Teenagers who were overweight at 17 were at significantly increased risk for developing colon cancer later in adulthood, and those who were obese were at increased risk for rectal cancer as well, according to a new report.

The study looked at nearly two million young Jewish Israelis who were weighed between 1967 and 2002, before induction to the Israeli army. Their weights were later checked against Israel’s national cancer registry.

Obesity is a known risk factor for colon cancer, but earlier studies have reported mixed results about whether being overweight during adolescence confers a risk. But while many of the earlier studies relied on recalled or self-reported body weight, the new study used actual weight measurements during health exams, said Dr. Gilad Twig, one of the study’s authors and a physician with the Israel Defense Forces.

The analysis, published in the journal Cancer, included just over one million males and 707,212 females. Over a median follow-up of 23 years, the investigators identified 1,977 cases of colon and rectal cancer among the men, and 990 cases among the women.

“Even if you are perfectly healthy at 17, but you are overweight or obese, we saw pretty much a 50 percent increased risk of cancer,” Dr. Twig said. “This should be a red flag for fighting adolescent obesity.”