In my series of articles about nitrogen in the food system, this article about the effect of Nitrogen fertilizers on human health is the most challenging to write. Soils and human health are both very complex systems and the systems are dynamic, which means that it is very difficult to establish a direct causality between the use of nitrogen and any, positive or negative, effects on human health. I believe the effects are spread on four different pathways: 1) the effect of nitrogen fertilizers on the global food system and its effect on health, 2) the effect of nitrogen fertilizers on the nutritional composition of crops, 3) the effect of nitrogen fertilizers on the soil microbiome and the subsequent effects on human health and 4) the effect on nitrogen pollution on human health.
The global food system and health
There is a strong link between the kinds of food eaten and the human health. And the kinds of food eaten are very much a result of how we farm. Let’s start there.
The use of nitrogen fertilizer has undoubtedly changed how and what we farm. One major effect is that it has supported large scale commodity farming of a few important crops such as soybean, maize, wheat and palm oil. This, in turn has changed the food system in two major ways. First, the ample supply of cheap grain and soy has transformed animal farming. Earlier, livestock was integrated in the food system in many ways. In farms, the main role of animals was to convert things that humans could not eat into meat, milk, eggs and other useful products. The manure was recycled to the fields and from slaughterhouses, bones and other waste was brought back as fertilizers. The number of animals in each farm or held by pastoralists was regulated by the capacity of the land.
With commodity crops these linkages were broken and land-less livestock farming became more common. Those industrial farms fed animals with commodity crops, sometimes or often, imported from afar. It also changed which animals were produced. The incredible increase in chicken consumption is a direct result of these changes as chicken production very often is land-less and chicken are superior transformers of grain and soy into meat, especially compared with ruminants. In the earlier agriculture system, in most places, chicken was luxury food (there is a reason for why Dolly Parton sings about Sunday chicken).
Chicken every day!
Cheap grain, starting with maize, also formed the basis for a whole new food industry oriented to stripping apart agriculture raw materials into single components which were reconfigured into a flood of new “products”. This paved the way for the replacement of traditional foods with ready-made foods and ultra-processed foods. In addition, the globalized and linear food system for which synthetic fertilizers are a prerequisite has led to the abandonment of local food systems and the local foods that was the core of the local food system (Khoury et al 2014). The dietary changes which are a direct result of this are causing major health problems: “Greater exposure to ultra-processed food was associated with a higher risk of adverse health outcomes, especially cardiometabolic, common mental disorder, and mortality outcomes” according to a recent review published in the BMJ (Lane et al 2024). The Western diet* has negative effects on the metabolism, antioxidant status, as well as on the gut microbiota and mitochondrial fitness, cardiovascular and mental health, and increases cancer and inflammation (Clemente-Suárez et al 2023). A diet with coarser, less processed foods, fermented foods and foods with a lot of antioxidants, i.e. the opposite to the Western diet, is supposedly best for your health (Sariola and Gilbert 2020, Blum et al 2019).
Well, I think I don’t need to waste more ink (pixels?) on explaining that the modern diet is not resulting in healthy outcomes. I believe I have provided a credible web of interactions here and in an earlier article, that shows a strong link, but as there are so many interactions and feedbacks in the system, I don’t think it will ever be possible to prove causality.
Food is losing oomph
It is equally hard to prove causality between the use of nitrogen fertilizers and direct effects on the food we eat, i.e. effects that are not related to the industrial processes and the overall diets which are caused by the architecture of the food system. But what can be ascertained is that there have been changes in the nutritional content of many foods and that these changes are linked to the application of “green revolution” technologies such as the use of N-fertilizers, high yielding varieties etc.
Mayer (1997) observed declines in the nutrient levels of twenty vegetables from 1936 to 1991, including calcium, magnesium and copper, as well as in the nutrient levels of twenty fruits, including sodium, magnesium, iron, copper and potassium. Studies on rice and wheat in India show a downward trend in concentrations of essential and beneficial elements, but an upward in toxic elements in past 50 years in both rice and wheat (Debnath et al 2023). Research on maize in China showed that the nitrogen, copper, iron, potash, magnesium, phosphorous and zinc concentrations were significantly reduced with the newer varieties. With increasing levels of N-fertilization, nitrogen, copper, iron, magnesium and manganese concentrations rose, while potash, phosphorous and zinc decreased (Guo et al 2020). Also from China, an experiment showed that fertilizer inputs lowered zinc and iron but not selenium in wheat, relative to a control (Han et al 2022).
A comparison of 1,600 compounds in white cabbage from rigidly controlled long-term field trials in Denmark showed that conventional and organic management practices have a systematic impact on the relative content of compounds (Mie et al 2014). Cheap grain produced with N-fertilizers also paved the way for feeding ruminants grain, supplemented with soy or other protein feeds. This in turn has changed the nutritional profile of beef, lamb and milk. Grass-fed beef shows a more favorable profile of saturated fatty acids. Grass-fed beef also contains an increased level of omega-3 fatty acids which can offer benefits (Nogoy et al 2022). Grass-fed dairy and organic dairy cows provide milk significantly higher in beneficial fatty acids and lower in omega-6 (Davis et al 2020). And we haven’t even talked about taste.
The soil-gut connection
Soils harbor the most complex and diverse microbiome on Earth, often with more than 50 000 species and 0.5 mg biomass per gram of soil. This means that the weight of the microorganisms is around 1.5 ton per hectare of land – a lot more than any animals or humans that could ever live off the land. A few of the microorganisms are pathogens or parasites, most are indifferent to us. Many are beneficial for humans and we are exposed to them via dust or direct contact with soils, via animals or via food brought into houses and ingested. In some cultures eating soil, geophagy, has also been rather common, especially among pregnant women (Banerjee and van der Heijden 2022).
Contact with nature makes us healthier
There are several studies showing that children that are in regular contact with animals, soil and nature have less risk of developing autoimmune diseases. A Finnish team of researchers studied groups of children 3-5 years old attending 10 different nursery schools. They got the same food but in four of the schools the yards, which earlier were covered in gravel or tarmac, were turned into natural areas. The children were also encouraged to help with growing vegetables. After four weeks the children that had been playing in natural surroundings and soil had more than 30 percent more microorganisms on their skin and also more in the internal microflora. Blood samples showed increased levels of proteins and other substances associated with the immune system (Roslund et al 2020).
Considering the apparent links between the soil microbiome and the human microbiome and the link between the human microbiome and human health, it is obvious that changes in the soil microbiome influence human health. The intensification of farming has caused a homogenization of soil microbial communities with dominance of a few taxonomic or trophic groups and a decrease in the overall diversity (Gossner et al 2016, Gámez-Virués, S. et al 2015).
As there are hardly any studies on how the application of nitrogen fertilizers alone influence the microbiome of the soil, most relevant research has been conducted on comparisons of organic and conventional farms, and in a few cases also the consumption of organic versus conventional food. Because the use of nitrogen fertilizers, as well as the non-use is part of a “package”, it is reasonable to refer to such research even if some of the effects may very well be the result of the use of synthetic pesticides or modern varieties rather than of the N-fertilizer. When making comparisons between organic and conventional farming one must consider time. Researchers have showed that the increase of biodiversity caused by the transition to organic agriculture progress for at least 30 years (Carrie et al 2024). Changes in microbiome are likely quicker, though.
Comparisons between people eating an organic or a conventionally produced diet tend to show that an organic diet is associated with reduced incidence of infertility, birth defects, allergies, otitis media, pre-eclampsia, metabolic syndrome, high BMI, and non-Hodgkin lymphoma (Vigar et al 2019). Admittedly, it is very hard to draw any definitive conclusions as consumption of organic food might be associated with other healthy lifestyle factors (better dietary choices, exercise etc.). It is even harder to derive how much of these differences that can be associated with N-fertilizers in particular, and if the differences are caused by changes in the microbiome or differences in nutritional contents.
Pollution
More direct and easier to show causal relation, are some of the effects of nitrogen pollution on human health.
Most evident and most discussed is nitrate pollution of drinking water. The strongest evidence for a relationship between nitrates in drinking water and adverse health outcomes is for methemoglobinemia (blue baby syndrome), for colorectal cancer, thyroid disease, and neural tube defects. Many studies have observed increased risk with ingestion of water nitrate levels that were below regulatory limits for maximum contaminant level (MCL). In the US it was estimated that 2% of public-supply wells and 6% of private wells exceeded the MCL; whereas, in agricultural areas, 21% of private wells exceeded the MCL. Nitrates in drinking water supplies is particularly high in rural areas in India, where levels often are more than double the WHO recommendation (Ward et al 2018). Another effect is the occurrence of toxic algal blooms which are stimulated by excess nitrate and dissolved organic nitrogen in run-off from fields (Einarsson 2024).
As mentioned before, nitrogen fertilizers is a major driver of climate change both trough the huge use of fossil fuels in the production, transport and application of the fertilizers and through the emissions of nitrous oxide. Climate change, in turn, has major impacts on human health. The WHO estimates that between 2030 and 2050, climate change will cause approximately 250 000 additional deaths per year, from undernutrition, malaria, diarrhea and heat stress alone (WHO 2023a). Nitrous oxide is also the most important stratospheric ozone-depleting emission (de Vries 2021), which increases the amount of ultraviolet light (UVB) that reaches the Earth’s surface. UVB causes non-melanoma skin cancer and plays a major role in malignant melanoma development (EPA 2024). Meanwhile, emission of N compounds to air is also responsible for an increased production of tropospheric ozone. Worldwide, the number of premature deaths from exposure to ground-level ozone is estimated to increase from 142,000 to 358,000 between 2010 and 2050 (de Vries 2021).
Emissions of nitrogen dioxide (NO2) are associated with respiratory illness and increased mortality (de Vries 2021). N-fertilizers contribute to those emissions via processes in the soil as well as from the combustion of fossil fuels for the production, distribution and application of the fertilizers. Finally, ammonia from manure handling or from N-fertilizers, can react with other pollutants and cause respiratory disease and cancer. When eventually deposited on land or water it will also contribute to eutrophication (Einarsson 2024).
The food system based on N-fertilizers is also based on the use of pesticides with apparent health risks. The industrial livestock systems enabled by cheap grain and N-fertilizers use huge amounts of antibiotics, which in turn cause a development antibiotic resistant bacteria, one of the biggest health risk in our times.
It is the civilization, stupid
Clearly there is a correlation, but not necessarily any causation, between the increase in use of N-fertilizers and a number of human health issues. There is also such a correlation between N-fertilizers and an increase in food production and a decrease in food prices, supposedly saving a number of people from hunger – but causing poverty among others.
By and large, N-fertilizers is so strongly embedded in the whole agri-food system that it is not possible to disentangle the impacts of the N-fertilizers from the impact of many other aspects of the system. And the agri-food system, in turn, is both a driver and a result of the modern industrial civilization which in turn is linked to numerous changes in environment and lifestyles. In the end, health is an emergent state and the health of soil, plants, animals and humans are strongly interrelated, almost indivisible as expressed by the concept of “One health” (WHO 2023b).
In my next, and last, N-article, I will discuss how an agri-food system without, or with very little N-fertilizers would look like.
* The Western diet in the cited research is defined as a dietary pattern characterized by high intakes of pre-packaged foods, refined grains, red meat, processed meat, high-sugar drinks, candy, sweets, fried foods, conventionally raised animal products, high-fat dairy products, and high-fructose products.
Previous N-articles:
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