Diets Heavy in Meat Boost Risk for Certain Cancers
A new multicountry study provides more evidence that certain dietary and lifestyle factors can increase the risk of developing cancer.

Results from the study, published in the January issue of Nutrients, show that smoking and diets rich in animal products have the strongest correlations with cancer incidence rates.

The strongest correlation with animal products was seen in cancers of the female breast, corpus uteri, kidney, ovaries, pancreas, prostate, testicles, and thyroid, and in multiple myeloma.

In contrast, alcohol consumption was found to be significantly correlated with only colorectal cancer.

Added sweeteners were also associated with an increased incidence of brain cancer in women, and in corpus uteri, pancreatic, and prostate cancers. Cereals were associated with a decreased incidence of kidney and prostate cancer.

"My study reiterates that animal products are an important risk factor for many types of cancer," said study author William B. Grant, PhD, from the Sunlight, Nutrition, and Health Research Center in San Francisco.

The data also show that there is a 15- to 30-year lag between diet and cancer incidence. Eating animal products is as important as smoking for increasing the incidence of all cancer types (apart from lung cancer), Dr. Grant noted.

Interestingly, animal fat is an important risk factor for lung cancer. "This helps explain why lung cancer rates have increased dramatically in Japan, even though smoking rates have not," he told Medscape Medical News. "As they made the nutrition transition from the traditional Japanese diet to the Western diet, rates of cancers common in Western developed countries, such as breast, colorectal, and prostate, increased with about a 20-year lag."

In addition, Alzheimer's disease rates for those older than 65 years increased in Japan from 1% in 1985 to 7% in 2008.

Dietary Guidelines

According to the World Health Organization (WHO), approximately 30% of cancer mortality is due to the 5 leading behavioral and dietary risks: high body mass index, low fruit and vegetable intake, lack of physical activity, tobacco use, and alcohol use. Tobacco use is the most important risk factor, accounting for 22% of global cancer deaths and 71% of global lung cancer deaths.

In recent years, several organizations, including the American Institute for Cancer Research (AICR), the American Cancer Society (ACS), and the US Department of Agriculture, have issued dietary guidelines aimed at encouraging healthier eating habits, increasing physical activity, and curbing rising obesity rates.

Some of the most extensive data on diet/lifestyle and cancer risk have been released by the AICR, which recommends limiting the intake of red and processed meat, eating mostly foods of plant origin, and limiting the consumption of energy-dense foods, which includes sugary drinks.

Dr. Grant's work is an ecological study that examines the association between incidence rates for 21 types of cancer in both men and women from 157 countries (87 with high-quality data) and various risk-modifying factors.

The rates are from 2008 and were obtained from the International Agency for Research on Cancer of the WHO, and dietary supply data came from the Food and Agriculture Organization. Data going back to 1980 were included because there is typically a lag time of up to 20 years between dietary changes and peak cancer rates.

The animal products index in this study included meat, milk, fish, and eggs. Dr. Grant also used lung cancer incidence rates as an index for the effects of smoking and air pollution, because this integrates the effect of all factors contributing to lung cancer and other cancers linked to lifetime smoking.

Latitude, Vitamin D, and Cancer

Dr. Grant explained that he looked at latitude as a an index of solar UVB doses, which is an important source of vitamin D. In fact, he found that higher latitudes (where there is less UVB, and consequently less vitamin D absorption) were associated with higher rates of cancer.

In the 87 countries for which high-quality data were available, there was an increase in the incidence of bladder, colorectal, kidney, and lung cancers, and in melanoma and Hodgkin's lymphoma. When all 157 countries studied were taken into account, an increased incidence of brain and breast cancer was seen at higher latitudes.

"These results add support to other studies that have found that UVB and vitamin D reduce the risk of many types of cancer," he said.

For melanoma, however, the result are primarily related to changes in skin pigmentation with latitude, Dr. Grant explained. "There is very little melanoma within 30 degrees of the equator for people living in their ancestral homelands, since they have dark enough skin to protect against melanoma."

Conversely, Australia and New Zealand have the highest rates of melanoma and are populated primarily by individuals with Northern European ancestry.

Hypothesis-Generating

Marji McCullough, ScD, RD, strategic director of nutritional epidemiology at the ACS, noted that this study is consistent with other studies that show that healthy lifestyle behaviors can modify the risk for many types of cancer.

However, she explained, the problem with ecological studies such as this one is that they are hypothesis-generating, and estimates are made across large groups. In this case, diet is measured over populations rather than individuals. Thus, there can be issues with measuring the data and accounting for confounders.

"The ACS recommends limiting red and processed meats, and instead choosing poultry, fish, nuts, and legumes as alternative sources of protein," Dr. McCullough told Medscape Medical News. "But in this study, all animal products are grouped together, so we can't be sure what it means or which ones are being consumed."

"It would be valuable to have a breakdown of the types of animal products so we know what people were actually eating," she said, "and to have more detail."

Dr. Grant acknowledged that he was unable to separate the types of animal products, so he could not show a relation between any specific item and cancer incidence. "The limitation of my study is that different countries have different amounts of animal products from the various types of animal products," he said. "However, all animal products are high in fat and/or protein, and both increase the production of insulin-like growth factor 1, which encourages bodies — and tumors — to grow."

Dr. Grant has disclosed no relevant financial relationships.

Nutrients. 2014;6:163-189. Abstract