Source:
Author: Laurie Barclay, MD & Hien T. Nghiem, MD

Calcium intake may protect against cancer, particularly gastrointestinal tract cancer, according to the results of a prospective study reported in the February 23 issue of the Archives of Internal Medicine.

“Dairy food and calcium intakes have been hypothesized to play roles that differ among individual cancer sites, but the evidence has been limited and inconsistent,” write Yikyung Park, ScD, from the National Cancer Institute in Bethesda, Maryland, and colleagues. “Moreover, their effect on cancer in total is unclear.”

In the National Institutes of Health-AARP (formerly known as the American Association of Retired Persons) Diet and Health Study, the investigators evaluated the association of dairy food and calcium intakes with incidence of total cancer and cancer at individual sites. A food frequency questionnaire was used to determine intakes of dairy food and calcium from foods and supplements.

Linkage with state cancer registries allowed identification of incident cancer cases. Relative risks and 2-sided 95% confidence intervals (CIs) were determined with a Cox proportional hazard model.

During follow-up (average, 7 years), 36,965 cancer cases were identified in men and 16,605 in women. In men, calcium intake was not associated with total cancer. However, calcium intake was nonlinearly associated with total cancer in women, with the risk decreasing up to approximately 1300 mg/day but with no further risk reduction above those levels.

Dairy food and calcium intakes were inversely associated with cancers of the digestive system in both men and women. Multivariate relative risk for the highest quintile of total calcium vs the lowest was 0.84 in men (95% CI, 0.77 – 0.92) and 0.77 in women (95% CI, 0.69 – 0.91). This reduction in the risk for gastrointestinal tract cancer was especially prominent for colorectal cancer, and supplemental calcium intake was also inversely associated with the risk for colorectal cancer.

“Our study suggests that calcium intake is associated with a lower risk of total cancer and cancers of the digestive system, especially colorectal cancer,” the study authors write.

Limitations of this study include failure to examine whether associations with intakes of dairy food and calcium differed by tumor subtype or tumor aggressiveness of site-specific cancers, possible residual confounding by unknown or unmeasured risk factors, limited statistical power to examine an association for some low-incidence cancers, and diet evaluated only once at baseline.

“Nevertheless, our study is one of the first cohort studies to examine dairy food and calcium intakes in relation to total cancer as well as low-incidence cancers,” the study authors concluded. “Moreover, our prospective design avoids the recall and selection biases that can affect results from case-control studies.”

The Intramural Research Program of the National Cancer Institute, National Institutes of Health, supported this study. The study authors have disclosed no relevant financial relationships.

Arch Intern Med. 2009;169:391-401.

Clinical Context
Because of the known effects of calcium on bone health, currently dietary guidelines recommend intakes of both calcium and dairy food. The Institute of Medicine recommends 1200 mg/day of calcium for adults 50 years and older, and the 2005 dietary guidelines for Americans recommend 3 cups per day of fat-free or low-fat dairy food. In addition, dairy food and calcium intakes have been hypothesized to play roles that differ among individual cancer sites, but the evidence has been limited and inconsistent. Moreover, their effect on cancer in total has been difficult to assess.

The aim of this study was to examine whether intakes of dairy food and calcium were associated with the risk for total cancer as well as cancer at multiple individual sites.

Study Highlights

  • In this large prospective cohort study, dairy food and calcium intakes in relationship to total cancer as well as cancer at individual sites were examined in the National Institutes of Health-AARP Diet and Health Study.
  • In 1995 and 1996, intakes of dairy food and calcium from foods and supplements were assessed with a baseline food frequency questionnaire.
  • Demographic characteristics were also assessed with the baseline questionnaire. Compared with participants in the lowest quintile of dairy food or total calcium intake, participants in the highest quintile were more likely to be white, non-Hispanic; college educated; physically active; current menopausal hormone therapy users if female, and to have a lower body mass index, but they were less likely to smoke cigarettes and to drink alcohol.
  • During follow-up from 1995 to 2003, incident cancer cases were identified through linkage with state cancer registries.
    A Cox proportional hazard model was used to estimate relative risks and 2-sided 95% CIs.
  • During an average of 7 years of follow-up, 36,965 and 16,605 cancer cases were identified in men and women, respectively.
  • Results demonstrated that total calcium intake was not related to total cancer in men but was nonlinearly associated with total cancer in women: the risk decreased with total calcium intake up to approximately 1300 mg/day, above which no further risk reduction was observed.
  • In addition, dairy food and dietary, supplemental, and total calcium intakes were not associated with total cancer mortality rates in both men and women.
  • In both men and women, dairy food and calcium intakes were inversely associated with cancers of the digestive system (multivariate relative risk for the highest quintile of total calcium vs the lowest, 0.84; 95% CI, 0.77 – 0.92 in men and 0.77; 95% CI, 0.69 – 0.91 in women), especially with colorectal cancer.
  • Supplemental calcium intake was also inversely associated with colorectal cancer risk.
  • Calcium intake was not related to breast, endometrial, ovarian, or prostate cancer.
  • Limitations to this study were that there was lack of evaluation for associations with intakes of dairy food and calcium on tumor subtype or tumor aggressiveness of site-specific cancers; residual confounding by unknown or unmeasured risk factors may exist for some cancers; in the analysis of low-incidence cancers, there was limited statistical power to examine an association; and because the diet was only assessed at baseline, it may not account for long-term usual intake as accurately as repeated measurements of diet during follow-up.

Pearls for Practice

  • Current dietary guidelines recommend 1200 mg/day of calcium for adults 50 years and older and 3 cups per day of fat-free or low-fat dairy food according to the Institute of Medicine and the 2005 dietary guidelines for Americans, respectively.
  • Calcium intake is associated with a lower risk for total cancer in women and cancers of the digestive system, especially colorectal cancer, in both men and women.