# Vitamin D: Bone Health and Immune Function

Essential Vitamins | By Longevity Futures Research Team | Updated November 2025

Vitamin D is perhaps the most critical nutrient for optimal health that most people are deficient in. Often called the "sunshine vitamin," this fat-soluble nutrient functions more like a hormone than a traditional vitamin, influencing the expression of over 1,000 genes throughout your body.

Unlike other vitamins that must come primarily from food, vitamin D can be synthesized in your skin when exposed to UVB sunlight. However, modern indoor lifestyles, sunscreen use, and northern latitudes have created a global epidemic of vitamin D deficiency—with profound implications for bone health, immune function, and longevity.

The Deficiency Epidemic: Estimates suggest that over 1 billion people worldwide have insufficient vitamin D levels. In the US alone, 42% of adults are deficient. This deficiency is linked to increased risks of osteoporosis, autoimmune diseases, infections, cardiovascular disease, and certain cancers. Correcting vitamin D status may be one of the simplest and most impactful interventions for improving health.
600-800 IU
RDA (Adults)
42%
US Adults Deficient
1,000+
Genes Influenced
40-60 ng/mL
Optimal Blood Level

## Bone Health

Vitamin D's most well-known role is maintaining bone health. It's absolutely essential for calcium and phosphorus absorption—without adequate vitamin D, you absorb only 10-15% of dietary calcium versus 30-40% with sufficient levels:

- Calcium absorption: Vitamin D regulates intestinal calcium uptake
- Bone mineralization: Directs calcium into bone tissue
- Parathyroid regulation: Prevents excessive PTH release that would leach calcium from bones
- Phosphorus balance: Maintains proper phosphorus levels for bone matrix
- Muscle function: Supports muscle strength to prevent falls

Fracture Prevention: Meta-analyses show that vitamin D supplementation (700-800 IU daily or more) combined with calcium reduces hip fracture risk by 20% and non-vertebral fractures by 14% in older adults. The effect is most pronounced in those who are deficient at baseline.

## Immune Function

Vitamin D is a powerful immune modulator, affecting both innate and adaptive immunity:

- Antimicrobial peptides: Vitamin D stimulates production of cathelicidin and defensins—natural antibiotics
- Macrophage activation: Enhances the ability of these immune cells to destroy pathogens
- T-cell regulation: Modulates T-cell responses to prevent excessive inflammation
- Autoimmune protection: Helps maintain immune tolerance and prevent self-attack

Respiratory Infections: A comprehensive review of 25 randomized controlled trials found that vitamin D supplementation reduced the risk of acute respiratory infections by 12% overall, and by 42% in those who were severely deficient at baseline. Daily or weekly supplementation was more effective than large intermittent doses.

## Beyond Bones and Immunity

Research continues to reveal vitamin D's influence on numerous body systems:

- Cardiovascular health: Low vitamin D is associated with higher cardiovascular disease risk
- Mood and cognition: Deficiency linked to depression and cognitive decline
- Muscle function: Important for muscle strength and reducing fall risk
- Cancer risk: Higher vitamin D levels associated with lower risk of certain cancers
- Metabolic health: May influence insulin sensitivity and diabetes risk
- Pregnancy outcomes: Adequate levels support healthy fetal development

## Forms of Vitamin D

### Vitamin D3 (Cholecalciferol)

The form produced in human skin and found in animal sources. More effective at raising blood levels than D2. This is the preferred form for supplementation.

### Vitamin D2 (Ergocalciferol)

Plant-derived form found in mushrooms exposed to UV light. Less potent and shorter-lasting than D3. Sometimes used in prescription supplements.

## Sources of Vitamin D

### Sunlight

The most natural source. Just 10-30 minutes of midday sun on arms and legs (without sunscreen) can produce 10,000-20,000 IU. Factors affecting synthesis include skin tone, latitude, season, time of day, and age.

#### Salmon (Wild)

600-1,000 IU per 3 oz serving. Wild salmon contains significantly more than farmed.

#### Cod Liver Oil

1,360 IU per tablespoon. Traditional source with omega-3s and vitamin A.

#### Fortified Milk

100-120 IU per cup. Standard fortification level in most countries.

#### Egg Yolks

40-50 IU per yolk. Pasture-raised eggs may contain more.

#### Mushrooms (UV-exposed)

400-1,000 IU per serving. Only significant plant source.

#### Fortified Foods

Variable. Orange juice, cereals, and plant milks often fortified.

## Optimal Blood Levels

Vitamin D status is measured by 25-hydroxyvitamin D [25(OH)D] blood levels:

- Below 20 ng/mL: Deficient—associated with increased disease risk
- 20-30 ng/mL: Insufficient—many experts consider this suboptimal
- 30-40 ng/mL: Adequate for bone health
- 40-60 ng/mL: Optimal range suggested by vitamin D researchers
- 60-80 ng/mL: Upper optimal—may provide additional benefits
- Above 100 ng/mL: Potentially excessive—monitor closely

Testing Recommendation: Everyone should know their vitamin D level. Testing is inexpensive and provides crucial information for optimizing supplementation. Retest 2-3 months after starting or adjusting supplementation to ensure you've reached optimal levels.

## Supplementation Guidelines

Most people need supplements to achieve optimal vitamin D status, especially during winter months:

- General maintenance: 1,000-2,000 IU daily for most adults
- Deficiency correction: 4,000-5,000 IU daily until levels normalize
- Obesity: May require 2-3x higher doses due to sequestration in fat tissue
- Malabsorption: May need higher doses or sublingual forms

The upper limit of 4,000 IU is very conservative; many researchers consider up to 10,000 IU daily safe for adults. However, monitoring blood levels is wise with higher doses.

## Vitamin D and Longevity

Optimal vitamin D status is associated with multiple longevity-related benefits:

- Reduced mortality: Multiple studies link higher vitamin D with lower all-cause mortality
- Healthier aging: Associated with better physical function in older adults
- Cognitive preservation: May protect against dementia and cognitive decline
- Reduced inflammation: Vitamin D has anti-inflammatory properties
- Telomere length: Some evidence suggests vitamin D supports telomere maintenance

## Recommended Vitamin D Supplement

### Thorne Vitamin D-5000

Vitamin D3 supplement supporting healthy bones, teeth, muscles, cardiovascular and immune function. NSF Certified for Sport. Gluten-free, soy-free. 60 capsules from Thorne Research - pharmaceutical-grade quality.

$19.00
View on Amazon

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