Vitamin D is often called the “sunshine vitamin,” and for good reason. Our bodies produce it when exposed to UVB rays from sunlight. But despite this built-in biological advantage, 42% of U.S. adults are estimated to have vitamin D insufficiency. Among certain populations—such as people with darker skin, those who live in northern latitudes, or individuals who spend most of their time indoors—the number may be even higher.
We usually think of vitamin D in the context of bone health. But in recent years, researchers have uncovered a broad range of health conditions linked to vitamin D deficiency, from mood disorders to autoimmune disease to hormone-driven conditions like fibroids. Many symptoms—chronic fatigue, muscle aches, or poor immunity—can often be traced back to low vitamin D levels that go undetected.
“Vitamin D plays a far more systemic role than most people realize,” says Ali Anderson, RN and FNTP Functional Nutritional Therapy Practitioner. “It affects cellular health, hormone regulation, and even how we handle stress. It’s not just a bone issue—it’s a whole-body issue.”
Why Vitamin D Matters So Much
Vitamin D isn’t technically a vitamin—it’s a hormone precursor that influences over 2,000 genes in the human body. It plays a central role in:
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Calcium absorption and bone mineralization
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Immune system modulation
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Muscle function
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Hormonal signaling
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Inflammation regulation
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Mood and brain health
When levels are low, the impact is widespread—and often subtle at first. Fatigue, pain, low mood, and frequent infections are common early signs.
Bone Health and Fracture Risk
Vitamin D’s role in calcium absorption is well established. Without enough vitamin D, calcium can't be effectively absorbed from the intestines, leading to weakened bones over time.
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Osteomalacia, or soft bones, is a classic sign of deficiency in adults.
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Long-term deficiency contributes to osteoporosis, increasing fracture risk—especially in women over 50.
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A study found that people with vitamin D levels under 20 ng/mL had a 33% higher risk of hip fractures.
“Bone loss isn’t just about age,” Anderson explains. “I’ve seen women in their 30s and 40s with early signs of bone weakening tied to low vitamin D. It’s something we need to monitor earlier and more often.”
Chronic Fatigue: A Common Yet Overlooked Symptom
Many people experiencing chronic fatigue are tested for iron or thyroid issues but never think to check their vitamin D levels. Yet research suggests a strong link.
A 2014 study in North American Journal of Medical Sciences found that fatigue scores were significantly higher in people with low vitamin D, and energy levels improved after supplementation.
Why? Vitamin D affects mitochondrial function—the energy-producing centers in your cells. Without enough, your energy production slows, and you’re left feeling sluggish, no matter how much you rest.
Muscle Weakness and Falls in Older Adults
Vitamin D plays a role in neuromuscular coordination, which is critical for stability and mobility.
A meta-analysis found that older adults who supplemented with vitamin D had a 23% lower risk of falls compared to those who didn’t—especially when their baseline levels were low.
“Muscle weakness in older adults is often dismissed as aging, but vitamin D deficiency is a huge piece of that puzzle,” Anderson says. “Correcting it can improve strength and reduce fall risk within weeks.”
Fibroids: A Surprising Hormonal Connection
Uterine fibroids are noncancerous tumors affecting between 50% and 70% of women by age 50, according to the National Institutes of Health. New research suggests that vitamin D may play a role in their development.
A observational study found that women with lower vitamin D levels may be more likely to develop fibroids. Laboratory research also suggests that vitamin D may help slow fibroid cell growth by affecting certain cellular pathways.
“We’re learning that vitamin D may have broader effects on hormone-sensitive conditions than previously thought,” says Anderson. “In the case of fibroids, it’s not a guaranteed solution, but supporting healthy vitamin D levels might be one small piece of the puzzle.”
Autoimmune and Inflammatory Diseases
Low vitamin D is consistently linked to increased risk of autoimmune conditions such as:
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Multiple sclerosis
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Rheumatoid arthritis
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Type 1 diabetes
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Inflammatory bowel disease
Vitamin D helps regulate the innate and adaptive immune systems, promoting tolerance and reducing chronic inflammation. A deficiency may tip the immune system toward overreaction, potentially triggering autoimmune responses.
Breast Cancer
Emerging research continues to explore the connection between vitamin D status and breast cancer risk, outcomes, and recurrence. While not a cure or guaranteed preventive measure, vitamin D appears to play a meaningful role in modulating cell growth, immune response, and inflammation—factors that can influence cancer development.
A 2018 study published in PLOS ONE found that women with vitamin D levels above 60 ng/mL had an 80% lower risk of breast cancer compared to women with levels under 20 ng/mL. The protective mechanisms are believed to stem from vitamin D’s ability to regulate cell differentiation and reduce angiogenesis, or the formation of new blood vessels that feed tumor growth.
Low vitamin D levels have also been associated with poorer prognosis in breast cancer survivors. A study in JAMA Oncology reported that women with higher serum vitamin D at the time of diagnosis had better overall survival rates.
Given that breast cancer is the most common cancer in women worldwide, maintaining adequate vitamin D may be a simple, modifiable factor that supports broader risk-reduction strategies.
Cardiovascular and Metabolic Health
Research shows that vitamin D may influence:
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Insulin sensitivity
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Blood pressure regulation
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Lipid profiles
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Inflammatory cytokine activity
A study found that vitamin D deficiency was associated with a 64% higher risk of heart attack. Another study found that people with low vitamin D levels were more likely to develop type 2 diabetes, even after adjusting for other risk factors.
Depression and Mood Disorders
Vitamin D receptors are present in areas of the brain involved in mood regulation. Low levels have been linked to both seasonal affective disorder (SAD) and major depressive disorder.
A 2019 study found that people with low vitamin D levels were more than twice as likely to experience depressive symptoms.
While supplementation isn’t a replacement for therapy or medication, it may help support mood—especially in winter or in people with known deficiencies.
How to Know If You're Deficient
Symptoms of low vitamin D can be vague:
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Persistent fatigue
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Muscle or bone aches
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Frequent colds
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Low mood
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Hair thinning
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Brain fog
The only reliable way to know is through a 25-hydroxyvitamin D blood test. Work directly with your Health Care Provider for your own optimal levels!
How to Raise Your Vitamin D Levels
1. Sunlight
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Aim for 10–20 minutes of direct midday sun exposure to face, arms, or legs (without sunscreen) 3–4 times per week
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Factors like skin tone, latitude, and cloud cover affect synthesis
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In winter months or northern regions, sunlight alone may not be enough
2. Diet
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Fatty fish (salmon, sardines, mackerel, cod livers)
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Egg yolks
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Maitake & portobello mushrooms
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UV-exposed mushrooms
3. Supplements
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Vitamin D3 (cholecalciferol) is the most effective form
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Typical maintenance dose: 1,000–2,000 IU/day
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Higher doses (5,000 IU or more) may be used short-term under medical supervision
Always take vitamin D with fat-containing food to improve absorption.
Final Thoughts
Vitamin D deficiency is more common—and more impactful—than many people think. It touches nearly every system in the body: immune, hormonal, skeletal, cardiovascular, and neurological.
From fatigue and fibroids to bone loss and mood issues, low vitamin D levels can quietly drive symptoms that are often dismissed or misdiagnosed.
“Don’t wait for obvious signs,” Anderson says. “If you’re dealing with low energy, hormone imbalances, or chronic pain, it’s worth testing your vitamin D. It’s a simple fix that can make a huge difference.”
