Professor Reinhold Vieth wrote the seminal paper on the safety of vitamin D back in 1999. As readers know, I believe Vieth’s visionary 1999 paper led directly to the current vitamin D revolution. His paper is free to read in its entirety, which I highly recommend.
Remember to divide vitamin D blood levels that are expressed in nmol/L by 2.5 to get ng/ml. For example, vitamin D blood levels of 100 nmol/L are the same as levels of 40 ng/ml.
Recently, Dr Vieth updated his review of vitamin D’s safety profile and reached much the same conclusions as he did thirteen years ago.
Vieth R. Implications for 25-hydroxyvitamin D testing of public health policies about the benefits and risks of vitamin D fortification and supplementation. Scand J Clin Lab Invest Suppl. 2012 Apr;243:144-53.
More recently, Professor Armin Zittermann and colleagues of the Ruhr University Bochum in Germany also looked at safety issues and reached conclusions quite similar to Professor Vieth.
Dr Zittermann makes the following points:
- Data on skin synthesis of vitamin D indicate oral intake of 10,000 IU/day is safe.
- The half-life of 25(OH)D3 is about 3 weeks. That is, it takes about 3 weeks for the body to metabolize one-half of a dose of vitamin D. However, this depends on initial levels, the lower the initial level the longer the half-life.
- Vitamin D intoxication is rare, while vitamin D deficiency is widespread and a “re-emerging global health problem.”
- Excessive vitamin D intake leads to low blood phosphate levels.
- Vitamin D toxicity is probably caused by the effects of elevated 25(OH)D itself, not by elevated 1,25(OH)D levels.
- Professor Hector DeLuca of the University of Wisconsin, recently concluded that high blood calcium might occur in some people after prolonged daily doses of 25,000 IU/day.
- 10,000 IU/day for six months in young men led to mean 25(OH)D levels of 90 ng/ml.
- 5,000 IU/day for six months in young men led to mean 25(OH)D levels of 53 ng/ml.
- Dr John Hathcock’s analysis of 20 publications showed no ill effects of using 10,000 IU/day.
- Most reports indicate the toxicity threshold is somewhere between 10,000 and 40,000 IU/day.
- High intermittent doses of vitamin D, such as monthly doses, should be avoided.
- A high initial serum 25(OH)D level somehow allows the body to rid itself of most of an additional dose of vitamin D.
- When 25(OH)D3 is used as a drug, it is five times more potent than vitamin D3. It used to be available in the USA as Calderol and Delakmin.
- According to the Institute of Medicine’s Food and Nutrition Board, the upper safe blood level for 25(OH)D is 50 ng/ml. However, according to Dr Zimmermann, this recommendation “is not well documented.” Certainly, much higher levels are seen with sun exposure alone.
- How much vitamin D one needs depends on body weight, with 20-40 IU/kg/day needed to increase blood levels by 20 ng/ml.
Because of the individual variation in response to vitamin D, Professor Zimmermann recommends that people obtain an initial vitamin D level, then take individualized doses, then check another 25OH)D level in 3-6 months.
In contrast, we recommend that you simply take 5,000 IU/day and consider getting a 25(OH)D blood level in four to six months, especially if you are overweight. We believe natural levels, about 50 ng/ml, are the most conservative choice, while we wait for the results of ongoing studies.