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Higher antioxidant and lower cadmium concentrations and lower incidence of pesticide residues in organically grown crops: a systematic literature review and meta-analyses

Abstract

Demand for organic foods is partially driven by consumers' perceptions that they are more nutritious. However, scientific opinion is divided on whether there are significant nutritional differences between organic and non-organic foods, and two recent reviews have concluded that there are no differences. In the present study, we carried out meta-analyses based on 343 peer-reviewed publications that indicate statistically significant and meaningful differences in composition between organic and non-organic crops/crop-based foods. Most importantly, the concentrations of a range of antioxidants such as polyphenolics were found to be substantially higher in organic crops/crop-based foods, with those of phenolic acids, flavanones, stilbenes, flavones, flavonols and anthocyanins being an estimated 19 (95 % CI 5, 33) %, 69 (95 % CI 13, 125) %, 28 (95 % CI 12, 44) %, 26 (95 % CI 3, 48) %, 50 (95 % CI 28, 72) % and 51 (95 % CI 17, 86) % higher, respectively. Many of these compounds have previously been linked to a reduced risk of chronic diseases, including CVD and neurodegenerative diseases and certain cancers, in dietary intervention and epidemiological studies. Additionally, the frequency of occurrence of pesticide residues was found to be four times higher in conventional crops, which also contained significantly higher concentrations of the toxic metal Cd. Significant differences were also detected for some other (e.g. minerals and vitamins) compounds. There is evidence that higher antioxidant concentrations and lower Cd concentrations are linked to specific agronomic practices (e.g. non-use of mineral N and P fertilisers, respectively) prescribed in organic farming systems. In conclusion, organic crops, on average, have higher concentrations of antioxidants, lower concentrations of Cd and a lower incidence of pesticide residues than the non-organic comparators across regions and production seasons.

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Alex’s Notes: The war between organic and conventional farming methods has raged for decades. Organic agriculture prohibit the use of synthetic pesticides and certain mineral fertilizers in an attempt to reduce environmental impacts such as soil run-off and pesticide contamination, but also to minimize the pesticide residues of the plant crops themselves because of arising health concerns with their consumption. Instead, organic agriculture relies on organic fertilizers like manure and composts, the use of crop rotation, and the application of non-chemical crop protection methods.

Over the last 20-something years, countless studies have been published comparing the amounts of pesticides, toxic metals, and nutritionally relevant compounds between organic and conventional crops. However, up to now only three systemic reviews have analyzed these studies. However, they all used different study inclusion criteria and methodologies, did not cover the last five years of research, provided no structured assessment of the strength of the evidence, and had contradicting results. One found no difference between organic and conventional crops. Another confirms this but notes that organic foods reduce exposure to pesticides. And the last concluded that merely eating organic instead of conventional could increase lifespan by 25 days in men and 17 days for women. The study at hand aims to silence all the conflicting conclusions by conducting a systemic review focused on plant secondary metabolites (antioxidants and polyphenols), chemical pesticides, toxic metals, macronutrients, and minerals, and then conducting several meta-analyses and sensitivity analyses.

The initial literature search returned over 17,000 results. After a bunch of scrutinizing by some unlucky researchers, 343 peer-reviewed publications reporting crop composition data were selected for inclusion. About 70% of the studies were conducted in Europe, 50% reported data for vegetables, 32% reported for fruits, 18% reported for cereal grains, and 10% reported for other crop-based foods like herbs and spices. A total of eight different meta-analyses were undertaken with the following results.

  • Organic produce had an average of 17% greater antioxidant activity. When fruits and vegetables were reported separately, the significance held for fruits but approached significance in vegetables (p=0.06; 0.05 is significant).
  • Organic crops had consistently higher amounts of all reported antioxidants, ranging from 18% to 69% more polyphenols, flavanones, anthocyanins, and carotenoids.
  • Organic crops had 25% more carbohydrates, 7% more sugars, 11% less fiber, 15% less protein, 11% less amino acids than conventional crops.
  • Organic crops had 48% less cadmium (a toxic heavy metal), 30% less nitrate, and 87% less nitrites. No differences were found for the other toxic heavy metals arsenic and lead.
  • Conventional crops had a four-fold increase in the frequency of detectible pesticides, with the difference most noticeable in fruits, vegetables, and the “other” crops (herbs/spices).
  • There were no major differences in the mineral content of organic and conventional crops.

Now it must also be noted that about half the studies had publication bias and the study heterogeneity was extremely high (indicating that excessive variation in study results occurred). This only speaks to the severity of the conventional vs. organic war. You have to get funding from somewhere, and half the studies probably received theirs from an organic organization or conventional industry. Regardless,

“The results of meta-analyses of the extensive data set of 343 peer-reviewed publications indicated that organic crops and processed crop-based foods have a higher antioxidant activity and contain higher concentrations of a wide range of nutritionally desirable antioxidants/(poly)phenolics, but lower concentrations of the potentially harmful, toxic metal Cd.”

With regard to the lower protein and higher carbohydrate amounts, the clinical relevance is very low. When a serving of broccoli has only 2 grams of protein, and 15% difference is negligible, especially considering that plants were never meant to meet protein requirements to begin with. The greater carbohydrate content is also negligible, but does speak to the superiority of organic farming for plant growth, given that plants photosynthesize to produce carbohydrates. Obviously lower exposure levels of heavy metals and pesticides are beneficial for health, although the benefits are impossible to estimate. This applies to the reduced exposure to nitrites as well, which is a risk factor for stomach cancer. Finally, something I found fascinating was that,

“A switch from conventional to organic crop consumption would result in a 20–40 % (and for some compounds more than 60 %) increase in crop-based antioxidant/(poly)phenolic intake levels without a simultaneous increase in energy, which…  would be equivalent to the amount of antioxidants/(poly)phenolics present in one to two of the five portions of fruits and vegetables recommended to be consumed daily.”

If anything, that is the main takeaway. Merely eating organic rather than conventional produce is the equivalent of adding 1-2 additional servings of fruits and vegetables to your diet.

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