Why I Check Every Patient’s Vitamin D Level

By Dr. Kevin Passero

In my clinical practice, there are a handful of lab markers I consider truly foundational, influencing multiple systems in the body and shaping long-term health. One of them is vitamin D.

Despite being labeled a “vitamin,” vitamin D behaves more like a hormone. Its active form interacts with receptors found throughout the body, influencing gene expression and cellular signaling in ways that extend far beyond any other vitamin. 

In fact, vitamin D signaling has been shown to regulate over 1,000 genes in the human body, which helps explain why inadequate levels can have such a dramatic impact on your health. 

Vitamin D: More Than a Bone Nutrient

Most people associate vitamin D with calcium and bones, and that role is certainly important. But vitamin D’s influence goes far beyond that. 

Through its involvement in gene expression, immune regulation and cellular communication, vitamin D helps support brain function, cardiovascular health, mood regulation, immune balance and inflammatory control.

This is why I view adequate vitamin D as a baseline requirement for optimal physiology impacting all aspects of healthy aging. 

Why So Many People Run Low

A substantial portion of adults (up to 50% by some estimates) have vitamin D levels below commonly accepted reference ranges.

There are a few reasons for this, including:

• Limited exposure to UVB sunlight due to indoor lifestyles
• Higher latitudes and seasonal changes that reduce vitamin D synthesis
• Aging, which reduces the skin’s ability to produce vitamin D
• Increased body fat, which can sequester vitamin D
• Minimal dietary sources capable of meeting daily needs
  

For many people—especially during winter months or those with predominantly indoor lifestyles—sunlight and diet alone are not enough to reliably maintain optimal vitamin D levels year-round.

When Supplementing, the Right Kind of Vitamin D is Critically Important

When it comes to supplementation, vitamin D3 (cholecalciferol) is the form I recommend and use in my clinic.

Human studies show that vitamin D3 is up to 87% more effective than vitamin D2 at raising and maintaining blood levels of 25-hydroxyvitamin D, the primary marker used to assess vitamin D status. 

It’s important to note that vitamin D needs are highly individual. Factors such as body weight, baseline levels, sun exposure, absorption and genetics all influence requirements.

For this reason, I measure and analyze the blood level of 25-hydroxyvitamin D for each of my patients at regular intervals.

Why Taking Vitamin D3 Alone Isn’t Enough

Many people assume that taking a vitamin D3 supplement on its own will be sufficient. But in reality, vitamin D is only one part of a larger metabolic system that cannot function properly without the essential co-nutrients required to activate it, move it through the body and signal where it’s needed.

Here are the three critical co-nutrients that I recommend and why: 

#1 Vitamin K2: Directing Calcium Where It Belongs
Vitamin D increases calcium absorption and raises circulating calcium levels in the bloodstream. That’s a key reason it supports bone health—but increasing calcium without proper regulation can create problems over time.

This is where vitamin K2 comes in.

Vitamin K2 activates specific calcium-binding proteins that help drive calcium into bone tissue and inhibit calcium deposition in soft tissues such as arteries. 

Human clinical studies show that vitamin K2 supplementation not only supports bone mineralization, it also improves measures of arterial stiffness and vascular elasticity, making it a vital nutrient for maintaining a healthy cardiovascular system. 

In short, vitamin D helps make calcium available and vitamin K2 helps make sure the calcium goes where it belongs.

The form of vitamin K2 also matters. Only the all-trans form of MK-7 is biologically active, yet many products on the market contain mixed isomers—meaning you are only getting the benefit of a fraction of the labeled dose. 

I prefer patented K2VITAL™ because it is 100% all-trans, clinically studied, and stabilized to remain potent as needed. This matters because standard MK-7 is highly unstable and can quickly degrade and become ineffective. 

#2 Magnesium: Essential for Vitamin D Metabolism
Magnesium is another essential—but often missing—piece of the vitamin D puzzle.

Magnesium is needed to:

• Convert vitamin D into its active forms
• Support liver and kidney metabolism of vitamin D
• Regulate calcium balance at the cellular level
  

Low magnesium status is common, and when magnesium is insufficient, vitamin D metabolism can be impaired—even when vitamin D intake is adequate.

I often see patients whose vitamin D levels respond poorly until their magnesium status is addressed to support effective activation and utilization of vitamin D. 

#3 Zinc: Supporting Vitamin D Receptor Function
Zinc plays a quieter but still important supporting role.

The vitamin D receptor (VDR) contains zinc-specific structures that allow it to bind DNA and influence gene expression. This structural relationship highlights why including zinc is critical for proper vitamin D signaling.

Inadequate zinc status can compromise vitamin D signals at the cellular level, making zinc a critical co-factor.

Comprehensive and Synergistic Durable D3 COMPLEX

I hope I have made it clear that vitamin D is critically important, but not a standalone nutrient. It is part of a system—and that system only works when it is combined with specific co-nutrients. 

After seeing so many patients with low vitamin D levels—even when supplementing—I created Durable D3 COMPLEX®, a formula that supports the entire vitamin D pathway.

What makes this formula different is not just the ingredients—it’s the philosophy behind it. Instead of focusing on “raising vitamin D levels,” Durable D3 COMPLEX supports what this means for your health: clearer arteries, more stable immune responses, a more balanced inflammatory response and support for cognitive vitality.

Durable D3 COMPLEX is a comprehensive, synergistic vitamin D formula designed to ensure your body can activate, utilize and benefit from vitamin D.

It brings together the essential components required for vitamin D to work at its full potential:

• Bioavailable D3 125 mcg (5,000 IU)—the natural form produced through exposure to the sun.
  
  
• K2VITAL™ MK-7 (180 mcg)—100% active, trans-form and stabilized K2 at a clinically proven dose to help direct calcium into your bones while inhibiting calcium buildup in your arteries.
  
  
• Bioavailable Magnesium + Zinc—to promote proper vitamin D absorption, activation and bone mineralization. 
  

It’s a formula built from my clinical experience to provide you with a vitamin D supplement that reflects how human physiology actually works.

To learn more about Durable D3 COMPLEX, click here.