Everything You Need to Know About Vitamin D

As a functional medicine nurse practitioner, vitamin D is one of the most common deficiencies I see in my practice. Despite being called the “sunshine vitamin,” most of my patients are surprisingly low in it — and it has a big impact on their health.

Let’s dive into why vitamin D is so important, what deficiency looks like, and how to safely correct it.

What Vitamin D Actually Is

Vitamin D is technically not just a vitamin — it acts more like a hormone in your body. Your skin makes vitamin D when exposed to sunlight, and then your liver and kidneys convert it into its active form.

It plays a crucial role in:

• Bone health – It helps you absorb calcium and keep bones strong.

• Immune support – A well-balanced immune system depends on vitamin D.

• Mood regulation – Low levels are linked to seasonal affective disorder and depression.

• Hormone balance – Vitamin D receptors are present in nearly every cell, including thyroid, adrenal, and reproductive tissues.

• Inflammation control – It helps keep inflammation in check, supporting heart, brain, and metabolic health.

Sources of Vitamin D

It is difficult to get adequate vitamin D from food alone. For the average person with light to medium skin tone, you need about 15–20 minutes of midday sun exposure (between 10 AM and 2 PM), three times per week, with arms and legs exposed and no sunscreen. People with darker skin may need 2–3 times longer to make the same amount of vitamin D. Factors like season, latitude, cloud cover, and even air pollution can all lower how much vitamin D your skin produces — which is why deficiency is so common, especially in the fall and winter months.

Symptoms of Vitamin D Deficiency

Many people with low vitamin D don’t have obvious symptoms — but there are subtle clues.

Common signs include:

• Fatigue or low energy

• Frequent colds or infections

• Muscle aches or weakness

• Bone or joint pain

• Low mood or seasonal depression

• Slow wound healing

• Hair loss

Because these symptoms can overlap with other conditions, I always recommend a simple blood test (25-hydroxy vitamin D) to know where you stand.

How to Supplement Safely

This is where most people get confused — or overdo it. Here’s what I tell my patients:

1. Test before you start. Always check your blood level first. Optimal levels are generally 50–80 ng/mL (not just “normal”).

2. Take it with K2. Vitamin K2 helps direct calcium into your bones — not your arteries. Look for a D3 + K2 combo supplement.

3. Pair with fat. Vitamin D is fat-soluble, so take it with a meal that contains healthy fats for best absorption.

4. Re-check your level. After a few months, retest to make sure you’re in the optimal range (and not too high).

5. Don’t forget magnesium. Magnesium helps activate vitamin D — if you’re deficient in magnesium, you won’t get the full benefit.

Vitamin D is one of the easiest nutrients to correct — but you need to do it thoughtfully. A simple blood test, the right form (D3 + K2), and regular rechecks can make a dramatic difference in energy, mood, and immune health.

If you’ve never had your vitamin D level checked, make it a priority this season — it’s one of the simplest and most impactful ways to support your health.