Vitamin D is one of the most widely studied nutrients in functional medicine. It is so much more than just another vitamin and actually behaves like a fat soluble steroid hormone and as such it plays critical roles in nearly every system of the body. We see vitamin D deficiency very frequently in our patients and it often explains low energy, poor immunity, mood struggles and slow recovery from stress or illness.

In this guide, we will break down what vitamin D does, the best vitamin D sources, and how deficiency affects the body.

What Is Vitamin D?

Vitamin D is both a nutrient and a hormone precursor. Once your body absorbs it from sunlight, food or supplements, it must then convert it into an active form. This active form interacts with receptors throughout the body, influencing thousands of genes involved in immunity, bone health, gut integrity and cognitive function.

Health Benefits of Adequate Vitamin D

Research continues to show how deeply vitamin D affects whole body wellness. Adequate levels may:

Improve calcium absorption for strong bones
Support a balanced and well functioning immune system
Reduce risk of viral and respiratory tract infections
Improve athletic performance and muscle recovery
Lower the risk of chronic diseases including diabetes, cardiovascular disease, cancer and digestive disorders
Strengthen the intestinal barrier and positively influence the gut microbiome
Reduce risk of many autoimmune conditions
Support healthy neurotransmitter production for better mood and cognitive function

The Best Sources of Vitamin D

Sunshine

Vitamin D is often called the sunshine vitamin because your skin can synthesize it when exposed to UVB rays. The challenge is that UVB exposure varies greatly with latitude. Anyone living north of Dallas or Atlanta only receives meaningful UVB from roughly March through October.

Face exposure alone is not enough. You absorb only about 9 percent of vitamin D through the face which means arm and leg exposure provides more opportunity for natural vitamin D production. Aim for 5-30 minutes in the sun a few times a week. Those with darker skin will need more time in the sun to produce sufficient vitamin D.

Food

Food sources of vitamin D are limited. The richest dietary sources include:

Fatty fish like salmon, sardines and mackerel
Whole fat dairy (if tolerated)
Maitake mushrooms that have been grown in the sun contain an inactive form of vitamin D that needs to be converted into bioavailable form

These foods contribute helpful amounts but rarely enough to correct a deficiency.

Supplements

Most people will need supplemental vitamin D3 to achieve optimal levels. Vitamin D3 is more bioavailable than vitamin D2 and is the form recommended in functional and integrative medicine. We always recommend supplementing vitamin K2 with your D3. More on that later.

The Science Behind Vitamin D

Vitamin D deficiency is among the most common nutritional deficiencies worldwide.

Bone Health

Vitamin D regulates calcium and phosphorus absorption which support healthy bone remodeling. Low vitamin D can lead to low calcium levels, soft or weak bones, increased fracture risk and symptoms such as muscle weakness, cramps and fatigue.

Magnesium is essential because it activates vitamin D. People with low magnesium often struggle to raise their vitamin D levels until magnesium is corrected.

Mental Health

Vitamin D receptors are found throughout the brain and central nervous system. Adequate levels may:

Help clear amyloid plaque
Regulate genes involved in dopamine and serotonin production
Support healthier mood and emotional resilience

Many people report feeling calmer and more focused once they reach optimal vitamin D levels.

Immune Function

Vitamin D influences both the innate and adaptive immune systems. It supports healthy gut barrier integrity, helps regulate macrophages and natural killer cells and may reduce risk of respiratory infections including influenza A and other viral illnesses. Optimal levels are also associated with lower risk of cancer and other chronic inflammatory conditions.

Why Vitamin D Levels Are Unique to You

Several factors determine how well you absorb and activate vitamin D:

Latitude and seasonal sunlight
Amount of skin exposed
Sunscreen use
Age
Skin pigmentation
Genetic differences in vitamin D conversion
Digestive, liver, kidney or autoimmune conditions
Fat malabsorption
Obesity
Gastric bypass
Low magnesium levels
Indoor lifestyle or covered clothing

This is why two people taking the exact same dose may have very different blood levels.

Signs You May Be Low in Vitamin D

Testing is the only reliable way to know, although symptoms can offer clues. Low vitamin D may contribute to:

Fatigue and feeling run down, dizziness
Getting sick a lot and having a hard time getting better
Asthma, pneumonia
Muscle cramps and aching muscles, reduced endurance
Low mood, depression, anxiety
ADHD, poor memory and focus
Poor bone density, breaking bones easily, osteoporosis
Sleep disruptions
Chronic inflammation
Infertility and miscarriage
Weight gain or binge eating

People can also have no obvious symptoms yet still test low.

Raising Your Vitamin D Levels

The current Dietary Guidelines recommend only 600 IU per day. This amount prevents bone disease but usually does not help patients reach optimal blood levels for immunity, metabolic health, gut health or mood. Many patients require 2000 to 5000 IU per day or more depending on deficiency, absorption and genetics. This is why it is so important to get your levels tested and work with a clinician to find the right dose for you.

What Blood Levels Should You Aim For?

Many functional medicine clinicians and vitamin D researchers suggest:

Minimum level for basic bone health: 20 to 30 ng/ml
Optimal level for immune support, gut function and chronic disease prevention: 50 to 70 ng/ml
Avoid levels over 150 ng/ml

Key Cofactor: Vitamin K2

When supplementing vitamin D3, we recommend pairing it with vitamin K2. This is because vitamin D helps absorb calcium from the gut, and then the K2 helps direct that calcium into the bones to make new bone. If K2 is not there in sufficient amounts, that calcium could contribute to plaque deposition in the arteries.

This is why combination formulas of D3 plus K2 are so valuable. For best results, it is also important to optimize your magnesium levels as magnesium is needed to activate vitamin D and support calcium metabolism.

Bottom Line: Personalized Vitamin D Support Matters

Working with a knowledgeable provider ensures your vitamin D plan is personalized to your health history, genetics and current bloodwork.

Here are the essentials to remember:

Always test rather than guess
Dose based on your lab results
Spend time outdoors when possible
Supplement vitamin D3 with K2
Dose daily for the best immune and metabolic benefits

Vitamin D is one of the most powerful tools we have for improving long term health. With the right testing and personalized dosage, you can optimize your levels and support stronger immunity, better mood, healthier bones and a more resilient body.

Vitamin D: Why This Essential Nutrient Matters More Than You Think