No one looks forward to that first colonoscopy, but this glimpse into the gut is one of the most powerful existing weapons against colon cancer. Yet current protocol for when to start checking for the disease may be too late for many men and may put many women through an expensive and unnecessary ordeal, a new study suggests.

Doctors currently advise men and women with no family history of colon cancer or other risk factors to start undergoing screening at age 50, and sooner for those deemed more at risk. But this sweeping guideline does not account for individual genetic and lifestyle differences. To calculate the ideal age for the first screening, researchers at the Fred Hutchinson Cancer Research Center in Seattle and their colleagues analyzed patient data detailing 19 behavioral patterns—including exercise, alcohol and red meat consumption, body mass index and aspirin use—and 63 genetic markers associated with colorectal cancer.

The results suggest that 15 percent of men with no family history of the disease should start getting scoped before age 45, whereas half of women with no family history could wait until they are at least 56—and 10 percent of those could start as late as 64. Thirteen years of data from participants of European descent showed that hormone replacement therapy reduced women's cancer risk, that men were more likely to engage in risky behaviors such as drinking and smoking, and that being overweight was a higher risk for men than women.

The study's findings, published in June in Gastroenterology, also call into question the assumption that a family history of the disease always calls for early screening. The researchers found that more than half of women—and 15 percent of men—with a family history could wait until 50 for a first colonoscopy. These findings are a step toward individualized screening guidelines but should not be considered medical advice, says Jihyoun Jeon of the University of Michigan, the study's lead author.

“The study is significant because [disease] models don't usually combine both genetics and habits to predict colon cancer risk,” says Brian Wells, a biostatistician at the Wake Forest School of Medicine, who was not involved in the work. “But the authors did not tell us how many colonoscopies could be avoided and how many colorectal cancers could be prevented using this model and how this compares with the current guidelines. This comparison is needed to evaluate risks versus benefits for the real world.”