Eating red meat and processed meat – what do the scientists say?

Marianne Sødring, stipendiat ved Institutt for mattrygghet og infeksjonsbiologi, NMBU

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Several reports warn about a link between eating red and processed meat and the risk of developing cancer in the gut. These reports have led to new dietary recommendations which advise people to limit their intake of red and processed meat. A recent perspective paper, authored by 23 scientists, published in the latest issue of the journal Meat Science, brings to light the uncertainties in the scientific evidence. The authors stress that more research is needed to clarify these issues and improve the basis for future recommendations regarding the intake of red meat.

The definition of what is considered red meat can be confusing. In some cases red meat is limited to meats that are red in color prior to cooking, such as beef, lamb, and sometimes pork, while in other cases, red meat refers to meat from mammals. Occasionally there is even a distinction made simply based on the flavor of the meat; bland flavor in white meat, more intense flavor in red meat. The fact that there are no clear lines separating white meat from red meat makes the consumers‘ job that much harder when trying to follow dietary guidelines relating to red meat intake.

Although meat, including poultry, is the primary protein source in a westernized diet, the consumers are becoming more critical when it comes to the health and safety aspect of meat products. Meat products that are labeled ‘light and healthy’ have shown a noticeable growth during the last 10-15 years. Also, the consumers’ opinions are often negatively influenced by messages in the media claiming that unhealthy diets are (among other things) characterized by high meat intake. This image of meat products is further affected by reports on the potential health risks linked to meat preservatives such as nitrite and by the notion that meat may be associated with cancer, in particular colorectal cancer (CRC).

Meat and colorectal cancer

CRC is the third most common cancer worldwide, and the number one cause of cancer-related death in European non-smokers. The World Cancer Research Fund’s expert panel has concluded that the link between red- and processed meat intake and CRC is convincing; however, there is still some uncertainty among scientist regarding this link. This relationship between CRC and meat intake appears to be a complex one, depending on many different variables besides just the meat itself. How meat intake and CRC is linked is still not completely clear, but it may be related to nitrite or heme iron in the meat, an abnormally functioning gut, growth of harmful  bacteria in the gut, and/or eating too much of a diet insufficient in nutrients, i.e. an unbalanced diet.

Humans, animals and cells – different research methods

Studies on the effects meat consumption in humans may have on the development of CRC (epidemiological studies) show findings from both side of the spectrum; no significant risk to significant risk. However, conducting such studies with human participants often poses several problems since the participants themselves may be from different regions of the world, may be of opposite genders, or may not fully recall what they have actually eaten, just to name a few.  Randomized controlled trials would be ideal, but, since CRC may take upwards of 10-20 years to develop, such trials are just not practical.

As an alternative to trials involving human participants, animals may be used to investigate the potential link between meat intake and CRC. Animal studies are clearly much easier to control, cost less money, and can produce results much quicker. Rodent models are among the most used in cancer research; rodents are small, have short life cycles, and can reproduce in large litters. In order to speed up results when doing animal studies, the animals can be made more sensitive to developing cancer when faced with an environmental challenge. Such a challenge may for instance be a basic “standard” diet that is high in meat, but doesn’t contain any ingredients that protect and can help the gut stay healthy. This means no vegetables, no fiber, no milk or other sources of calcium. One such rodent model is the APCmin mouse. This particular mouse has a mutation in a gene that normally limits the growth of tumors. Each mouse has two copies of the gene, but it is only when both copies are damaged that tumors begin to form in the intestines. This mouse model is very sensitive to factors that may damage this gene, as well as factors that may compensate for the consequence of two damaged gene copies. Alternatively, mice and rats can be injected with cancer-causing agents to induce cancer. In recent years scientists have shown that beef meat promotes the development of CRC in rats that were given cancer-causing agents prior to the meat diet, but only if the diet was low in calcium. The effects that were seen after feeding the rats meat were mimicked when the animals were given a low calcium diet with a comparable dose of heme iron, the component of hemoglobin that give blood its red color. These findings suggest that the heme alone could explain why meat leads to development of cancer in animals. However, it is important to keep in mind that the “standard” diet of the lab animal is not very comparable to that of humans, which may explain why the results seem to differ: in humans, the observed association between the intake of red and processed meat and cancer is relatively small in magnitude, but consistent, and may still present a serious public health impact. But, on the other hand, a burger meal is most likely accompanied by a soda rather than a glass of milk, which might then show a stronger association between red meat intake and CRC, because the meal lacks the protective effects of calcium.

Working with cultured cancer cells grown in a laboratory is a third approach to studying cancer. These cells are used to investigate the basic functions of tumors, and can produce results in a much shorter time than both human and animal studies. However, interactions between different tissues cannot be studied in cells, and the cells do not have direct cancer-related endpoint such as tumor formation or mortality. Therefore, cell models can currently only support human or animal models for determining the effect of food consumption on health outcomes such as CRC.

Can we protect ourselves against CRC?

Improving the bacteria in the gut may be a step towards protecting against CRC. Both pre-and probiotics have the ability to change the type of bacteria present in the colon, and the substances they produce. This can help improve gut functions and reduce inflammation, and in turn protect against red-meat induced damage to the colonic genetic material. Also, the diet can be altered to try to cancel out the potentially harmful effects of eating red meat; for example by eating more calcium, selected vitamins and antioxidants, and getting enough fiber from fiber-rich fruit and vegetables. Thirdly, meat preparation may also have an impact. Seasonings such as garlic or curcumin are suggested to have protective effects against cancer, while frying and grilling meat has been associated with the formation of cancer-causing chemicals that have been linked with a higher risk for CRC development in humans. It must be noted that these chemicals are also formed when grilling or frying chicken or fish and are therefore not specific to red meat. Lastly, production and packaging of meat may also impact the risk for CRC. The risk may be reduced by adding certain compounds to the meat prior to packing,  processing and packing the meat under conditions devoid of oxygen, or improving the feed given to the pigs and cattle from which the meat is produced.

What should we do?

Intake of red meat may be associated with an increased risk of CRC in ‘Western’ food cultures. This could be genuine cause-and-effect relationship, but it should be kept in mind that any links between meat consumption and CRC may be misleading because of the association of the cause with other factors that influence the effect. Meat contains a number of compounds that are beneficial to us, and may not be cancer-causing per se, but, when consumed in very high amounts, it may result in an imbalanced diet and thus increase the risk of developing CRC. These imbalances may possibly result from the lack of certain agents in the diet, such as calcium or antioxidants, which may be able to offset the damaging products that result from the interactions of heme with an unfavorable gut environment. The interactions between meat, gut, and health outcomes such as CRC are very complex and are not clearly pointing in one direction. Science does not yet have a full understanding of how the food we eat affects our gut and our health. To get a better grip on this complex issue, there is a need for further studies on both the epidemiological relation between red meat and health, and the mechanisms behind cancer development. It is also necessary to establish methods that can better measure how much meat people eat, types and composition of the meat they eat, and how it is processed. There is also a need to adopt new methods for studying diet and gut bacteria. At the same time research and development for making meat products healthier must continue.



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