8 Sources of Vitamin D

As we start the transition into seasons with less sunlight, it's crucial to address one of the most under-discussed yet vital aspects of health, vitamin D. Often referred to as the sunshine vitamin, vitamin D plays a central role in immune function, bone health, mood regulation, brain function, and more. And yet, a staggering number of people worldwide are deficient, which can have widespread consequences on overall well-being.

From a functional medicine perspective, we always ask why symptoms are occurring and how we can address root causes, not just manage symptoms. Vitamin D deficiency is one of those root causes that often goes unnoticed, yet silently contributes to fatigue, low mood, poor immunity, and hormonal imbalances.

In this blog, we’ll explore:

• Why vitamin D is so important
  
  

• The symptoms of deficiency
  
  

• Who’s at risk
  
  

• 8 sources of vitamin D (including diet, lifestyle & supplementation)
  
  

• Functional tools to assess and optimise your levels
  
  

Why Vitamin D Matters

Vitamin D is a fat-soluble vitamin that functions more like a hormone in the body. It helps regulate calcium absorption for strong bones and teeth, supports muscle and nerve function, influences brain development, and activates the immune system. Both T and B cells, your immune system’s frontline defenders, require vitamin D to function optimally.

But here’s the catch: the active form (vitamin D3) is primarily produced in the skin via direct sunlight exposure. With increased indoor lifestyles and geographic limitations, especially in the UK and Northern Europe, most people simply aren’t getting enough.

In fact, up to 60% of Europeans are not receiving sufficient vitamin D. In the UK alone, 1 in 5 adults and children are deficient. And certain groups are at even higher risk (more on that later).

Symptoms of Vitamin D Deficiency

Could you be deficient? Here are some common warning signs:

• Fatigue: Tired all the time, even after rest? Vitamin D may be a missing piece.
  
  

• Brain fog: Struggling to focus or feeling mentally sluggish.
  
  

• Sleep issues: Vitamin D plays a role in melatonin production, affecting sleep quality.
  
  

• Bone or muscle aches: Without vitamin D, calcium isn’t properly absorbed, leading to weak bones and muscle pain.
  
  

• Low mood or depression: It helps regulate serotonin, the “feel-good” neurotransmitter.
  
  

• Hair loss: Deficiency can disrupt the growth cycle of hair follicles.
  
  

• Frequent illness: A weakened immune response is a hallmark of low vitamin D.
  
  

• Loss of appetite: An often-overlooked but real symptom of deficiency.
  
  

From a functional medicine lens, we don’t simply suppress symptoms with a quick fix or a generic pill, we seek to understand why those symptoms are showing up in the first place. Often, what may appear as unrelated issues, chronic fatigue, low mood, poor sleep, or recurring illness, can all stem from something as foundational as a micronutrient imbalance. Nutrient sufficiency is not just about preventing deficiency; it's about giving the body the raw materials it needs to heal, regulate, and thrive. Vitamin D, in particular, is a cornerstone nutrient that influences countless biological processes. When it's lacking, your body begins to operate in a state of compensation rather than optimal function. Functional medicine aims to correct these imbalances at the root.

Who’s at Risk?

Certain groups have a higher likelihood of deficiency:

• Pregnant women
  
  

• Children under 5
  
  

• The elderly
  
  

• People with darker skin tones (due to higher melanin levels, which reduce vitamin D synthesis)
  
  

• Those with digestive/malabsorption issues like IBS, Crohn’s, colitis, or coeliac disease
  
  

• People with autoimmune conditions
  
  

• Vegetarians & vegans
  
  

• Overweight or obese individuals (vitamin D can become sequestered in fat tissue)
  
  

For these groups, testing and supplementation isn’t just helpful, it’s essential.

8 Sources of Vitamin D

Here’s how to naturally and effectively boost your levels:

1. Sunlight

Nature’s original source. Aim for 15–20 minutes of sunlight exposure on bare skin, 3–4 times a week (depending on your skin tone). But remember, during autumn and winter, the UK sun simply isn’t strong enough for this to be reliable.

2. Fatty Fish

Fish like salmon, sardines (with skin), mackerel, and trout are among the best dietary sources. Wild-caught is preferred for optimal nutrient profile.

3. Egg Yolks

Pasture-raised egg yolks contain small but valuable amounts of vitamin D. Pair with healthy fats for improved absorption.

4. Cod Liver Oil

A time-tested supplement rich in both vitamin D and vitamin A. Functional practitioners often recommend this for immune and skin health.

5. Beef Liver

Nutrient-dense and high in vitamin D, though not commonly eaten. Consider it as part of a nose-to-tail approach to nutrition.

6. Fortified Foods

Plant-based milks, cereals, and some orange juices are fortified, but amounts vary and absorption can be inconsistent. Always read labels.

7. Vitamin D Supplements

Since dietary and sunlight sources alone often aren’t enough, supplementation is the most reliable route, especially in winter months.

We recommend Balance Beam Vitamin D3 + K2, a high-quality, easily absorbed liquid spray. It contains vitamin D3 (the most bioavailable form) paired with vitamin K2, which supports calcium transport into the bones rather than the arteries, a crucial combination for long-term bone and heart health.

8. Lab Testing & Practitioner Support

The most accurate way to assess your vitamin D status, and truly understand what your body needs, is through functional lab testing. At Synergised, we recommend the Metabolism Test, a comprehensive at-home assessment designed to go beyond the basic and access a full spectrum of thyroid markers ,including vitamin D, cholesterol and thyroid markers. 

This test goes beyond basic bloodwork by offering deep insight into the systems that drive your overall wellbeing. Whether you're dealing with persistent fatigue, low mood, hair loss, or immune challenges, this lab helps connect the dots and identify what’s really going on beneath the surface.

Armed with these results, our certified Integrative Health Practitioners can create a fully personalised wellness protocol, addressing not just symptoms, but the root causes. Because when we test, we don’t guess. And that’s the cornerstone of true healing.

Why We Pair Vitamin D with K2

Nutrients don’t work in isolation, they operate as part of a delicate biochemical symphony. Vitamin D is essential for enhancing calcium absorption in the gut, but without proper guidance from vitamin K2, that calcium can end up in the wrong places, like soft tissues and arteries, rather than where it’s truly needed: your bones and teeth.

Vitamin K2 acts as a critical co-factor, directing calcium out of the bloodstream and into the skeletal structure, thereby protecting both bone density and cardiovascular integrity. When vitamin D is taken without K2, we risk tipping the balance and unintentionally creating calcification issues over time.

That’s why the Bare Biology Balance Beam formula is thoughtfully crafted with both vitamin D3, the most bioavailable form, and K2, in precisely calibrated ratios to support synergy, safety, and optimal absorption. This combination not only strengthens the skeletal system but also supports brain health, hormonal balance, and vascular resilience, making it a truly holistic tool for long-term wellbeing.

Want to Learn More?

We've discussed this vital topic in-depth on several podcast episodes. Tune in here to learn how vitamin D ties into hormonal balance, energy, immune strength, and more:

🎧 Episode 312 – The Sunshine Vitamin Uncovered
🎧 Episode 141 – Immune Resilience & Vitamin D
🎧 Episode 136 – Hidden Deficiencies You Need to Know

Vitamin D deficiency is more common than you think, but it's also entirely preventable and reversible. As we head into seasons with less sunlight, it’s time to prioritise this essential nutrient for a strong immune system, vibrant energy, better sleep, and a more stable mood.

Key Takeaways:

• Get tested to know your levels.
  
  

• Focus on real food sources but don’t rely on them alone.
  
  

• Supplement wisely, with D3 + K2, ideally in a spray form for optimal absorption.
  
  

• Consider functional testing like the Metabolism Test to see the full picture.
  
  

• Support your health holistically with the guidance of an Integrative Health Practitioner.
  
  

Because when we address nutrient deficiencies at the root, we unlock the body’s natural ability to heal, thrive, and feel its best, no matter the season.