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Resistant starch may reduce colon cancer risk from red meat.

A diet high in red meat increases risk of colon cancer. But eating resistant starch—a carbohydrate that acts like fiber—may reduce that risk, according to a study in August’s Cancer Prevention Research. “Red meat and resistant starch have opposite effects on the colorectal cancer– promoting microRNAs,” said first author Karen Humphreys, Ph.D., a research associate at the Flinders Centre for Innovation in Cancer at Australia’s Flinders University. “This finding supports consumption of resistant starch as a means of reducing the risk.” In a 2011 study, colon cancer risk increased by 29% for every 100 g of red or processed meat eaten per day, plateauing around 140 g. But no one knew exactly why. This new study offers insights, said Sonia Kupfer, M.D., director of the Gastrointestinal Cancer Risk and Prevention Clinic at the University of Chicago Medical Center. “It may be that protective factors and risk factors work through the same pathways.” She noted other possible mechanisms: “Heme iron and heterocyclic amines from red meat cooked at high temperature alter gene expression and increase proliferation. Butyrate from microbes decreases proliferation. This paper starts to provide intriguing ways in which we can think about these dietary factors mechanistically altering the colonic epithelium. But there might be other effects as well,” she said. “Dietary substances can be complex.” Unlike most carbohydrates, resistant starch passes undigested to the colon. There, gut microbes ferment it, yielding short-chain fatty acids, such as butyrate, which promote colon health. Those shortchain fatty acids, the study suggests, also reduce expression of microRNAs that are associated with severe colon cancer and that increase cell proliferation. The study involved 23 healthy volunteers, aged 50–75 years. Participants were randomly assigned to either a high-redmeat diet (300 g raw per day of lean beef or lamb) or that same diet plus a resistantstarch supplement called StarPlus (40 g per day of butyrylated high-amylose maize starch). After 4 weeks on one diet, participants switched to the other for another 4 weeks. For 4 weeks before each intervention, participants ate normally. After each phase, researchers took fecal and pinch biopsy samples of rectal mucosa. They measured levels of butyrate and other short-chain fatty acids; proliferation; microRNA expression; and target genes of those microRNAs, including cell cycle inhibitor CDKN1A and proapoptotic genes PTEN and BCL2L11. A high-red-meat diet statistically significantly increased cell proliferation in the mucosa. This increase corresponded with higher expression of oncogenic microRNAs (the miR17-92 cluster and miR21). After 4 weeks on the highred-meat diet, miR17-92 levels increased by a mean of 30%. Adding resistant starch to the meat diet reversed the increase in the miR17-92 cluster, but not in miR21. in Stanford, Calif. Her lab is also in the early stages of growing and characterizing patients’ CTCs in a dish. Jeffrey sees researchers combining CTC assays with circulating tumor DNA (ctDNA) assays to make diagnosis and treatment decisions. “We can use ctDNA to monitor patients and predict tumor recurrence, but this doesn’t tell you which drugs should then be used for treatment. That is where CTCs come in—the ability to culture and directly do drug sensitivity testing, while not currently possible, is an attractive future idea,” she said. “[Drug sensitivity, CTC and ctDNA assays] are a nice union of many fields,” said Luis Diaz, M.D., associate professor of oncology at Johns Hopkins University in Baltimore. Diaz develops genomic diagnostic and therapeutic approaches for cancer, including ctDNA assays. He is cautiously optimistic about cultured CTCs and would like to see a comparison of CTC cultures with those grown directly from tumor samples. “The major questions I have are, what do these cells really represent in terms of the tumor, and what fraction of patients will have CTCs from which cultures can be formed?” Diaz said. According to Diaz, cultured CTCs and CTC-derived xenografts are another version of mouse avatars—xenografts grown from individual tumors for drug screening: “The jury is still out on whether this will work well in the clinic and the use of these assays will depend on patients’ clinical outcomes.” So far, such studies are also costly. But researchers working on culturing CTCs are optimistic. “We are convinced that CTCs will be used to identify effective therapies for a patient and as a real-time biopsy,” Marchetti said. Jeffrey agreed: “There is a lot we still don’t know about this technique, but it’s extraordinarily exciting. This is going to be an important tool for precision-based medicine and drug testing.”