The high-protein, low-carbohydrate diets many people turn to for weight loss might have potentially harmful long-term effects on the colon, a small study hints.

In a study of 17 obese men, UK researchers found that a protein-heavy, low-carb diet created certain changes in the colon that could, over time, contribute to colon cancer risk.

The study looked only at short-term shifts in certain compounds that are byproducts of metabolism, and not actual disease risk. So it does not show whether high-protein diets really raise the risk of any colon diseases.

But the findings raise that possibility, the researchers report in the American Journal of Clinical Nutrition.

“The concern raised by our studies is that the risk of colorectal cancer might be raised by long-term adherence to diets that are high in protein and low in carbohydrate, especially fiber,” senior researcher Dr. Harry J. Flint, of the University of Aberdeen, told Reuters Health in an email.

So what does that mean for people who want to lose weight?

Diets relatively high in protein and lower in carbs have been shown to help heavy people shed pounds. And, Flint and his colleagues point out, obesity is thought to be a risk factor for a number of diseases, including colon cancer.

“People should not be discouraged from losing weight, as this offers important health benefits,” Flint said.

However, he added, they should make sure that any weight loss plan they follow includes adequate amounts of fiber. People should also be aware, Flint said, that a high protein intake over months to years might have ill effects in the colon.

The findings are based on 17 obese men who each followed three short-term diets: a one-week menu plan designed to maintain their weight; a four-week high-protein diet with moderate amounts of carbohydrates; and a four-week high-protein diet low in carbs.

The first diet, which allowed about 360 grams of carbs per day, typically offered cereal, eggs and toast for breakfast; a sandwich and salad for lunch; and chicken, fish or soy, along with pasta, for dinner.

The low-carb diet - which allowed just 22 grams of carbs each day - generally consisted of eggs-and-bacon breakfasts, and lunches and dinners heavy in meat, poultry and fish, along with some vegetables and cheese.

The moderate-carbohydrate diet allowed 181 grams of carbs each day. Both high-protein diets contained just less than 140 grams of protein per day.

At the end of each diet period, Flint’s team analyzed fecal samples from the men to look at levels of certain metabolic byproducts.

On average, the study found, when the men were on the high-protein diets, they had higher levels of substances known as N-nitroso compounds, and certain other metabolites that have been linked to cancer.

And when they were on the high-protein, low-carb diet, they had lowered concentrations of fiber-derived compounds thought to be protective against cancer.

Exactly what those changes might mean for a person’s long-term health is not clear. But Flint said that the findings suggest that people should be cautious about consuming too much protein and too little fiber over a prolonged period.

People seeking to lose weight, he and his colleagues say, should be sure to get enough fiber in their diets. In general, experts recommend that adults get about 28 grams of fiber per day - though it’s not known whether that’s enough for someone on a high-protein weight-loss diet.

Flint and his colleagues are currently studying that question.

SOURCE: American Journal of Clinical Nutrition, online March 9, 2011.