Meat is animal flesh that is eaten as food. : 1 Humans have hunted and killed animals for meat since prehistoric times. The advent of civilization allowed the domestication of animals such as chickens, sheep, rabbits, pigs and cattle. This eventually led to their use in meat production on an industrial scale with the aid of slaughterhouses.
Meat is mainly composed of water, protein, and fat. It is edible raw, but is normally eaten after it has been cooked and seasoned or processed in a variety of ways. Unprocessed meat will spoil or rot within hours or days as a result of infection with and decomposition by bacteria and fungi.
Most often, meat refers to skeletal muscle and associated fat and other tissues, but it may also describe other edible tissues such as offal. : 1 Meat is sometimes also used in a more restrictive sense to mean the flesh of mammalian species (pigs, cattle, lambs, etc.) raised and prepared for human consumption, to the exclusion of fish, other seafood, insects, poultry, or other animals.  
The word meat comes from the Old English word mete , which referred to food in general. The term is related to mad in Danish, mat in Swedish and Norwegian, and matur in Icelandic and Faroese, which also mean 'food'. The word mete also exists in Old Frisian (and to a lesser extent, modern West Frisian) to denote important food, differentiating it from swiets (sweets) and dierfied (animal feed).
Paleontological evidence suggests that meat constituted a substantial proportion of the diet of even the earliest humans. : 2 Early hunter-gatherers depended on the organized hunting of large animals such as bison and deer. : 2
The domestication of animals, of which we have evidence dating back to the end of the last glacial period (c. 10,000 BCE), : 2 allowed the systematic production of meat and the breeding of animals with a view to improving meat production. : 2 The animals which are now the principal sources of meat were domesticated in conjunction with the development of early civilizations:
- Sheep, originating from western Asia, were domesticated with the help of dogs prior to the establishment of settled agriculture, likely as early as the 8th millennium BCE. : 3 Several breeds of sheep were established in ancient Mesopotamia and Egypt by 3500–3000 BCE. : 3 Today, more than 200 sheep-breeds exist.
- Cattle were domesticated in Mesopotamia after settled agriculture was established about 5000 BCE, : 5 and several breeds were established by 2500 BCE. : 6 Modern domesticated cattle fall into the groups Bos taurus (European cattle) and Bos taurus indicus (zebu), both descended from the now-extinct aurochs. : 5 The breeding of beef cattle, cattle optimized for meat production as opposed to animals best suited for draught or dairy purposes, began in the middle of the 18th century. : 7
- Domestic pigs, which are descended from wild boars, are known to have existed about 2500 BCE in modern-day Hungary and in Troy ; earlier pottery from Jericho and Egypt depicts wild pigs. : 8 Pork sausages and hams were of great commercial importance in Greco-Roman times. : 8 Pigs continue to be bred intensively as they are being optimized to produce meat best suited for specific meat products. : 9
Other animals are or have been raised or hunted for their flesh. The type of meat consumed varies much between different cultures, changes over time, depending on factors such as tradition and the availability of the animals. The amount and kind of meat consumed also varies by income, both between countries and within a given country. 
- Horses are commonly eaten in France,  Italy, Germany and Japan, among other countries. Horses and other large mammals such as reindeer were hunted during the late Paleolithic in western Europe.
- Dogs are consumed in China,  South Korea  and Vietnam.  Dogs are also occasionally eaten in the Arctic regions.  Historically, dog meat has been consumed in various part of the world, such as Hawaii,  Japan,  Switzerland and Mexico.
- Cats are consumed in Southern China, Peru  and sometimes also in Northern Italy.  
- Guinea pigs are raised for their flesh in the Andes. 
- Whales and dolphins are hunted, partly for their flesh, in Japan, Alaska, Siberia, Canada, the Faroe Islands, Greenland, Iceland, Saint Vincent and the Grenadines and by two small communities in Indonesia. 
Modern agriculture employs a number of techniques, such as progeny testing, to speed artificial selection by breeding animals to rapidly acquire the qualities desired by meat producers. : 10 For instance, in the wake of well-publicised health concerns associated with saturated fats in the 1980s, the fat content of United Kingdom beef, pork and lamb fell from 20–26 percent to 4–8 percent within a few decades, due to both selective breeding for leanness and changed methods of butchery. : 10 Methods of genetic engineering aimed at improving the meat production qualities of animals are now also becoming available. : 14
Even though it is a very old industry, meat production continues to be shaped strongly by the evolving demands of customers. The trend towards selling meat in pre-packaged cuts has increased the demand for larger breeds of cattle, which are better suited to producing such cuts. : 11 Even more animals not previously exploited for their meat are now being farmed, especially the more agile and mobile species, whose muscles tend to be developed better than those of cattle, sheep or pigs. : 11 Examples are the various antelope species, the zebra, water buffalo and camel, : 11ff as well as non-mammals, such as the crocodile, emu and ostrich. : 13 Another important trend in contemporary meat production is organic farming which, while providing no organoleptic benefit to meat so produced,  meets an increasing demand for organic meat. 
Meat consumption varies worldwide, depending on cultural or religious preferences, as well as economic conditions. Vegetarians choose not to eat meat because of ethical, economic, environmental, religious or health concerns that are associated with meat production and consumption.
According to the analysis of the FAO the overall consumption for between 1990 and 2009 has dramatically increased. For example, poultry meat has increased by 76.6% per kilo per capita and pig meat by 19.7%. However, on the contrary, bovine meat has decreased from 10.4 kilograms (23 lb)/capita in 1990 to 9.6 kilograms (21 lb)/capita in 2009. 
Growth and development of meat animals
Agricultural science has identified several factors bearing on the growth and development of meat in animals.
Several economically important traits in meat animals are heritable to some degree (see the adjacent table) and can thus be selected for by animal breeding. In cattle, certain growth features are controlled by recessive genes which have not so far been controlled, complicating breeding. : 18 One such trait is dwarfism ; another is the doppelender or " double muscling " condition, which causes muscle hypertrophy and thereby increases the animal's commercial value. : 18 Genetic analysis continues to reveal the genetic mechanisms that control numerous aspects of the endocrine system and, through it, meat growth and quality. : 19
Genetic engineering techniques can shorten breeding programs significantly because they allow for the identification and isolation of genes coding for desired traits, and for the reincorporation of these genes into the animal genome. : 21 To enable such manipulation, research is ongoing (as of 2006) to map the entire genome of sheep, cattle and pigs. : 21 Some research has already seen commercial application. For instance, a recombinant bacterium has been developed which improves the digestion of grass in the rumen of cattle, and some specific features of muscle fibres have been genetically altered. : 22
Experimental reproductive cloning of commercially important meat animals such as sheep, pig or cattle has been successful. The multiple asexual reproduction of animals bearing desirable traits can thus be anticipated, : 22 although this is not yet practical on a commercial scale.
Heat regulation in livestock is of great economic significance, because mammals attempt to maintain a constant optimal body temperature. Low temperatures tend to prolong animal development and high temperatures tend to retard it. : 22 Depending on their size, body shape and insulation through tissue and fur, some animals have a relatively narrow zone of temperature tolerance and others (e.g. cattle) a broad one. : 23 Static magnetic fields, for reasons still unknown, also retard animal development. : 23
The quality and quantity of usable meat depends on the animal's plane of nutrition , i.e., whether it is over- or underfed. Scientists disagree, however, about how exactly the plane of nutrition influences carcase composition. : 25
The composition of the diet, especially the amount of protein provided, is also an important factor regulating animal growth. : 26 Ruminants, which may digest cellulose, are better adapted to poor-quality diets, but their ruminal microorganisms degrade high-quality protein if supplied in excess. : 27 Because producing high-quality protein animal feed is expensive (see also Environmental impact below), several techniques are employed or experimented with to ensure maximum utilization of protein. These include the treatment of feed with formalin to protect amino acids during their passage through the rumen, the recycling of manure by feeding it back to cattle mixed with feed concentrates, or the partial conversion of petroleum hydrocarbons to protein through microbial action. : 30
In plant feed, environmental factors influence the availability of crucial nutrients or micronutrients, a lack or excess of which can cause a great many ailments. : 29 In Australia, for instance, where the soil contains limited phosphate, cattle are being fed additional phosphate to increase the efficiency of beef production. : 28 Also in Australia, cattle and sheep in certain areas were often found losing their appetite and dying in the midst of rich pasture; this was at length found to be a result of cobalt deficiency in the soil. : 29 Plant toxins are also a risk to grazing animals; for instance, sodium fluoroacetate, found in some African and Australian plants, kills by disrupting the cellular metabolism. : 29 Certain man-made pollutants such as methylmercury and some pesticide residues present a particular hazard due to their tendency to bioaccumulate in meat, potentially poisoning consumers. : 30
Meat producers may seek to improve the fertility of female animals through the administration of gonadotrophic or ovulation -inducing hormones. : 31 In pig production, sow infertility is a common problem — possibly due to excessive fatness. : 32 No methods currently exist to augment the fertility of male animals. : 32 Artificial insemination is now routinely used to produce animals of the best possible genetic quality, and the efficiency of this method is improved through the administration of hormones that synchronize the ovulation cycles within groups of females. : 33
Growth hormones, particularly anabolic agents such as steroids, are used in some countries to accelerate muscle growth in animals. : 33 This practice has given rise to the beef hormone controversy, an international trade dispute. It may also decrease the tenderness of meat, although research on this is inconclusive, : 35 and have other effects on the composition of the muscle flesh. : 36ff Where castration is used to improve control over male animals, its side effects are also counteracted by the administration of hormones. : 33
Sedatives may be administered to animals to counteract stress factors and increase weight gain. : 39 The feeding of antibiotics to certain animals has been shown to improve growth rates also. : 39 This practice is particularly prevalent in the USA, but has been banned in the EU, partly because it causes antimicrobial resistance in pathogenic microorganisms. : 39
Numerous aspects of the biochemical composition of meat vary in complex ways depending on the species, breed, sex, age, plane of nutrition, training and exercise of the animal, as well as on the anatomical location of the musculature involved. : 94–126 Even between animals of the same litter and sex there are considerable differences in such parameters as the percentage of intramuscular fat. : 126
Adult mammalian muscle flesh consists of roughly 75 percent water, 19 percent protein, 2.5 percent intramuscular fat, 1.2 percent carbohydrates and 2.3 percent other soluble non-protein substances. These include nitrogenous compounds, such as amino acids, and inorganic substances such as minerals. : 76
Muscle proteins are either soluble in water ( sarcoplasmic proteins, about 11.5 percent of total muscle mass) or in concentrated salt solutions ( myofibrillar proteins, about 5.5 percent of mass). : 75 There are several hundred sarcoplasmic proteins. : 77 Most of them – the glycolytic enzymes – are involved in the glycolytic pathway, i.e., the conversion of stored energy into muscle power. : 78 The two most abundant myofibrillar proteins, myosin and actin, : 79 are responsible for the muscle's overall structure. The remaining protein mass consists of connective tissue ( collagen and elastin) as well as organelle tissue. : 79
Fat in meat can be either adipose tissue, used by the animal to store energy and consisting of "true fats" ( esters of glycerol with fatty acids), : 82 or intramuscular fat, which contains considerable quantities of phospholipids and of unsaponifiable constituents such as cholesterol. : 82
Red and white meat
Meat can be broadly classified as "red" or "white" depending on the concentration of myoglobin in muscle fibre. When myoglobin is exposed to oxygen, reddish oxymyoglobin develops, making myoglobin-rich meat appear red. The redness of meat depends on species, animal age, and fibre type: Red meat contains more narrow muscle fibres that tend to operate over long periods without rest, : 93 while white meat contains more broad fibres that tend to work in short fast bursts. : 93
|fish.||110–140||20–25 g||0 g||1–5 g|
|chicken breast||160||28 g||0 g||7 g|
|lamb||250||30 g||0 g||14 g|
|steak (beef top round)||210||36 g||0 g||7 g|
|steak (beef T-bone)||450||25 g||0 g||35 g|
All muscle tissue is very high in protein, containing all of the essential amino acids, and in most cases is a good source of zinc, vitamin B12, selenium, phosphorus, niacin, vitamin B6, choline, riboflavin and iron.  Several forms of meat are also high in vitamin K.  Muscle tissue is very low in carbohydrates and does not contain dietary fiber.  While taste quality may vary between meats, the proteins, vitamins, and minerals available from meats are generally consistent.
The fat content of meat can vary widely depending on the species and breed of animal, the way in which the animal was raised, including what it was fed, the anatomical part of the body, and the methods of butchering and cooking. Wild animals such as deer are typically leaner than farm animals, leading those concerned about fat content to choose game such as venison. Decades of breeding meat animals for fatness is being reversed by consumer demand for meat with less fat. The fatty deposits that exist with the muscle fibers in meats soften meat when it is cooked and improve the flavor through chemical changes initiated through heat that allow the protein and fat molecules to interact. The fat, when cooked with meat, also makes the meat seem juicier. However, the nutritional contribution of the fat is mainly calories as opposed to protein. As fat content rises, the meat's contribution to nutrition declines. In addition, there is cholesterol associated with fat surrounding the meat. The cholesterol is a lipid associated with the kind of saturated fat found in meat. The increase in meat consumption after 1960 is associated with, though not definitively the cause of, significant imbalances of fat and cholesterol in the human diet. 
The table in this section compares the nutritional content of several types of meat. While each kind of meat has about the same content of protein and carbohydrates, there is a very wide range of fat content.
Meat is produced by killing an animal and cutting flesh out of it. These procedures are called slaughter and butchery, respectively. There is ongoing research into producing meat in vitro, that is, outside of animals.
Upon reaching a predetermined age or weight, livestock are usually transported en masse to the slaughterhouse. Depending on its length and circumstances, this may exert stress and injuries on the animals, and some may die en route . : 129 Unnecessary stress in transport may adversely affect the quality of the meat. : 129 In particular, the muscles of stressed animals are low in water and glycogen, and their pH fails to attain acidic values, all of which results in poor meat quality. : 130 Consequently, and also due to campaigning by animal welfare groups, laws and industry practices in several countries tend to become more restrictive with respect to the duration and other circumstances of livestock transports.
Animals are usually slaughtered by being first stunned and then exsanguinated (bled out). Death results from the one or the other procedure, depending on the methods employed. Stunning can be effected through asphyxiating the animals with carbon dioxide, shooting them with a gun or a captive bolt pistol, or shocking them with electric current. : 134ff In most forms of ritual slaughter, stunning is not allowed.
Draining as much blood as possible from the carcass is necessary because blood causes the meat to have an unappealing appearance and is a breeding ground for microorganisms. : 1340 The exsanguination is accomplished by severing the carotid artery and the jugular vein in cattle and sheep, and the anterior vena cava in pigs. : 137
Dressing and cutting
After exsanguination, the carcass is dressed; that is, the head, feet, hide (except hogs and some veal), excess fat, viscera and offal are removed, leaving only bones and edible muscle. : 138 Cattle and pig carcases, but not those of sheep, are then split in half along the mid ventral axis, and the carcase is cut into wholesale pieces. : 138 The dressing and cutting sequence, long a province of manual labor, is progressively being fully automated. : 138
Under hygienic conditions and without other treatment, meat can be stored at above its freezing point (–1.5 °C) for about six weeks without spoilage, during which time it undergoes an aging process that increases its tenderness and flavor. : 141
During the first day after death, glycolysis continues until the accumulation of lactic acid causes the pH to reach about 5.5. The remaining glycogen, about 18 g per kg, is believed to increase the water-holding capacity and tenderness of the flesh when cooked. : 87 Rigor mortis sets in a few hours after death as ATP is used up, causing actin and myosin to combine into rigid actomyosin and lowering the meat's water-holding capacity, : 90 causing it to lose water ("weep"). : 146 In muscles that enter rigor in a contracted position, actin and myosin filaments overlap and cross-bond, resulting in meat that is tough on cooking : 144 – hence again the need to prevent pre-slaughter stress in the animal.
Over time, the muscle proteins denature in varying degree, with the exception of the collagen and elastin of connective tissue, : 142 and rigor mortis resolves. Because of these changes, the meat is tender and pliable when cooked just after death or after the resolution of rigor , but tough when cooked during rigor. : 142 As the muscle pigment myoglobin denatures, its iron oxidates, which may cause a brown discoloration near the surface of the meat. : 146 Ongoing proteolysis also contributes to conditioning. Hypoxanthine, a breakdown product of ATP, contributes to the meat's flavor and odor, as do other products of the decomposition of muscle fat and protein. : 155
When meat is industrially processed in preparation of consumption, it may be enriched with additives to protect or modify its flavor or color, to improve its tenderness, juiciness or cohesiveness, or to aid with its preservation. Meat additives include the following:
- Salt is the most frequently used additive in meat processing. It imparts flavor but also inhibits microbial growth, extends the product's shelf life and helps emulsifying finely processed products, such as sausages. Ready-to-eat meat products normally contain about 1.5 to 2.5 percent salt. Salt water or similar substances may also be injected into poultry meat to improve the taste and increase the weight, in a process called plumping.
- Nitrite is used in curing meat to stabilize the meat's color and flavor, and inhibits the growth of spore-forming microorganisms such as C. botulinum . The use of nitrite's precursor nitrate is now limited to a few products such as dry sausage, prosciutto or parma ham.
- Phosphates used in meat processing are normally alkaline polyphosphates such as sodium tripolyphosphate. They are used to increase the water-binding and emulsifying ability of meat proteins, but also limit lipid oxidation and flavor loss, and reduce microbial growth.
- Erythorbate or its equivalent ascorbic acid (vitamin C) is used to stabilize the color of cured meat.
- Sweeteners such as sugar or corn syrup impart a sweet flavor, bind water and assist surface browning during cooking in the Maillard reaction.
- Seasonings impart or modify flavor. They include spices or oleoresins extracted from them, herbs, vegetables and essential oils.
- Flavorings such as monosodium glutamate impart or strengthen a particular flavor.
- Tenderizers break down collagens to make the meat more palatable for consumption. They include proteolytic enzymes, acids, salt and phosphate.
- Dedicated antimicrobials include lactic, citric and acetic acid, sodium diacetate, acidified sodium chloride or calcium sulfate, cetylpyridinium chloride, activated lactoferrin, sodium or potassium lactate, or bacteriocins such as nisin.
- Antioxidants include a wide range of chemicals that limit lipid oxidation, which creates an undesirable "off flavor", in precooked meat products.
- Acidifiers, most often lactic or citric acid, can impart a tangy or tart flavor note, extend shelf-life, tenderize fresh meat or help with protein denaturation and moisture release in dried meat. They substitute for the process of natural fermentation that acidifies some meat products such as hard salami or prosciutto.
With the rise of complex supply chains, including cold chains, in developed economies, the distance between the farmer or fisherman and customer has grown, increasing the possibility for intentional and unintentional misidentification of meat at various points in the supply chain. 
In 2013, reports emerged across Europe that products labelled as containing beef actually contained horse meat.  In February 2013 a study was published showing that about one-third of raw fish are misidentified across the United States. 
Various forms of imitation meat have been created for people who wish not to eat meat but still want to taste its flavor and texture. Meat imitates are typically some form of processed soybean ( tofu, tempeh), but they can also be based on wheat gluten or even fungi ( quorn).
Various environmental effects are associated with meat production. Among these are greenhouse gas emissions, fossil energy use, water use, water quality changes, and effects on grazed ecosystems.
The livestock sector may be the largest source of water pollution (due to animal wastes, fertilizers, pesticides), and it contributes to emergence of antibiotic resistance. It accounts for over 8% of global human water use. It is by far the biggest cause of land use, as it accounts for nearly 40% of the global land surface.  It is a significant driver of biodiversity loss, as it causes deforestation, ocean dead zones, land degradation, pollution, and overfishing.      It is reportedly responsible for 14.5% of the world's anthropogenic greenhouse gas emissions.  Although, a revised study of the environmental impact of livestock states that they account for 51% of worldwide greenhouse gas emissions. 
The occurrence, nature and significance of environmental effects varies among livestock production systems.  Grazing of livestock can be beneficial for some wildlife species, but not for others.   Targeted grazing of livestock is used as a food-producing alternative to herbicide use in some vegetation management.  Meat-producing livestock can provide environmental benefits through waste reduction, e.g. conversion of human-inedible residues of food crops.   Manure from meat-producing livestock is used as fertilizer; it may be composted before application to food crops. Substitution of animal manures for synthetic fertilizers in crop production can be environmentally significant, as between 43 and 88 MJ of fossil fuel energy are used per kg of nitrogen in manufacture of synthetic nitrogenous fertilizers. 
According to a report produced by United Nations Environment Programme's (UNEP) international panel for sustainable resource management, a worldwide transition in the direction of a meat and dairy free diet is indispensable if adverse global climate change were to be prevented.  Meat consumption is considered one of the primary contributors of the sixth mass extinction.    A 2017 study by the World Wildlife Fund found that 60% of global biodiversity loss is attributable to meat-based diets, in particular from the vast scale of feed crop cultivation needed to rear tens of billions of farm animals for human consumption puts an enormous strain on natural resources resulting in a wide-scale loss of lands and species.  In November of 2017, 15,364 world scientists signed a Warning to Humanity calling for, among other things, drastically diminishing our per capita consumption of meat and "dietary shifts towards mostly plant-based foods". 
Spoilage and preservation
The spoilage of meat occurs, if untreated, in a matter of hours or days and results in the meat becoming unappetizing, poisonous or infectious. Spoilage is caused by the practically unavoidable infection and subsequent decomposition of meat by bacteria and fungi, which are borne by the animal itself, by the people handling the meat, and by their implements. Meat can be kept edible for a much longer time – though not indefinitely – if proper hygiene is observed during production and processing, and if appropriate food safety, food preservation and food storage procedures are applied. Without the application of preservatives and stabilizers, the fats in meat may also begin to rapidly decompose after cooking or processing, leading to an objectionable taste known as warmed over flavor.
Methods of preparation
Fresh meat can be cooked for immediate consumption, or be processed, that is, treated for longer-term preservation and later consumption, possibly after further preparation. Fresh meat cuts or processed cuts may produce iridescence, commonly thought to be due to spoilage but actually caused structural coloration and diffraction of the light.  A common additive to processed meats, both for preservation and because it prevents discoloring, is sodium nitrite, which, however, is also a source of health concerns, because it may form carcinogenic nitrosamines when heated. 
Meat is prepared in many ways, as steaks, in stews, fondue, or as dried meat like beef jerky. It may be ground then formed into patties (as hamburgers or croquettes), loaves, or sausages, or used in loose form (as in "sloppy joe" or Bolognese sauce).
Some meat is cured by smoking, which is the process of flavoring, cooking, or preserving food by exposing it to the smoke from burning or smoldering plant materials, most often wood. In Europe, alder is the traditional smoking wood, but oak is more often used now, and beech to a lesser extent. In North America, hickory, mesquite, oak, pecan, alder, maple, and fruit-tree woods are commonly used for smoking. Meat can also be cured by pickling, preserving in salt or brine (see salted meat and other curing methods). Other kinds of meat are marinated and barbecued, or simply boiled, roasted, or fried.
Meat is generally eaten cooked, but many recipes call for raw beef, veal or fish ( tartare). Steak tartare is a meat dish made from finely chopped or minced raw beef or horse meat.   Meat is often spiced or seasoned, particularly with meat products such as sausages. Meat dishes are usually described by their source (animal and part of body) and method of preparation (e.g., a beef rib).
Meat is a typical base for making sandwiches. Popular varieties of sandwich meat include ham, pork, salami and other sausages, and beef, such as steak, roast beef, corned beef, pepperoni, and pastrami. Meat can also be molded or pressed (common for products that include offal, such as haggis and scrapple) and canned.
A study of 400,000 subjects conducted by the European Prospective Investigation into Cancer and Nutrition and published in 2013 showed "a moderate positive association between processed meat consumption and mortality, in particular due to cardiovascular diseases, but also to cancer." 
A 1999 metastudy combined data from five studies from western countries. The metastudy reported mortality ratios, where lower numbers indicated fewer deaths, for fish eaters to be 0.82, vegetarians to be 0.84, occasional meat eaters to be 0.84. Regular meat eaters and vegans shared the highest mortality ratio of 1.00. 
In response to changing prices as well as health concerns about saturated fat and cholesterol, consumers have altered their consumption of various meats. A USDA report points out that consumption of beef in the United States between 1970–1974 and 1990–1994 dropped by 21%, while consumption of chicken increased by 90%.  During the same period of time, the price of chicken dropped by 14% relative to the price of beef. In 1995 and 1996, beef consumption increased due to higher supplies and lower prices.
The 2015–2020 Dietary Guidelines for Americans asked men and teenage boys to increase their consumption of vegetables or other underconsumed foods because they eat too much protein. 
Various toxic compounds can contaminate meat, including heavy metals, mycotoxins, pesticide residues, and polyaromatic hydrocarbons. Often, these compounds are not very dangerous themselves but can be metabolized in the body to form harmful by-products, so any actual toxicological effects may depend on the individual genome, diet, and history of the consumer. 
Meat and meat products may contain substances such as dioxins, polychlorinated biphenyl (PCBs), and cooked meat may contain carcinogens, that are toxic to the consumer, although any chemical's toxicity is dependent on the dose and timing of exposure. Toxins may be introduced to meat as part of animal feed, as veterinary drug residues, or during processing and cooking. 
Carcinogenesis is the main long-term toxic response of consuming meat and meat byproducts. 
Health concerns have been raised about the consumption of meat increasing the risk of cancer.  In particular, red meat and processed meat were found to be associated with higher risk of cancers of the lung, esophagus, liver, and colon, among others — although also a reduced risk for some minor type of cancers. 
The International Agency for Research on Cancer (IARC) is the specialized cancer agency of the World Health Organization (WHO). IARC classified processed meat (e.g., bacon, ham, hot dogs, sausages) as, " carcinogenic to humans (Group 1), based on sufficient evidence in humans that the consumption of processed meat causes colorectal cancer." IARC also classified red meat 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."   
Another study found an increase risk of pancreatic cancer for red meat and pork. That study noted that, "findings suggest that intakes of red meat and processed meat are positively associated with pancreatic cancer risk and thus are potential target factors for disease prevention. [...] Future analyses of meat and pancreatic cancer risk should focus on meat preparation methods and related carcinogens." 
That study also suggests that fat and saturated fat are not likely contributors to pancreatic cancer. Animal fat, particularly from ruminants, tends to have a higher percentage of saturated fat vs. monounsaturated and polyunsaturated fat when compared to vegetable fats, with the exception of some tropical plant fats;  consumption of which has been correlated with various health problems. The saturated fat found in meat has been associated with significantly raised risks of colon cancer,   although evidence suggests that risks of prostate cancer are unrelated to animal fat consumption. 
Other research does not support significant links between meat consumption and various cancers. Key et al. found that "There were no significant differences between vegetarians and nonvegetarians in mortality from cerebrovascular disease, stomach cancer, colorectal cancer, lung cancer, breast cancer, prostate cancer or all other causes combined." Truswell reviewed numerous studies, concluding that the relationship of colorectal cancer with meat consumption appeared weaker than the "probable" status it had been given by the World Cancer Research Foundation in 1997. A study by Chao et al. (2005) found an apparent association of colorectal cancer with red meat consumption after adjustment for age and energy intake. However, after further adjustment for body mass index, cigarette smoking and other covariates, no association with red meat consumption was found. Alex' ander conducted a meta-analysis which found no association of colorectal cancer with consumption of animal fat or protein.  Based on European data (EPIC-Oxford study), Key et al. found that incidence of colorectal cancer was somewhat lower among meat eaters than among vegetarians. However, they concluded that 'the study is not large enough to exclude small or moderate differences for specific causes of death, and more research on this topic is required'. A study within the European Prospective Investigation into Cancer and Nutrition (EPIC) found that association between esophageal cancer risk and total and processed meat intake was not statistically significant. Another recent study of EPIC  found a significant correlation between eating ' processed meat and cardiovascular diseases...also to cancer. In this population, reduction of processed meat consumption to less than 20 g/day would prevent more than 3% of all deaths.'
The correlation of consumption to increased risk of heart disease is controversial. Some studies fail to find a link between red meat consumption and heart disease  (although the same study found statistically significant correlation between the consumption of processed meat and coronary heart disease), while another study, a survey, conducted in 1960, of 25,153 California Seventh-Day Adventists, found that the risk of heart disease is three times greater for 45- to 64-year-old men who eat meat daily, versus those who did not eat meat.  A major Harvard University study  in 2010 involving over one million people who ate meat found that only processed meat had an adverse risk in relation to coronary heart disease. The study suggests that eating 50 g (less than 2oz) of processed meat per day increases risk of coronary heart disease by 42%, and diabetes by 19%. Equivalent levels of fat, including saturated fats, in unprocessed meat (even when eating twice as much per day) did not show any deleterious effects, leading the researchers to suggest that "differences in salt and preservatives, rather than fats, might explain the higher risk of heart disease and diabetes seen with processed meats, but not with unprocessed red meats."
The EPIC-PANACEA study, published in 2010 in the American Journal of Clinical Nutrition closely tracked 373,803 people over a period of 8 years across 10 countries. It concluded that meat consumption is positively associated with weight gain in men and women.  The National Cattlemen's Beef Association countered by stating that meat consumption may not be associated with fat gain.  In response, the authors of the original study controlled for just abdominal fat across a sample of 91,214 people and found that even when controlling for calories and lifestyle factors, meat consumption is linked with obesity.  Additional studies and reviews have confirmed the finding that greater meat consumption is positively linked with greater weight gain even when controlling for calories, and lifestyle factors.  
A 2011 study by the Translational Genomics Research Institute showed that nearly half (47%) of the meat and poultry in U.S. grocery stores were contaminated with S. aureus , with more than half (52%) of those bacteria resistant to antibiotics. 
Several studies published since 1990 indicate that cooking muscle meat creates heterocyclic amines (HCAs), which are thought to increase cancer risk in humans. Researchers at the National Cancer Institute published results of a study which found that human subjects who ate beef rare or medium-rare had less than one third the risk of stomach cancer than those who ate beef medium-well or well-done.  While eating muscle meat raw may be the only way to avoid HCAs fully, the National Cancer Institute states that cooking meat below 212 °F (100 °C) creates "negligible amounts" of HCAs. Also, microwaving meat before cooking may reduce HCAs by 90%. 
Nitrosamines, present in processed and cooked foods, have been noted as being carcinogenic, being linked to colon cancer. Also, toxic compounds called PAHs, or polycyclic aromatic hydrocarbons, present in processed, smoked and cooked foods, are known to be carcinogenic. 
Meat in society
Meat is part of the human diet in most cultures, where it often has symbolic meaning and important social functions.  Many people, however, choose not to eat meat (this is referred to as vegetarianism) or any food made from animals ( veganism). The reasons for not eating all or some meat may include ethical objections to killing animals for food, health concerns, environmental concerns or religious dietary laws.
Ethics of eating meat
Ethical issues regarding the consumption of meat include objecting to the act of killing animals or to the agricultural practices used in meat production. Reasons for objecting to killing animals for consumption may include animal rights, environmental ethics, or an aversion to inflicting pain or harm on other sentient creatures. Some people, while not vegetarians, refuse to eat the flesh of certain animals (such as cows, pigs, cats, dogs, horses, or rabbits) due to cultural or religious traditions.
Some people eat only the flesh of animals that they believe have not been mistreated, and abstain from the flesh of animals raised in factory farms or else abstain from particular products, such as foie gras and veal. Some people also abstain from milk and its derivatives for ethical reasons, because the production of veal is a byproduct of the dairy industry. The ethical issues with intensive agriculture have to do with the concentration of animals, animal waste, and the potential for dead animals in a small space.
Some techniques of intensive agriculture may be cruel to animals: foie gras is a food product made from the liver of ducks or geese that have been force fed corn to fatten the organ; veal is criticised because the veal calves may be highly restricted in movement, have unsuitable flooring, spend their entire lives indoors, experience prolonged deprivation (sensory, social, and exploratory), and be more susceptible to high amounts of stress and disease. 
The religion of Jainism has always opposed eating meat, and there are also schools of Buddhism and Hinduism that condemn the eating of meat. Jewish dietary rules ( Kashrut ) allow certain ( kosher ) meat and forbid other ( treif ). The rules include prohibitions on the consumption of unclean animals (such as pork, shellfish including mollusca and crustacea, and most insects), and mixtures of meat and milk. Similar rules apply in Islamic dietary laws: The Quran explicitly forbids meat from animals that die naturally, blood, the meat of swine (porcine animals, pigs), and animals dedicated to other than Allah (either undedicated or dedicated to idols) which are haram as opposed to halal. Sikhism forbids meat of slowly slaughtered animals (" kutha ") and prescribes killing animals with a single strike (" jhatka "), but some Sikh groups oppose eating any meat. 
Research in applied psychology has investigated practices of meat eating in relation to morality, emotions, cognition, and personality characteristics.  Psychological research suggests meat eating is correlated with masculinity,  support for social hierarchy,  and reduced openness to experience.  Research into the consumer psychology of meat is relevant both to meat industry marketing  and to advocates of reduced meat consumption.