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The higher your BMI, the greater your risk of developing cancer

More body fat dramatically raises the risk of cancers of the digestive system, warn Cambridge scientists.

StudyFinds.org, Oct 3, 2021

The likelihood of a tumor developing in the stomach or liver soars by 13 percent for every additional unit of body mass index (BMI), according to new findings.

Researchers also warn the risk of esophageal cancer cancer increases by ten percent, and pancreatic cancer by six percent. Obesity is a major factor in all four types of cancer, and shedding the pounds can prevent them, research shows.

According to the CDC, obesity affects nearly half (over 40 percent) of Americans.

"The key message to the public should focus less on physical size, which people can often do little about, and more on managing the amount of fat that they carry," says study co-author Dr. Amy Mason, a statistician at the University of Cambridge University, in a statement.

The study is based on 367,561 older people in the biomedical database UK Biobank. Participants' health data has been tracked since 2006 when their DNA was mapped. Researchers identified a link between the illnesses and genes that predispose an individual to a higher BMI.

How is BMI calculated and what's the connection to cancer?

The BMI calculation divides an adult's weight in kilograms by their height in meters squared. A BMI over 25 is considered overweight and a BMI over is 30 signals obesity.

"It's well known being large is linked with having a greater risk of cancer," adds study co-author Dr. Stephen Burgess, also of Cambridge. "But what was not known is whether the increased risk is an inevitable result of being a big person, or whether it is caused by a specific component of obesity that people can change."

Potbellies or muffin tops may lead to digestive cancers caused by eating too many carcinogens in processed food. Fat could also trigger inflammation in the digestive tract, said the researchers.

Each unit increase in BMI increases the risk of womb cancer by ten percent, lung cancer by eight percent, and the risk of ovarian and cervical cancer by four percent. Interestingly, it reduced the risk of breast cancer by one percent. The study also finds it decreased prostate cancer risk in men by three percent.

Links between obesity and sex-specific forms of the disease are likely driven by the production of reproductive hormones in fatty tissue.

"This result has important clinical implications," explains study lead author Dr. Mathew Vithayathil, a gastroenterologist at Imperial College London."While our research supports a causal role of obesity in driving and protecting against certain cancers, it suggests differential effects of BMI for different malignancies which should be explored further. Rather than presenting obesity as a generic cancer risk factor, a more nuanced public health message with regards to obesity as a risk factor for digestive system cancers may be more appropriate."

'Fat is main risk for digestive system cancer'

The research team analyzed two dozen different types of cancer. There was a consistent connection between genes that make a person taller. It backs the theory they have more cells in their bodies in which dangerous mutations can occur.

The latest findings are more targeted than previous research that has linked increased body size to the disease. They indicate fat is the main risk for digestive system cancer. It also influences certain other types of cancer, but not all.

The international team, including experts at the University of Bristol in England and the Karolinska Institute in Sweden, also compared genes that influence an individual's fat mass and fat-free mass. The former is the portion of an individual's weight attributable to fat; the latter is an individual's weight excluding body fat. The increased risk of various digestive cancers was primarily attributable to fat mass.

Comparing genes rather than directly measured height or weight avoided pitfalls such as "reverse causation," where patients often lose weight due to cancer or treatment. It can distort the link between body size and cancer risk.

Genes are fixed for life. They are not affected by the presence of cancer, or any other risk factor that may lead to a correlation that is unrepresentative of the true causal relationship between body size and cancer risk.

"Our results show that the evidence for BMI as a causal risk factor for cancer is mixed. We find that BMI has a consistent causal role in increasing risk of digestive system cancers and a role for sex-specific cancers within consistent directions of effect. In contrast, increased height appears to have a consistent risk-increasing effect on overall and site-specific cancers," added Dr. Vithayathil.

Per the CDC, in 2018, a total of almost two million new cancer cases were reported in the United States.

The study is published in PLOS Medicine.

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