Higher consumption of trans fatty acids can increase a person's risk of precancerous colorectal tumors by 86 percent, researchers have found.

"These results suggest that consumption of high amounts of trans-fatty acid may increase the risk of colorectal neoplasia [abnormal cell growth], and they provide additional support to recommendations to limit trans-fatty acid consumption," the researchers wrote in the American Journal of Epidemiology.

Between 2001 and 2002, researchers from the University of North Carolina-Chapel Hill interviewed 622 people about their diet, lifestyle and demographic information between, then gave each of them a complete colonoscopy. They found that participants with the highest intake of trans fats were 86 percent more likely to have colorectal adenomas than those with the lowest trans fat consumption.

Colorectal adenomas are small, benign tumors or polyps in the colon and rectum that can progress into cancer if not treated. While consumption of trans fats increased the risk of developing adenoma, it did not appear to affect the number, size or location of the tumors.

Trans fats are a particular type of fat that occurs naturally only in very trace amounts in meat and dairy products. The bulk of trans fats in the modern diet are synthetically produced by the partial hydrogenation of vegetable oils. Unlike other fats, trans fats have no nutritional value to the human body. Nevertheless, they have come to be widely used in restaurants and packaged foods, because they hold flavor longer and have a greater shelf life than non-hydrogenated oils.

The current study is not the first to link trans fats to colorectal cancer, which affects 150,000 new people per year in the United States, making it the country's third most commonly diagnosed cancer. Prior research has linked markers of trans fat consumption to an elevated colorectal cancer risk.

While research into trans fats' contribution to cancer risk is a fairly new line of investigations, the fats' damaging effects on cardiovascular health are well documented. Trans fats raise the body's levels of LDL ("bad") cholesterol while lowering levels of HDL ("good") cholesterol, and increase the risk of other factors involved in cardiovascular disease, including inflammation and irregular heartbeat.

Studies have also linked the fats to reduced fertility in women, suggesting that eating even four grams of the fats per day (an amount easily consumed in a single serving of French fries) could double the risk of infertility due to ovulation failure.

Leading food and health expert Walter Willett of Harvard Medical School has called trans fats the "worst single specific problem" facing health in the United States today, as well as "the biggest food processing disaster in U.S. history."

Concern over the health effects of trans fats has led cities such as New York and Philadelphia to prohibit their use in restaurants, and other major cities such as Boston and Chicago are considering similar bans. A ban on the use of trans fats by all food facilities is due to take effect in California in 2010, and to be complete by 2011.

Willett has called for the ban to be made nationwide, and to extend to all kinds of food products, not only restaurants. He cited the example of Denmark, which imposed a de facto trans fat ban in 2004 by mandating that any local or imported food product sold in the country contain no more than 2 percent artificially hydrogenated oils.

A handful of restaurant chains have pledged to remove trans fats from their products, and the new study is intended to increase the pressure on the food industry as a whole to phase them out.

Currently, U.S. law requires that packaged foods declare the trans fat content per serving on nutritional labels, but quantities below 0.5 grams per serving do not have to be reported. Another way to tell if foods contain trans fats is to look for hydrogenated oils in the ingredients list.