Increased calcium intake is associated with a reduced risk for primary hyperparathyroidism in women, according to an analysis of data from the Nurses’ Health Study published on Oct. 18.
In what the researchers called the first prospective study to assess the link between hyperparathyroidism and calcium intake, Dr. Julie M. Paik of Brigham and Women’s Hospital in Boston and her colleagues looked at 58,354 women from the Nurses’ Health Study I, an ongoing, prospective cohort study that began in 1976. Participants in the study complete questionnaires every 2 years.
Dietary intake was assessed using the food frequency questionnaire, which asked about participants’ average intake of more than 130 individual food items and 22 beverages during the previous year.
The survey also asked about calcium supplements, vitamin D supplements, and multivitamins, with calcium intake determined by the brand, type, and frequency of use.
Only cases of primary hyperparathyroidism diagnosed between the date on which the 1986 questionnaire was returned and May 31, 2008, were included in the study.
Overall, during 1,475,978 person-years of follow-up, there were 277 cases of incident primary hyperparathyroidism.
After the data were adjusted for age, women in the highest quintile of dietary intake had a relative risk for developing hyperparathyroidism of 0.61, compared with women in the lowest quintile (95% confidence interval, 0.42-0.90; P = .03), the investigators reported (BMJ 2012;345:e6390).
Further adjustments for body mass index, race, smoking, calcium supplement use, intake of vitamin D, dietary intake of vitamin A and protein, alcohol intake, and diuretic use only strengthened the effect: The relative risk among the highest quintile of intake was 0.56, by this calculation, compared with patients in the lowest intake category (95% CI, 0.37-0.86; P = .009).
The authors also looked at supplemental calcium intake, although "because of an insufficient number of cases of primary hyperparathyroidism among the different categories of supplemental calcium use in the earlier time periods, our analysis on the relation between supplemental calcium intake and primary hyperparathyroidism began in 1994 with follow-up until 2008," wrote Dr. Paik and her coinvestigators.
In this analysis, which included 985,628 person-years of follow-up, there were a total of 257 cases of incident primary hyperparathyroidism.
When the findings were adjusted for age, "the relative risk for women taking more than 500 mg/day of calcium supplements compared with those not taking calcium supplements was 0.69 (95% CI, 0.50-0.94; P less than .001)," wrote the authors.
As with dietary intake, the protective effect was only strengthened in multivariable analysis, including adjustment for dietary calcium: The higher-intake group’s risk was 0.41, compared with the lower-intake group (95% CI, 0.29-0.60; P less than .001).
The authors conceded that the study population was exclusively female and almost entirely white, making the findings not necessarily generalizable to men or to women of other races.
Additionally, "although the food frequency questionnaires have been well validated, calcium intake was not perfectly assessed in this study," they wrote.
"However, because of the prospective design, any misclassification would be random with respect to case status, and therefore would probably underestimate the magnitude of the inverse association between calcium intake and risk of primary hyperparathyroidism."
The study investigators stated that they had no relevant financial disclosures. The study was supported by grants from the National Institutes of Health.