(RxWiki News) Vitamin D supplements are often suggested to help prevent a variety of conditions. But there have been concerns that too much vitamin D might be harmful.
A recent study looked at the connection between vitamin D levels in the blood and the risk for kidney stones. NIH guidelines currently recommend 600 IU per day of vitamin D which is smaller than the doses used in this study.
The researchers found that vitamin D was not linked to an increased risk for kidney stones. Older age, being male and a higher body mass index (BMI) were found to be risk factors associated with kidney stones.
BMI is a measure used to determine if someone is overweight or obese.
The researchers concluded that different doses of vitamin D can result in different levels of vitamin D in the blood, depending on a participant's BMI. The researchers suggested that previous findings linking vitamin D intake to increased risk of kidney stones may have been due to differences in participants' BMI.
"Tell your doctor if you're taking vitamin D supplements."
This study was conducted by Cedric F. Garland, DrPH, FACE, from the Department of Family and Preventive Medicine at the University of California, San Diego, and colleagues.
The researchers got participant data from a previously existing database of people who were taking a daily vitamin D supplement of approximately 3,600 IU per day and had an average 25(OH)D level of 45 nanograms per milliliter (measure of vitamin D in the blood). The database included information on vitamin D blood levels, demographic characteristics (race, sex, age, etc.) and health status.
Previous studies have suggested that a 25(OH)D level of 40 to 60 nanograms per milliliter can help reduce the risk for many conditions, such as breast cancer, colorectal cancer and diabetes. However, the authors of the current study explained that most people need vitamin D supplements to achieve a level of 40 nanograms per milliliter or higher.
This study included 2,012 participants from the database. These participants completed two questionnaires and provided two blood samples between December 2008 and March 2012. All ages and both sexes were eligible.
Kidney stone cases were determined by participant self-reporting, then confirmed by medical records, email correspondence or phone interviews. The researchers found a total of 13 cases of kidney stones in this study population.
The results determined that the rate of new kidney stones for the whole study population was four per 1,000 person-years (the number of participants multiplied by the length of time each participated in the study). The rate was three per 1,000 person-years among women and six per 1,000 person-years among men.
The researchers discovered that the average 25(OH)D level was 47 nanograms per milliliter for the cases of kidney stones and 50 nanograms per milliliter for the participants with no kidney stones.
The association between 25(OH)D levels and incidence of kidney stones was not found to be significant.
The researchers found that the participants who were male, were 55 years and older and had a high BMI were at greater risk for kidney stones than the rest of the participants. The participants with a BMI of 30 or higher (typically considered obese) were found to be 3.5 times more likely to develop kidney stones than the participants with a lower BMI.
Overall, the researchers did not find a significant association between kidney stones and 25(OH)D levels of 20 to 100 nanograms per milliliter.
The researchers believe that previous studies linking high vitamin D intake and kidney stones could have been due to study participants' having a high BMI and therefore needing to take more vitamin D.
The authors noted a few limitations of their study.
First, the researchers used self-reported data from a small group of people, and there was not a control group. Therefore, the findings may not be applicable to the general population.
Second, the people from the database who decided to send in the two questionnaires and take part in this study may have been more likely to be taking larger doses of vitamin D and more likely to stay on a specific supplement plan than the general population.
Third, the findings of this study may not be extremely significant because the incidence rate of kidney stones in the United States is already very low. Lastly, the follow-up period of this study was very short.
This study was published online on October 17 in the American Journal of Public Health.
Funding was provided by donations to GrassrootsHealth, a nonprofit public health research organization.