Vitamin D: What It Actually Does to Your Muscles and Bones

Most people know Vitamin D as the "sunshine vitamin."

Good for bones. Take a supplement in winter. Done.

But here's what most people miss…

Vitamin D directly affects how your muscles function.

And in New Zealand — with all our outdoor culture — deficiency is far more common than you'd think.

It's Not Really a Vitamin

Technically, Vitamin D is a hormone precursor.

UVB radiation hits your skin. Your liver and kidneys convert it into its active form. And that active form works on receptors found in virtually every tissue in your body.

Including your muscles. Including your bones.

This isn't a background nutrient. It's a signalling molecule with effects across your whole system.

What It Does for Your Bones

Vitamin D controls how much calcium you absorb from food.

Without enough of it, you absorb roughly 10–15% of the calcium you eat.

With adequate levels? That rises to 30–40%.

Over years, chronic deficiency contributes to bone pain, stress fractures, and accelerated bone loss. It's one of the key drivers of osteoporosis.

But here's the thing — supplementation alone doesn't fix it. Bone health is a system. Vitamin D is a critical piece. It doesn't do the job by itself.

What It Does for Your Muscles

This is where it gets really interesting.

Vitamin D receptors exist directly inside skeletal muscle tissue. Your muscles are biologically responsive to your Vitamin D status.

Research consistently links low Vitamin D to:

• Reduced strength — especially in the lower limbs

• Impaired Type II muscle fibre development (the fibres that generate power and speed)

• Slower recovery after training

• Higher rates of muscle pain and fatigue

• Increased fall risk as we age

A 2023 review in the Journal of Cachexia, Sarcopenia and Muscle identified low Vitamin D as an independent risk factor for sarcopenia — age-related muscle loss.

That matters. Because sarcopenia isn't inevitable. It's preventable. And Vitamin D is one of the variables you can actually do something about.

The New Zealand Reality

Here's something most people don't realise.

In Wellington, from around April to August, the sun angle is too low to produce meaningful Vitamin D — even on clear days.

Add sun protection (the right call for skin cancer prevention), time spent indoors, and the fact that darker-skinned individuals need significantly more sun exposure to produce the same amount — and you start to see the problem.

Studies of NZ adults show insufficiency rates of 30–50%.

If you're not testing, you genuinely don't know where you stand.

So Where Does Strength Training Fit In?

Vitamin D does not build muscle on its own.

But the research is clear — the benefits of supplementation are strongest in people who are deficient and doing resistance training.

Here's why.

Think of it like this:

Strength training sends the signal to your muscles to get stronger. Vitamin D helps your body actually receive and act on that signal.

It supports protein synthesis, calcium signalling inside muscle fibres, and the function of those Type II fibres we mentioned.

If your levels are low, that whole process is running below capacity. You're training. But your body isn't getting as much back from it as it should.

Both matter. And they work better together.

What to Actually Do

Food sources help — fatty fish, egg yolks, fortified foods — but getting enough from diet alone is difficult for most people.

For supplementation:

• Choose D3 (cholecalciferol) — it raises blood levels more effectively than D2

• General adult recommendations sit around 600–1000 IU/day, with higher doses needed to correct an existing deficiency

• Vitamin D is fat-soluble, so very high doses without monitoring can accumulate over time

The most practical step?

Get a 25-hydroxyvitamin D blood test through your GP. Know your actual number. The NZ optimum range for bone health is 50–150 nmol/L — with many researchers suggesting 75+ nmol/L for broader muscle and metabolic health.

Then supplement based on what your results show — not a guess.

The Bottom Line

If you're training consistently but your Vitamin D is chronically low, you're working harder than you need to for the results you're getting.

Get your levels checked.

Supplement if you need to.

And if strength training isn't part of your routine yet — that's the other piece.

Because one without the other only gets you so far.

Want to talk about how your training, nutrition, and recovery all work together? Get in touch. That's exactly what we do at LifeForce.

References

1. Health New Zealand | Te Whatu Ora — Waikato Laboratory. *Vitamin D (25-OH Vitamin D) Test Guide.* Optimum range for bone health: 50–150 nmol/L. https://lab.waikatodhb.health.nz/test-guide/view/1016/vitamin-d 2. BPAC NZ (2025). *Vitamin D supplementation: an update.* https://bpac.org.nz/2025/vitamind.aspx 3. Royal College of Pathologists of Australasia (RCPA). *Use and Interpretation of Vitamin D Testing — Position Statement.* Target level >50 nmol/L at end of winter. https://www.rcpa.edu.au/Library/College-Policies/Position-Statements/Use-and-Interpretation-of-Vitamin-D-Testing

4. Holick MF. (2007). Vitamin D deficiency. *New England Journal of Medicine*, 357(3):266–281.

5. Bischoff-Ferrari HA et al. (2009). Prevention of nonvertebral fractures with oral Vitamin D and dose dependency. *Archives of Internal Medicine*, 169(6):551–561.

6. Ceglia L & Harris SS. (2013). Vitamin D and its role in skeletal muscle. *Calcified Tissue International*, 92(2):151–162.

7. Kupisz-Urbańska M, Płudowski P, Marcinowska-Suchowierska E. (2021). Vitamin D deficiency in older patients — problems of sarcopenia, drug interactions, management in deficiency. *Nutrients*, 13(4):1247.