Nitrite in Cured Meat Products
Sodium nitrite is a salt and an anti-oxidant. Salt (sodium chloride or table salt) containing some sodium nitrate and lesser amounts of sodium nitrite as impurities was used by the ancient Egyptians and Romans as well as the early Europeans to preserve meat. From these early experiences evolved products such as hams, bacon, sausages, bologna, salami and pepperoni.
Today, the use of sodium nitrite is acknowledged by regulatory authorities around the world as an important contributor to food safety. In addition, a rapidly expanding body of scientific research is demonstrating that the advantages of nitrite extend well beyond food safety to include a diverse and critical array of potent benefits for human health.
Sources of Nitrate and Nitrite Intake by Humans
Nitrate and nitrite are widespread in the environment. Dietary sources are mainly from vegetables, cured meats and drinking water.
Nitrate can be found at high concentrations, ranging from 200 to 2500 milligrams per kilogram (mg/kg), in vegetables and fruits. Vegetables containing high levels of nitrate include red beets, spinach, radishes, celery, lettuce, cabbage and broccoli. The concentration of nitrite in vegetables and fruits (on average, 10 mg/kg) is lower than that of nitrate and rarely exceeds 100 mg/kg.
Fresh meat normally contains low levels of nitrate and nitrite. In the case of cured meat products, federal food safety regulations require the addition of at least 100 mg/kg of nitrate and nitrite, but not more than 200 mg/kg, during the preparation process.
Referred to as the “depletion phenomenon,” most of nitrite added during the manufacture of cured meat products is depleted in the course of a series of nitrogen oxide reactions during processing and storage. Between 10 % and 20 % of the added nitrite may remain after the manufacturing process. This residual nitrite declines slowly during the storage of cured meat products.
Levels of residual nitrate and nitrite remaining in food commodities were measured as part of a total diet study sponsored by Health Canada in Ottawa in 2000. Nitrate was found in wieners and sausages (34.7 mg/kg) and in luncheon meats and cold cuts (41.2 mg/kg). The highest levels of nitrate were found in soya-based infant formula (45.9 mg/kg) and in dinners prepared with meat, poultry and vegetables (43.7 mg/kg). Nitrite was found in wieners and sausages (15.1 mg/kg), luncheon meats and cold cuts (11.6 mg/kg) and in hot dogs (11.1 mg/kg).
A similar total diet study was conducted by Health Canada in St. John's in 2001. Although nitrate was detected in luncheon meats and cold cuts (4.46 mg/kg), the highest levels were in frozen entrees (6.68 mg/kg) and processed cheese (5.11 mg/kg). Nitrite was measured in luncheon meats and cold cuts (6.78 mg/kg) and in wieners and sausages (5.20 mg/kg).
The results disclosed by the Canadian cities are similar to U.S. findings reported in 2012 by the Journal of Agricultural and Food Chemistry. In this study, 470 cured meat products were acquired from retail outlets in five major metropolitan cities across the United States. The overall weighted mean residual concentration was 37 mg/kg for nitrate and 4.5 mg/kg for nitrite.
Human Metabolism of Nitrate and Nitrite
Vegetables constitute, by far, the major source of nitrate in the human diet, providing over 85% of the average daily intake. In addition, nitrate and nitrite are synthesized by the body’s metabolism as bi-products of nitric oxide production during the nitrogen oxide cycle. Nitrate produced by body metabolism is secreted in saliva. Overall, approximately 25% of the nitrate that is swallowed is recycled back to the mouth via the salivary glands.
Ingested nitrate is reduced chemically to nitrite by symbiotic bacteria in the oral cavity. Normally, about 20% of the nitrate in the mouth from saliva and food is converted to nitrite. This conversion accounts for over 90% of total human ingestion of nitrite, the remainder coming directly from sources outside the body.
Nitrite Essential in Cured Meats
The preservation or curing of meat, poultry, fish, seafood and vegetables by salting predates written history. Nitrite provides both food safety and food quality benefits.
Nitrite is an essential curing ingredient that fixes the characteristic colour associated with cured meats, creates a unique flavour profile, controls the oxidation of lipids, and, most importantly, serves as an effective antimicrobial, particularly against Clostridium botulinium. Botulism is rare today because processing methods and preservatives such as sodium nitrite protect consumers.
Of the 100 to 200 mg/kg of nitrate and nitrite added to meats during the manufacturing process, the lion’s share is required for the control of C. botulinum. As nitrite levels increase, so does the inhibition of C. botulinum growth and toxin production. A small portion (25 mg/kg, or less) acts as a catalyst for colour development.
In recent decades, the North American market has experienced a marked reduction in the residual nitrite content of cured meats. A 1981 report by the U.S. National Academy of Sciences identified from 10-31 mg/kg in cooked sausages (hot dogs), 12-42 mg/kg in bacon, and 16-37 mg/kg in hams. An independent U.S. study published in 2009 reported residual nitrite levels of 7 mg/kg in hot dogs, bacon and hams. The latter amounted to an approximately 80% reduction in nitrite levels compared to 1975.
The Nitrite Debate
During the 1950s and 1960s, observational studies indicated the potential for nitrites to form carcinogenic N-nitrosamines in food. N-nitrosamine exposure can occur also through other foods, occupational settings, cosmetics, tobacco products and agricultural chemicals.
In the case of cured meats, N-nitrosamine formation may occur when secondary amines react with nitrous acid produced from nitrite at very high temperatures (for example, during the frying of bacon at 170 degrees Celsius). As potential public health concerns are related to the formation of N-nitrosamines rather than to the nitrite itself, regulations were introduced in the 1970s that both limited the addition of nitrite to cured meat products and required the inclusion of N-nitrosamine formation inhibitors in bacon. When added to cured meats, ascorbic acid (vitamin C), erythorbic acid (a mirror image of vitamin C) and alpha-tocopherol (vitamin E) inhibit the potential formation of nitrosamine.
In 1980, an Interagency Working Group convened by the U.S. Food and Drug Agency concluded there was no evidence that increased incidences of cancer tumors of the lymphatic system were induced by the ingestion of sodium nitrite. In 1981 and 1982, this finding was supported in two reports issued by a special committee of the U.S. National Academy of Sciences.
During the 1990s, epidemiological investigations into nitrite, nitrate and nitrosamine exposure and health outcomes were published. Epidemiological studies use public health databases in an endeavour to establish statistical relationships between many factors and human health. One series of studies reported that the consumption of cured meats could be related to cancer and leukemia. These conclusions were reported by the media without complete explanations of the study limitations, uncertainty, or conflicting evidence.
The U.S. National Toxicology Program, a multiagency program supported by many agencies in the U.S. Department of Health and Human Nutrition Services, and the world’s leading authority on the toxicological safety of chemicals, conducted a multi-year study to evaluate the safety of nitrite. Constituting original experimentation that assessed the potential of a direct cause and effect relationship, this study became known as the “gold standard” of research on this subject. The study, peer reviewed by a panel of independent experts in May, 2000, determined that nitrite is safe at the levels used in the meat industry and at levels consumed through the diet.
In 2006, a review of various epidemiological investigations that endeavoured to assess the potential carcinogenicity of nitrate and nitrite was conducted by a working group convened by the International Agency for Research on Cancer (IARC). The working group concluded that: “ingested nitrate or nitrite under conditions that result in endogenous nitrosation is probably carcinogenic to humans.“ In other words, under certain specified conditions, ingested amines and amides can be nitrosated to form carcinogenic nitrosamines. This report did not acknowledge that, as most ingested nitrite is formed in saliva, the swallowing of saliva in combination with virtually any food could be considered to result in the potential formation of nitrosated compounds.
In 2007, a report from the World Cancer Research Fund (WCRF) included a recommendation to limit red meat and eliminate processed meat consumption. These recommendations were considered by others to have been based on weak epidemiological associations and were challenged by researchers both associated with and independent of the meat and poultry industry.
New Research Identifies the Critical Role of Nitrite in Human Physiology
A growing body of recent research has proven the health benefits of nitrite extend well beyond the food industry and reach the heart of cardiovascular medicine. Once considered to be a harmful food additive, nitrite is now being viewed increasingly by members of the scientific community as not only a beneficial molecule, but one with vital medicinal properties.
The new scientific discoveries are providing a better understanding of the profound and important roles nitrite plays in human physiology. The past decade has revealed that nitrite from dietary sources can form nitric oxide within the human body.
Identified as one of the most important cellular signalling mechanisms in the body, maintaining nitric oxide homeostasis is critical for optimal health and disease prevention. The profound and far-reaching significance of this revelation was of such importance that, in 1998, the Nobel Prize in Physiology or Medicine was awarded for its discovery.
Evidence now demonstrates that nitric oxide insufficiency is the earliest event in the onset and progression of various illnesses including cardiovascular disease, the number one killer of both men and women worldwide. Aging is considered the single most important factor associated with cardiovascular related diseases and deaths. Cardio-protection decreases with increasing age and is attributed to a decline in nitric oxide. The lack of nitric oxide production can lead to hypertension, atherosclerosis, peripheral artery disease, heart failure, and thrombosis resulting in heart attack and stroke. All of these conditions have been shown to be affected positively by dietary nitrite interventions.
Enriching dietary intake of nitrite and nitrate has been demonstrated to reduce significantly injury from heart attack and stroke. When given in the diet, nitrite has been proven to combat vascular inflammation from an unhealthy diet. Additionally, experiments in primates have revealed a beneficial effect of the long-term application of nitrite on cerebral vasospasm (the physical narrowing of the central "lumen" of a blood vessel in the brain due to over contraction of the vessel wall which, in the worst-case scenario, can cease to permit blood flow). Moreover, inhalation of nitrite by sheep selectively dilates pulmonary circulation under hypoxic conditions (the failure to transport sufficient oxygen because of inadequate blood flow). Topical application of nitrite improves skin infections and ulcerations. Furthermore, in the stomach, nitrite-derived nitric oxide may play an important role in host defense and in regulation of gastric mucosal integrity.
In brief, scientific research has now demonstrated that nitrite serves a critical function in the protection of the cardiovascular system from injury.
In the context of these dramatic new and evolving discoveries, the historical classification of nitrite as a “cure” is acquiring new meaning.
The emerging health benefits of nitrite represent a profound change in paradigm from the debate that occurred during the previous five decades. Dietary nitrite may be necessary not only for food safety, but essential also for public health.
Recent research reports on the role of nitric oxide in human health include:
- Jeffrey J. Sindelar, A. L. Milkowski. Human safety controversies surrounding nitrate and nitrite in the diet, Nitric Oxide (2012). ELSEVIER Inc.
- Deepa K. Parthasarathy, Nathan S. Bryan. Sodium nitrite: The “cure” for nitric oxide insufficiency. Meat Science, Volume 92, Issue 3, November 2012
Third-party experts on the role of nitric oxide in human health include:
- Andrew L. Milkowski, PhD, Adjunct Professor, Muscle Biology Laboratory, Department of Animal Sciences, University of Wisconsin
- Nathan S. Bryan, Ph.D., Institute of Molecular Medicine, The University of Texas Health Science Center at Houston
- Dominik Alexander, Ph.D, MSPH, Principal Scientist, Exponent Health Scientists Centre for Epidemiology, Biostatistics and Computational Biology, Boulder, Colorado