Vitamin D: flexible supplementation for optimal health
Is vitamin D really a vitamin?
Vitamin D occupies a special place among vitamins. Unlike most vitamins, which we must obtain exclusively through diet, our body can synthesize vitamin D under the influence of sunlight. A smaller portion comes from food, particularly fatty fish (salmon, herring, sardines), organ meats (liver, kidneys), eggs, and certain mushrooms (the only non-animal source). It is also found in fortified products such as dairy or breakfast cereals.
Sun exposure remains the main source, as it stimulates the skin’s production of vitamin D. In Belgium, this exposure is often insufficient, especially during winter, limiting natural synthesis. It is estimated that 90–95% of the Belgian population has insufficient vitamin D intake during this period, depending on age and lifestyle (1). Prolonged deficiency can lead to bone problems such as rickets in children, osteomalacia, or osteoporosis in adults.
What is the recommended optimal dose?
Although national recommendations vary and there is no European consensus, several authoritative sources – such as the Superior Health Council, EFSA, and RIZIV (especially for older adults) – propose similar doses. Generally, a daily intake of 800–2,000 international units (IU) is recommended for adults, with adjustment based on sun exposure (sufficient sun may allow lower doses).
Note: 1 µg of vitamin D = 40 IU.
In certain situations, such as severe deficiency requiring rapid correction, higher initial doses can be given, followed by maintenance doses to maintain optimal blood levels (30–50 ng/mL, or 75–125 nmol/L) (2). In patients with chronic kidney disease, it is sometimes recommended to start with 5,000–7,000 IU per day for 12 weeks, adjusted for initial levels and body weight (3).
Daily, weekly, or monthly: little difference
Vitamin D is stored in fat tissue and released gradually into the bloodstream. This allows flexible supplementation: daily, weekly, or monthly. Clinical studies show these schedules are equivalent in effectiveness and safety, as long as the dose is not excessive (4).
However, it is up to the physician, in consultation with the patient, to determine the most appropriate dosing schedule to ensure good adherence: some patients are very consistent with daily intake, while others benefit more from a single monthly dose.
Specific patient groups
Certain populations are at higher risk of deficiency: obese individuals, patients with liver disease, malabsorption syndromes, the elderly or polymedicated patients, pregnant women, and children.
Better adherence with flexible schedules
The effectiveness of vitamin D supplementation strongly depends on patient adherence. Patients generally follow monthly schedules better than daily ones, as daily intake can be burdensome (5). Therefore, it is essential to adapt the frequency to patient preference and clinical context.
Conclusion
While supplementation provides clear benefits, excessive intake can be harmful. Use all supplements with caution.
Vitamin D deficiencies can be corrected effectively and safely through flexible dosing schedules. When following the recommended daily, weekly, or monthly intake, there is no risk of overdose.
Excessive vitamin D may cause side effects such as high calcium levels, cardiac arrhythmias, confusion, loss of appetite, nausea, kidney stones, etc.
In case of doubt, always consult your doctor or pharmacist.
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1.Sciensano (2017). Intake of Fat-Soluble Vitamins in the Belgian Population. Nutrients, 9(8): 860.
2. Pludowski P. et al. (2022). Clinical Practice in the Prevention, Diagnosis and Treatment of Vitamin D Deficiency: A Central and Eastern European Expert Consensus Statement. Nutrients, 14(7): 1483.22
3. Jørgensen HS et al. (2025). The role of nutritional vitamin D in chronic kidney disease–mineral and bone disorder in children and adults with chronic kidney disease [...] Nephrology Dialysis Transplantation, 40(4): 797‑822.
4. Ish‑Shalom S. et al. (2008). Comparison of daily, weekly, and monthly vitamin D3 in elderly hip fracture patients. Journal of Clinical Endocrinology & Metabolism, 93(9): 3430–3435. & Toth P. et al. (2017). Safety and Efficacy of Weekly 30,000 IU Vitamin D Supplementation as a Slower Loading Dose Administration Compared to a Daily Maintenance Schedule in Deficient Patients: A Randomized, Controlled Clinical Trial. J Pharmacovigil, 5: 233.
5. Bruyère O. et al. (2015). A phase IV, two‑armed, randomized, cross‑over study comparing compliance with once‑a‑month administration of vitamin D₃ to compliance with daily administration of a fixed‑dose combination of vitamin D₃ and calcium during two 6‑month periods. Osteoporosis International, 26(12): 2863–2868.