By Steven Reinberg
And for rectal cancer, obesity -- but not overweight --was tied to more than double the risk for girls, and 71 percent higher odds for boys, compared to normal-weight teens.
"This study is additional evidence that risk factors for colon cancer operate through the life course," said Dr. Andrew Chan, an associate professor of medicine at Harvard Medical School.
The findings "highlight the importance of maintaining a healthy body weight even in childhood," added Chan, who wasn't involved in the study.
According to the American Cancer Society, colon cancer is the third most common cancer diagnosis in U.S. men and women, excluding skin cancer.
About 95,500 new cases of colon cancer, and nearly 40,000 new cases of rectal cancer will be diagnosed in the United States this year, the society says.
With so many young Americans overweight or obese, concerns have been growing about the effect of excess weight on chronic disease, including cancer, later in life.
"When you are young, obesity is a disease that puts you at risk for many medical problems," said Dr. David Bernstein, chief of hepatology at Northwell Health's Center for Liver Diseases in Manhasset, N.Y.
"There is a well-documented link between obesity and colon cancer in adults," said Bernstein. "It makes sense that if you are obese when you are young, then you are going to have more problems when you are older."
Bernstein said it takes years to develop cancers, so it's not surprising that the effects of obesity in adolescents are seen in adulthood.
The new study was led by Dr. Zohar Levi, of Rabin Medical Center in Petah Tikva, Israel. Levi's team collected data on nearly 1.1 million Israeli men and more than 707,000 Israeli women. They had weight assessments at ages 16 to 19 between 1967 and 2002. Follow-up continued until 2012.
The final sample included almost 1.8 million participants, according to the study. The results were published online July 24 in the journal Cancer.
Over an average follow-up of 23 years, nearly 3,000 participants developed colon cancer, the researchers found.
Among men, about 1,400 had colon cancer, and nearly 600 had cancer of the rectum. Among women, more than 760 had colon cancer, and more than 220 had rectal cancer.
"This is a huge cohort with a minimum follow-up of 10 years, and all individuals had measured BMI [body mass index], not just reported or recalled," Levi said in a journal news release.
One limitation of the study is that participants were only an average age of 49 when their cancer was diagnosed, well before most colon cancer develops, the researchers said.
Also, while the study found a link between teenage obesity and adult colon cancer, it doesn't show a direct causal relationship.
In addition, the researchers had no data on diet, physical activity and smoking, which might have affected risk estimates. Nor did they have family medical histories, which might have shown a predisposition to colon cancer.
Bernstein said it hasn't been shown scientifically that losing weight can help reduce the risk. Still, "one should treat obesity," he said. "If obesity is a risk, then the only way to modify that risk is to lose weight. I don't know if it will help; it certainly can't hurt."