Food scare leads to design discovery
The idea that nitrates cause stomach cancer gained credence in the 1980s. Environmentalists made a tenuous connection between nitrates, nitrites and nitrosamines, the latter considered to be carcinogenic. The resulting public health scare resulted in many governments substantially reducing the legal limits on the amount of nitrates and nitrites allowed in food and water. Nitrites were the major preservative for all sorts of foods, particularly manufactured meats.
Are nitrates/nitrites dangerous? Epidemiological research in the mid-1980s failed to find any evidence of health risks.1 People who eat lots of vegetables (a major source of dietary nitrate), or drink water high in nitrates, do not have elevated stomach and bowel cancer rates. Indeed, workers in a factory producing ammonium nitrate fertilizer had no indications of elevated cancer risks and were actually healthier than other factory workers in the area.2
Since the big scare of the 1980s, research has filled in the picture in a remarkable way. It would now appear that, rather than being bad for healthy people, nitrates/nitrites are actually part of our body's defence systems against disease-causing micro-organisms. It works in the following way.3
Nitrate from food (leafy vegetables, especially) is released into the mouth through chewing. Nitrate is also produced within the body and circulates in the blood. If insufficient is released from eating, extra is excreted in the saliva. Anaerobic microbes, such as Staphylococcus sciuri and S. intermedius, in deep pockets in the back of the tongue, reduce the nitrate to nitrite. The nitrite is swallowed, ending up in the stomach. The acidity of the stomach results in the conversion of the nitrite to form large amounts of nitric oxide (NO) and other oxides of nitrogen. The conversion to NO is so rapid that there is no measurable rise in nitrite concentrations in the stomach following ingestion of nitrate.4 This means that carcinogenic nitrosamines are not formed from nitrite in the stomach.
Acidified nitrite and NO have been shown to have strong anti-microbial activity on a wide range of micro-organisms, including pathogens such as Salmonella and Yersinia, common causes of gastro-intestinal disease following the consumption of contaminated food ('food poisoning').
Statistics on food poisoning show that there has been a substantial increase in reported cases of food poisoning in Britain, for example, since 1987, about the time that maximum nitrite and nitrate levels allowed in food and water began to be reduced in earnest.2 The reduction in the allowable levels of nitrite has undoubtedly made it more difficult to control microbes that cause food poisoning and has probably contributed to the increase in reported cases of food poisoning. The food scare of the 1980s has almost certainly indirectly resulted in the deaths of people from severe food poisoning.
NO also has vasodilatory properties (reducing blood pressure) and is involved in controlling platelet activity,3 and so could be a (positive) factor in the heart disease story. Many functions of NO in cell signalling are being discovered.
Far from being poisons, nitrates and nitrites are part of normal mammalian physiology. Nitrites are generated by microbes in symbiotic relationship with us, and are converted to gaseous oxides of nitrogen that have a sterilizing effect in our stomachs. The system is therefore a non-immunological line of defence against invasion by microbes. It seems like a nice example of highly integrated (intelligent) design that involves symbiosis between microbes and mammals. McKnight et al., conclude from their review of the subject, that 'dietary nitrate may have an important therapeutic role to play'.3
Some of the most important bacteria responsible for this symbiosis involving nitrates are forms of Staphylococcus. This may be relevant to the origins of pathogenesis in microbes. Many human pathogens are very similar to free-living or saprophytic (feeding on dead organic matter) forms that do not cause disease. Here is a case. S. aureus is probably the major cause of infections in hospitals, but S. sciuri and S. intermedius help us ward off gastro-intestinal infections! Another example is E. coli. Normally an abundant harmless resident of our bowels, E. coli's presence helps us by suppressing harmful microbes, as well as synthesising vitamins. However, the O157:H7 strain has caused deaths from food poisoning. There are many other examples of bacteria that have harmless forms and pathogenic forms. It is quite possible that pathogenic forms arose by degenerative changes to the harmless forms.
- Gangolli, S.D., van den Brandt, P.A., Feron, V.J., et al., Nitrate, nitrite and N-nitroso compounds, Eur. J. Pharmacol. 292(1):1–38, 1994.
- Addiscott, T., Making a meal of it, New Scientist 165(2224):48–49, 2000.
- McKnight, G.M., Duncan, C.W., Leifert, C. and Golden, M.H., Dietary nitrate in man: friend or foe? Br. J. Nutr. 81(5):349–358, 1999.
- McKnight, G.M., Smith, L.M., Drummond, R.S., Duncan, C.W., Golden, M. and Benjamin, N., Chemical synthesis of nitric oxide in the stomach from dietary nitrate in humans, Gut 40(2):211–214, 1997.