Vitamin D is more than just a vitamin — it also functions as a hormone in the body, helping to regulate calcium absorption, support immune health, and even influence mood and brain function. However, despite its importance, many people don’t get enough. In fact, about 42% of U.S. adults and around 1 billion people worldwide are deficient in vitamin D, making this a serious public health concern. Low vitamin D levels have been linked to an increased risk of osteoporosis, weakened immunity, and even chronic diseases like heart disease and diabetes. Additionally, deficiency can contribute to fatigue, muscle weakness, and mood disturbances, including depression.
The good news is that recognizing the warning signs of vitamin D deficiency can help prevent long-term health complications and ensure your body functions optimally, which is why it’s more important than ever to remain educated and aware. Ahead, learn why vitamin D is important, the signs of deficiency, and how to make sure you are getting an adequate amount.
Vitamin D2 vs. D3: What’s The Difference?
When it comes to supplementation, you may notice two forms of vitamin D: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). While both help increase vitamin D levels, they are not equally effective. Here’s what you need to know:
Vitamin D2 (Ergocalciferol)
Source: Found in plant-based sources, such as mushrooms and fortified foods.
Effectiveness: Less effective at raising and maintaining blood vitamin D levels compared to Vitamin D3.
Stability: More prone to degradation over time, meaning its potency decreases faster.
Best For: People following a strict plant-based diet who prefer non-animal-derived supplements.
Vitamin D3 (Cholecalciferol)
Source: Found in animal-based sources like fish oil, egg yolks, and liver; also synthesized by the skin when exposed to sunlight.
Effectiveness: More efficient at raising and sustaining blood vitamin D levels compared to vitamin D2.
Stability: Remains potent longer and is better absorbed.
Best For: Most people, as it is the preferred and most effective form of Vitamin D supplementation.
8 Signs You May Be Deficient in Vitamin D
A vitamin D deficiency can lead to a range of symptoms, some of which may be subtle or mistaken for other health conditions. Here are some key signs to watch for:
#1 You’re Frequently Sick or Prone to Getting Infections
Vitamin D plays a critical role in immune function, helping the body fight off bacteria and viruses. If you’re getting sick often — especially with colds, flu, or respiratory infections — it may be a sign your vitamin D levels are too low. Research shows that vitamin D supplementation can reduce the risk of respiratory infections and even act protectively, particularly in those who are deficient.
RELATED: 9 Things to Do Daily to Avoid Getting Sick
#2 You’re Constantly Tired or Fatigued
Persistent fatigue and low energy levels can have many causes, but studies suggest that low vitamin D may be one of them. Individuals with vitamin D deficiency often report feeling unusually tired, and supplementation has been shown to improve energy levels in those who are deficient.
RELATED: Always Tired? Your Diet May Be Missing These Nutrients
#3 You’re Experiencing Bone Pain & Weakness
Since vitamin D is crucial for calcium absorption, low levels can lead to bone pain, osteoporosis, and even increased fracture risk. This is especially concerning for older adults, as vitamin D deficiency has been linked to a higher risk of falls.
RELATED: 3 Benefits of Resistance Training For Stronger Bones & A Lower Risk of Osteoporosis
#4 You Have Muscle Weakness or Cramps
While muscle soreness can be due to many factors, such as a hard workout or excess time on your feet, muscle weakness and frequent cramps, on the other hand, is often more concerning. Likely signaling a nutrient insufficiency, symptoms such as muscle weakness, slow recovery, and cramps or spasms can occur when vitamin D levels are low. This essential nutrient helps regulate calcium and phosphorus, both of which are critical for proper muscle contraction and nerve function. Without enough vitamin D, muscles may struggle to perform efficiently, leading to persistent fatigue, decreased strength, and a higher risk of cramps or spasms. Research has also linked vitamin D deficiency to chronic muscle pain.
#5 You’re Experiencing Mood Changes, Including Depression
There’s a strong connection between vitamin D and mental health. Studies suggest that low vitamin D levels are linked to an increased risk of depression, anxiety, and mood disorders. Some research even indicates that supplementing with vitamin D (including spending more time in the sunshine) may help improve symptoms in those with deficiency-related depression.
RELATED: What Research Shows About Vitamin D Status & Mental Health
#6 Your Wounds Are Slower to Heal
If cuts or injuries take longer than usual to heal, vitamin D deficiency could be a factor. Vitamin D plays a role in tissue repair and inflammation control, and low levels may impair the body’s ability to heal properly. If you’ve noticed slower recovery from surgeries, injuries, and even minor wounds, it may be a sign that your vitamin D levels are less than optimal.
#7 You’re Experiencing Hair Loss
Severe vitamin D deficiency has been linked to hair loss, particularly in conditions like alopecia areata. While hair loss can have multiple causes, some studies suggest that maintaining adequate vitamin D levels may support hair follicle health.
RELATED: 9 Essential Nutrients to Focus on When Dealing With Hair Loss
#8 You Have Brain Fog or Are Experiencing Cognitive Decline
Feeling mentally sluggish or foggy-headed? Some people with vitamin D deficiency report difficulty concentrating, memory problems, or “brain fog.” Research also suggests that low vitamin D levels are associated with an increased risk of cognitive decline and dementia in older adults.
RELATED: Food as Medicine: These Are The Best Nutrients For Mental Health
How to Get Enough Vitamin D
#1 Daily Sunlight Exposure
Spending time in the sun is one of the most effective ways to boost your vitamin D levels naturally. The good news is that when your skin is exposed to sunlight, it synthesizes vitamin D3, the most bioavailable form of the vitamin. However, there are several factors to consider, including sunscreen use, skin tone, age, and location.
Aim for 10–30 minutes of midday sun exposure on bare skin (such as arms, legs, or face) several times per week, and if sunlight exposure is limited due to lifestyle, weather, or sunscreen use, dietary sources, and supplementation may be necessary.
RELATED: 5 Powerful Benefits of Sunlight For Better Health
#2 Vitamin D-Rich Food Sources
While sunlight is the best natural source of vitamin D, certain foods can help boost your intake, especially if sun exposure is limited. Fatty fish like salmon, mackerel, and sardines are among the richest natural sources, providing a significant dose of vitamin D3. Other good sources include egg yolks, beef liver, and fortified foods such as dairy products, and plant-based milk. Mushrooms exposed to UV light can also provide vitamin D2, though it is not as easily absorbed as D3. Incorporating these foods into your diet can help support healthy vitamin D levels, particularly during winter months or for those who spend little time outdoors.
RELATED: 5 Delicious Vitamin D-Rich Foods to Add to Your Diet
#3 Supplements (If Recommended)
If you’re at risk for deficiency, a vitamin D supplement may be necessary. There are two main types: Vitamin D2 (found in plant sources) and vitamin D3 (found in animal sources). Considering its enhanced bioavailability and absorption, most experts recommend vitamin D3 over D2 for supplementation, especially if you have low levels. It’s also important to know that vitamin D is a fat-soluble vitamin, meaning taking vitamin D with a fat-containing meal enhances absorption.
However, supplementation is not a one-size-fits-all process — it’s best to get your levels tested by a doctor before supplementing.
How to Test For A Vitamin D Deficiency
If you suspect you’re deficient, the most accurate way to confirm is through a blood test measuring 25-hydroxyvitamin D (25(OH)D) levels. Your doctor can order this test to assess whether your levels are adequate, insufficient, or deficient.
Final Thoughts
Vitamin D deficiency is, unfortunately, all too common and can lead to a range of symptoms affecting your immune system, energy levels, mood, and overall health. Keep these deficiency symptoms in mind, and if you suspect you’re low in vitamin D, consider getting tested and increasing your intake through safe sun exposure, diet, or supplements if doctor recommended.
