Diet's role in cancer unclear

1st Part in Series: Science 'totality' finds no link 2nd Part in Series: Diet's role in cancer unclear 3rd Part in Series: Dietary advice complex

CANCER may originate from virtually any cell in one's body and is a diverse family of diseases with more than 100 kinds, according to a new report that discusses the relationship between cancer and diet.

Although the specific cause of the majority of cancers is yet to be identified, it is known that cancers are multifactorial -- involving a complex interaction of genetic, lifestyle and infectious factors -- and that progression from initiation to diagnosis takes several years, which compounds the difficulty in pinpointing the underlying cause or causes, according to the report, compiled by the health sciences practice at Exponent Inc.

-Second in a series-

In 2008, there were 1.437 million new cancer cases and 565,650 deaths from cancer, but the good news is that cancer incidence in the U.S. has remained stable while cancer deaths have declined over the last 10 years, according to lead author Dr. Dominik Alexander.

Breast and prostate are the most commonly diagnosed forms of cancer, followed by lung and colorectal cancers (Tables 1 and 2), he reported.

Lung cancer is responsible for one-third of all cancer deaths among men and one-fourth of all cancer deaths among women, a statistic largely attributable to smoking, Alexander said.

The report, "Red Meat & Processed Meat Consumption & Cancer," was commissioned by the beef and pork checkoffs in response to an epidemiologic study that associated consuming red meat and processed meat with cancer (Feedstuffs, Nov. 5, 2007). An epidemiologic study deals with the incidence, distribution and control of disease in a population.

Exponent is a scientific study group that performs risk assessments across several environmental and public health issues.

Inconclusive

Diet's role as a cause of cancer has received considerable attention since the 1970s, and a number of researchers have suggested that 35% to as many of 70% of all cancer deaths may be linked to dietary factors, which has consequently created a major public health issue, Alexander acknowledged.

Historically, nutrition studies in animals have led to hypotheses that the same biological processes that occur in animals also occur in people, and one of the first theories involving diet and cancer was that animal fat may be involved in carcinogenesis, he said.

Accordingly, researchers demonstrated in the 1950s that increased intake of dietary fat in rodents increased mammary tumors, and subsequent work determined that populations that consumed higher levels of dietary fat tended to have higher levels of some common cancers such as breast and colorectal cancers, Alexander said.

In the 1980s, fat consumption continued to be incriminated in causing cancer, as well as contributing to cardiovascular disease, and at the same time, fruit and vegetable consumption emerged as a cancer prevention, he said.

However, "there is no conclusive evidence" regarding the roles of foods and food groups in cancer causation, Alexander emphasized, adding that what was thought to be "convincing" evidence linking fruit and vegetable consumption to cancer reduction has been diluted by recent large-scale and well-conducted studies.

This is not to say that consuming fruits and vegetables may not reduce some kinds of cancer, he said, but the available scientific evidence "is not as clear today as was thought 10-20 years ago."

The same can be said about fat intake, Alexander noted: Large-scale studies in recent years "have not supported" positive associations between fat consumption and the more common kinds of cancer.

It is noteworthy, he added, that juxtaposed with evaluations of diet and cancer have been examinations of exercise/fitness and cancer. "It's becoming increasingly clear that obesity and physical activity are modifiable factors" that can increase or reduce cancer risk, he said, "this is recognized" by scientists worldwide.

'Underwhelming'

The diet-cancer relationship has been the subject of "thousands" of epidemiologic studies, but the scientific evidence surrounding dietary factors and cancer "has been somewhat underwhelming" considering the amount of research, Alexander said.

As such, many questions remain regarding the link between cancer and diet generally and red meat specifically, he said, and answering these questions is "of utmost significance" as beef and pork are both nutrient-rich foods that are critical to health and nutrition -- to achieving recommendations found in the Dietary Guidelines for Americans.

Diets include a large variety of foods and food intake patterns, and analyses of both have emerged as an important method to evaluate diet and health outcomes, Alexander said.

Many of these analyses have looked at diets characterized by higher intakes of red meat and processed meat and cancer outcomes, he said, and although associations have been inconsistent collectively, the majority of these studies have observed an increased risk for cancer.

However, Alexander noted that meat consumption is typically central to a "western" diet that is also characterized by consumption of other high-fat foods, sugar and alcohol, and people who consume western diets also are normally more sedentary, have a higher body mass and are normally more likely to smoke.

Therefore, the interpretation of the causative role of red and processed meats lacks "specificity" because these foods are not isolated from but included with other dietary factors, as well as confounding factors that may be indicative of unhealthy lifestyles, he said.

The purpose of the Exponent report, Alexander said, is to provide a comprehensive summary of the epidemiologic studies that have sought to understand the relationship between consumption of red and processed meats and cancer.

A copy of the report is available from the National Cattlemen's Beef Assn. for $30 by calling (303) 368-3138, and downloadable chapters will be posted soon at www.porkandhealth.org.