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 / Updated  / Source: NBC News
By Maggie Fox and Judy Silverman

Just a two-week diet swap shows just how bad a Western diet of junk food is for us.

Americans who ate a traditional South African menu for two weeks showed big changes in their digestive system. And, frighteningly, South Africans who ate the foods usually eaten by a group of African-Americans from Pittsburgh showed digestive changes that could, in theory, lead to colon cancer.

It’s a small study and covered just a short period of time. But the researchers say they are struck at how clear the changes were and how short a time it took to change the inner workings of the gut with a change of diet.

“It will be important to know whether these types of dietary changes will have a meaningful effect not just on biomarkers but on actual rates of colon cancer over time,” said Dr. Andrew Chan, a gastroenterologist at Massachusetts General Hospital and an associate professor at Harvard Medical School, who was not involved in the study.

“Animal protein and fat intake was two to three times higher in Americans."

Colon cancer is the No. 2 cancer killer of men and women in the U.S, with 136,000 new cases a year, and 50,000 deaths, according to the American Cancer Society.

Rates are far higher in the U.S. than in South Africa, Colon cancer affects five out of every 100,000 rural South Africans. It’s diagnosed in 65 out of every 100,000 African Americans.

Doctors also know diet is a huge factor. Immigrants to the U.S. quickly develop high risks of colon cancer. But is it diet, or some other lifestyle factor such as exercise, smoking or even pollution?

Stephen O’Keefe of the University of Pittsburgh teased this out by comparing Americans to South Africans head to head.

First, they looked at the diets of 20 Pittsburgh-area black Americans and 20 rural South Africans.

“Animal protein and fat intake was two to three times higher in Americans, whereas carbohydrate and fiber, chiefly in the form of resistant starch, were higher in Africans,” they wrote in their report, published in the journal Nature Communications.

They did colonoscopies. As expected, the Americans had more of the pre-cancerous growths called polyps that can lead to cancer.

They looked at the microbes living in the gut, as well as compounds linked with digestion and metabolism. Americans had more factors associated with breaking down fat, while the Africans had more bacteria associated with fermentation and compounds called butyrates, which are known to affect colon health.

“This suggests that a move to a fiber rich, low-fat diet may impact the high levels of colon cancer in the African American population."

Then came the big test.

“We performed two-week food exchanges in subjects from the same populations, where African Americans were fed a high-fiber, low-fat African-style diet and rural Africans a high-fat, low-fiber western-style diet, under close supervision,” the researchers wrote.

“In comparison with their usual diets, the food changes resulted in remarkable reciprocal changes,” they added. Just two weeks of eating different food changed the types of bacteria living in the colon and what they did.

“This suggests that a move to a fiber rich, low-fat diet may impact the high levels of colon cancer in the African American population,” they concluded.

Chan wasn’t surprised. “Certainly there have been other studies in the past that have shown dietary changes can result in specific changes in colon that could influence colon cancer risk,” he told NBC News.

“There is a lot of biological plausibility to what they found. “

But O’Keefe and Chan both said it’s too small and too short a study to know for sure.

They’ll have to do a bigger, longer study to see.

Want to know what the volunteers ate?

The American diet:

  • Beef sausage, pancakes, breakfast steak, hash brown and Rice Krispies for breakfast.
  • Hamburgers, French fries, spaghetti and meatballs, hot dogs and chili for lunch
  • Meatloaf, Salisbury steak, noodles, mashed potatoes, roast beef, rice, macaroni and cheese for dinner.

The African diet:

  • Corn fritters, salmon croquettes, cheese grits, bananas, biscuits for breakfast
  • Catfish, mango, tater tots, kale salad, hush puppies for lunch
  • Okra, grits, lentils, pineapple, fish taco for dinner.