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Diet swap study highlights bowel effects of western-style diet

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“Diet swap experiment reveals junk food’s harm to guts,” BBC News reports.

20 American volunteers were asked to eat an African-style diet (high fibre and low fat) while 20 Africans were asked to eat a typical American-style diet (low fibre and high fat). The western diet seemed to contain more red and processed meat.

The researchers found that after just two weeks, both diets led to biological changes in the guts of both groups, such as changes in the microbes present and levels of inflammation.

The African-style diet led to changes that were suggested to possibly contribute to reduced bowel cancer (also known as colon cancer) risk in the long term, while the opposite was true of the western-style diet.

However, it was a very short-term study, which only looked at biological changes in the gut, and the authors say they can’t be certain that these led to the changes in bowel cancer risk.

That said, there is the striking figure that Americans are around 13 times more likely to develop bowel cancer than Africans, with similar rates existing in most western countries. There is also evidence that when non-western populations adopt a more westernised diet, there is a corresponding rise in bowel cancer cases.

The Department of Health advises people who eat more than 90 grams (g) of red and processed meat (cooked weight) a day to cut down to 70g, to help reduce their bowel cancer risk.

Why is fibre important?

Fibre is an important part of a healthy diet. It can help to prevent heart disease, diabetes, weight gain and some cancers, and can also improve digestive health.

However, many people don’t get enough fibre. On average, most people in the UK get about 14g of fibre a day. You should aim for at least 18g a day.

Fibre is only found in foods that come from plants. Foods such as meat, fish and dairy products don’t contain any fibre.

There are two different types of fibre – soluble and insoluble. Each type of fibre helps your body in different ways, so a normal, healthy diet should include both types.

Read more about the importance of fibre.

Where did the story come from?

The study was carried out by researchers from the University of Pittsburgh and other research centres in the US, Europe and South Africa. It was funded by the US National Institutes of Health, the UK National Institute for Health Research, the Academy of Medical Sciences, the Netherlands Organization (de Vos) for Scientific Research, the European Research Council and the Academy of Finland. The study was published in the peer-reviewed journal Nature Communications.

The news headlines generally focus on the effects of these diets on cancer risk – not making it clear that this study was not looking at cancer directly. Instead, it was looking at a range of indicators – biomarkers – that may provide an indication of how healthy a person’s digestive system is.

The BBC bucks this trend, with a more representative headline “Diet swap experiment reveals junk food’s harm to gut”, although the study was not specifically looking at junk food.

Some sources took a positive interpretation of the results, such as The Independent, which told us that “Adopting high-fibre diet could dramatically cut risk of bowel cancer”. Others took a more negative approach, such as the Daily Express, whose headline was “Western diets can raise your cancer risk after just two weeks”. While the study did find bowel changes after two weeks, we don’t know if these changes directly raise cancer risk or whether they remained after people changed back to their normal diet.

What kind of research was this?

This was an experimental study looking at the effects of two different diets – those of African-Americans and rural Africans – on the gut. Rural South Africans have much lower rates of bowel cancer than African-Americans – with fewer than 5 people per 100,000 affected, as opposed to 65 per 100,000 African-Americans.

Dietary differences are likely to be responsible for this difference, and the researchers wanted to see what the effect of the typical diets of these groups had on the gut. They did this by getting these two groups to effectively switch diets for two weeks and seeing what happened.

This study is appropriate for looking at short-term effects of diet on the gut, which might be related to cancer risk if the diet was maintained in the long term.

However, a long-term study would be unethical, as you would be exposing some people to a diet that you know, or at least strongly suspect, is unhealthy.

What did the research involve?

The researchers recruited 20 healthy African-Americans aged 50 to 65 years old, living in the US, and an age and sex-matched group of 20 South Africans living in a rural area. They were first assessed over a two-week period, where they ate their normal diet at home. They then switched to the “opposite” diet – either a western-style diet or a rural African-style diet provided by the researchers. The researchers then assessed what affect this had on their gut.

The rural African-style diet increased average fibre intake among the African-Americans from 14g to 55g per day, and reduced their fat from 35% to 16% of their total calorie intake. The western-style diet reduced fibre intake among the rural Africans from 66g to 12g per day, and increased their fat intake from 16% to 52% of their total calorie intake.

During this part of the study, the participants lived in research facilities and had their meals prepared for them. The meals were also designed to be appealing to participants. While there was some “junk food” in the western-style diet used in the study (hamburger, fries and hot dogs), there were also some healthier meals, such as chilli, rice and stuffed bell peppers. The rural African-style diet also included some foods that would not be traditionally served in Africa – such as vegetarian corn dogs and hushpuppies (a fried or baked ball of cornmeal batter). From the sample menus reported in the study, the western-style menus appeared to include more red and processed meat than the African-style meals – with the latter including more fish.

The investigations the researchers performed included collecting faecal samples to test them for bacteria and chemical by-products of digestion, and carrying out colonoscopies (where a small tube containing a light and a camera is inserted via the rectum to observe the bowel wall).

What were the basic results?

In their normal diet, the African-Americans ate two to three times more protein and fat than the rural Africans. In contrast, fibre intake was higher in the rural Africans’ diets. The cells in the walls of the African-Americans’ colons were dividing more than those in the rural Africans.

The researchers found that switching African-Americans to the high-fibre, low-fat diet led to an increase in the fermentation of sugars in their gut. This indicated a change in the microbes in the gut that are responsible for this process, and this was supported by testing which microbes were present.

There was also a reduction in the production of certain bile acids in the rural African diet. Some animal studies have suggested that these bile acids can promote cells to become cancerous, and human studies were also reported to have found that higher levels are linked with increased colon cancer risk. There was also a reduction in signs of inflammation of the walls of the colon, and the cells in the colon wall stopped dividing as quickly. Again, these changes could potentially predict lower cancer risk.

The opposite changes were observed in the rural Africans when they switched to a western-style diet.

How did the researchers interpret the results?

The researchers concluded that “in individuals from high-risk and from low-risk cancer populations, changes in the food content of fibre and fat had remarkable effects on their [bowel bacteria and metabolites] within two weeks, and, critically, that these changes were associated with significant changes in inflammation and proliferation [of the bowel lining]”. They say these changes may not lead to changes in bowel cancer risk, but state that other studies suggest there could be links.


This study aimed to investigate various biological changes to the gut that occur when switching from a western-style low-fibre, high-fat diet to an African-style high-fibre, low-fat diet, and vice versa. These changes may partly explain why African-Americans living in the US have over 10 times the bowel cancer rate of rural Africans.

The differences seen may not solely have been due to the differences in fibre and fat. The western-style diet also appeared to contain more red and processed meat, which have also been linked to increased bowel cancer risk. It is also worth bearing in mind that this study only took place over two weeks, and the longer-term effects of these diets on the colon were not studied. The authors themselves acknowledge that they can’t be sure the changes they saw would directly lead to changes in cancer risk. However, other research suggests they might be if they were present in the long term.

The other limitations are that the study was relatively small and only included healthy middle-aged and older adults of African origin, so may not apply to the wider population. 

Overall, the results do not contradict current advice that consuming a high-fibre diet can reduce your bowel cancer risk. Meanwhile, obesity and a diet high in red and processed meat have been shown to increase bowel cancer risk.

Read more about bowel cancer prevention.

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