Thursday August 4 2011
Beans and lentils are good sources of fibre
“Lentil-loving hippies have the right idea when it comes to beating bowel cancer,” according to the Daily Express. The newspaper says that a diet rich in beans, pulses and brown rice cuts the risk of developing bowel cancer by up to 40%.
The news is based on a study that assessed people’s diets and examined the risk of developing colorectal polyps (small growths in the lining of the bowel that can become cancerous) over the next 26 years. It found that diets high in cooked green vegetables, dried fruit and brown rice were associated with a significantly lower risk of colorectal polyps. Legumes such as beans and other pulses were also linked to a lower risk, although results in this area were less robust.
The research had some limitations that make the results less reliable, including the fact that it relied on people reporting their diets on only one occasion during the lengthy study, and because the participants were self-reporting whether or not they had developed polyps. The participants were also Seventh Day Adventists, a religious group who may not represent the wider population due to their beliefs about avoiding harmful activities such as smoking and drinking. However, despite these limitations the main findings are in line with current advice that a diet rich in plant-based foods can reduce the risk of cancer. These foods are good sources of fibre, which helps maintain healthy bowels, as well as important nutrients.
Where did the story come from?
The study was carried out by researchers from Loma Linda University, California. It was funded by the US National Institutes for Health.
The study was published in the peer-reviewed journal Nutrition and Cancer.
The research was reported fairly in the media although the Daily Express’ claim that this was a “hippy diet” was perhaps misleading. These days, you don’t need to be a “lentil-loving hippy” to eat foods such as pulses, vegetables and brown rice.
What kind of research was this?
This was a prospective cohort study, which looked at the relationship between specific foods and the risk of colorectal polyps among 2,818 participants over 26 years. The researchers point out that colorectal cancer is a leading cause of cancer deaths and that the majority of cases originate from adenatomous (benign) polyps. Although previous research suggests diet plays a part in the risk of colorectal cancer, they wanted to look at how diet affects the risk of both polyps and CRC, as this remains unclear.
What did the research involve?
The study drew its participants from a Californian population of Seventh Day Adventists, a Christian religious group that places particular emphasis on healthy diet and lifestyle. For example, members of the church tend to avoid alcohol and smoking, and often limit their consumption of meat. The group is considered to be of scientific interest for dietary research as their lifestyle means they are likely to be largely unaffected by habits such as smoking and drinking, thereby helping to isolate the effect diet would have on diseases such as cancer.
The research was based around an analysis of two phases of a large, ongoing study examining Adventists. In the first phase, which took place between 1976-7, (known as AHS-1), participants were given a lifestyle questionnaire which included a dietary section asking them 55 questions on food frequency. People were asked how often on average they consumed different foods and drinks, with frequency of consumption mostly recorded using an eight-point scale ranging from “never or almost never” to “more than once a day”. The questionnaire also included comprehensive questions on lifestyle, medical and family history.
The second phase of the study (AHS-2) was carried out from 2002-4. In this part, participants were given a lifestyle questionnaire which asked if they had ever had a colonoscopy and whether they had ever been told by a doctor that they had specific conditions, including rectal or colon polyps. The participants in the two studies were linked, meaning that the data from the two studies was matched to ensure the questionnaire from 1976 matched the participants in 2002-4. They were also asked to specify the approximate amount of time since they were first diagnosed. To ensure higher validity of this self-reported outcome, only cases diagnosed after a colonoscopy were used in the study.
Of the 5,095 original study participants, they excluded those who had polyps or a history of colorectal cancer or inflammatory intestinal conditions before the study began. They also excluded those who had never had a colonoscopy and those who reported having one after their diagnosis. After these exclusions the researchers had information on 2,818 participants available for analysis.
Researchers used validated statistical methods to analyse the relationship between different foods and the risk of polyps, adjusting their findings for possible confounders such as family history of CRC, education, alcohol intake and smoking habits. As there were so few people who had ever drank or smoked in this population the researchers excluded these known influences from their analysis.
What were the basic results?
During an average 26-year follow-up period, the researchers identified a total of 441 cases of rectal or colon polyps, a figure which represents between 15%-16% of the study population. They found that:
- People who ate cooked green vegetables one or more times a day had a 24% reduced risk, compared to those eating them less than five times a week (OR 0.76, 95% CI 0.59 to 0.97).
- People who ate dried fruit three times a week or more had a 24% reduced risk compared to those who ate less than one portion a week (OR 0.76, 95% CI 0.58 to 0.99).
- People who ate brown rice at least once a week had a 40% reduced risk compared to those who never ate it (OR 0.60, 95% CI 0.42 to 0.87).
- People who ate legumes at least three times a week reduced their risk by 33% compared to those who ate them less than once a month (OR 0.67, 95% CI 0.44 to 1.01) However, this reduction was not statistically significant.
In the case of both legumes and brown rice, there was a “dose-response effect”, which means that the more people ate, the more their risk was lowered.
No significant association was found between the risk of polyps and other foods, including red meat (which other studies have found increases the risk), fish and salad.
How did the researchers interpret the results?
High consumption of cooked green vegetables, dried fruit, legumes and brown rice was associated with a lower risk of colorectal polyps, the researchers said. This type of diet contains fibre and types of chemicals called phytochemicals that may inhibit the development of colon cancer, they added.
This study had several strengths. It had a long follow-up period and it was also “prospective” as it assessed diet and followed the participants over time, rather than asking them to recall what they had eaten years previously. The researchers also pointed out that the Adventist population has a “unique lifestyle”, with lower levels of alcohol consumption and smoking. This limits the effect these factors would have had on the participants’ risk of polyps and cancer.
However, the study also some significant limitations:
- The study relied on people self-reporting their diets on only one occasion. It is possible, even probable, that people’s diets changed over the 26-year period.
- The researchers stated that about 80% of participants did not make changes in their dietary habits during the years of follow-up, but how they arrived at this estimate was not published.
- The self-reported diet information may not be accurate as estimating food intake is difficult to do accurately.
- The study relied on people self-reporting whether they had had a colonoscopy and whether they had been diagnosed with polyps. It is entirely possible that some people misunderstood, forgot or got confused about their medical history, including whether they had polyps or not. Studies of this type would typically verify this type of medical information using hospital/physician records and other independent data.
Also, the researchers’ decision to use a mostly vegetarian population who tend to adopt a stricter lifestyle is open to question. On one hand, the fact that few of the participants drank or smoked meant that the results are largely free from the influence of these known risk factors. However, on the other hand, this lifestyle and other differences, mean that on the other hand the outcomes seen in this group may not be applicable to the wider population.
However, despite these limitations it is accepted that a plant-based diet high in fibre reduces the risk of cancer, and this type of diet is already recommended in a major report from the World Cancer Research Fund [http://www.wcrf-uk.org/research/cp_report.php]. This report is useful for putting some figures against the reduced risk of specific foods, and for indicating how much of these foods people need to eat to reduce their risk.