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Colon cancer: Higher risk if you’re too thin

  Tuesday, 03 l 05 l 2011  Source:  The Straits Times   
By: Salma Khalik

Skinny folk 33% more likely to be hit than those of ideal weight: Study 

THE skinny on colon cancer is this – not only fat people have a higher risk of getting it. Those who are underweight are in the same boat. This surprising finding has emerged from the long-term Singapore Chinese Health Study, which involves more than 50,000 participants. The study began in 1993 with more than 63,000 people between the ages of 45 and 74. The study, funded by a grant from the National Cancer Institute in the United States, has already yielded many interesting results, mainly to do with cancer. Next month, the paper on its latest findings will be published in Cancer, the journal of the American Cancer Society. The results surprised the research team, whose members included researchers from the University of Minnesota in the US.

Associate Professor Koh Woon Puay, the principal investigator of this study, told The Straits Times: “We’re trying to understand how this is biologically plausible.” In its paper, the team suggests that the higher risk of colon cancer among underweight people could be due to something called “oxidative DNA stress”. These mild inflammations hit those who are underweight, damaging their immune system and allowing cancer cells to proliferate. This is unlike what causes cancer in overweight people. They are believed to have more insulin in their system, which might be responsible for decreasing the body’s ability to prevent tumours from forming.

The team studied colorectal cancer, which has become the top cancer here, and discovered that the risks faced by these two weight groups are significantly bigger, compared with that faced by people of normal weight. They grouped the study population according to their body-mass indexes (BMIs), calculated by dividing weight by the square of height – all in metric form. Among those with a good BMI of 21.5 to 24.4, it found that only 89 out of 100,000 had colon cancer. Among the “shoulder” group, with BMIs of 18.5 to 21.4 and from 24.5 to 27.4, the number of cases went up to 103.

But among those who were underweight, the incidence shot up to 119 – or 33 per  cent higher than the ideal group. This group, comprising those with a BMI of 18.5 or less, made up about 7.5 per cent of the study group. Typically, a person who is 1.65m tall and weighs 50kg would fall in this category. While skinny people were found to have a higher risk of colon cancer than those with normal weight, the amount of risk was still less than that faced by the obese group. Among those with a BMI of 27.5 and higher – about 10.4 per cent of those studied – the incidence of colon cancer was 130 per 100,000, or about 46 per cent higher than normal. But while researchers found a strong link between being underweight and getting colon cancer, there was hardly a difference in the cases of rectal cancer – for which people who are thin, fat or at a normal weight all face similar risks.

The two types of cancer together make up colorectal cancer – with rectal cancer roughly accounting for a third of cases. Other significant findings include the risks faced by smokers, which were higher across all groups than for non-smokers. Prof Koh, who is from the department of epidemiology and public health at the National University of Singapore, said the results probably could apply across races, as studies in Japan and South Korea also found a link between being obese and a higher risk of colon cancer. Figures from Singapore’s cancer registry show that of the three main races here, the Chinese are more likely to get colorectal cancer. Chinese men are twice as likely as Malay men, and three times as likely as Indian men, to get this form of cancer. Prof Koh is not calling for underweight people to eat more, but is urging them to screen for the disease. From this year, up to $950 can be used from Medisave if a colonoscopy is used to screen for this form of cancer. 

Results of the study

  • BMI from 21.5 to 24.4: In this ideal group, 89 out of 100,000 had colon cancer.
  • BMI from 18.5 to 21.4 and from 24.5 to 27.4: The number of cases went up to 103 in this “shoulder” group.
  • BMI of 18.4 or less: In this group of underweight people, it shot up to 119 – 33 per cent higher than the ideal.
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