Monday October 20 2008
No evidence that yoghurt fights cancer
“Yoghurt can help to beat cancer,” reports the Daily Express, suggesting that people who eat two pots of yoghurt a day reduced their risk of cancer by 40% compared to those who rarely eat yoghurt. The newspaper suggests that researchers believe that the “bacteria in yoghurt gives the protection”.
The newspaper story is based on a large Swedish study which found that consumption of cultured milk products (sour milk, yoghurt) was linked with a reduced risk of bladder cancer. Eating two or more servings a day reduced bladder cancer risk by about 40%, but this was not significant when the researchers looked at the female participants only. It is inaccurate to suggest that yoghurt can “fight bladder cancer”, as the premise that it can prevent cancer needs further study. The most well-established risk factor for bladder cancer is smoking, therefore smokers should consider quitting to reduce their risk before turning to dairy products.
Where did the story come from?
Dr Susanna Larsson and colleagues from the Karolinska Institute in Stockholm, Sweden carried out this study. The research was funded by the Swedish Cancer Foundation, Oredbro County Council Research Committee, and the Swedish Research Council Committee for Infrastructure. It was published in the peer-reviewed medical journal, American Journal of Clinical Nutrition.
What kind of scientific study was this?
The study was a prospective cohort study in which 82,002 men and women who did not have cancer were followed for an average of 9.4 years to see whether there were any links between their consumption of cultured milk and other dairy foods. Food intake was recorded using a food frequency questionnaire, which was completed in 1997 at the beginning of the study. The researchers identified cases of bladder cancer during follow-up by linking the study population to the National Swedish Cancer Register and the Regional Cancer Register for the study area.
Participants were recruited from two other cohort studies – the Swedish Mammography Cohort and the Cohort of Swedish Men. In 1997, they were sent a questionnaire which included 350 questions about their diet and other lifestyle factors (including smoking). The dietary questions looked at the consumption of 96 common foods and beverages over the preceding 12 months. Participants filled in their exact number of servings of milk, cheese, sour milk and yoghurt per day or week.
Using the cancer registers, the researchers assessed the new cases of bladder cancer that occurred during follow-up. Cancer was categorised as either superficial bladder cancer or invasive/advanced bladder cancer, and as lower grade or higher grade.
The participants were divided into four groups according to the number of servings per day of total dairy products, with each group containing 25% of the participants (quartiles). The researchers then divided the groups again for each of the different dairy components (milk, cheese, cultured milk). The researchers compared the rate of new bladder cancers across the different groups of consumption of cultured milk, yoghurt and dairy products. In their analyses, they took into account various factors that could affect these links, e.g. education, smoking status, smoking history, total energy intake and age.
What were the results of the study?
Overall, the researchers found that 485 of the 82,002 participants were diagnosed with bladder cancer; 76 of these were women and 409 of them were men. There was no link between total dairy intake and risk of bladder cancer. However, intake of cultured milk – a category that includes yoghurt – significantly reduced the risk of bladder cancer. People who consumed two or more servings a day were about 0.6 times as likely to get bladder cancer (a 40% reduction in risk).
When they broke the population down by gender, this reduction in risk wasn’t significant for women (though this was probably due to the small number of women with bladder cancer overall), but it was for men. These results took into account (i.e. adjusted for) other factors that may be responsible for this link.
When the researchers looked at the risk associated with each different type of bladder cancer, some results became non-significant. Another analysis suggested that each increase in one serving per day reduced the risk by 13% (though this was only just statistically significant).
Intake of cheese or milk was not significantly related to risk of bladder cancer. When they analysed the results separately for smokers and non-smokers, they found that the protective effect of cultured milk products on bladder cancer was only significant in non-smokers.
What interpretations did the researchers draw from these results?
The researchers conclude that their large study of Swedish men and women has shown that a high intake of cultured milk is associated with a reduced risk of bladder cancer.
What does the NHS Knowledge Service make of this study?
This prospective cohort study suggests that consumption of cultured milk products may protect against bladder cancers. There are two main issues to keep in mind when interpreting the results of this study:
- Firstly, the study design (an observational study) means that the researchers cannot rule out that another unmeasured variable was responsible for the links they saw. In their statistical analyses, they adjusted for the main risk factors for bladder cancer – smoking and age – but other factors may play a part. These include fluid intake, occupational exposure to some chemicals, etc.
- Secondly, data on food intake were collected through a food frequency questionnaire, which asked about intake over the previous 12 months. Food frequency questionnaires can be unreliable because people are expected to recall their past food consumption.
Given these potential shortcomings of their study, the researchers suggest that the association between cultured milk intake and the risk of bladder cancer merits further investigation. Smoking is the most important risk factor for bladder cancer, and quitting this habit will reduce the risk of the disease.
This study suggests that yoghurt may have a role to play in the development of bladder cancer, but it is not yet clear which component of yoghurt might be responsible, or if the effect is due to other related dietary or environmental components, which may themselves be linked to yoghurt eating. Until these sorts of questions are answered in other studies, people should not change their diets.
Sir Muir Gray adds...
I like yoghurt and it is a healthy food that is low in fat. This finding may be a bonus, but for people who smoke the best method of reducing bladder cancer risk is to stop smoking.