Vitamin D: dosing, deficiency, and the safety limits of high doses

27 Mar Vitamin D: dosing, deficiency, and the safety limits of high doses

Safety note. This page is educational. Vitamin D is fat-soluble and accumulates in the body. High doses can cause harm. Do not begin high-dose vitamin D without a blood test and clinical supervision.

Vitamin D deficiency is common and worth correcting. “Extremely high doses” are a different question, and the honest answer is that they carry real risk. This page sets out what vitamin D does, what the evidence supports, and where the safety limits are.

What does vitamin D do?

Vitamin D is a fat-soluble vitamin essential for calcium homeostasis and bone health. It is obtained from diet (oily fish, some mushrooms) and formed in the skin through UV-B conversion of 7-dehydrocholesterol. It also has a role in immune function: vitamin D influences the activity of monocytes and macrophages, and modulates inflammatory signalling.

Does vitamin D prevent respiratory infections?

The evidence is mixed, and it is weaker than the internet suggests.

A 2017 individual-participant-data meta-analysis in the BMJ (Martineau et al., BMJ 2017;356:i6583) found a modest protective effect of vitamin D supplementation against acute respiratory infection — but the benefit was concentrated in people who were genuinely deficient at baseline, and was largest with regular daily or weekly dosing rather than large infrequent boluses.

Subsequent large randomised trials, including VITAL (New England Journal of Medicine, 2019), did not find that supplementation reduced infection or major disease outcomes in generally vitamin-D-replete populations.

The reasonable summary: correcting a real deficiency may help. Taking large doses when you are already replete has not been shown to help, and is not risk-free. Vitamin D is not an antiviral, and it is not a treatment for any infection.

How much is too much?

The Tolerable Upper Intake Level for adults is 4,000 IU per day (Institute of Medicine, 2011). That is the level above which the risk of harm begins to rise for the general population.

Doses in the range of 100,000 IU are not a routine supplement dose. Single high-dose boluses are used in some clinical protocols under supervision, but sustained intake at that level causes vitamin D toxicity, which presents as hypercalcemia — elevated blood calcium — and can cause nausea, vomiting, weakness, frequent urination, kidney stones, kidney damage, and in severe cases cardiac arrhythmia. Vitamin D toxicity is real, it is documented in the literature, and people do harm themselves with over-the-counter high-dose products.

Because vitamin D is fat-soluble, it accumulates. You cannot simply excrete an excess the way you can with water-soluble vitamins.

What should you actually do?

• Test first. Serum 25-hydroxyvitamin D is a simple, inexpensive blood test. Dose to a measured deficiency, not to a headline.
• Retest. If you are supplementing at any meaningful dose, your level should be monitored.
• Do not self-administer high doses. Particularly if you have kidney disease, sarcoidosis, hyperparathyroidism, or take thiazide diuretics or digoxin — all of which change how your body handles calcium.

Vitamin D at Advanced Rejuvenation

We test vitamin D levels and, where a deficiency is identified, we correct it — including by IV where absorption is a problem. That is done on the basis of a blood test, with monitoring. It is not a protocol we recommend anyone run on themselves.

References

• Martineau AR, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583.
• Manson JE, et al. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease (VITAL). N Engl J Med. 2019;380:33-44.
• Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press; 2011. (Tolerable Upper Intake Level: 4,000 IU/day for adults.)
• Marcinowska-Suchowierska E, et al. Vitamin D Toxicity — A Clinical Perspective. Front Endocrinol. 2018;9:550.

Medical disclaimer. Educational content only; not medical advice. Vitamin D is not a treatment or preventive for COVID-19 or any other infection. Consult a qualified clinician before starting supplementation.