Understanding the Impact of Nitrates in Vegetables on Human Health: Accumulation, Effects, and Mitigation Strategies” – Delve into the intricate relationship between nitrogen fertilizers, nitrate accumulation in vegetables, and their effects on human health. Explore processing methods, storage conditions, and strategies for reducing nitrate content. Learn how sustainable agriculture practices and careful food processing can contribute to safe and nutritious consumption
Naveen Kumar Mahanti1, Sai Prasanna N2, B. Baburao3, and D.V. Swami4
With the onset of the Green Revolution in the 1960s, higher crop yields and varieties were achieved by introducing different inorganic fertilizers. Nitrogen (N₂) is one of the primary nutrients that improves soil fertility, controls crop growth, metabolic and regulatory processes, and leads to wholesome improvements in agriculture production. Since N₂ supply in the inorganic form to plants from the soil/ earth is limited, fertilizers should be applied for enhancing crop productivity. Synthetic fertilizers are used widelyin agriculture due to their high nutrient density and low cost. In India, the consumption of nitrogenous fertilizers reported approximately20,404 thousand tons during the year 2021-22 (https://www.faidelhi.org/statistics/statistical-database). The extensive usage of nitrogenous fertilizers in agriculture when far exceeds assimilation by crops, nitrate residues accumulated inside the tissues of horticultural crops (i.e., vegetables, herbs, fruits, and vines). Nitrates (NO3¯) constituted the most important nitrogen form taken up readily into the human body, thus affecting public health upon consuming nitrate-accumulated crops.
Effects of nitrate compounds on human health
Nitrate uptake of human body was mainly through the consumption of raw vegetables (80%), drinking water (15%), animal products (meat and cheese), and grains (5%) (Colla et al., 2018). Nitrate intake is not harmful to human life but when it is reduced to reactive products and metabolites, i.e., nitric oxide, nitrite, and N-nitroso compounds found detrimental to public health. The salivary enzymes, oral bacteria (e.g., Staphylococcus sciuri and Streptococcus intermedius), and acidic conditions in the human digestive system endogenously reduce nitrates (NO3–) to nitrites (NO2–), and form nitrosamines on interacting with amines and amides. These nitrites and nitrosamines are the major cause of gastric and bladder cancers (Colla et al., 2018; Bivolarska, & Gatseva, 2015). The nitrites react with haemoglobin present in blood cells forming methemoglobinemia which converts its heme iron configuration from Fe+2 into Fe+3 state. This makes blood lose its oxygen-carrying ability to the tissues. The higher level of methemoglobin leads to methemoglobinemia or blue baby syndrome, where the body or parts of the body are deprived of sufficient oxygen amount to maintain adequate homeostasis (Colla et al., 2018; Bahadoran et al., 2016). Higher nitrate content obstructs the uptake of iodine and results in hypertrophic alterations in the thyroid gland (Bivolarska, & Gatseva, 2015).
Figure:Formation of N-nitroso compounds in the gastrointestinal tract (Kobayashi, 2018).
The detrimental effect of nitrates on human health made many countries and policymakers issue strict regulations for food producers on commercializing nitrate-rich vegetables and processed cereal-based foods to protect vegetarians, infants, and the elder population from high health risks. So far, several clinical and pre-clinical studies have not thoroughly correlated dietary intake and toxicity effects of nitrates, thereby their influence on human health is still uncertain. Some studies reported upon low-concentrated intake of nitrate/nitrites accumulated foods presented beneficial effects such as reduced cardiovascular risks, myocardial infarction, stroke, gastric ulcer, renal failure, and metabolic syndrome (Bahadoran et al., 2016; Li et al., 2022; Ding et al., 2018). As per organizations, European Commission Scientific Committee on Food, Joint Expert Committee of the Food and Agriculture and World Health Organization (WHO), the acceptable daily intake range of nitrate is 0-3.7 mg/kg body weight (i.e., equivalent to 222 mg of NO3¯ per day for a 60 kg individual). However, U.S. Environmental Protection Agency recommended a daily nitrate intake of 7.0 mg/kg body weight, while nitrate dosage higher than 7-35 g is considered fatal to adult humans. European Commission regulations recommended up to 200 mg/kg fresh weight for processed cereal-based and baby foods for infants and children [Colla et al., 2018].
Factors affecting nitrates accumulation in crops
The accumulation of nitrates in horticultural crops depends upon preharvest practices (i.e., plant species/genotype), nitrogen fertilizer application (i.e., time of application, concentration and form), environmental conditions during the plant growth period (i.e., light intensity, spectral quality, photoperiod, air and root zone temperature, and CO₂ concentration), pre-harvesting stage and period, harvest maturity, and postharvest storage conditions (i.e., temperature, light, and duration). Notably, genetic background, nitrate supply and light conditions are predominant factors affecting nitrate levels in crops. The concentration of nitrate accumulated in horticultural crops and vegetables are generally ordered as culinary herbs, leaves, stems, roots and tubers, fruit vegetables, seeds, inflorescences and buds, bulbs, fruits, and fungi. Prominent nitrate-accumulating species are green leafy vegetables (i.e., rocket, Swiss chard, spinach, lettuce, celery, and parsley) [Colla et al., 2018].
Effect of nitrate on Quality attributes of vegetables
Application of nitrogenous fertilizers results in enhanced carbohydrate, protein, oleoresin, and essential oil contents in ginger; protein content in cereals and pulses; oil content in oil seeds; starch content on tuber crops; acid/sugar ratio, total antioxidant activity, titratable acidity, carotenes and vitamin B1 in fruits and vegetables. The accumulated nitrate improved carotenoid and vitamin C contents and mineral contents (such as Ca, Mg, S, B, Cu, and Zn) in green leaves (spinach, swiss chard, and cauliflower) but decreased mineral contents (Fe, Mn, and Mo) in parsley leaves. The postharvest quality and safety of fresh vegetable crops are related to their residual nitrate content which is responsible formation of carcinogenic N-nitroso compounds in humans. Postharvest practices such as processing methods (cooking, boiling, steaming, frying, baking, etc.) and storage conditions (time, temperature, etc.,) will elicit or inhibit the endogenous conversion of nitrates to nitrites in human digestion.
Processing methods (Ding et al., 2018)
- washing of leafy vegetables (like lettuce, spinach, and cabbage) with water reduces nitrate content.
- The nitrate content in vegetables can be reduced by removing the nitrate-rich parts like under the peels of potato, beetroot, and midrib of leafy vegetables (spinach and lettuce). Peeling reduced nitrate content in vegetables like green bananas, potatoes, carrot, lemon, pumpkin, and beetroot.
- Traditional boiling at high water temperatures for longer durations reduced soluble nitrate contents in vegetables such as carrots, onion, potato, beans, tomato, parsley-root, celery etc., due to their leaching into the cooking liquids (water). However, the nitrate reduction in microwave boiling was found low as compared to traditional boiling.
- Blanching of vegetables like green beans, spinach etc., can also lower nitrate content.
- The formation rate of nitrite increased when vegetables were pureed due to the release of endogenous nitrate reductase that forms nitrite, especially in vegetables containing a large amount of nitrates.
- Cooking vegetables in water removed at least 50 % of accumulated nitrates.
- Frying increased nitrate content in vegetable tubers such as dalo, potato, carrot, and sweet potato. Baking shows a slight increase in nitrate content.
- Fermented fruits and vegetables in pickles showed consistent nitrate levels with that of fresh ones, but low nitrite levels are detected.
Storage conditions
The nitrate content of raw and fresh vegetables was high after harvesting while nitrite content was low. During the storage of fresh vegetables at high/ ambient temperatures, the conversion rate of nitrate to nitrite increases, besides degrading the overall quality of vegetables. This increase in nitrite content during storage depends upon the species, bacterial contamination, and nitrate reductase activity. Vegetables when stored under refrigerated conditions (0-10°C), the nitrate reductase activity was inhibited and thus prevented nitrite accumulation along with preserving overall quality. Nitrate and nitrite contents of leafy, stalk, root, and tuberous vegetables remained unaffected even after 12 weeks of storage under refrigerated conditions.
The infant foods should be consumed immediately after preparation or frozen conditions, but consumption time should not be delayed more than 12 h. However effective temperature ranges seem a function of species and storage time. Since nitrite reductase activity is light dependent, this activity can be arrested in vegetables stored in dark conditions (Colla et al., 2019).
Conclusion
Nitrates in vegetables can be reduced by adopting new and sustainable or ecological agriculture practices such as organic farming (which includes usage of manure, biofertilizers, biopesticides, slow-release fertilizers and nano fertilizers), conservation tillage, cover cropping, reducing nitrate concentration in fertigation applications, replacing nitrates with other N-based fertilizers (ammonium and urea) and chlorides (e.g., calcium chloride), and use of genotypes with low nitrate accumulation capacity can be adopted for improving crop productivities. Pre-processing of vegetables such as washing, peeling, blanching and boiling before cooking can reduce the concentration of nitrate content of the end product. Establishing a worldwide database of the food composition of horticultural vegetables with frequent updating will help for robust assessment of dietary nitrate intakes and empirical evaluation of health implications. The up-to-date database and further research on nitrate/nitrites role in human nutrition should be justified for safe consumption.
References
Colla, G., Kim, H. J., Kyriacou, M. C., & Rouphael, Y. (2018). Nitrate in fruits and vegetables. Scientia Horticulturae, 237, 221-238.
Kobayashi, J. (2018). Effect of diet and gut environment on the gastrointestinal formation of N-nitroso compounds: a review. Nitric Oxide, 73, 66-73.
Bahadoran, Z., Mirmiran, P., Jeddi, S., Azizi, F., Ghasemi, A., & Hadaegh, F. (2016). Nitrate and nitrite content of vegetables, fruits, grains, legumes, dairy products, meats and processed meats. Journal of Food Composition and Analysis, 51, 93-105.
Li, X., Zhang, W., Laden, F., Curhan, G. C., Rimm, E. B., Guo, X., … & Wu, S. (2022). Dietary nitrate intake and vegetable consumption, ambient particulate matter, and risk of hypertension in the Nurses’ Health study. Environment International, 161, 107100.
Ding, Z., Johanningsmeier, S. D., Price, R., Reynolds, R., Truong, V. D., Payton, S. C., & Breidt, F. (2018). Evaluation of nitrate and nitrite contents in pickled fruit and vegetable products. Food Control, 90, 304-311.
Bivolarska, A., & Gatseva, P. (2015). Thyroid status in pregnant women and association with nitrates as an environmental factor stimulating the manifestation of iodine deficiency. Trace Elements & Electrolytes, 32(2).
Contact Details
NAVEEN KUMAR MAHANTI
Scientist, Post-Harvest Technology Research Station,
Dr. Y.S.R Horticultural University, Venkataramannagudem,
Andhra Pradesh – 534101.
Email: naveeniitkgp13@gmail.com
SAI PRASANNA
Research Scholar,
Department of Chemical Engineering,
Indian Institute of Technology Tirupati
Andhra Pradesh
Email: sprasanna557@gmail.com
Mobile no: 8333028033
Baburao
Scientist,
Post-Harvest Technology Research Station,
Dr. Y.S.R Horticultural University, Venkataramannagudem,
Andhra Pradesh – 534101.
Email id: babuhorty10@gmail.com
D.V. Swami
Principal Scientist & Head,
Postharvest Technology Research Station,
Dr. Y.S.R Horticultural University, Venkataramannagudem,
Andhra Pradesh – 534101.
Email id: swamihorti@gmail.com
