Vitamin D and Strength Training After 50 — Why Most Older Trainees Are Deficient and What to Do About It

During the COVID pandemic, like many people, I found myself with time I hadn't expected and decisions I hadn't planned for. One of them was getting a comprehensive blood panel done — the kind of full picture that tends to get deprioritised in ordinary life when you feel broadly well and training is going reasonably. I wasn't expecting any significant findings. I was wrong.

My vitamin D levels had tanked. Not borderline low — genuinely deficient. And what made the result land so hard was that I'm an outdoorsy type. I walk, I get out in the British summer, I'm not someone who spends every daylight hour behind glass. I had assumed, without ever consciously examining the assumption, that my lifestyle was covering me. It was not. The blood test said otherwise, clearly and unambiguously.

I've thought about that result a great deal since. Because if vitamin D deficiency caught me — someone who was actively trying to live and train in a way that supports long-term health — it can catch anyone. And for the strength trainee over 50, the consequences go considerably further than the bone health story most people associate with vitamin D. This is about muscle function. Recovery. Neuromuscular performance. And, for those for whom it matters, the testosterone connection that the research has been quietly building for over a decade.

This page covers what the evidence actually says, why older trainees in the UK and Northern Europe are structurally disadvantaged when it comes to vitamin D status, what deficiency looks like in a training context, and what a sensible supplementation approach looks like. Today I take a high-strength vitamin D3 supplement year-round and feel markedly better for it. I want to give you the information to make the same decision from a position of knowledge rather than assumption.

The mechanism most people don't know about is this: the skin's capacity to synthesise vitamin D from UVB sunlight decreases significantly with age. By some estimates, the skin of a 70-year-old produces less than half the vitamin D from the same sun exposure as a 20-year-old. The machinery still works — it just works much less efficiently. Which means the outdoor lifestyle that may have kept vitamin D status adequate at 35 is no longer doing the same job at 55, even if nothing else has changed.

Then there is the geography. The UK sits above 51 degrees north latitude. At that latitude, the angle of the sun is too low for meaningful UVB to reach the skin's surface from approximately October through to March. During those six months, regardless of how much time is spent outdoors, vitamin D synthesis from sunlight is negligible. The summer window — April to September, ideally between 11am and 3pm — is the only period when meaningful skin synthesis is possible, and even then cloud cover, clothing, and sunscreen (all entirely reasonable choices) reduce it further.

Dietary sources fill some of the gap but not enough. Oily fish, egg yolks, and fortified foods contain vitamin D, but the quantities available from a typical diet fall well short of what's needed to maintain optimal status without supplementation. A serving of salmon contains roughly 400–600 IU of vitamin D; most researchers studying the subject consider 1,000–4,000 IU daily appropriate for adults seeking to maintain optimal blood levels. The maths doesn't add up without a supplement.

There is one further factor worth mentioning: vitamin D is fat-soluble, meaning it is stored in adipose tissue. Higher body fat is associated with lower circulating vitamin D levels, because the vitamin becomes sequestered in fat stores rather than circulating freely in the blood. For the over-50 trainee managing the body composition changes that come with age, this is one more reason why the assumption that outdoor activity is sufficient may not hold.

The bone density association is real and well-established — vitamin D facilitates calcium absorption, and inadequate vitamin D is a direct contributor to osteoporosis and fracture risk in older adults. But for the strength trainee, the more immediately relevant story is in the muscle tissue itself.

Vitamin D receptors are present in skeletal muscle cells. This is not a peripheral finding — it means that muscle tissue is an active target of vitamin D action, and that the functioning of that tissue is directly affected by vitamin D status. The clinical evidence is now substantial: low vitamin D status is consistently associated with reduced muscle strength, impaired neuromuscular function, reduced power output, and slower recovery from training. For the over-50 trainee already managing the age-related muscle loss that begins in the fourth decade of life, impaired muscle function from an easily correctable vitamin D deficiency is a headwind nobody needs.

The recovery dimension is particularly relevant. Vitamin D plays a role in modulating the inflammatory response following training — a function that becomes more important as we age, because the inflammatory response to exercise becomes both more pronounced and slower to resolve. Adequate vitamin D status appears to support faster resolution of exercise-induced inflammation and, with it, faster readiness to train again.

The insidious quality of vitamin D deficiency is that its symptoms are non-specific. Every one of them can be convincingly explained by something else — which is exactly why deficiency goes unidentified for years in otherwise health-conscious adults who assume they'd know if something was wrong.

Persistent fatigue that doesn't resolve with adequate sleep. Muscle weakness that doesn't match training load — the sensation that you should be recovering faster, lifting more, feeling stronger than you do. Joint aches and a general muscular discomfort that sits below the threshold of pain but above the threshold of comfort. Low mood and reduced motivation, particularly through the winter months. A training malaise that's hard to name but impossible to ignore.

For the over-50 trainee, all of these are easy to attribute to age, overtraining, life stress, or seasonal variation — which is exactly what makes vitamin D deficiency so easy to miss. I attributed some of these things to exactly those causes for longer than I should have. The blood test was the corrective that nothing else was going to provide.

The COVID lockdown context made the picture starker. Restricted outdoor time pushed already marginal vitamin D levels across the UK population lower during 2020–21. The pandemic-era blood test caught what years of outdoor activity had not — not because the lockdown created the problem from nothing, but because it tipped an already compromised status below a threshold the previous baseline had been hovering near for some time. It was a clarifying moment.

The reference range matters here. Most UK labs flag deficiency below 50 nmol/L of 25(OH)D — the standard blood marker for vitamin D status. But many researchers working specifically in this area consider 75–100 nmol/L optimal for muscular and immune function in active adults. Being above the lab's deficiency threshold does not necessarily mean being at an optimal level for training performance and recovery. These are different targets, and the distinction is worth understanding when interpreting a blood test result.

If I were starting again at 50 with what I know now, vitamin D3 supplementation would be on the list from day one — not as an experiment, but as a year-round baseline safeguard. The case for it is too strong and the cost too low for it to be anything else.

The first distinction worth making is between vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). D3 is the form the skin produces in response to sunlight and the form most efficiently converted to the active hormone in the body. D2 is the plant-derived form, used in some supplements and fortified foods, but with inferior bioavailability and a shorter duration of action in the body. When choosing a supplement, D3 is the form to look for.

Vitamin D is fat-soluble and absorbs significantly better when taken alongside dietary fat. The practical implication: take it with the largest meal of the day — lunch or dinner — rather than first thing in the morning on an empty stomach. A small but consistent improvement in absorption across months and years adds up.

Two co-factors are worth knowing about. Vitamin K2 (specifically the MK-7 form) works in partnership with vitamin D to direct calcium to bone rather than soft tissue — including arterial walls. When supplementing with meaningful vitamin D doses over the long term, K2 is a sensible companion. Many high-quality D3 supplements now include K2 in the formulation; if yours doesn't, it's worth considering separately. Magnesium is required for the enzymatic conversion of vitamin D to its active hormonal form in the body — and magnesium deficiency, also common in older adults, can blunt the effect of vitamin D supplementation significantly. If magnesium intake from food is unreliable, a magnesium supplement alongside D3 is worth considering.

On dosage: the UK government's official recommendation is 400 IU (10mcg) daily during autumn and winter. Speaking from experience, and from reading the clinical literature on this: most researchers studying vitamin D in active older adults consider 400 IU insufficient for meaningful status correction or maintenance. Common supplementation doses in the clinical literature range from 1,000 to 4,000 IU daily, with 2,000 IU a commonly cited middle-ground dose for maintenance in adults without deficiency and higher doses used for correction under medical supervision. I take a high-strength D3 year-round. That is my personal approach; the appropriate dose for any individual depends on their baseline status, which only a blood test can establish.

The blood marker is 25(OH)D — 25-hydroxyvitamin D — and it's the standard test for vitamin D status. A single blood draw gives you a clear picture of where you currently stand. Everything else — whether to supplement, what dose to take, whether a current supplement is working — is easier to answer with that number in hand.

In the UK, a GP will often test for vitamin D if you present with symptoms consistent with deficiency — fatigue, bone aches, muscle weakness — but routine vitamin D testing for asymptomatic adults is not standard NHS practice. Many deficient adults are therefore never tested, because they don't present with symptoms that prompt a test, and the test isn't offered routinely. Private testing closes this gap cheaply: several home finger-prick blood tests are now available for well under £30, with results returned within a few days. It is a small investment for a potentially significant piece of information.

The best time to test is in autumn — October or November — because this captures the end of the summer accumulation period, when levels should be at their annual peak. If levels are low in October after a British summer of outdoor activity, they're going to be considerably lower by February. Autumn testing gives you the most useful baseline before the deficiency months begin.