New study findings by medical scientist in the United States say there is a significant decline in incidence of people aged over 50 years developing colon cancer. The same study raises alarm over rising colon cancer incidence among younger people, although the new phenomenon is attributed to increased screening.

The study findings that appear in BMC Medicine found that a diet high in healthy plant-based foods – whole grains, fresh fruit, and vegetables – is associated with a lower risk of colorectal cancer in men.

Significantly, the findings were published barely days before Brazil soccer icon Edson Arantes do Nascimento Pele died of cancer, which doctors at Albert Einstein Israelite Hospital in Sao Paulo, where he had been admitted described it as colorectal cancer. The cancer is also known as bowel, colon or rectal cancer, described as the third most commonly diagnosed and the second deadliest cancer in the United States.

Most people who receive a colorectal cancer diagnosis are over the age of 50, although it can affect younger people, too, the study says. The risk of colorectal cancer increases with age. Other risk factors people cannot influence are a family history of colorectal cancer, inflammatory bowel diseases — such as Crohn’s disease – and certain genetic syndromes.

In recent years, it adds, cases in older people have started to decline, but the incidence among younger people is increasing. However, these changes may be due to more effective cancer screening.

There are, however, many lifestyle factors that also influence a person’s risk of colorectal cancer. Factors that are likely to increase the risk include: a diet low in fiber, fruit and vegetables, lack of physical activity, a diet high in fat and red or processed meat, overweight and obesity, tobacco use and heavy alcohol consumption.

Several studies have investigated the relationship between diet and colorectal cancer, finding that the typical Western diet that is high in fat, red meat and processed meat increases the risk. Reducing these foods and increasing foods high in dietary fibre is associated with a reduction in risk. Plant-based foods tend to be high in dietary fibre, but only in an unprocessed state, the study shows.

Unhealthy plant-based foods – refined grains, fruit juices, and added sugars – had no beneficial effect on cancer risk.

“This American study adds to lots of existing evidence on the benefits of eating a balanced diet high in fruit, vegetables and fibre for both men and women,” explains Beth Vincent, health information manager at Cancer Research UK (CRUK)

The study group included 79,952 men and 93,475 women who were followed up for an average of 19.2 years. All participants were from Hawaii or the Los Angeles area and were aged between 45 and 75 years at enrolment. The group included people of African American, Japanese American, Native Hawaiian, Latinx and white volunteers.

At the start of the study, researchers assessed participants’ usual diet with a self-reported questionnaire. Participants had to report how often and how much they ate out of more than 180 different foods and beverages. They chose from four portion size options, and frequencies ranging from never to four times a day.

From the responses, the researchers calculated daily energy and nutrient intakes, then calculated three plant-based diet indices – overall (PDI), healthful (hPDI) and unhealthful (uPDI).

The researchers defined whole grains, fruits, vegetables, vegetable oils, nuts, legumes, tea and coffee as healthy plant-based foods. Less healthy plant-based foods included refined grains, fruit juices, potatoes and added sugars.

To achieve a high hPDI score, participants had to have a high intake of healthy plant-based foods and a low intake of less healthy plant-based foods. Overall, plant-based diets, particularly healthy plant-based diets, were associated with a reduced risk of colorectal cancer in men, but not in women. Unhealthy plant-based diets did not appear to reduce the risk.

For healthy plant-based diets, the association was stronger in Japanese American, Native Hawaiian, and white men than in those from other groups.

The researchers suggest that “the benefits from plant-based diets may vary by sex, race and ethnicity, and anatomic subsite of tumor.”

The study had a large sample size, long follow-up time and racial and ethnic diversity in the study population. However, the authors acknowledge some limitations of the study, including possible selection bias in who responded to the questionnaires and the negative scoring of all animal-based foods.

Several other studies have shown that some animal-based foods may actually be beneficial. Two reviews have found that both fish and dairy products may reduce the risk of colorectal cancer.

Beth Vincent argued that the study findings should be viewed with caution: “The research tried to compare ‘healthy plant foods’ and ‘unhealthy plant foods’ and found a link with bowel cancer in men. But because of the design of the study, the authors themselves acknowledge we can’t read too much into their results. The study relied on people remembering what they had eaten up to a year ago. It also made the assumptions that participants’ diets stayed the same over many years, and that all meat and animal products were unhealthy, which isn’t the case.”

This study adds to the growing evidence that diet and lifestyle play a key role in cancer risk. Vincent agreed, giving the following advice, “Eating a well-balanced diet can help with maintaining a healthy weight, which reduces the risk of cancer. Not smoking, cutting down on alcohol, and staying safe in the sun are other important ways to reduce your cancer risk.”

One study suggests that up to 35 per cent of cancers are linked to diet. And diet can greatly affect the risk of colorectal cancers.

The American Cancer Society recommends that to reduce colorectal cancer risk, a person should include lots of vegetables, fruits and whole grains, and limit the amount of meat they eat. Prof Jihye Kim, from Kyung Hee University, who is one of the study authors, says, “We speculate that the antioxidants found in foods such as fruits, vegetables, and whole grains could contribute to lowering colorectal cancer risk by suppressing chronic inflammation, which can lead to cancer. As men tend to have a higher risk of colorectal cancer than women, we propose that this could help explain why eating greater amounts of healthy plant-based foods was associated with reduced colorectal cancer risk in men but not women.”

The authors’ conclusion that “improving the quality of plant foods and reducing animal food consumption can help prevent colorectal cancer” may be a little optimistic, but their study certainly adds to the evidence that a healthy diet can help to reduce overall cancer risk.

• A Tell / Children’s Health Defense report