In the small study of 17 obese men, UK researchers found that a protein-heavy, low-carb diet created certain changes in the colon that could potentially contribute to colon cancer risk.
To investigate, researchers assigned 17 obese men to 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 allowed just 22 grams of carbs each day and 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, researchers 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.
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.
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