Vitamin D: One of the Most Overlooked Foundations of Health

Vitamin D deficiency is incredibly common—far more common than most people (and even many clinicians) realize. In my practice, I routinely see patients with low or suboptimal vitamin D levels, even those who “eat well” and consider themselves healthy.

Despite the name, vitamin D is not really a vitamin at all. It functions more like a steroid hormone, influencing gene expression throughout the body. In fact, vitamin D receptors are found in nearly every tissue, which helps explain why deficiency is linked to such a wide range of health problems.

When vitamin D levels are optimized, we see benefits far beyond bone health—supporting immune function, cardiovascular health, metabolic balance, and even cancer prevention.

Why Vitamin D Matters So Much

Vitamin D plays a role in regulating hundreds of genes involved in:

Immune defense against bacteria and viruses
Inflammation control
Cell growth and repair
Calcium balance and bone strength
Muscle function
Cardiovascular health

Low vitamin D levels have been associated with increased risk of infections, autoimmune disease, cardiovascular disease, mood disorders, chronic pain, and several types of cancer.

Vitamin D Deficiency Is Widespread

Large population studies have shown that vitamin D deficiency is the norm—not the exception:

Roughly one-third of U.S. adults and children are deficient
Up to 70% of school-aged children are deficient or insufficient
About half of the general population is considered at risk

One important point I emphasize with patients:
“Normal” does not necessarily mean “optimal.”

Many lab reference ranges are based on average values from a population that is already deficient.

What Is an Optimal Vitamin D Level?

Vitamin D status is measured by a blood test called 25-hydroxyvitamin D.

Based on populations with regular sun exposure and current research, most experts agree:

Below 20 ng/mL – deficient
20–30 ng/mL – insufficient
40–60 ng/mL – optimal for general health
50–70 ng/mL – often targeted in chronic disease prevention

This is why testing matters. The “right” dose of vitamin D varies widely from person to person.

Sunlight: The Most Natural Source of Vitamin D

Whenever possible, sun exposure is the best way to optimize vitamin D.

When your skin is exposed to UVB rays, your body produces vitamin D in a form that lasts longer in the bloodstream than supplements. Sun exposure also triggers other beneficial compounds in the skin that we don’t get from pills.

General guidelines for sensible sun exposure:

Expose arms, legs, abdomen, and back
2–3 times per week
About half the time it would take to burn
Best window: late morning to early afternoon

Important notes:

You cannot make vitamin D through glass
Early morning and late afternoon sun produces little to no vitamin D
During daylight saving time, peak sun exposure is closer to 1:00 pm, not noon

If you live in northern latitudes, meaningful vitamin D production from sunlight is usually not possible from late fall through early spring.

What About Tanning Beds?

If sunlight is truly unavailable, certain tanning beds that emit UVB (not just UVA) can increase vitamin D levels. However, not all tanning beds are created equal, and this approach should be used cautiously and selectively.

Protecting the face and limiting exposure time is critical.

Vitamin D Supplements: When Sun Isn’t Enough

If adequate sun exposure isn’t realistic, vitamin D3 supplementation can be very helpful.

General dosing ranges often used:

Children: 600–1,000 IU daily
Adults: 1,500–2,000 IU daily (minimum)
Many adults require 3,000–4,000 IU daily to reach optimal levels

People with obesity, chronic illness, or malabsorption often need higher doses.

A commonly used estimate is ~35 IU per pound of body weight, but testing is still the best guide.

Vitamin D can be taken daily, weekly, or monthly, but I generally prefer daily dosing for consistency.

Don’t Forget Vitamin K2

If you supplement with vitamin D—especially at higher doses—vitamin K2 is essential.

Vitamin D increases calcium absorption. Vitamin K2 ensures that calcium goes into bones and teeth, not into arteries or soft tissues.

Without adequate K2, calcium can end up in the wrong places, which is why vitamin K2 deficiency—not vitamin D itself—is often behind concerns about calcification.

Think of them as a team:

Vitamin D increases calcium availability
Vitamin K2 directs calcium where it belongs

Fatigue, Aches, and Chronic Pain? Check Vitamin D

Low vitamin D can cause deep, aching bone pain, muscle weakness, and fatigue—symptoms often mislabeled as fibromyalgia or chronic fatigue.

Correcting vitamin D deficiency (along with adequate magnesium and dietary calcium) can be life-changing for some patients.

Vitamin D Across the Lifespan

Vitamin D is critical from pregnancy through old age.

Research links deficiency to:

Pregnancy complications such as pre-eclampsia
Increased risk of C-section
Childhood asthma and autoimmune disease
Higher lifetime risk of diabetes, MS, and inflammatory bowel disease

Optimizing vitamin D status early may have profound long-term health effects.

The Bottom Line

Vitamin D is one of the simplest, most affordable, and most powerful tools we have for supporting long-term health.

Whether through smart sun exposure, supplementation, or a combination of both, maintaining optimal vitamin D levels should be a foundational part of any preventive or functional medicine approach.

If you haven’t checked your vitamin D level recently—or ever—it’s worth discussing with your healthcare provider.

Sometimes, small changes truly make a big difference.

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Vitamin D: One of the Most Overlooked Foundations of Health