People who are obese have a greater risk of developing and dying of some forms of cancer, including malignant tumors of the breast, ovary, kidney, pancreas, colon, rectum, and bone marrow, a research review confirms.
Cancer is a major cause of death in the world, and the prevalence of obesity has more than doubled in the course of the past generation. Although obesity is believed to influence the risk of developing and dying from a wide range of tumors, the study found “strong evidence” to support this connection for only 11 types of cancer.
“Other associations can also be genuine, but there is still great uncertainty about them,” lead study author Dr. Maria Kyrgiou of the Imperial College in London, said by e-mail.
Worldwide 1.9 billion adults are overweight or obese, according to the World Health Organization. About four in 10 adults are overweight, and more than one in 10 are obese, a condition that increases the risk of heart disease, diabetes, kidney problems and joint disease in addition to certain forms of cancer.
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For the current study, researchers analyzed results of 204 previously published studies on the relationship between obesity, weight gain, waist circumference and 36 different types of cancer.
The researchers looked in particular, the proof of the previous results may have overstated the link between obesity and cancer, a connection that was too small to rule out the possibility that it was due to chance.
They found the strongest links between obesity and malignant diseases of the digestive organs and for hormone-related tumors in women, according to the report in The BMJ.
When researchers have looked at what is known as the body mass index (BMI), a ratio of weight to length, they found an increase in BMI tied to a higher risk of developing cancer in the esophagus, bone marrow, biliary tract system, pancreas and kidneys. Upticks in BMI were also associated with a greater risk of colon cancer and rectal cancer in both men and endometrial malignant tumors in younger women.
A BMI between 18.5 and 24.9 is considered a healthy weight, while 25 to 29.9 is overweight, 30 or higher is obese and 40 or higher is what is known as morbid obesity. (The National Heart, Lung, and Blood Institute has an online BMI calculator here: bit.ly/1D0ZqDv.)
The increased risk of cancer for every 5-unit gain in BMI ranged from 9 percent for rectal cancer among men and 56 percent for tumors in the bile duct system.
For women, weight gain and extra abdominal fat, a measurement known as the waist-to-hip circumference ratio, were also associated with an increased risk of certain forms of cancer.
After menopause, women’s risk of breast cancer increased by 11 percent for every 5 kilos (11 pounds) of weight they gained during adulthood. This was in women who are not taking hormones to relieve symptoms of menopause, a treatment which has been independently linked to an increased risk of breast cancer.
If women have more fat around the abdomen, every 0.1 unit increase in waist-to-hip ratio was associated with a 21 percent jump in the odds of endometrial cancer.
The researchers found strong evidence linking weight gain in colorectal cancer. They also found a strong relation between the BMI increase and the get of a cancer of the gall bladder, stomach, and ovaries, as well as the death of bone marrow tumors.
A limitation of the study is that the researchers not to investigate, in controlled experiments designed to prove that obesity directly causes cancer, the authors note.
More research is needed that assesses changes in body fat in the time to better understand how obesity directly affects the risk of getting cancer or of dying from the disease, the authors conclude.
The way obesity impacts the risk of cancer also varies in other parts of the body, Dr. Graham Colditz, a researcher at the Washington University School of Medicine in St. Louis, said by e-mail.
But the take-home message is the same for every patient, added to Colditz, author of an editorial supervision of the study.
“Avoiding weight gain through adult years is important,” Colditz said. “Even if you are overweight, focus first on not gaining more weight; for those who are overweight or obese the take of a few pounds can lower cancer risk.”