Quick Comparison: D3 Approaches for Men

Approach	Daily D3 Dose	K2 Included?	Absorption Form	Male-Specific Design	Outcome
Sunlight only	Variable (0 IU in winter at northern latitudes)	No	Natural synthesis	No	❌ Unreliable for 4–8 months per year depending on latitude
Budget D3 tablet	400 IU (10mcg)	No	Standard	No	⚠ Often used as a baseline daily intake in some guidance; may be insufficient for some men depending on blood levels and sun exposure
Standalone D3 supplement	1,000–2,000 IU	No	Standard	No	⚠ A higher D3 dose option; K2 is separate; suitability depends on diet, blood levels and medical context
Generic men’s multivitamin	400–600 IU	Rarely	Oxide minerals	Partial	⚠ Lower D3 dose with mixed mineral forms; check label details and whether K2 is included
MenTools One A Day	1,000 IU (25mcg)	Yes – 75mcg MK-7	Chelated bisglycinate	Yes – iron-free, zinc-optimised	✅ A single-capsule option combining D3 + K2 with a broader male-focused formula

Summary: Common Approaches to Addressing Low Vitamin D

If you want the short version, here are three common approaches people use, depending on diet, preferences and blood-test results:

1. MenTools One A Day: 1,000 IU D3 + 75mcg K2 MK-7 inside a chelated male formula. One capsule includes D3 plus zinc, magnesium and active B-vitamins.
2. Standalone D3 + K2 supplement: A common option if you already take other supplements. Look for at least 1,000 IU D3 with MK-7 form K2, not MK-4.
3. Higher-dose standalone D3 (medical context): Sometimes used if blood tests confirm severe deficiency, typically under medical supervision.

Why Are So Many Men Vitamin D Deficient?

A Global Problem, Not a Local One

Vitamin D deficiency is not limited to any single country or climate. A pooled analysis of 308 studies covering 7.9 million participants across 81 countries found that 15.7% of the global population has serum 25(OH)D below 30 nmol/L (severe deficiency) and 47.9% falls below 50 nmol/L (insufficiency)[1]. In absolute terms, subclinical vitamin D deficiency affects approximately 1 billion people worldwide[2].

The problem is not confined to cold or cloudy regions. In the United States, 35% of adults are vitamin D deficient[3]. In Europe, 18% of the population has severe deficiency (below 30 nmol/L) and 53% falls below 50 nmol/L[1]. In South Asia, the numbers are worse: more than 80% of adults in Pakistan, India and Bangladesh experience deficiency[2]. Even in sun-rich regions like Saudi Arabia, 87.8% of men tested had suboptimal vitamin D levels[4].

The Latitude and Season Factor

Between October and March in countries above 40 degrees north latitude (covering most of Europe, Canada, northern United States, and northern Asia), the sun angle is too low for your skin to produce meaningful vitamin D3, regardless of how much time you spend outdoors[5]. This creates a four-to-eight-month window where deficiency is almost guaranteed without supplementation or food fortification.

But latitude alone does not explain the full picture. Even at equatorial and tropical latitudes, high deficiency rates persist due to cultural clothing practices, indoor work, air pollution and darker skin pigmentation reducing UVB conversion[4]. The global meta-analysis found that deficiency prevalence in winter-spring is 1.7 times higher than in summer-autumn[1].

Why Men Are Particularly at Risk

The global data reveals specific risk factors that hit men hard:

• Indoor work patterns: The majority of working-age men spend peak sunlight hours indoors. Office workers, factory workers and remote workers all miss the UVB exposure window needed for skin synthesis. A Canadian study of workers in northern Alberta found that male employees were significantly more likely to be vitamin D deficient and insufficient than females[6].
• Lower supplement uptake: Men are historically less likely to take vitamin D supplements than women, partly due to the perception that supplementation is unnecessary with a healthy diet.
• Higher body fat: Vitamin D is fat-soluble and gets sequestered in adipose tissue. Individuals with obesity have a 35% higher prevalence of vitamin D deficiency, regardless of latitude or age[2]. Men who carry visceral fat have reduced circulating vitamin D even at the same dietary intake levels as leaner individuals.
• Higher metabolic demand: Men have greater muscle mass and higher baseline metabolic rates, which increases the demand for vitamin D as a cofactor in energy production and muscle function.
• Working-age men hardest hit: The global meta-analysis found that the highest prevalence of severe deficiency (below 30 nmol/L) was among adults aged 19 to 44 at 18.2%, precisely the demographic most likely to be working indoors and neglecting supplementation[1].

How Vitamin D3 Regulates Serotonin and Mood

Does Low Vitamin D Cause Low Mood?

Vitamin D does not merely correlate with mood. It directly controls the molecular machinery that produces serotonin, the neurotransmitter responsible for emotional stability, motivation, focus and sleep-wake regulation. The active form of vitamin D (1,25-dihydroxyvitamin D) activates gene expression of the enzymes that synthesise serotonin in the brain, meaning without adequate vitamin D your brain physically cannot produce serotonin at normal rates[7].

Vitamin D receptors are widely distributed throughout the brain, including regions that regulate mood, reward and executive function. These receptors influence not just serotonin but also dopamine, the neurotransmitter tied to motivation and drive[8].

Vitamin D Influences Similar Biological Pathways to Some Antidepressant Targets

A landmark study published in Genes & Nutrition reported that vitamin D can affect expression of the serotonin reuptake transporter (SERT) and monoamine oxidase A (MAO-A) in serotonergic neurons[9]. SERT is also a target of some antidepressant medicines, while MAO-A is involved in serotonin breakdown.

In practical terms, adequate vitamin D status may influence serotonin signalling. This helps explain why vitamin D deficiency is associated with mood disturbance in some research, and why correction of deficiency is investigated for wellbeing outcomes. Vitamin D supplements are foods, not medicines, and are not intended to treat depression or other medical conditions.

The Seasonal Mood Connection

For men, the mood impact of vitamin D deficiency creates a compounding problem. Reduced serotonin directly impairs motivation, decision-making and cognitive sharpness. When these decline, physical activity often drops too, which further reduces testosterone production and deepens the mood deficit. Maintaining adequate vitamin D through consistent daily supplementation during low-sunlight months is one of the simplest, most evidence-backed ways to interrupt this cycle before it takes hold.

A systematic review and meta-analysis found a significant association between low vitamin D status and depression, with supplementation showing benefit particularly in individuals with baseline deficiency[10]. This affects men across the Northern Hemisphere, from North America to Europe to Northern Asia, for four to six months every year.

Why Vitamin D Deficiency Drains Your Energy

The Mitochondrial Connection

Fatigue is typically the first symptom men notice when vitamin D levels drop, and the underlying mechanism is now well documented. Research presented at the Society for Endocrinology annual conference demonstrated that vitamin D levels directly correlate with muscle efficiency at the cellular level. Low vitamin D was associated with reduced mitochondrial function (r=-0.41, p=0.009), meaning your muscles literally produce ATP energy less efficiently when you are deficient[11].

After just 10 to 12 weeks of vitamin D supplementation, phosphocreatine recovery time in muscles improved significantly from 34.4 seconds to 27.8 seconds (p<0.001). Many participants reported subjective improvement in fatigue symptoms[11]. The researchers noted this was the first time a direct link between vitamin D and mitochondrial function had been established in humans.

The Fatigue and Muscle Weakness Data

A 2025 cross-sectional study of 300 adults quantified the fatigue relationship more precisely. Among participants, 54% were vitamin D deficient (below 20 ng/mL). The deficient group scored an average of 5.6 on the Fatigue Severity Scale compared to 3.8 in the sufficient group (p<0.001). Handgrip strength was also significantly reduced in the deficient group at 18.4 kg versus 26.1 kg (p<0.001)[12].

A strong inverse correlation was observed between vitamin D levels and both fatigue severity (r=-0.61) and muscle weakness (r=-0.58)[12]. For men who train regularly, work physically demanding jobs, or simply want to stop feeling drained by mid-afternoon, correcting vitamin D deficiency is one of the highest-leverage interventions available.

Does Vitamin D Affect Testosterone in Men?

What the Clinical Research Shows

Large observational studies consistently demonstrate that men with low vitamin D have lower testosterone. A 2023 review found that men with 25(OH)D levels below 25 nmol/L had significantly lower total testosterone compared to men with levels above 75 nmol/L[13]. The vitamin D receptor has been identified in the male reproductive tract, including Leydig cells (where testosterone is produced), confirming a direct biological mechanism[14].

The most widely cited intervention study (Pilz et al., 2011) gave vitamin D deficient men 3,332 IU of D3 daily for one year. The results were significant:

• Total testosterone increased from 10.7 to 13.4 nmol/L (p<0.001), a 25% increase
• Bioactive testosterone increased from 5.21 to 6.25 nmol/L (p=0.001), a 20% increase
• Free testosterone increased from 0.222 to 0.267 nmol/L (p=0.001), a 20% increase
• The placebo group showed no change in any testosterone measure[14]

The Important Nuance Men Should Understand

Honesty matters more than marketing hype. Subsequent randomised controlled trials in men with normal vitamin D levels and normal baseline testosterone found no significant effect from D3 supplementation on testosterone[15][16].

This is critical context: vitamin D3 supplementation supports testosterone most reliably in men who are already vitamin D deficient. It corrects a hormonal drag rather than artificially boosting levels beyond your natural baseline. Given that roughly half the global male population falls below the 50 nmol/L sufficiency threshold[1], this correction is relevant to a very large number of men worldwide.

If you are already vitamin D sufficient (above 75 nmol/L) and your testosterone is within the normal healthy range, D3 supplementation alone will not dramatically elevate testosterone further. But if you are among the roughly half of men globally who are insufficient, correcting this gap removes a proven obstacle to healthy hormone production.

How Vitamin D3 Powers Your Immune Function

The Antimicrobial Peptide Mechanism

When immune cells detect a bacterial or viral threat, they can convert circulating vitamin D into its active hormonal form and use it to trigger production of cathelicidin, an antimicrobial peptide studied for its role in immune responses. Liu et al. demonstrated that vitamin D-mediated antimicrobial activity against intracellular Mycobacterium tuberculosis is dependent on cathelicidin induction[17].

The vitamin D receptor is expressed across many immune cell types, including B cells, T cells, monocytes, macrophages and dendritic cells. This is one reason vitamin D status is widely studied in immune health research[19].

Respiratory Infection Research: What Studies Report

A 2026 investigation led by the University of Surrey, analysing health data from 36,258 individuals, reported that people with severe vitamin D deficiency (below 15 nmol/L) were 33% more likely to be hospitalised for respiratory infections including bronchitis and pneumonia, and that higher vitamin D levels were associated with lower hospital admissions[18].

The Martineau et al. individual participant data meta-analysis of 25 randomised controlled trials (11,321 participants) found vitamin D supplementation was associated with a reduced risk of acute respiratory tract infection overall (adjusted OR 0.88, 95% CI 0.81–0.96). Among those deficient at baseline (below 25 nmol/L), regular supplementation was associated with a larger reduction in risk[19].

An important detail in the literature: some analyses report that regular dosing schedules may differ from infrequent bolus dosing in observed outcomes[20]. This article summarises research on vitamin D status and does not claim that any supplement prevents or treats infection.

Why D3 Alone Is Not Enough: The K2 MK-7 Synergy

The Calcium Direction Problem

Vitamin D3 increases calcium absorption from your gut by up to 500%. This is beneficial for bone health but creates a serious secondary risk: without proper molecular signalling, that extra calcium can deposit in your arteries rather than your bones. Arterial calcification is a major contributor to cardiovascular disease.

Vitamin K2, specifically the MK-7 form, activates two proteins that solve this problem:

• Osteocalcin: Binds calcium to the bone matrix, directly increasing bone mineral density[21]
• Matrix Gla-Protein (MGP): Prevents calcium from depositing in blood vessel walls and soft tissues[21]

Clinical Evidence for the D3 + K2 Combination

In a three-year study, D3 + K supplementation maintained healthy vessel wall characteristics of the carotid artery. The group receiving D3 alone showed significant worsening of arterial health over the same period[21]. In patients with chronic kidney disease, the K2 + D3 group showed significantly lower progression of carotid intima-media thickness compared to D3 alone over 9 months[21].

The Danish AVADEC trial reported outcomes on coronary artery calcification progression and cardiovascular endpoints in the studied population[22]. MenTools uses the MK-7 form of K2, which has a half-life exceeding 72 hours for sustained calcium regulation throughout the day.

How We Chose the MenTools D3 + K2 Formula

The MenTools One A Day formula was designed around five evidence-based criteria to address vitamin D deficiency effectively for men:

1. D3 dose: 1,000 IU (25mcg) sits within the range often used to support vitamin D intake and is below the 4,000 IU upper limit cited for most adults[2].
2. K2 MK-7 pairing: 75mcg of the MK-7 form to support vitamin K’s role in normal blood clotting and the maintenance of normal bones, alongside vitamin D’s roles in bone and muscle function[21].
3. Daily delivery format: Research compares regular dosing schedules and bolus dosing in vitamin D studies[20].
4. Chelated mineral base: Magnesium and zinc delivered as bisglycinate chelates. Magnesium contributes to the reduction of tiredness and fatigue and to normal psychological function; zinc contributes to the maintenance of normal testosterone levels, normal cognitive function and the normal function of the immune system (authorised claims).
5. Male-specific design: Iron-free formula with 10mg chelated zinc (100% NRV), active B-vitamins (5-MTHF folate, P5P vitamin B6), and a set of authorised health claims. In the UK/EU, benefits should be described using authorised claim wording.

Common Mistakes Men Make With Vitamin D

Are You Making These D3 Supplementation Errors?

Even men who are aware of vitamin D deficiency often undermine their efforts through avoidable mistakes. Understanding what not to do is almost as important as knowing the right approach.

• Taking D3 without fat: Vitamin D3 is fat-soluble. Taking it on an empty stomach or with a fat-free meal reduces absorption significantly. Always take your D3 supplement alongside a meal that contains dietary fat, even a small amount such as eggs, avocado, butter, or olive oil.
• Relying on food sources alone: Very few foods contain meaningful vitamin D. Fatty fish like salmon and mackerel provide some, but you would need to eat around 400g of salmon daily to reach 1,000 IU. Fortified foods vary enormously between countries, and most men do not consistently consume enough fortified products to maintain adequate levels year-round.
• Assuming summer sun fixes the problem: Even in sunny months, modern lifestyles keep most men indoors during peak UVB hours (10am to 3pm). Sunscreen with SPF 30 reduces vitamin D synthesis by approximately 95%. A study in tropical Brazil found that 31.5% of elderly men attending a primary care clinic were vitamin D deficient despite living at low latitude with abundant sunshine year-round[23].
• Taking D3 without K2: As detailed above, D3 alone increases calcium absorption but does not direct where that calcium goes. Over months and years, this can contribute to arterial calcification. Any D3 supplement protocol should include K2 MK-7 unless you are already obtaining substantial K2 from diet (natto, hard cheeses, egg yolks).
• Using cheap oxide mineral forms: Many budget multivitamins include vitamin D3 alongside magnesium oxide and zinc oxide. These oxide forms have absorption rates of only 4 to 10%, meaning the companion minerals that vitamin D needs for activation and metabolism are barely reaching your bloodstream. Chelated bisglycinate forms absorb at 80 to 90%, making them dramatically more effective.
• Sporadic high-dose supplementation: Some men take weekly or monthly high-dose vitamin D capsules (20,000 to 50,000 IU). While this can raise blood levels, some analyses report differences between daily/regular dosing schedules and bolus dosing in observed outcomes[20].
• Not testing blood levels: Symptoms of vitamin D deficiency overlap with dozens of other conditions. The only way to confirm your status is a 25(OH)D blood test. If your level is below 30 nmol/L, you are severely deficient. Between 30 and 50 nmol/L is insufficient. Above 75 nmol/L is considered optimal by the Endocrine Society[2]. Testing annually, ideally at the end of winter, gives you actionable data rather than guesswork.

The Global Economic Impact of Vitamin D Deficiency

Why This Matters Beyond Individual Health

Vitamin D deficiency is not just a personal health issue. It carries significant economic weight. The global vitamin D therapy market was valued at USD 2.64 billion in 2025 and is projected to reach USD 5.62 billion by 2032, growing at a compound annual growth rate of 11.3%[24]. This explosive growth reflects both increasing awareness of deficiency and the rising cost of treating its downstream health consequences, from osteoporosis fractures to immune-related hospitalisations.

For individual men, the cost calculation is simpler. A single day off work due to illness costs significantly more than a month of supplementation. The University of Surrey study reported that vitamin D status was associated with respiratory infection hospitalisation outcomes in the analysed population[18]. At £17.99 per month, MenTools One A Day costs approximately 60p per day, less than a cup of coffee, for consolidated nutritional coverage as part of a balanced diet and lifestyle.

The over-the-counter segment dominates the vitamin D therapy market, accounting for 58.2% of the global share[24]. This means the majority of people addressing deficiency are doing so through self-directed supplementation rather than prescription treatment, making the quality and formulation of the supplement you choose critically important.

The Working Man’s Dilemma

The irony of modern male vitamin D deficiency is this: the men most at risk are the ones least likely to address it. Working-age men (19 to 44 years) have the highest rate of severe deficiency globally at 18.2%, yet they are the demographic least likely to visit a doctor for routine screening, least likely to take supplements, and most likely to dismiss fatigue as “just being busy”[1].

This creates a cycle where deficiency worsens performance, which increases stress, which reduces the motivation to address the underlying cause. Breaking this cycle starts with a single daily action: one capsule with breakfast. The data supports it, the mechanism is clear, and the cost is negligible relative to the performance impact of ongoing deficiency.

How MenTools One A Day Helps Address the D3 Gap

More Than Just Vitamin D: A Complete Male Formula

Most standalone D3 supplements fix one problem: low vitamin D. But for men, D3 deficiency rarely exists in isolation. Low vitamin D typically co-occurs with suboptimal zinc, magnesium and B-vitamin status, all of which compound the fatigue, mood disruption and hormonal drag that deficiency causes.

MenTools One A Day brings together D3 + K2 dosing with a male-specific mineral and vitamin base in a single daily capsule:

What MenTools Delivers	Dose	Notes (Authorised Claims Where Relevant)
Vitamin D3	25mcg (1,000 IU, 500% NRV)	Vitamin D contributes to normal muscle function and the maintenance of normal bones (authorised claims)
Vitamin K2 MK-7	75mcg	Vitamin K contributes to normal blood clotting and the maintenance of normal bones (authorised claims)
Zinc bisglycinate	10mg (100% NRV)	Zinc contributes to the maintenance of normal testosterone levels, normal cognitive function, and the normal function of the immune system (authorised claims)
Magnesium bisglycinate	100mg	Magnesium contributes to the reduction of tiredness and fatigue and to normal psychological function (authorised claims)
5-MTHF folate	200mcg	Folate contributes to normal psychological function and to the reduction of tiredness and fatigue (authorised claims)
Vitamin B12	25mcg (1,000% NRV)	Vitamin B12 contributes to the reduction of tiredness and fatigue and to normal psychological function (authorised claims)
Selenium	55mcg (100% NRV)	Selenium contributes to normal spermatogenesis and the normal function of the immune system (authorised claims)

Price: £17.99 for 30 vegan capsules (60p per day). Compared to buying standalone D3, K2, zinc and magnesium supplements separately (typically £35 to £50 per month combined), MenTools consolidates everything into one capsule at significantly lower total cost.

Who This Is Best For

MenTools One A Day is the best fit for:

• Men who spend most daylight hours indoors: Office workers, remote workers, factory workers and shift workers who miss peak sunlight exposure and face near-certain D3 depletion during winter months.
• Men who experience seasonal energy and mood dips: If autumn and winter reliably bring fatigue, brain fog and lower motivation, D3 deficiency is a likely contributing factor.
• Active men: Gym-goers, runners and athletes who want to support their routine and maintain normal health markers through consistent nutrition.
• Men living above 40 degrees latitude: This includes most of Europe, Canada, the northern United States and northern Asia, where UVB radiation is insufficient for 4 to 8 months annually.
• Men who want one solution, not five bottles: If buying separate D3, K2, zinc, magnesium and B-vitamin supplements feels overcomplicated and expensive, one capsule covers all of it.

You may not need MenTools specifically if: You already have confirmed vitamin D levels above 75 nmol/L year-round and currently take individual high-quality chelated mineral supplements. In that case, a standalone D3 + K2 product may be sufficient.

FAQ

How much vitamin D should men take daily?

The recommended daily intake for adults ranges from 400 to 800 IU, with 1,000 IU being appropriate for men with limited sun exposure, darker skin, higher BMI or confirmed low levels[2]. The safe upper limit is 4,000 IU (100mcg) per day for most adults. MenTools provides 1,000 IU (25mcg) per capsule, one quarter of the upper limit.

Does vitamin D actually affect mood and energy?

Yes. Vitamin D directly regulates serotonin synthesis by activating gene expression of the enzymes that produce serotonin in the brain[7]. It also affects expression of proteins involved in serotonin signalling in serotonergic neurons[9]. For energy, vitamin D status is linked to mitochondrial function in muscles, with supplementation improving muscle efficiency significantly (p<0.001) within 10 to 12 weeks in one study[11]. Vitamin D supplements are foods, not medicines, and are not intended to treat depression or fatigue as medical conditions.

Can vitamin D boost testosterone in men?

In men who are vitamin D deficient, supplementation has been shown to increase total testosterone by 25% (from 10.7 to 13.4 nmol/L) over one year in a randomised controlled trial[14]. However, trials in men with normal D3 and normal testosterone found no significant additional increase[15]. The primary benefit is correcting a deficiency that can be associated with lower testosterone in some men.

Why do I need vitamin K2 with D3?

Vitamin D3 increases calcium absorption from food. Vitamin K contributes to normal blood clotting and the maintenance of normal bones (authorised claims). K2 is studied for its role in activating proteins involved in calcium handling, including osteocalcin and Matrix Gla-Protein[21]. Clinical trials report different outcomes across populations and study designs.

How common is vitamin D deficiency in men globally?

Very common. A 2023 meta-analysis of 7.9 million participants across 81 countries found that 15.7% of people globally have severe deficiency (below 30 nmol/L) and 47.9% are insufficient (below 50 nmol/L)[1]. In the US, 35% of adults are deficient[3]. In South Asia, over 80% of adults are affected[2]. Working-age men (19 to 44) have the highest rate of severe deficiency at 18.2%[1].

Is 1,000 IU of vitamin D3 safe every day?

Yes. The safe upper limit for adults is 4,000 IU (100mcg) per day[2]. MenTools provides 1,000 IU, which is one quarter of this upper limit. Toxicity is extremely rare and typically only occurs with chronic intake exceeding 10,000 IU daily. Hypervitaminosis D with clinical symptoms is generally reported when serum levels exceed 88 ng/mL, which requires sustained very-high-dose intake[2].

When should I take vitamin D3 for best results?

Take it with a meal containing dietary fat, as vitamin D3 is fat-soluble and requires fat for optimal absorption[5]. MenTools One A Day is designed to be taken with breakfast. Some analyses report differences between regular dosing schedules and infrequent bolus dosing in observed outcomes[20].

How long before I notice results from vitamin D?

Energy and fatigue improvements are typically noticed within 2 to 4 weeks in some studies as muscle mitochondrial function changes[11]. Mood-related changes are often evaluated over 4 to 8 weeks as vitamin D status improves[7]. Testosterone outcomes are usually evaluated over 3 to 12 months when starting from deficiency[14]. Not everyone notices a clear change, and results vary between individuals.

Options For Men

Understanding the science behind vitamin D deficiency is the first step. Turning that knowledge into consistent daily action is where results come from. Most men know they should supplement, but without a system and accountability it tends to fall off after a few weeks.

The MenTools One A Day helps simplify daily supplementation. One capsule, taken with breakfast, covering D3, K2, chelated zinc, magnesium and active B-vitamins in a single male-specific formula.

• Challenge: Take MenTools One A Day consistently for 90 days and track your energy, mood and recovery. Not everyone notices a clear change, and results vary between individuals.
• App access: Use the MenTools app to log daily habits and monitor your progress.
• AI accountability coach: Get personalised check-ins and reminders to keep your health routine on track.
• Toolkit PDF: Download the MenTools framework covering sleep optimization, mindful practices, training recovery and more.
• Routine: Create a winning routine that includes hydration, light exposure and movement.

If you are serious about addressing common vitamin D-related gaps in diet and lifestyle, get MenTools One A Day here.

Last updated: 2026-02-21 v1.1

References

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2. Sizar O, Khare S, Goyal A, Givler A. Vitamin D Deficiency. StatPearls. Updated 2025. NBK532266
3. University Hospitals. Why You Should Pay Attention to Low Vitamin D. 2025. Link
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12. Healthcare Bulletin. Evaluation of vitamin D deficiency and its association with fatigue and muscle weakness. 2025. Link
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15. Santos HO, Teixeira FJ, Schoenfeld BJ. Reviewing the evidence on vitamin D supplementation in the treatment of testosterone deficiency. Clinical Therapeutics. 2020;42(8):e162-e177. doi:10.1016/j.clinthera.2020.05.009. Link
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17. Liu PT, Stenger S, Tang DH, Modlin RL. Cutting edge: vitamin D-mediated human antimicrobial activity against Mycobacterium tuberculosis is dependent on the induction of cathelicidin. Journal of Immunology. 2007;179(4):2060-2063. doi:10.4049/jimmunol.179.4.2060. PubMed 17675463
18. BBC News / University of Surrey. Vitamin D deficiency linked to respiratory infection hospitalisation. 2026. Link
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