Colorectal cancer is the 4th most commonly diagnosed cancer in the United States and causes the 2nd highest number of cancer-related deaths each year (1).
Colorectal cancer often begins as polyp on the inner lining of the colon (aka large intestine) or the rectum (the last 6 inches of the large intestine). Polyps are detected during a colonoscopy and if detected, they can be removed.
Two common types of polyps are adenomatous and serrated. Adenomatous tend to grow slowly. Serrated may grow quickly such that someone with a “clean” colonoscopy could develop a large polyp or even cancer in the interval between colonoscopies (2). Serrated polyps are believed to be involved in 10-30% of cases of colorectal cancer (3).
A recent study raises questions about the effect of calcium supplements on the risk of serrated polyps (4). Researchers studied 2,058 people who had already been diagnosed with a conventional adenoma. They were randomized to one of four groups:
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- 1,200 mg supplement of calcium per day
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- 1,000 IU supplement of vitamin D per day
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- 1,200 mg of calcium plus 1,000 IU of vitamin D per day
- Placebo with neither calcium or vitamin D
They received their assigned treatment for 3 to 5 years, had a follow-up colonoscopy, and then stopped taking the supplement. They were then observed for at least 3 more years, at the end of which they had another colonoscopy. At the end of the placebo period, there was no significant difference between the groups in terms of serrated polyps. But at the end of the study—at least 3 years after the supplements were discontinued—subjects who had received either calcium supplements, either alone or in combination with vitamin D, had a higher risk of serrated polyps, 2.65 (1.43–4.91) for calcium and 3.81 (1.25–11.64) for calcium plus vitamin D.
Women and current smokers who were in the groups receiving the calcium supplements had the highest risk. Dietary calcium intake did not appear to affect risk.
It’s important to keep several things in mind:
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- This is a single study and others have had different results. A meta-analysis of observational and interventional studies found higher overall intakes of calcium to be associated with a decreased risk of serrated polyps (3) and an observational study found no association of calcium intake with risk of serrated polyps (5)—neither specifically studied calcium supplements.
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- There’s relatively strong evidence that tobacco smoking, higher alcohol intake, a higher body mass index, high fat intake, and high red meat intake increases risk for serrated polyps (3). Addressing these issues may help reduce an individual’s risk.
- The researchers only looked at a calcium supplement providing 1,200 mg of calcium per day—we generally recommend much lower amounts if supplementing.
It’s important to see if other researchers are able to replicate this study’s results. The authors suggest that those with serrated polyps, especially women and smokers, should avoid calcium supplements (4).
Because of calcium’s role in preventing osteoporosis, those who aren’t using calcium supplements should be aware of good sources and strive for an adequate calcium intake—see Daily Needs for our most up-to-date calcium recommendations and Calcium Part 2—Research for a more thorough discussion of calcium needs of vegans.
References
1. SEER Cancer Stat Facts: Colorectal Cancer. National Cancer Institute. Bethesda, MD.
