On a population wide basis, red meat is definitely associated with cancer. The association has been shown numerous times, but is fairly weak in relevance. The most well-controlled study noted an 0.2-fold increase.
Please note association; this has not yet been shown through interventions nor is the cause known. If we are to answer 'does red meat cause cancer', the answer is 'we do not know'
Recently, a study published controlled for other dietary confounders as well as activity and the basics (smoking, alcohol, social class) and found a Hazard Ratio of 1.16 (+16% risk) for red meat and cancer. Past studies of the same manner find Hazard ratios around 1.2 as well, although this latter study was criticized for its controls. It should be noted that the 20% increased risk was derived from comparing the top quintile (20%) against the bottom quintile of meat consumption, and means a 0.2-fold increase.
A select few studies look at risk of cancer in general, whereas others look at specific forms of cancer. This is due to red meat intake being weakly but confidently associated with various forms of cancer.
A netherlands cohort found an association with a hazard ratio of 2.66 for unprocessed red meat and 3.47 in regards to esophageal squamous cell carcinoma risk in men. No association was found with gastric cancers or esophageal adenocarcinoma, nor esophageal squamous cell carcinoma risk in women.
Colorectal cancer risk is associated with inactivity and low plant consumption
Some studies that measure fiber intake do not find a strong association with distal colorectal cancer risk and red meat.
Interestingly, chicken skin and eggs have been found to confer more risk than lean meats (in a population of Hawaiins), although the strongest correlation was found with total caloric intake. The Relative Risk ratios tended to not exceed 2.0 overall however.
Difference between processed and unprocessed?
Many studies define 'processed' red meats as those that have had added nitrates, due to the correlation between nitrates in meats and nitrosamine formation in the body.
These meats tend to refer to sausages and hot dogs, bacon and other pink meat such as ham, cold cuts, and croquettes.
Compounding and Contributing factors
Risk of colorectal cancer from red meat rises to an 8.8-fold risk when the subject also smokes, and may be due to carcinogens found in the meats as measured by a preference for well-done meat (higher source of the carcinogens Heterocyclic Amines and Polyaromatic Hydrocarbons).
In persons with overactive NAT1/NAT2 enzymes, there is an increased risk of colorectal cancer from meats. This may be due to rapid bioactivation of Heterocyclic Amines, which are known colonic carcinogens. One study noted that this phenotype did not associated with an increased cancer risk (OR=1.43) until HCA intake was measured (OR=6.50); those without the phenotype had a lesser increase in risk (OR=2.32). Other P450 enzymes may also contribute to a lesser extent.
As NAT1/NAT2 enzymes metabolize caffeine only after CYP1A, a caffeine tolerance test can be used to assess personal susceptibility to HCA-induced carcinogenesis risk via CYP1A. This has been used in some studies and, informally, might be a good indicator of risk if taken with some skepticism (with more caffeine tolerance correlating with more risk).
Compounding factors tend to be unhealthy habits like smoking and alcohol; things that are either pro-oxidant in nature or interact with liver enzymes
Many studies note that people who consume more red meat also tend to consume more energy overall and have a higher BMI, eat less fruits and vegetables, and have higher rates of smoking and alcohol consumption with a slightly lower education level. Socioeconomic class has also been related to red meat and morality (lower socioeconomic classes eat more red meat) but this does not seem to fully explain the link either. Exercise is also a factor to be considered, specifically cardiorespiratory fitness, as an indicator of diet.
If not otherwise controlled for, these common cofounders can influence the results of survey research.
As there have been no interventions, it cannot be concluded that red meat causes cancer or mortality. It is associated, but causation has not yet been established.
If it were to cause cancer or increased mortality, suspect areas could be as follows.
Various compounds produced in meats during cooking and processing have possible pro-carcinogenic effects when consumed by humans. These compounds (Heterocyclic Amines, Polyaromatic Hydrocarbons, and Nitrosamines in particular) and preventing their formation can be read up about here.
The content of HCAs and PAHs are more indicative of preparation method than of meat selected, and tend to be associated with preparation methods of red meats rather than chicken or fish (barbecuing, flame grilling). Nitrosamines tend to be associated with pink meats such as ham or bacon, although their levels are even higher in fermented fish products.
In all studies that controlled for vegetable intake, a greater risk was seen in people not consuming vegetables. On a whole, vegetable intake seems to confer a protective effect. In addition, one research group noted that although red meat causes increases in a variety of cancers that fruits and veggies seemed to confer protective effects against just as wide a variety, and were more protective in cohorts that were more at risk. Fruits appeared to be more protective than vegetables in this study.
Preparation techniques of the meat can also confer protective effects, and can be read up more on this FAQ page.