Tan and healthy with vitamin D

Initial note: this article was previously published and has been updated according to scientific and informative criteria. It is for informational purposes only and does not replace medical advice: to evaluate blood values or therapies, consult a healthcare professional.

IN BRIEF

• Most vitamin D is synthesized in the skin under the action of UV-B rays; some comes from food and supplements.
• Adequate vitamin D levels are associated with better bone health and, in some elderly groups, a lower risk of fractures if taken at sufficient doses.
• There is conflicting evidence on effects on muscles, prevention of respiratory infections, metabolism, and hormones: benefits are more robust when deficiency is present.
• The risk/benefit ratio depends on the starting status (deficiency vs. normal type), form and dose of vitamin D, and the clinical context: not all populations benefit from supplementation.

Abstract: what does science say?

Vitamin D is a fat-soluble nutrient that acts as a precursor to steroid hormones and is primarily obtained through skin synthesis induced by the sun. Reviews and clinical studies indicate that maintaining adequate levels of 25-OH-vitamin D is important for bone health and can reduce the risk of fractures in elderly people if the dose is sufficient. For other effects (muscle strength, prevention of respiratory infections, weight control, or hormone modulation), the evidence is heterogeneous: the most reliable signals appear in deficient subjects. The effect of supplementation therefore depends on the initial status, the dose, the frequency of administration, and the concomitant use of calcium. Methodological limitations remain, as does the need to interpret observational results cautiously compared to causal evidence.

Vitamin D: skin production and food sources

Vitamin D is primarily produced in the skin when ultraviolet B (UV-B) rays convert 7-dehydrocholesterol into previtamin D, which then becomes cholecalciferol (vitamin D3). Skin production depends on environmental and individual factors: latitude, season, time of day, skin pigmentation, age, and use of sunscreens. The amount derived from the diet is generally modest: fatty fish (e.g., salmon, mackerel), egg yolks, and fortified foods are the main sources, while many leafy green vegetables contain minimal amounts of vitamin D. To determine nutritional status, 25-hydroxyvitamin D (25[OH]D) is measured in the laboratory. Low levels indicate deficiency or insufficiency; target values and diagnostic thresholds may vary according to national guidelines and measurement methods [1].

Bone health and fracture risk

The classic role of vitamin D is the regulation of calcium and bone metabolism: it contributes to intestinal calcium absorption and parathyroid hormone modulation, relevant elements for bone mineral density. Evidence from clinical studies indicates that supplementation is more effective in reducing fractures when the effective dose is sufficiently high and when subjects have a deficiency or increased risk (e.g., institutionalized elderly) [2].

Doses and population dependence

Analyses of large studies and pooled data show that the protective effect on fracture hospitalizations is dose-dependent: risk reductions have been observed especially for effective intakes equivalent to approximately ≥800 IU/day (or for high intake quartiles), while lower doses or intermittent high-bolus regimens do not always show benefit [2][3]. The contribution of calcium in combination with vitamin D has been evaluated in many trials with variable results; in some meta-analyses, the combination showed a small advantage in fragile populations [3].

Muscles, balance, and fall prevention

Vitamin D also exerts effects at the muscular level through the vitamin D receptor present in muscle cells. Observational studies report associations between higher 25(OH)D levels and greater strength and physical function, but evidence from randomized trials is heterogeneous [4].

Evidence on functional outcomes

Some RCTs have shown improvements in specific parameters (e.g., type II muscle fiber diameter in elderly women with severe deficiency), while other trials in adults with non-deficient levels have not found significant clinical benefits on strength or physical performance [5]. In summary, the benefit is more likely in subjects with initial deficiency.

Fall prevention

Meta-analyses in the elderly indicate a reduction in falls with supplementation at moderate doses (700–1000 IU/day), especially if co-administered with calcium; however, the heterogeneity of populations and protocols means that the results are not universal and that very high dosages administered intermittently may have different effects [4].

Immune system and inflammation

Vitamin D acts on various components of the immune system and can modulate inflammatory responses. Observational studies have associated lower 25(OH)D levels with a higher risk of infections and higher inflammatory markers, but the relationship does not prove causality. An important individual participant data meta-analysis showed that vitamin D supplementation modestly reduces the overall risk of acute respiratory infections, with a more evident effect in subjects deficient at the start of the study [6].

Metabolism, weight, and hormones

Numerous observational studies correlate low vitamin D levels with obesity, metabolic alterations, and unfavorable lipid profiles; however, clinical interventions have yielded conflicting results. Trials in overweight/obese individuals have observed modest or no effects on weight loss or body composition in most cases, although some studies report small reductions in BMI or waist circumference in subgroups or in per-protocol analyses [8].

Hormones and testosterone

Small studies have found increases in testosterone levels after vitamin D supplementation in deficient subjects; however, the evidence is limited and does not allow for definitive conclusions on a direct and generalizable role of vitamin D in male hormone production [7].

What the evidence does NOT show

Large-scale studies and well-conducted trials do not support clear effects of vitamin D supplementation on cardiovascular disease or cancer in the general unselected population; large randomized trials in non-deficient populations have generally reported null results for these primary endpoints [9]. Even the prevention of diabetes with vitamin D has not provided consistent evidence of benefit in the general high-risk population, except for possible signals in deficient subgroups [10].

What it means in practice

For most people, the practical message is cautious and personalized: reasonable sun exposure (without sunburn), a diet that includes vitamin D sources, and laboratory verification of status (25[OH]D) when there are risk factors for deficiency are the most sensible approaches. Supplementation is justified in subjects with documented deficiency or in at-risk groups (fragile elderly, institutionalized individuals, individuals with limited sun exposure or malabsorption), always under medical indication and with periodic checks [1][2][3].

KEY POINTS TO REMEMBER

• Most vitamin D is produced in the skin from exposure to UV-B rays; diet contributes only partially. [1]
• For bone protection in the elderly, clinical benefits are observed mainly with adequate intake (≈≥800 IU/day) or in the presence of deficiency. [2]
• Benefits on muscle strength, infection risk, and metabolism are conditioned by the initial status (greater evidence in those who are deficient). [4][5][6]
• Large trials in non-deficient populations have generally shown no benefit on cardiovascular events and diabetes prevention. [9][10]

Limitations of the evidence

It is important to distinguish sources of evidence: observational studies show associations but do not prove causality; randomized trials are necessary to support causal effects. The most frequent limitations include heterogeneity of dosages, frequency of administration (daily vs. intermittent boluses), study duration, variability of participants' baseline 25(OH)D levels, and different outcome measures. These factors explain why different results can coexist and why recommendations must be tailored to the individual patient.

Editorial conclusion

Vitamin D remains an essential element for health, especially for bone function and, in the presence of deficiency, for some muscular and immune aspects. Practical recommendations must be based on updated evidence, blood measurements, and individual risk assessment. Avoid excessive sun exposure that causes skin damage and do not use high-dose supplements without medical supervision. Research continues to clarify where supplementation offers real benefits and where observational signals do not translate into measurable clinical effects.

Editorial note

This update has been prepared with attention to transparency, balance, and verifiability of sources. The scientific statements reported are accompanied by references to peer-reviewed literature with verifiable DOIs in the "Scientific Research" section. The article aims to inform and not to replace individual clinical evaluations.

SCIENTIFIC RESEARCH

1. Holick MF. Vitamin D deficiency. New England Journal of Medicine. 2007. https://doi.org/10.1056/NEJMra070553
2. Bischoff‑Ferrari HA et al. A pooled analysis of vitamin D dose requirements for fracture prevention. New England Journal of Medicine. 2012. https://doi.org/10.1056/NEJMoa1109617
3. US Preventive Services Task Force evidence review / meta‑analysis: Association Between Calcium or Vitamin D Supplementation and Fracture Incidence in Community‑Dwelling Older Adults. JAMA. 2017. https://doi.org/10.1001/jama.2017.19344
4. Bischoff‑Ferrari HA et al. Effect of vitamin D on falls: a meta‑analysis. JAMA. 2004. https://doi.org/10.1001/jama.291.16.1999
5. Ceglia L et al. A randomized study on the effect of vitamin D3 supplementation on skeletal muscle morphology and vitamin D receptor concentration in older women. Journal of Clinical Endocrinology & Metabolism. 2013. https://doi.org/10.1210/jc.2013-2820
6. Martineau AR et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta‑analysis of individual participant data. BMJ. 2017. https://doi.org/10.1136/bmj.i6583
7. Pilz S et al. Effect of vitamin D supplementation on testosterone levels in men. Hormone and Metabolic Research. 2011. https://doi.org/10.1055/s-0030-1269854
8. Castaneda‑García (review) / Vitamin D supplementation in obesity and during weight loss: a review of randomized controlled trials. Metabolism. 2019. https://doi.org/10.1016/j.metabol.2018.12.010
9. Manson JE et al.; VITAL Research Group. Vitamin D supplements and prevention of cancer and cardiovascular disease. New England Journal of Medicine. 2019. https://doi.org/10.1056/NEJMoa1809944
10. Pittas AG et al.; D2d Research Group. Vitamin D supplementation and prevention of type 2 diabetes. New England Journal of Medicine. 2019. https://doi.org/10.1056/NEJMoa1900906