Studies have shown that vitamin D levels have an inverted U-shaped curve, where low levels (95.9 nmol/L). Population-based studies and randomized clinical trials have shown a U- or J-shaped curve and suggested an increased risk of adverse outcomes in those with the highest serum Vitamin D levels, including falls, fractures, and frailty. Most studies have reported a higher risk in those participants with serum levels above 100 nmol/L. Are the deleterious effects noted in these studies caused by elevated vitamin D? If so, your support for taking 5,000 to 10,000 IU of vitamin D3 daily seems risky. What is most important for us to know?

What you are referencing here are blood levels but not the corresponding intakes with respect to total amounts of vitamin D needed to obtain those average circulating concentrations of the vitamin. The correlations will be highly varied due to individual makeup so it will not be possible to estimate with great precision the vitamin D3 dose needed to reach a particular target range in circulating vitamin D. You can trust us to know what is typical and we have factored in the likelihood of getting a benefit while staying within a safe concentration range as an outcome that would provide a benefit without risk. It is true that too high a concentration in the blood, and hence the tissues, can have a number of downsides and this is a good thing to be aware of because it would be most unfortunate if, in the attempt to obtain the full potential benefits of high vitamin D exposure, it gets overdone through excess enthusiasm or ignorance and an intake of a toxic level is produced. At the present time, medical science is much more inclined to err on the side of caution because it have learned the hard way that, in ignorance, there are dangers in proceeding because false assumptions can lead to tragedy. This is the reason that recent guidelines for vitamin D intake continue to discourage clinical testing of vitamin D levels. We can tell you that this would lead to discovery of many, many subpar individuals who are suffering adverse consequences of inadequate vitamin D intake. There is wisdom in treating people as individuals and making a direct measurement to see where they are at and if there is a desire to take advantage of a higher exposure and get a greater benefit. This could be done with safe clinical assessment, through such testing, to stay within a safe range of serum levels. Given that vitamin D is only one of many, many things that can influence occurrence of active cancer and its worsening, there is a danger in people risking trouble by taking mega doses thinking it is akin to chemotherapy when it is not, it is a somewhat subtle influence, measurable statistically, and certainly a significant factor in individual cases where there is a good outcome that otherwise might not happen. But given the potential for harm, it is best to do high-dose ingestion with careful blood level determination monitoring. Such experience will show that daily doses of 5,000 to 10,000 IU vitamin D3 will rarely be harmful.