Meat which is red when raw, with high myoglobin content
In gastronomy, red meat is commonly red when raw (and a dark color after it is cooked), in contrast to white meat, which is pale in color before (and after) cooking. In culinary terms, only flesh from mammals or fowl (not fish) is classified as red or white. In nutritional science, "red meat" is defined as any meat that has more of the protein myoglobin than white meat. White meat is defined as non-dark meat from fish or chicken (excluding the leg or thigh, which is called dark meat).
Regular consumption of red meat, both unprocessed and especially processed types, has been associated with negative health outcomes.
Definition.
Under the culinary definition, the meat from adult or "gamey" mammals (for example, beef, horse, mutton, venison, boar, hare) is red meat, while that from young mammals (rabbit, veal, lamb) is white although sometimes rabbit meat is considered red meat. Poultry is white, excluding certain birds such as ostriches. Most cuts of pork are red, others are white. Game is sometimes put in a separate category altogether (French: "viandes noires" — "dark meats"). Some meats (lamb, pork) are classified differently by different writers.
According to the United States Department of Agriculture (USDA), all meats obtained from mammals (regardless of cut or age) are red meats because they contain more myoglobin, which gives them their red color, than fish or white meat (but not necessarily dark meat) from chicken. The USDA also classifies ratites, such as ostriches, emus, and rhea, as red meat. Some cuts of pork are considered white under the culinary definition, but all pork is considered red meat in nutritional studies. The National Pork Board has positioned it as "the other white meat", profiting from the ambiguity to suggest that pork has the nutritional properties of white meat, which is considered more healthful.
Nutrition.
Red meat contains large amounts of iron, creatine, minerals such as zinc and phosphorus, and B-vitamins: (niacin, vitamin B12, thiamin and riboflavin). Red meat is a source of lipoic acid.
Red meat contains small amounts of vitamin D. Offal such as liver contains much higher quantities than other parts of the animal.
In 2011, the USDA launched MyPlate, which did not distinguish between kinds of meat, but did recommend eating at least of fish each week. In 2011, the Harvard School of Public Health launched the Healthy Eating Plate in part because of the perceived inadequacies of the USDA's recommendations. The Healthy Eating Plate encourages consumers to avoid processed meat and limit red meat consumption to twice a week because of links to heart disease, diabetes, and colon cancer. To replace these meats it recommends consuming fish, poultry, beans, or nuts.
Health effects.
There are no long-term randomized controlled trials that have investigated red meat consumption and disease outcomes and are unlikely to be conducted due to ethical, financial and practical reasons. Most of the data on red meat and health effects is from long-term epidemiological studies. 
Overall, diets high in red and processed meats are associated with an increased risk of type 2 diabetes (T2D), cardiovascular disease (CVD), cancer (particularly colorectal cancer), and all-cause mortality. The body of epidemiological data showing their associations with T2D, CVD, and cancer is large and consistent.
The associations are strongest for processed meat, which is meat that has undergone salting, curing, fermentation, smoking, or other processes to enhance flavor or improve preservation, such as bacon, ham, salami, pepperoni, hot dogs, and some sausages. Also, meat that is cooked longer and at higher temperatures is associated with a 4.62 times greater risk of breast cancer compared with rare or medium-done meat.
A 2021 umbrella review reported an increase of 11–51% risk of multiple cancer per 100g/d increment of red meat, and an increase of 8-72% risk of multiple cancer per 50g/d increment of processed meat.
A 2022 study challenged the dose-response relationship using a newly developed burden of proof risk function (BPRF). It found weak evidence available regarding whether eating red meat increases the risk of death. The authors conclude that the quality of the available evidence is insufficient to make stronger or more conclusive recommendations regarding the health effects of eating red meat. However, the BPRF approach has been criticised for being overly simplistic.
Cancer.
The International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) classifies processed meat as carcinogenic to humans (Group 1), based on "sufficient evidence in humans that the consumption of processed meat causes colorectal cancer." Unprocessed red meat is categorised as "probably carcinogenic to humans (Group 2A), based on limited evidence that the consumption of red meat causes cancer in humans and strong mechanistic evidence supporting a carcinogenic effect." Positive associations have also been observed between red meat consumption and increased risks of pancreatic cancer and prostate cancer but the link is not as clear. The American Cancer Society have stated "evidence that red and processed meats increase cancer risk has existed for decades, and many health organizations recommend limiting or avoiding these foods."
The American Institute for Cancer Research, Cancer Research UK and World Cancer Research Fund International have stated there is strong evidence that consumption of processed red meat is a cause of colorectal cancer and there is probable evidence that unprocessed red meat is a cause of cancer. Put in perspective in the UK, adults eating processed and red meat at 79g per day on average had a 32% increased risk of colorectal cancer compared to those eating less than 11g per day.
Reviews have also found that high consumption of red meat is associated with increased risk of gastrointestinal cancer.
Mechanisms.
Heme iron in red meat has been associated with increased colorectal cancer risk. The American Institute for Cancer Research and World Cancer Research Fund have commented that "haem iron, which is present at high levels in red meat, has been shown to promote colorectal tumorigenesis by stimulating the endogenous formation of carcinogenic N-nitroso compound." N-Glycolylneuraminic acid (Neu5Gc) has also been suggested as a mechanism. 
A 2017 literature review indicated there are numerous potential carcinogens of colorectal tissue in red meat, particularly those in processed red meat products, such as N-nitroso compounds, polycyclic aromatic hydrocarbons (PAHs), and heterocyclic amines (HCAs). Cooking meat with "high-temperature methods, such as pan frying or grilling directly over an open flame", also causes formation of PAHs and HCAs.
Cardiovascular disease and stroke.
Red meat consumption is associated with an increased risk of coronary heart disease, high blood pressure and stroke. Factors associated with increased stroke risk from consuming red meat include saturated fats that increase levels of blood cholesterol, LDL cholesterol, triglycerides, and heme iron, which may precipitate atherogenesis in cerebral arteries, leading to stroke.
In 2020, the National Heart Foundation of New Zealand Expert Nutrition Policy (ENP) issued a position statement that concluded that high consumption of red meat increases risk of heart disease and stroke by 16% therefore one should aim to reduce consumption of red meat below 350g per week and replace meat with plant sources of protein.
Diabetes.
Processed and unprocessed red meat consumption is a risk factor for developing type 2 diabetes across populations.
A 2017 review found that daily consumption of 85 grams of red meat and 35 grams of processed red meat products by European and American consumers increased their risk of type 2 diabetes by 18–36%, while a diet of abstinence of red meat consuming whole grains, vegetables, fruits, and dairy was associated with an 81% reduced risk of diabetes. One study estimated that "substitutions of one serving of nuts, low-fat dairy, and whole grains per day for one serving of red meat per day were associated with a 16–35% lower risk of type 2 diabetes". A 2022 umbrella review found that consuming an additional 100g of red meat per day was associated with a 17% increased risk of type 2 diabetes. 
The European Association for the Study of Diabetes recommends that diabetics minimise the consumption of red meat.
Inflammatory bowel disease.
High red and processed meat consumption is associated with an increased risk of inflammatory bowel disease. The American Gastroenterological Association have stated that a diet low in red and processed meat may reduce ulcerative colitis flares.
References.
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