When you think of vitamin D, you probably picture sunshine and strong bones — and you wouldn’t be wrong. But what if I told you that vitamin D is involved in almost every system in your body — and that low levels could be contributing to issues like fatigue, poor immunity, low mood, and even chronic disease?
As a functional medicine nutritionist, I see this all the time in my practice. A client comes in feeling tired, struggling with joint pain, catching every cold that’s going around — and their vitamin D levels are sitting at the low end of “normal.” The problem? “Normal” doesn’t always mean optimal — and that’s where mainstream medicine often misses the mark.
Vitamin D isn’t just a nutrient; it’s more like a hormone that regulates over 1,000 genes in the body. It influences everything from how your immune system responds to infections to how well your cells handle insulin. Low levels have been linked to a higher risk of autoimmune diseases, cardiovascular issues, mental health problems, and even certain cancers.
So why is vitamin D deficiency so common — even in people who take supplements or spend time in the sun? And more importantly, what can you do to make sure your levels are where they need to be — not just for bone health, but for whole-body health?
Why Vitamin D Matters Beyond Bone Health
Vitamin D regulates calcium and phosphate balance in the body, which is why it’s long been associated with bone health. But it also acts like a hormone, influencing systems such as:
✅ Immune system – Vitamin D helps regulate immune responses, reducing inflammation and supporting the body’s ability to fight infections.
✅ Cardiovascular health – It helps regulate blood pressure and reduces arterial stiffness.
✅ Mental health – It influences neurotransmitter production, including serotonin and dopamine, which affect mood and emotional balance.
✅ Metabolism – Vitamin D improves insulin sensitivity and glucose metabolism.
✅ Cell growth and repair – It supports healthy cell turnover and reduces the risk of cancer development.
In other words, vitamin D is involved in far more than just your bones — and low levels can have a ripple effect throughout the body.
Signs and Symptoms of Low Vitamin D
Vitamin D deficiency can show up in a variety of ways — and many of the symptoms are easy to overlook or attribute to other causes:
- Fatigue and low energy – Feeling tired even after a good night’s sleep.
- Muscle weakness and joint pain – Aching muscles and slow recovery from exercise.
- Low mood and depression – Persistent low mood, irritability, or brain fog.
- Frequent infections – Getting sick often or taking longer to recover.
- Hair loss – Linked to poor calcium regulation and weakened hair follicles.
- Skin conditions – vitamin D has a role in skin growth, repair and immune function.
- Poor sleep quality – Difficulty falling or staying asleep.
Because these symptoms are so non-specific, it’s easy to miss the underlying cause — which is why a “test don’t guess” approach is better.
Why Supplementing Alone Isn’t Always Enough
Many people assume that taking a vitamin D supplement is enough to correct low levels — but it’s not always that simple. Several factors can affect how well your body absorbs and uses vitamin D:
🌿 1. Cofactor Deficiencies
Vitamin D doesn’t work in isolation — it relies on key nutrients to be properly activated and utilized:
- Magnesium – Needed for vitamin D conversion and activation.
- Vitamin K2 – Directs calcium into bones and away from soft tissues.
- Boron – Enhances the half-life of vitamin D in the body.
- Zinc – Supports immune function and cell signaling.
🍽️ 2. Poor Fat Absorption
Since vitamin D is fat-soluble, it requires a healthy digestive system and adequate fat intake for proper absorption. Conditions like IBS, Crohn’s disease, and gallbladder issues can reduce absorption.
🧬 3. Genetic Factors
Variations in the VDR gene (vitamin D receptor) can affect how efficiently your body converts and utilizes vitamin D. This is why some people need higher doses than others to achieve the same blood levels.
☀️ 4. Sunlight Exposure
While sun exposure triggers vitamin D production in the skin, factors like:
- Season and latitude – Low UVB levels in winter months.
- Sunscreen use – Blocks UVB rays needed for vitamin D synthesis.
- Skin tone – Higher melanin levels reduce vitamin D production.
What Are Optimal Vitamin D Levels?
Mainstream medicine typically considers anything above 50 nmol/L as “normal” — but functional medicine takes a more nuanced view. The optimal range for vitamin D is generally between 75–125 nmol/L.
- Below 50 nmol/L – Deficient
- 50–75 nmol/L – Insufficient
- 75–125 nmol/L – Optimal
- Over 150 nmol/L – Excessive (can lead to calcium buildup)
Testing is key — and ideally, you’d check vitamin D levels twice a year (once after winter and once after summer) to adjust for seasonal changes.
How to Optimise Your Vitamin D Levels
Once you know your vitamin D status, you can take targeted steps to improve it:
☀️ 1. Safe Sun Exposure
- Aim for 10–20 minutes of midday sun (without sunscreen) on bare skin, several times a week.
- Avoid burning — just enough for a light pink flush is ideal.
🍳 2. Increase Dietary Sources
- Fatty fish (salmon, mackerel, sardines)
- Egg yolks
- Liver
- Fortified dairy and plant milks
💊 3. Supplement Wisely
- Most people need between 2,000–4,000 IU of vitamin D3 daily — but individual needs vary based on genetics and absorption.
- Consider combining with vitamin K2 (MK-7) to ensure calcium is directed to bones, not arteries.
- Ensure adequate magnesium intake to support vitamin D metabolism.
The Functional Medicine Approach to Vitamin D
From a functional medicine perspective, vitamin D is part of a bigger picture. Correcting low levels can have a profound impact on overall health — but it’s not just about taking a pill. Addressing underlying issues like gut health, nutrient absorption, and cofactor balance is key to unlocking vitamin D’s full potential.
Take the Next Step
If you’re struggling with fatigue, low mood, or recurring health issues, it’s worth exploring your vitamin D status. A simple blood test could reveal whether low vitamin D is contributing to your symptoms — and with the right guidance, optimising your levels could make a real difference.
👉 Interested in learning more or getting your levels tested? As a trained nutrigenomics practitioner, I can also help you understand whether your body has genetic variations that affect how well you absorb and use vitamin D. Together, we can create a personalised plan to help you feel your best.