The Optimal Vitamin D Level for Men: Testosterone, Muscle, and the Deficiency Epidemic

Vitamin D deficiency affects an estimated 1 in 6 UK adults, and the proportion is considerably higher in men who work indoors, are darker-skinned, or live above the 52nd parallel (which includes most of England, all of Scotland, and all of Wales).

But the problem goes beyond general health. Low vitamin D has a direct, documented, mechanistic relationship with testosterone production. And most men who are deficient have no idea.

What Vitamin D Actually Is

Vitamin D is not a vitamin in the conventional sense. It's a secosteroid hormone that functions as a nuclear receptor ligand in virtually every cell type in the body. More than 1,000 genes are directly regulated by the vitamin D receptor, including genes involved in immune function, cell proliferation, calcium metabolism, and hormone synthesis.

Your skin produces vitamin D3 (cholecalciferol) when exposed to UVB radiation from sunlight. This is converted in the liver to 25-hydroxyvitamin D (25-OH-D) - the form measured in blood tests - and then further converted in the kidneys and other tissues to the active hormone 1,25-dihydroxyvitamin D (calcitriol).

The blood test measures 25-OH-D as a proxy for vitamin D status, since it reflects total body stores better than the active form.

Deficiency vs Insufficiency vs Optimal

The terminology causes a lot of confusion:

Deficient: Below 25 nmol/L (10 ng/dL). Severe deficiency associated with rickets in children and osteomalacia in adults. This is a clinical emergency by any standard.

Insufficient by NHS standards: 25–50 nmol/L. The NHS classifies below 50 nmol/L as deficient, but GPs often don't treat until below 25 nmol/L.

Sufficient by NHS standards: Above 50 nmol/L.

Insufficient by research-based standards: Many researchers and clinicians who focus on vitamin D function treat below 75 nmol/L as functionally insufficient, particularly for non-skeletal health outcomes (immune function, muscle function, cognitive performance, hormone production).

Optimal: 75–150 nmol/L (30–60 ng/mL). This is the range where most research shows peak vitamin D receptor activity and full expression of vitamin D's non-skeletal benefits.

Potentially excessive: Above 200 nmol/L. True toxicity from vitamin D supplementation requires sustained levels above 250 nmol/L and is associated with hypercalcaemia. It's difficult to achieve through supplementation at normal doses.

The Testosterone Connection

Vitamin D receptors (VDR) are expressed in human testicular tissue, including Leydig cells - the cells responsible for testosterone production. Research has demonstrated that vitamin D directly regulates key enzymes in the testosterone synthesis pathway.

The clinical evidence is compelling. A 2011 randomised, double-blind, placebo-controlled trial by Pilz et al., published in Hormone and Metabolic Research, assigned 54 men to 3,332 IU of vitamin D3 daily or placebo for 12 months. The supplementation group showed a 25.2% increase in total testosterone, 20.2% increase in bioactive testosterone, and 19.2% increase in free testosterone compared to placebo. The placebo group showed no significant change.

Observational data consistently support this association. A large cross-sectional study of 2,299 men found that vitamin D levels were positively correlated with testosterone levels across the entire range of vitamin D - men with the highest vitamin D had the highest testosterone. The correlation persisted after adjustment for age, body mass, and season.

UK Sunlight and the Winter Problem

In the UK, UVB radiation from sunlight is only sufficient to produce vitamin D in the skin from approximately late April to early September, and only when the sun is above 45 degrees elevation - roughly between 10am and 3pm on clear days.

For the remaining 7–8 months of the year, skin vitamin D synthesis is essentially zero regardless of how much time you spend outside. This means most UK men rely entirely on dietary vitamin D (limited, unless eating oily fish 3–4 times a week) and supplementation to maintain adequate levels through winter.

Research tracking seasonal variation in UK men finds that vitamin D levels typically peak in August–September and reach their nadir in February–March. For men who don't supplement, this often means levels dropping into the insufficient range for half the year - with corresponding seasonal effects on testosterone, mood, immune function, and energy.

What Impacts Vitamin D Absorption

Not everyone absorbs supplemental vitamin D equally:

Body fat: Vitamin D is fat-soluble and sequesters in adipose tissue. Higher body fat percentage is associated with lower circulating 25-OH-D for a given dose.

Magnesium: Vitamin D conversion requires magnesium at multiple enzymatic steps. Magnesium deficiency (extremely common) impairs the body's ability to activate vitamin D, meaning supplementation may be ineffective until magnesium status is corrected.

Vitamin K2: Vitamin D increases calcium absorption significantly. Without adequate vitamin K2 (specifically MK-7), the calcium mobilised by vitamin D is not properly directed to bone and teeth, and may deposit in soft tissues and arteries. Co-supplementation with K2 is considered best practice when supplementing vitamin D at doses above 2,000 IU.

Genetic variation: The VDR gene has common polymorphisms that affect receptor sensitivity. Some men require higher 25-OH-D levels to achieve equivalent receptor activation.

Gut absorption: Conditions affecting fat absorption (coeliac disease, inflammatory bowel disease, bariatric surgery) reduce vitamin D absorption from both food and supplements.

The Right Protocol

Based on the research and clinical guidance:

Test first. The only way to know where you are is a blood test. Without a baseline, you're guessing at dose. The target is 75–100 nmol/L, and the dose required to get there varies significantly between individuals.

Standard maintenance dose (UK adults): The NHS recommends 400 IU daily for general population adults in winter. This is widely regarded by vitamin D researchers as woefully inadequate for individuals who are deficient. 400 IU is barely sufficient to prevent rickets in a child; it will not meaningfully raise a 40-year-old man's circulating vitamin D.

Evidence-supported supplementation dose:

• For maintaining sufficiency (if already above 75 nmol/L): 2,000 IU daily year-round
• For correcting insufficiency (50–75 nmol/L): 3,000–4,000 IU daily for 3 months, then retest
• For correcting deficiency (below 50 nmol/L): 5,000 IU daily for 3 months, then retest

Always take with vitamin K2-MK7 at 100–200mcg when supplementing above 2,000 IU.

Take with food. Vitamin D is fat-soluble and absorbs best when taken with a meal containing dietary fat.

Retest after 3 months to verify you've reached the target range. Annual testing is sufficient for maintenance.

Getting Your Vitamin D Tested

Vitamin D is one of the markers included in Lola Health's comprehensive male health panel, alongside testosterone, SHBG, cortisol, thyroid function, metabolic markers, and 35 other biomarkers. The at-home finger-prick kit means you can establish your baseline within 48 hours without waiting for a GP referral.

Testing in late winter (January–February) when levels are typically at their seasonal low gives you the most useful data point. If you're deficient in February after supplementing through winter, you need to increase your dose.

The data is what drives the decision. Without it, you're guessing.

Check your vitamin D and hormone levels with Lola Health →

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Reference ranges and dosing guidance are based on published research and should not be treated as personal medical advice. Consult a GP before supplementing at high doses, particularly if you have kidney disease or hypercalcaemia risk factors.