A new study has reported that intake of up to 4,000 IU vitamin D per day is unlikely to result in vitamin D toxicity in the general population and may be required for most people to reduce their cancer risk. The study was designed to investigate the relationship between vitamin D supplementation above 1,000 IU per day and circulating levels of vitamin D. The blood test for vitamin D measures the level of 25-hydroxyvitamin D (25(OH)D), which is the primary metabolic product of vitamin D3 in the blood. Studies have reported that intake of vitamin D from 1,100 to 4,000 IU per day and a serum 25(OH)D concentration from 60 to 80 ng/ml may be required to reduce cancer risk, including risk of breast cancer. However, few population studies have estimated the dose-response relationship between oral intake of vitamin D (i.e., vitamin D supplements) above 1,000 IU per day and corresponding circulating 25(OH)D.

The study included 3,667 participants with average age 51 years who were taking 1,000 to 10,000 IU of vitamin D per day. Study participants self-reported their vitamin D supplement intake and blood tests were performed to determine the level of circulating vitamin D. Serum 25(OH)D was found to rise as a function of vitamin D supplement intake. All intake levels up to 10,000 IU per day were consistently found to be safe, never resulting in 25(OH)D values above 200 ng/ml, which is considered a potentially harmful level. The supplemental dose estimated to ensure that 97.5% of this population achieve a serum 25(OH)D of at least 40 ng/ml was found to be 9,600 IU per day.

Please see our comprehensive article on vitamin D and breast cancer for more information on the relationship between vitamin D and breast cancer risk and prognosis.