Chewing tobacco, or smokeless tobacco, is placed between the cheek and gum, delivering nicotine and other chemicals without combustion. Users hold the tobacco in their mouth, but some portion of the accumulated juices and saliva is always swallowed. This raises questions about the product’s effect on the entire digestive system, particularly the lower gastrointestinal tract. While the link to oral and esophageal cancer is well-established, the specific connection between chewing tobacco and colon cancer remains an important area of scientific inquiry. This article explores the toxic agents in smokeless tobacco, their pathway to the colon, and the current scientific understanding of the associated colorectal cancer risk.
Carcinogenic Agents in Smokeless Tobacco
Smokeless tobacco products contain at least 28 different cancer-causing chemicals, with the most potent group being the Tobacco-Specific Nitrosamines (TSNAs). These harmful compounds form naturally during the curing, fermenting, and aging of the tobacco leaf, specifically from a reaction between tobacco’s natural alkaloids and nitrites. The levels of TSNAs vary widely depending on the specific product and its processing method. Among the TSNAs, N-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) are considered the most significant carcinogens. The concentration of these nitrosamines is directly related to the overall cancer risk posed by the product.
The Pathway to the Gastrointestinal Tract
The method of chewing tobacco use introduces a distinct exposure route for the body’s internal organs. Although nicotine is absorbed directly through the mucous membranes lining the mouth, the cancer-causing agents take a different path that involves the digestive system. As the user chews or holds the tobacco, it mixes with saliva, creating a highly concentrated juice of chemicals and carcinogens. A significant portion of this chemically-laden saliva is swallowed by the user, leading to the ingestion of TSNAs and other toxic substances. This swallowing process exposes the upper gastrointestinal tract, including the esophagus and stomach, to high concentrations of carcinogens before they continue their journey through the small intestine and ultimately into the large intestine, which includes the colon and rectum. This direct, prolonged exposure of the digestive lining to known carcinogens is the hypothesized mechanism for any associated lower gastrointestinal cancer risk.
Scientific Consensus on Colorectal Cancer Risk
The evidence linking smokeless tobacco use directly to colorectal cancer is not as definitive as the links to other cancers. While smoking is an established risk factor for colorectal cancer, studies examining smokeless tobacco have yielded mixed or inconclusive results. Some large-scale cohort studies, such as one involving American veterans, have suggested a significantly elevated risk specifically for rectal cancer among users of chewing tobacco or snuff. Another analysis of a large population of Swedish men found no clear association between smokeless tobacco use and colorectal cancer overall. However, that same study noted an increased risk for rectal cancer among exclusive current users. The distinction between colon and rectal cancer is sometimes seen in the data, with the rectum, being the final segment of the large intestine, showing a slightly more consistent, albeit modest, association.
Associated Gastrointestinal and Related Cancer Risks
Beyond colon cancer, the use of chewing tobacco is strongly associated with a range of other cancers, particularly those in the upper digestive tract. The most well-established health risk is for cancers of the oral cavity, including the tongue, gums, and cheek. The constant, direct contact between the tobacco product and the oral tissues causes cellular changes that can lead to malignancy. Smokeless tobacco also significantly increases the risk for cancers of the pharynx, esophagus, and pancreas. The risk for these cancers is often higher than for colorectal cancer because the upper parts of the digestive system have more immediate and prolonged contact with the concentrated carcinogens as they are ingested.