The amount of vitamin D that you should obtain through diet, supplements, and sun exposure depends on your vitamin D status, which is measured through serum 25(OH)D concentrations.

The Deutsche Gesellschaft für Ernährung (DGE - The German Nutrition Society) has suggested that 20 ng/mL is a sufficient concentration of 25(OH)D to prevent deficiency, and have recommended that adults obtain 800 IU of vitamin D in their diet or through supplements to achieve this goal.

Such recommendations are in contrast to the Endocrine Society who argue that less than 20 ng/mL is a deficiency, 20–29 ng/mL is an insufficiency, and more than 30 ng/mL is the goal (sufficiency). They suggest getting at least 1500 IU of vitamin D per day, with up to 10,000 IU per day being safe and effective for a short period of time to correct a deficiency if present.

From an evolutionary perspective, it's likely that we evolved with vitamin D concentrations similar to indigenous tribes in Africa: 25–68 ng/mL, averaging ~45 ng/mL. Solar exposure estimations have suggested that this equates to about 2000 IU per day of oral vitamin D.

Several large meta-analyses have also reported the lowest risk of death is among those with 25(OH)D levels above 30 ng/mL, particularly for cardiovascular disease. One of these meta-analyses has even suggested that 9.4% of all deaths in Europe and 12.8% of those in the United States could be attributed to vitamin D deficiency.

Conclusion: Official recommendations are very conservative. Many lines of logic suggest that a vitamin D status, as measured by 25(OH)D, of at least 30 ng/mL is ideal for optimal health, and that you may need to supplement 1500 to 10,000 IU of vitamin D per day to achieve this goal.

Can I Get Too Much Vitamin D?

The European Food Safety Authority (EFSA) has set official upper limits for oral vitamin D intake, called the No-Observed-Adverse-Effect-Level (NOAEL) and Upper Limit (UL). 

The NOAEL represents a dose for which no adverse effects have been observed over an extended period of time, and is set at 10,000 IU per day. The UL includes a safety factor that is believed to allow the dose to be safe when taken for life, and is set at 4,000 IU per day.

It’s important to keep in mind, however, that toxicity from vitamin D does not come from vitamin D itself, but from how that vitamin D affects your 25(OH)D concentrations. Toxicity occurs at levels exceeding 150 ng/mL, which takes substantial amounts of vitamin D over long periods of time to achieve.

In one case study, a 42-year-old man was hospitalized with 25(OH)D levels of 487 ng/mL after consuming anywhere from 156,000 to 2.6 million IU of vitamin D per day for two years. The primary adverse effect was hypercalcemia (elevated blood calcium levels), which quickly returned to normal upon ceasing supplementation, despite his 25(OH)D levels taking 10+ months to return to normal.

It’s worth noting that toxicity from sun exposure is not biologically possible for two primary reasons. First, previtamin D in the skin is photochemically converted into two biologically inert compounds, lumisterol and tachysterol, with continued sun exposure, which limits the amount of previtamin D available to convert to vitamin D. Second, sunlight destroys (photodegrades) vitamin D formed in the skin that can’t make it into circulation.

Conclusion: Vitamin D toxicity is possible but rare, as it requires enormous doses of oral vitamin D supplements that push 25(OH)D concentrations above 150 ng/mL. Taking up to 10,000 IU per day for extended periods of time is safe regardless of sun exposure.