(CBS NEWS) Overweight 20-somethings dramatically increase their risk of esophageal and stomach cancer if they become obese later in life, a new study suggests.

The research indicated that people who were overweight in their 20s had a 60 percent to 80 percent increased risk of developing these cancers, compared with those who maintained a normal weight throughout their life, researchers said.

And those who then gained more than 40 pounds by age 50 doubled their risk of esophageal cancer and moderately increased their risk for stomach cancer, the study found.

But, it was those individuals who progressed from overweight at age 20 to obese at age 50 and older who had three times or more increased risk for esophageal and stomach cancer, the study found.

“These findings underscore the potential of weight control programs for decreasing the likelihood of developing esophageal and stomach cancer, which both have extremely poor survival,” said lead researcher Jessica Petrick, from the U.S. National Cancer Institute in Bethesda, Md.

Excess weight can trigger acid reflux problems and heartburn that can lead to cancer, Petrick said.

It can also change the levels of such hormones as estrogen and testosterone and can cause levels of insulin to rise and lead to inflammation, she said, noting all have been associated with increased cancer risk.

The report, based on data on more than 400,000 people, was published recently in the British Journal of Cancer.

However, Dr. Patrick Okolo, chief of gastroenterology at Lenox Hill Hospital in New York City, cautioned that the study did not prove that weight gain causes either cancer. There only appears to be an association between putting on the pounds and an increased risk for the rare cancers.

But, he added, “Without a doubt, there is an association between increased weight and cancer.

“If you gain weight like most adults, typically 40 pounds or more, you approximate the risk for these cancers of someone who has been heavy all their life. So not only is being heavy bad, but weight gain is also bad,” Okolo said.

Everyone should do everything in their power to maintain a healthy weight, he added.

“To those people I look after, I often preach to them that your desire to lose weight should not be based on these studies, but on the benefits on your quality of life. Behavior should often be driven by the immediate benefits, and if you gain those immediate benefits, then other long-term benefits will accrue to you in terms of cancer prevention,” Okolo said.

In the United States, esophageal cancer is rare, accounting for only 1 percent of all new cancers diagnosed each year, according to the U.S. National Cancer Institute. But, the five-year survival rate averages only about 18 percent, because it’s a cancer often diagnosed at an advanced stage.

Stomach cancer, likewise, is also rare, accounting for fewer than 2 percent of all new cancer cases each year. The five-year survival rate for stomach cancer is also relatively low, at about 30 percent, the institute says.