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Nitrates in foods such as spinach, beet root and lettuce stimulate the production of nitric oxide in the body. Nitric oxide relaxes blood vessels. Ingested nitrate is reduced by oral, commensal, bacteria to nitrite, which can further be reduced to nitric oxide.
Vegetables are a major source of dietary nitrate. Green leafy and root vegetables, such as spinach and carrots, provide more than 85% of dietary nitrate. Foods in which nitrite are present are bacon, fermented sausage, hot dogs, bologna, salami, corned beef, ham and other products such as smoked or cured meat, fish and poultry. The conversion of dietary nitrate to nitrite has antimicrobial benefits in the mouth and stomach. Some epidemiological studies show a reduced rate of gastric and intestinal cancer in groups with a high vegetable-based nitrate intake.
Nitrate is totally harmless; however, it can be converted to nitrite and some portion of nitrite to nitrosamines, some of which are known to be carcinogenic. Heating increases the conversion rate. The longer the heat treatment, the more nitrosamines will be formed. Hence, the recommendation not to heat leafy vegetables twice.
Adding lemon juice to vegetables will reduce the formation of nitrosamines. It contains vitamin C, which also reacts with nitrite, thereby preventing the nitrosamine formation.