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Vegetables Guard the Gut... for a Startling Reason
5/19/2008
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Nitrites in cured meats can turn cancerous, but prove stomach-protective when produced from vegetables

by Craig Weatherby


Have the nitrites in cured meats been unfairly demonized?


Not exactly. But a study from Sweden paints a more nuanced picture of this widely feared preservative.


Meat processors use sodium nitrite to preserve the red color in cured meat, and lend it a salty flavor (In a reaction with meats’ myoglobin, nitrite yields a rich red color).


Key Points

  • Swedish study shows that the nitrates in vegetables help strengthen stomach walls by producing nitric oxide in the gut.
  • Oral bacteria turn nitrates into nitrites, which convert to nitric oxide in the stomach.
  • Nitric oxide produced from dietary nitrates protects animals’ stomachs from aspirin-induced damage.
  • Nitrate-rich vegetables may help keep stomachs healthy, but anti-bacterial mouthwash may block their benefits.

Absent added nitrite, hot dogs, bologna, and bacon would look gray. Sodium nitrate is used in dry cured meat, because it breaks down slowly into nitrite.


The real reason may be coloration, but the meat industry justifies its use of nitrite and nitrate on anti-bacterial grounds. In fact, freezing and refrigeration work just as well, and the USDA has developed an alternative method using lactic-acid-producing bacteria.


The nitrites in meat can form cancer-causing chemicals called nitrosamines, and this happens much more when nitrite-cured meats such as bacon are cooked.


Because lab evidence raises concern about peopleespecially children and prengnant motherswho might eat a lot of bacon, U.S. companies must add vitamin C or erythorbate to reduce nitrosamine formation.


Despite dire indications from lab tests, the epidemiological evidence hasn’t shown a convincing link between nitrites or nitrates and cancer risk (Eichholzer M et al. 1998; Blot WJ et al. 1999).


While the cancer risks associated with red and cured meats may stem as much from their iron and/or salt content as to nitrites, both foods are linked strongly to higher colon cancer risk, so it seems wise to minimize consumption.


We now know that people's oral bacteria convert the nitrates that occur naturally in common vegetables into nitrites.


And that unexpected fact leads us to today’s story.


Human mouths produce nitrites in abundance

Some fruits and vegetables are rich in nitrates, with the levels dependent to some extent on the amount of nitrogen-containing fertilizer applied to the crops.


Oral bacteria convert nitrates into nitrites, and when we swallow them, gastric acids convert nitrites into the essential metabolic chemical nitric oxide.

Nitric oxide can act as an antioxidant, but more important, the body uses it to keep arteries properly dilated. (Viagra™ works by raising nitric oxide levels and thereby easing blood flow to the penis.)


We’ve long known that nitric oxide is produced by bodily enzymes. But recently it was discovered that nitric oxide can also be formed in the stomach from nitrites produced in the saliva, without enzymatic assistance.


Nitrates in veggies bolster stomachs and block aspirin-induced bleeding

Doctoral candidate Joel Petersson of Sweden’s Uppsala University has published a study that encourages consumption of nitrate-rich vegetables such as spinach, lettuce, radishes and beetroot.


Earlier this month, his team published a study demonstrating that nitrate-rich vegetables strengthen the stomach’s mucous membranes.


Petersson’s research shows that when nitric oxide is formed in the stomach this stimulates protective mechanisms that exist keep the stomach from digesting itself along with food.


The stomach constantly renews the mucous layer of the mucous membrane and nitric oxide widens the blood vessels in the mucous membrane, thus increasing blood flow and aiding regeneration of mucus (Petersson J et al. 2007).


Last year, Petersson’s team reported that nitrates in food protected rats against the gastrointestinal damage associated with chronic use of aspirin, Aleve, and other non-steroidal anti-inflammatory drugs (NSAIDs).


Gastric bleeding kills tens of thousands of aspirin users annually by causing undiagnosed bleeding and ulcers in the gastrointestinal tract.


The key finding by Petersson is that oral bacteria are vital to supplying nitrates that yield the nitric oxide needed to strengthen the stomach's mucous membrane.


As he wrote, “We demonstrated that a nitrate-rich diet protects against NSAID-induced gastric damage, as a result of the increased formation of NO in the stomach. We also showed that the gastro-protective effect attributed to nitrate depended completely on conversion of nitrate to nitrite by the bacterial flora colonizing the tongue, and that the oral microflora is therefore important in regulating physiological conditions in the stomach.”


The fact that oral bacteria indirectly support gastrointestinal health was revealed last year by a study that Petersson and colleagues conducted in rats.


The Swedes fed rats a nitrate-rich chow, along with aspirin-type anti-inflammatory drugs. But only the animals that also received an antibacterial oral spray suffered damage to their mucous membrane (Jansson EA et al. 2007).


The oral spray had killed the animals’ oral bacteria, which otherwise would have converted the dietary nitrates into nitrites, to be converted to nitric oxide in their stomachs.


As Petersson said, “This shows how important our oral flora [microbes] is. It is an important issue, as antibacterial mouthwashes have become more and more common. If a mouthwash eliminates the bacterial flora in the mouth this may be important to the normal functioning of the stomach, as the protective levels of nitric oxide greatly decrease.”


He also thinks that his team’s results provide another important reason to enjoy fruits and vegetables in abundance: “If we followed the Swedish Food Administration's recommendation and ate 500 g (18 ounces) of fruit and vegetables per day it would definitely be better for our stomachs.”


In addition to their contribution to nitric oxide production in the stomach, prior evidence suggests that the antioxidants in fruits, vegetables, whole grains, and tea may help protect the gastrointestinal tract from free radicals.


Free radicals can damage DNA and thereby promote cancer, so food-borne antioxidants could help delay or deter the development of stomach, colon and rectal cancer.



Sources

  • Björne HH, Petersson J, Phillipson M, Weitzberg E, Holm L, Lundberg JO. Nitrite in saliva increases gastric mucosal blood flow and mucus thickness. J Clin Invest. 2004 Jan;113(1):106-14. Erratum in: J Clin Invest. 2004 Feb;113(3):490.
  • Blot WJ, Henderson BE, Boice JD Jr. Childhood cancer in relation to cured meat intake: review of the epidemiological evidence. Nutr Cancer. 1999;34(1):111-8. Review.
  • Eichholzer M, Gutzwiller F. Dietary nitrates, nitrites, and N-nitroso compounds and cancer risk: a review of the epidemiologic evidence. Nutr Rev. 1998 Apr;56(4 Pt 1):95-105. Review.
  • Jansson EA, Petersson J, Reinders C, Sobko T, Björne H, Phillipson M, Weitzberg E, Holm L, Lundberg JO. Protection from nonsteroidal anti-inflammatory drug (NSAID)-induced gastric ulcers by dietary nitrate. Free Radic Biol Med. 2007 Feb 15;42(4):510-8. Epub 2006 Nov 21.
  • Petersson J, Phillipson M, Jansson EA, Patzak A, Lundberg JO, Holm L. Dietary nitrate increases gastric mucosal blood flow and mucosal defense. Am J Physiol Gastrointest Liver Physiol. 2007 Mar;292(3):G718-24. Epub 2006 Nov 2.
  • Petersson J. Nitrate, Nitrite and Nitric Oxide in Gastric Mucosal Defense. ISBN: 978-91-554-7152-1
  • Uppsala University (UU). Nitrates in vegetables protect against gastric ulcers. May 7, 2008. Accessed online May 11, 2008 at http://www.uu.se/news/news_item.php?typ=pm&id=173

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