Researchers from the University of Pittsburgh asked 20 Americans to do a diet switch with 20 Africans for two weeks.

The Americans were given a high-fibre, low-fat African-style diet with plenty of pulses and beans, while the rural Africans were fed a high-fat, low-fibre Western-style diet, liberal in burgers and fries.

It is not hard to guess which diet is better for us, but the surprise was in the speed our bodies respond to diet.

In comparison with their usual diets, the food changes resulted in "remarkable" changes in biomarkers of bowel cancer risk.

"In just two weeks, a change in diet from a Westernised composition to a traditional African high-fibre, low-fat diet reduced these biomarkers of cancer risk, indicating that it is likely never too late to modify the risk of colon cancer," lead researcher Dr Stephen O'Keefe says.

Researchers estimate that up to one-third of bowel cancer cases could be avoided by improving your diet. And fibre is fabulous for more than just our bowels. One new study found that eating more of it is one simple way to lose weight and improve your overall health.

"By changing one thing, people in the fibre group were able to improve their diet and lose weight and improve their overall markers for metabolic syndrome," says the study's lead author.

That said, the study is small and the results need to be put into context.

A similar new study found that just five days of eating a high-fat diet can change the way your body's muscle processes food.

"It is important to realise that any change in diet will bring about changes in the way your body metabolises foods," Associate Professor Amanda Salis, of Sydney University's Boden Institute of Obesity, Nutrition, Exercise and Eating Disorders says. "In the short term those changes are generally adaptive, in that they help your body to maintain a healthy status quo, and the responses subside when the previous diet is resumed."

Still, they show the powerful effect diet can have on our health and that it's never too late to improve our nutrition.

"I found it very encouraging that just two weeks of dietary changes for the better can bring about changes in health markers that indicate improvements," Prof Salis says of the new study, published in the journal Nature Communication.

"As for the negative effects, it is always a case that those dietary changes need to be sustained long-term for the changes in health to translate to long-term benefits."