategy year-round.

What is the best time of day to take vitamin D?

There is no single best time. The most important factor is taking it with a meal that contains fat to enhance absorption. Some people prefer mornings to make it part of a routine, while others take it with dinner. There is anecdotal evidence that taking vitamin D at night may interfere with sleep by suppressing melatonin, but this has not been conclusively demonstrated in clinical studies. Choose whatever time helps you take it consistently.

Does sunscreen prevent vitamin D production?

Technically yes. Sunscreen with SPF 30 reduces vitamin D synthesis by about 95 percent. However, most people do not apply sunscreen thickly enough or consistently enough for this to completely block production. Dermatologists generally recommend that you should not skip sunscreen for the sake of vitamin D; instead, meet your vitamin D needs through diet and supplements while protecting your skin from UV damage.

Should I take vitamin D with vitamin K2?

There is growing interest in combining vitamin D with vitamin K2. The rationale is that vitamin D increases calcium absorption, while vitamin K2 helps direct that calcium into bones and teeth rather than allowing it to deposit in soft tissues like arteries. While the evidence is still evolving, taking K2 alongside vitamin D is considered safe and may provide additional benefits for bone and cardiovascular health. Many combination supplements are now available.

How do I know if I should get tested for vitamin D deficiency?

Consider requesting a 25(OH)D blood test if you have multiple risk factors (limited sun exposure, darker skin, age over 65, obesity, or a malabsorption condition), if you are experiencing symptoms like unexplained fatigue, bone pain, or frequent infections, or if you are about to start supplementation and want to know your baseline level. The test is simple, inexpensive, and widely available through standard blood panels.

Putting It All Together

Vitamin D deficiency is common, consequential, and correctable. The symptoms, from chronic fatigue and frequent infections to bone pain and mood changes, are your body's way of signaling that something important is missing. The good news is that once you understand your risk factors, get your levels tested, and implement a combination of dietary strategies and smart supplementation, you can bring your levels into the optimal range relatively quickly.

The key is not to treat this as a one-time fix. Vitamin D levels fluctuate with the seasons, your diet, your weight, and your lifestyle. Ongoing tracking is what separates people who correct a deficiency once and slide back into it from those who maintain healthy levels year-round. Whether you use Acai or another method, the habit of monitoring your micronutrient intake, including vitamin D, is one of the highest-leverage health habits you can build.

Start with a blood test. Know your number. Then build a plan around food, supplements, and sensible sun exposure to keep that number where it needs to be. Your bones, immune system, muscles, and mood will thank you.