Evaluation of Nitrate Status in Drinking Water and Vegetables of Savadkuh and Simorgh Counties and Its Relationship with the Prevalence of Gastrointestinal Cancers

Introduction: One of the most critical and controversial issues regarding the pollution of water resources and agricultural products is the challenging relationship between nitrate intake and the risk of gastrointestinal cancers. During the last two centuries, human activities have fundamentally changed the global nitrogen cycle and increased the amount of nitrate in the soil. Nitrate and nitrite move quickly in the environment due to the high solubility. Nitrate in wastewater or fertilizers used to increase crop production can be accessed to surface and groundwater by rainfall or irrigation. Nitrate in the diet is absorbed into the bloodstream and enters the oral fluid through the salivary glands. Approximately 25% of nitrate in the human body enters the salivary glands after absorption in the upper gastrointestinal tract, and it is actively concentrated up to 20 times in saliva. Nitrite disrupts young vertebrates by destroying hemoglobin in the blood and forming the toxic compound methemoglobin. Nitrite also causes hypotension (reduction of arterial pressure), a precursor to the formation of carcinogenic nitrosamines. However, there is no conclusive evidence that nitrate can cause cancer without amines containing nitrosamines.
Since accurate information about the amount of nitrate in drinking water sources and vegetables used in Mazandaran province is not available, this study aimed to obtain accurate and up-to-date data on nitrate concentration in drinking water and vegetables. Also, the relationship between nitrate concentration in drinking water and vegetables in areas of Mazandaran province with a high and low prevalence of gastrointestinal cancers was investigated.

Materials and Methods: In this study, two areas with high incidence and low incidence of gastrointestinal cancers (Savadkuh and Simorgh counties, respectively) were identified. Information on patients with gastrointestinal cancer was prepared between 2014-2016. Then the patients living in the studied counties were separated and coded. Nitrate concentration in 30 samples of vegetables (60 samples in total) and water was analyzed by spectrophotometry and compared with international standards.
The selection of vegetable samples was made according to the prevailing consumption pattern of the people. Selected vegetables included tomatoes, lettuce, potatoes, onions, cucumbers, garlic, cress, and spinach. According to the available instructions, plant samples from each region were quickly transferred to the laboratory. Nitrate concentration was measured by the semi-micro distillation method. The relationship between nitrate concentration and the incidence of gastrointestinal cancer and the ASR index of gastrointestinal cancer were determined using SPSS (26) software for Mazandaran province.
Discussion of Results: Statistical analysis of the data showed that nitrate concentration of all studied crops in the Savadkooh region showed a significant difference compared to the Simorgh region, except potato. In the Simorgh region, nitrate concentration in lettuce, cress, potato, and spinach was lower than the maximum limit, while other vegetables were significantly higher than the standard. The data showed that in Savadkuh and Simorgh regions, the highest nitrate concentration was observed in garlic and the lowest in onion.
According to the data, except for potatoes, all other vegetables in the Savadkooh region were significantly higher than the Simorgh region (p <0.01). The average concentration of nitrate in the drinking water of Savadkooh and Simorgh counties was significantly lower than the standards presented in the literature.
The effect of nitrate concentration variables (in the regression model presented) in tomatoes, potato and cucumber, and drinking water was more than the allowable error (0.05). Therefore, changes in the nitrate concentration of these products and the amount of nitrate in drinking water will not significantly affect changing the dependent variable (the incidence of cancer). On the other hand, the significance level of the t-test for the effect of nitrate concentration variables in spinach, garlic, cress, and onion was less than the allowable error (0.01). Therefore, changes in the amount of nitrate in these variables significantly affect changes in the dependent variable. Based on the coefficients related to β, the most effect on the changes of the dependent variable will be the change in the amount of nitrate in spinach and garlic with a coefficient of β of 0.372 and 0.367, respectively. The amount of nitrate in the variables of cress and onion are of the following ranks. Comparison of residual nitrate levels in vegetables in Savadkuh and Simorgh showed that nitrate levels in Simorgh city were lower than Savadkuh, and nitrate concentration in four vegetables i.e. lettuce, cress, onion, and spinach, was less than the maximum limit. However, other products far exceeded the maximum limit. According to the researchers of the Razi University of Kermanshah, the concentration of nitrate in all cases under investigation in Kermanshah was exceeded by the National Standard Institute of Iran.
The highest and lowest nitrate concentrations were observed in the Savadkuh region with an average of 3398 and 71 mg/kg in garlic and onion crops, respectively, and in the Simorgh region with an average of 2259 and 38 mg/kg in garlic and potato as well. The common denominator between the two regions is the lowest nitrate concentration observed in the onion crop.
Significant differences between the two counties in terms of nitrate concentration in the studied products, except potato, can be examined from various aspects. It seems that the management of fertilizer consumption, especially nitrogen fertilizers in the Simorgh region, has been done better. The result has been shown by reducing nitrate accumulation in agricultural products of the region. Nitrogen in chemical and organic fertilizers is converted to nitrate after use in soil by soil bacteria such as Nitrosomonas and Nitrobacter. Nitrate absorbed in the root is reduced and converted to ammonium compounds and transferred through the vascular system to different parts of the plant, and used in physiological processes of the plant. Suppose the amount of soil nitrate increases for any reason, including excessive consumption of nitrogen fertilizers, and the plant does not have the necessary ability to reduce it. This compound enters the plant organs through the mass transfer mechanism and the vascular system and accumulates there.
It has been found that there is a direct relationship between the concentration of nitrate in the environment and the amount of nitrate accumulation in agricultural and horticultural products, so that the most important environmental factor affecting the accumulation of nitrate in the product is the amount of available nitrate ions. In addition, harvest time (morning or evening) is also an essential factor in the amount of nitrate output through the harvested product. Receiving light energy during the day increases the plant's photosynthesis, and the production of more assimilates and increases the activity of the enzyme nitrate reductase in the plant.
The average nitrate concentration in drinking water was 10.4 mg/L in Savadkuh and 15.8 mg/L in Simorgh. Fortunately, this concentration is lower than the standard defined for drinking water (50 mg/L) in both areas.
Conclusions: Although the concentration of nitrate in the drinking water of Savadkuh county is significantly higher than Simorgh one, the concentration of nitrate in the drinking water of both counties is below the standard limit. Therefore, there is probably no significant relationship between the prevalence of gastrointestinal cancers and nitrate concentration in drinking water sources in these areas.
Nitrate concentration in vegetables consumed by patients with gastrointestinal cancers in the Savadkuh region was significantly higher than the Simorgh region and, to some extent, maximum contaminant level. The results showed that the consumption of spinach, garlic, cress, and onion had played an influential role in increasing the incidence of gastrointestinal cancers. Therefore, management practices in agriculture, especially changes in the consumption pattern of nitrogen fertilizers, are inevitable to reduce the accumulation of nitrate in vegetables.
It is strongly recommended that the vegetables be harvested in the afternoon to reduce nitrate accumulation in the plant tissues. It is suggested that new strategies to reduce nitrate accumulation in food, especially vegetables, be studied to reduce the incidence of gastrointestinal cancers in this province.