Vitamin D Deficiency in Adults
By Charles Kamen, MD, board-certified neurologist
Vitamin D deficiency affects an estimated 40-60% of the adult population worldwide, making it one of the most common nutrient deficiencies in medicine. Because vitamin D acts as a hormone that reaches nearly every tissue, inadequate levels quietly weaken bone, immunity, muscle, and energy long before obvious symptoms appear. [1]
Dr. Charles Kamen, MD, a board-certified neurologist at LiveNow Longevity in Las Vegas, screens for and treats vitamin D deficiency as part of comprehensive longevity care. Individual results vary with any approach.
What Does Vitamin D Actually Do in the Body?
Vitamin D functions as a hormone rather than a simple vitamin, binding receptors in virtually every tissue to regulate calcium absorption, immune function, inflammatory response, and cellular growth. This is why a single deficiency can affect so many systems at once — from the skeleton to the immune system to the brain. [1]
Beyond regulating calcium and bone, vitamin D has wide-ranging skeletal and extraskeletal actions across the cardiovascular, immune, and metabolic systems. [4] Optimal status is associated with lower risk of cardiovascular disease, certain cancers, autoimmune conditions, and all-cause mortality — the reason it belongs in a longevity workup, not just a bone-health checklist. [3]
What Are the Signs and Consequences of Vitamin D Deficiency?
Vitamin D deficiency often develops silently, but as it deepens it produces recognizable problems across several systems — weaker bones, more frequent infections, muscle weakness, low mood, and persistent fatigue. Because these symptoms are nonspecific, deficiency is frequently missed until it is measured directly. [1]
- Bone health: increased fracture risk, osteoporosis, osteomalacia
- Immune dysfunction: increased susceptibility to infections and autoimmune conditions
- Muscle weakness: impaired muscle function and increased fall risk in older adults
- Fatigue and mood: low vitamin D is associated with depression and chronic fatigue
- Cardiovascular risk: deficiency is linked to hypertension and cardiovascular disease
- Accelerated aging: low vitamin D is associated with increased all-cause mortality
How Is Vitamin D Deficiency Diagnosed?
Vitamin D status is measured with a serum 25-hydroxyvitamin D blood test. Dr. Kamen generally targets levels above 40-60 ng/mL for optimal health, higher than the conventional threshold of 30 ng/mL that many labs still label as merely sufficient. [2]
Testing matters because symptoms alone are a poor guide: two people can feel equally tired yet have very different levels. A number on a lab report turns a vague complaint into a target Dr. Kamen can correct and then re-check over time.
How Vitamin D Supports Immunity, Muscle, and Energy
Vitamin D supports the outcomes patients notice most — a stronger immune response, better-functioning muscle, and steadier daytime energy. Deficiency is repeatedly linked to more frequent infections, measurable muscle weakness with higher fall risk, and the low mood and chronic fatigue that many adults dismiss as normal. [1]
As a neurologist, Dr. Kamen pays particular attention to the fatigue-and-mood side of low vitamin D. Receptors for it are present throughout the nervous system, and correcting a true deficiency can help resolve the flat energy and low mood tied to it — while supporting the immunity and muscle strength that underpin a longer healthspan. [4]
How Is Vitamin D Deficiency Treated?
Most vitamin D deficiency is corrected with oral supplementation, but for patients with poor absorption or very low levels, IV vitamin D can bypass the digestive tract to restore status more reliably. Dr. Kamen matches the route to the severity of the deficiency and to any underlying absorption problem. [2]
- High-dose oral vitamin D for moderate deficiency
- IV vitamin D when absorption is compromised or levels are severely low
- Adjunctive IV therapies: magnesium supports vitamin D metabolism
- Sunlight exposure guidance for natural vitamin D production
- Regular monitoring to ensure optimal levels are achieved and maintained
Key Takeaways
- Vitamin D acts as a hormone, with receptors in virtually every tissue in the body.
- An estimated 40-60% of adults are deficient, one of the most common deficiencies in medicine.
- Deficiency affects bone strength, immune function, muscle, mood, and all-cause mortality.
- Dr. Kamen targets 40-60 ng/mL on the serum 25-hydroxyvitamin D test, above the conventional 30 ng/mL "sufficient" mark.
- IV vitamin D offers advantages for patients with absorption issues or severely low levels.
- Regular monitoring keeps levels in the optimal range and guards against toxicity.
Common Questions
What is the optimal vitamin D level?
Dr. Kamen generally targets a serum 25-hydroxyvitamin D level of 40-60 ng/mL for optimal health, higher than the conventional 30 ng/mL threshold many labs label as merely sufficient. The right target still depends on your overall health, so levels are interpreted alongside your full clinical picture.
Can vitamin D deficiency cause fatigue and low energy?
Low vitamin D is consistently associated with chronic fatigue and low mood, and receptors for it exist throughout the nervous system. Many adults dismiss this flat energy as normal. As a neurologist, Dr. Kamen checks vitamin D when unexplained fatigue and brain fog do not respond to rest or caffeine.
Can I get enough vitamin D from sunlight alone?
Sunlight can maintain vitamin D in some people, but latitude, skin tone, age, sunscreen use, and time indoors all limit how much your skin actually produces. In Las Vegas the sun is strong, yet many residents still test low, so most patients need supplementation to reach optimal levels.
Is vitamin D toxicity possible?
Yes. Excessive supplementation can raise blood calcium too far, causing nausea, hypercalcemia, and kidney problems. This is why Dr. Kamen doses based on your measured level and re-checks it rather than guessing. Monitoring lets you reach an optimal range safely without overshooting into the toxic zone.
Should I take vitamin D with K2 or magnesium?
Vitamin K2 helps direct calcium into bone rather than arteries, and magnesium is required to activate vitamin D in the body, which is one reason Dr. Kamen sometimes adds magnesium to IV protocols. He may include these cofactors in your plan so supplementation works as intended.
Can IV vitamin D help when oral supplements are not enough?
Yes. IV vitamin D bypasses digestive absorption, which helps patients with gut disorders, malabsorption, or levels so low that oral dosing corrects them too slowly. It is not necessary for everyone, but for the right patient it restores optimal status faster. Dr. Kamen determines the best route for you.
Vitamin D optimization is a foundational, measurable part of longevity-focused care — simple to correct, but far-reaching in its effects on bone, immunity, muscle, and energy. Schedule a consultation with Dr. Kamen to assess your vitamin D status and develop an appropriate optimization strategy.
References
- Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266-281.
- Giustina A, Bilezikian JP, Adler RA, et al. Endocr Rev. 2024;45(5):625-654. (Consensus statement on vitamin D status assessment and supplementation).
- Chowdhury R, et al. BMJ. 2024;386:q1427.
- Bouillon R, Marcocci C, Carmeliet G, et al. Endocr Rev. 2019;40(4):1109-1151. (Skeletal and extraskeletal actions of vitamin D).
Wondering what’s right for you?
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