In a study that followed more than 120,000 Dutch adults for 16 years, researchers found that smoking increased the risk of the two main forms of stomach cancer, as well as the two forms of esophageal cancer -- by anywhere from 60 percent to 263 percent versus non-smokers.
Alcohol, in contrast, affected only the risk of esophageal squamous cell carcinoma, the form found in the upper part of the esophagus.
People who, at the study's start, drank more than 30 grams of alcohol per day -- equivalent to two to three glasses of wine -- were nearly five times more likely to develop the cancer than abstainers were.
The findings, published in the journal Gut, underscore general health recommendations to drink only in moderation and to quit, or preferably never start, smoking.
In addition, they also suggest that smoking and drinking cannot account for the changing face of esophageal and stomach cancers in Western countries, according to Jessie Steevens of Maastricht University in the Netherlands and co-investigators.
In recent decades, the rate of esophageal adenocarcinoma, the other major form of esophageal cancer, has been rising in the U.S. and Europe. A similar trend has been seen with gastric cardia adenocarcinoma. (Most stomach cancers are adenocarcinomas, which arise in the stomach lining; gastric cardia adenocarcinoma and non-cardia adenocarcinoma are the two forms.)
However, because alcohol is not associated with either cancer, changes in drinking habits cannot explain these increases, Steevens and her colleagues point out. Nor can smoking, since rates of the habit have not been rising in Western countries.
"Therefore," the researchers write, "we suggest that further research should focus on other risk factors for (esophageal) and gastric cancer subtypes, to search for explanations for these increases."
The findings are based on a long-term study of 120,852 Dutch adults who were between the ages of 55 and 70 at the outset, in 1986. At that time, participants completed detailed questionnaires on their diets and other lifestyle habits.
Over the next 16 years, 491 people developed non-cardia adenocarcinoma of the stomach, and 164 were diagnosed with a cardia adenocarcinoma. Another 107 developed esophageal squamous cell carcinoma, while 145 developed esophageal adenocarcinoma.
People who, at the study's start, said they currently smoked had higher risks of all four cancers than those who had never smoked.
Former smokers also had elevated risks, though they were lower than current smokers' -- around 40 percent for each cancer, versus people who had never smoked.
Esophageal squamous cell carcinoma was the only cancer for which smoking and drinking were both risk factors. What's more, the two habits showed additive effects; current smokers who drank more than the equivalent of a glass or two of wine per day were eight times more likely to develop the cancer than non-smokers who drank little to no alcohol.
There was no such "synergistic" effect seen for the other three cancers, according to the researchers.
Other known or suspected risk factors for stomach cancer include family history of the disease, diets high in salted, smoked or pickled foods, and infection with the ulcer-causing bacterium H. pylori (though most people with this common infection never develop the cancer).
For esophageal cancer, other risk factors include obesity and chronic acid reflux. Men have higher rates of both stomach and esophageal cancers than women do.