Discussion
The studies presented, have shown a positive and linear relationship of unprocessed and especially, processed red meat consumption with the incidence of CVD, CHD, stroke, HF, T2DM, and all-cause mortality.
Cardiovascular disease: Several studies have demonstrated a positive association of increased consumption of unprocessed red meat and in particular processed meat with an increased incidence of CVD, CHD, and death [6,20-29], which in some studies were dose-dependent [6,22,23,24]. The association of high red meat consumption with CV complications has been attributed to its high content of cholesterol and saturated fats. Saturated fats are high in LDL-C, which has been considered the culprit for the development of ASCVD, CAD and CVD by genetic, epidemiologic, and clinical observational studies [2-12]. The association with consumption of foods high in saturated fat with the incidence of ASCVD has been demonstrated experimentally, first by the studies of Ignatowski [42] in 1908 and subsequently, by other studies [43-45]. In addition, the coexistence of type 2 diabetes mellitus (T2DM) and increased BP caused by the high salt content of processed meat contribute to the increased incidence of CVD and CHD [65]. Processed meat contains also nitrites, which are converted in the bowel into nitrosamines, which have a toxic effect on the pancreatic β-cells, and in addition, it contains advanced glycation and lipooxydation end products, that have diabetogenic effects. It also, contains high amounts of salt, which could increase the BP [66-68]. Therefore, people should decrease as much as possible the consumption of unprocessed red meat and particularly the consumption of processed red meat, since several studies have shown a dose-response relationship between red meat consumption and the incidence of CVD.
Stroke: Besides its association with ASCVD, red meat consumption and especially processed meat has also, been associated with an increased risk of ischemic stroke [20,30-32]. The increased incidence of stroke with the consumption of processed red meat is a direct consequence of ASCVD, hypertension, and T2D, which have been shown to be increased with the high consumption of processed red meat which is high in saturated fats and salt [61-67].
Heart Failure: Red meat consumption, especially processed red meat, is also, associated with an increased risk of HF [32- 35]. The increased risk of HF could be due to a combination of ASCVD, CVD, CHD, T2DM, and hypertension. Other studies have also shown that the increased consumption of processed meat with its high content of salt, could add to the increased incidence of HF [34]. However, the exact association of red meat consumption with the incidence of HF is not clear at present and more studies are needed to clarify this association.
Type 2 Diabetes Mellitus: Several studies have reported an association between unprocessed and processed red meat and increased incidence of T2DM [36-40], with some reviews and meta-analyses reporting a high quality of evidence [74-76], with the exception of a major review and meta-analysis of cohort studies, which found a poor association between red meat consumption and the incidence of T2DM, probably due to the poor quality of included studies [77]. The new onset of T2DM appears to be multifactorial and among the various factors are, the saturated fatty acids, sodium, advanced glycation end products (AGEs), nitrates/nitrites, heme iron, branched chain aminoacids, (BCAAs), endocrine disruptor chemicals (EDCs), and the trimethylamine N-oxide (TMAO) derived from the conversion of choline in the gut [78-80].
Recent Trends in Food Selection: Red meat consumption is high the developed countries and is increasing in the developing countries. Of the 190 metric tons of meat produced globally each year, ½ of it will be consumed by less than 25% of the people living in developed countries [75]. People living in Australia, UK and the US are among those consuming the highest quantities of red meat [1,81]. However, recent studies show that there is a healthy trend in decreasing red meat consumption by the developed countries, but a worrisome trend in increasing red meat consumption by the developing economies [82,83]. Indeed, in the US there is a recent tendency for plant-based meat consumption due to increased awareness of the adverse CV effects of animal meat [84]. An analysis of results of a prospective cohort study of 131,342 subjects of 46,329 were men from the US Health Care Professionals Study and 85,013 were women from the Nurses’ Health Study, showed that plant-based meat consumption was associated with lower incidence of CV mortality than animal-based meat [85]. The public interest in plant-based protein has increased lately, and there is a resurgence in vegetarian and vegan diets [19] mostly by young people (38-65 years of age), usually female, educated (≥ high school education), physically active, and from the Western US regions [86]. Vegetarians usually follow a diet that does not include animal meat, fowl, seafood, or products containing these type of foods. Some vegetarians eat foods that contain dairy products (lacto-vegetarianism) or eggs (ovo-vetarianism), whereas the vegans eat foods that do not contain any type of animal meat, fish, dairy products, eggs, and sometimes even honey [86]. However, the percentage of vegetarians and vegans in the US was only 1.9% in 2012 and the great majority of people switching from red meat to white meat and plant-based meat were the previously red meat eaters. Based on this new trend, several companies have produced plant-based meat patties, which are sold by many fast food restaurants and supermarkets. Also, there are some studies demonstrating that animal meat from grassfed animals is leaner and healthier than meat from animals fed highly processed foods and it should be preferred for consumption [87]. The authors contend that this type of lean meat lowers the amount of saturated fats and is less prone to oxidation and peroxydation, which are implicated in the cause of CVDs and cancer [87]. Other studies contend that switching to consumption of plant-based meat and fiber-based products, although healthier, it could have significant economical consequences for the US, since the livestock industry employs 1.6x106 people and accounts for $31.8 billion kg in meat exports, and that the livestock recycle more than 43.2x103 kg of human inedible food converting it into human edible food, pet food, industrial products, and 4x109 kg of nitrogen fertilizer [88]. Another contention of this study is that plant-based foods do not provide the necessary nutrients and energy required for the normal functions of the body. However, this contention has been challenged by studies showing that the quality of life of female and male vegetarian and vegan endurance runners is not different compared to omnivore runners [20]. We believe that the adaption of the recommendations by American College of Cardiology/American Heart Association (ACC/AHA) [89], to eat plant-based diets high in fiber, eggs, dairy products and unsaturated fatty acids, will far exceed the income reduction from the decreased consumption of animal-based meat. The conviction that high consumption of red meat is unhealthy is so strong, that a recent article by the Annals of Internal Medicine [90], suggesting that red meat consumption is not associated with adverse CV effects, was severely challenged by several investigators for reporting unsubstantiated and misleading information to the point of exerting legal action against the journal [91]. Of interest, are two recent studies [92,93] suggesting that the food industry by the year 2050 will have to feed 10 billion subjects and this will necessitate certain dietary changes in order to be sustainable. To accomplish this, a drastic change in the population diet will have to be made that will include the use of much more plant-based diet, and little red meat. Alternatively, they propose the adoption of a holistic flexitarian diet, which is a mixture of plant-based and animal-based foods and has been shown to have significant health benefits [94]..
Conclusion: The analysis of results from the selected papers presented, demonstrates that the consumption of red meat, processed and unprocessed, but not the white meat are associated with increased incidence of ASCVD, CVD, CHD, Stroke, HF and T2DM. Red meat consumption Is still high in the developed countries, although there is a tendency towards a decreased consumption lately. In contrast there is a progressive increase in its consumption by the developing countries, and this is a worrisome sign. On the other hand, the consumption .of healthier foods, like fiber-based products, dairy products, eggs, and unsaturated fats, is still limited. However, there is a recent tendency for an increased consumption of these products including the consumption of plant-based protein. The ACC/AHA guidelines, hopefully, will enforce this tendency and result in the decreased incidence of CVD, HF, stroke, and T2DM.