If you’re balancing a demanding job, family life and regular exercise, chances are you’re training not to win medals, but to feel good, stay strong and keep doing the things you enjoy. Strength training, walking, running or fitness classes all help, but there’s one often-overlooked factor that can quietly undermine your efforts: vitamin D.

I’ve written about this vitamin before (although technically it is a pro-hormone, but hey?!), and that newsletter can be found here – https://www.powerofonefitness.com/vitamin-d/  While it is best known for its role in bone health and also in immune function as well as other bodily functions, new science is now showing that vitamin D also plays a crucial role in muscle strength, recovery and energy levels, particularly as we move through our forties, fifties and beyond.

Recent research, including a 2025 review by Dominguez and colleagues, helps explain why vitamin D – and the receptors it activates in our muscles – matter so much for active adults.

Why Vitamin D Becomes More Important After 40

As we age, several things happen that make vitamin D especially relevant. We naturally lose muscle mass and strength (a process known as sarcopenia). Recovery from exercise can take longer. Time outdoors often decreases due to work and family commitments. And then added to that the body becomes less efficient at producing vitamin D from sunlight.

Vitamin D deficiency and insufficiency are common in adults over 40, particularly in the UK, where sunlight is limited for much of the year (got to love our amazing weather!). This matters because vitamin D influences not just bones, but also how well your muscles function.

Vitamin D and Your Muscles: What’s the Connection?

Vitamin D works by binding to a protein inside muscle cells called the vitamin D receptor (VDR). These receptors allow vitamin D to “communicate” with your muscles, switching on processes that support strength, repair and efficient movement.

When vitamin D binds to these receptors, it helps:

• Support muscle protein synthesis (building and maintaining muscle)

• Improve muscle fibre function

• Enhance energy production within muscle cells

• Aid repair after exercise, reducing prolonged soreness

In practical terms, adequate vitamin D helps your muscles respond better to training – whether that’s lifting weights, doing Pilates, or simply staying active day to day.

What Happens When Vitamin D Is Low?

Low vitamin D levels don’t usually cause dramatic symptoms, but they can quietly affect how you feel and move. In adults who train regularly, insufficient vitamin D has been associated with:

• Reduced muscle strength

• Feeling weaker despite consistent training

• Slower recovery after workouts

• Increased aches, pains or niggles

• Higher risk of bone stress or injury over time

If you’ve ever wondered why training feels harder than it should, or why progress has stalled despite “doing everything right”, vitamin D status may be part of the picture.

Vitamin D, Strength Training and Everyday Life

For the typical personal training client, the goal isn’t maximal performance. It’s usually more about functional strength and vitamin D supports exactly that.

Healthy vitamin D levels help you:

• Get more out of strength training sessions

• Maintain muscle mass as you age

• Move more confidently and powerfully

• Reduce the risk of falls and fractures later in life

• Stay consistent with exercise by supporting recovery and resilience

Simply put, vitamin D helps your body use the training you’re already doing.

The Magnesium Link: An Important Supporting Player

The 2025 research also highlights the importance of magnesium, a mineral many busy adults are low in. Magnesium is essential for activating vitamin D and allowing it to do its job properly.

Without enough magnesium, vitamin D may be less effective, even if intake looks adequate on paper. Magnesium also supports:

• Muscle relaxation

• Sleep quality

• Energy production

• Stress regulation

This makes the vitamin D–magnesium combination particularly relevant for stressed, time-poor professionals.

Why Busy Adults Are Often Low in Vitamin D

Even if you train regularly, modern lifestyles work against optimal vitamin D levels:

• Most workdays are spent indoors

• Training often happens early morning or evening

• UK winters provide very little UV-B sunlight

• Diet alone rarely provides enough vitamin D

For many adults, especially from autumn through spring, sunlight and food simply aren’t sufficient.

Practical Takeaways for Personal Training Clients

• Consider getting tested: A simple blood test measuring 25-hydroxyvitamin D can show whether your levels are low, adequate or optimal.

• Don’t rely on sunshine alone: In the UK, supplementation is often necessary for much of the year – particularly for adults over 40.

• Think beyond bones: Vitamin D supports muscle strength, recovery and long-term mobility – not just skeletal health.

• Support vitamin D with magnesium: Ensuring adequate magnesium intake helps vitamin D work effectively and supports overall muscle function.

• Personalisation matters: Needs vary depending on body size, lifestyle, training load and baseline levels. 

The Bottom Line

Vitamin D may be small, but its impact on strength, movement and quality of life is significant, especially for busy adults who want to stay fit, capable and injury-free as we age.

If you’re investing time and effort into training, vitamin D helps ensure your body can fully benefit from that work. Think of it not as a performance supplement, but as a foundation nutrient for long-term health, strength and confidence in movement.