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Why Kale May Not Always Be a Superfood

Analysis by Dr. Joseph Mercola Fact Checked

kale pesticide residue

Story at-a-glance -

  • The government tests the pesticide levels on nearly 41,000 fruits and vegetables, and the Environmental Working Group (EWG) reports the least- and most-contaminated to consumers
  • In 2014, the government reported that 41 percent of tested food samples contained no detectable pesticide residues; in 2015, it was 15 percent, but in 2019, 92 percent showed two to 18 different pesticides on kale alone
  • EWG uses government data and ranks pesticide contamination in fruits and vegetables on an annual basis to submit its annual “Dirty Dozen” list, and in the latest one, kale ranked No. 3
  • While governmental agencies may contend that pesticide residues found in produce are safe and don’t pose risk to consumers’ health, multiple studies show they can cause health problems, particularly in children
  • Children are uniquely susceptible to toxins associated with pesticide residues, with exposure linked to pediatric cancers, decreased cognitive function and behavioral problems

Antioxidants, phytonutrients, omega-3s and other benefits that relate to disease resistance, anti-inflammatory effects and other advantages are wonderful features in any food, and all have been observed in kale, the cruciferous vegetable known for its ability to thrive in cold temperatures.

The leafy green vegetable that seemed to win so many positive reviews from health advocates due to kale's incredible list of nutrients — impressive amounts of vitamins A, C and K, antioxidants, sulforaphane, lutein, zeaxanthin and DNA-repairing indole-3-carbinol — has come under some not-so-positive scrutiny.

A Time headline recently stated the problem: "Kale is one of the most contaminated vegetables you can buy."1 The upshot is, 92 percent of the conventionally grown kale samples that were tested by the U.S. Department of Agriculture (USDA) and Food and Drug Administration (FDA) contained at minimum two pesticide residues, and some concealed as many as 18.

In fact, the Environmental Working Group (EWG)2 peruses government data and ranks pesticide contamination in fruits and vegetables to submit its annual "Dirty Dozen" list.3 Kale earned the third slot from the top — right behind strawberries, which has ranked as the No. 1 most pesticide-ridden plant-based food for three years in a row4 — and spinach, another popular green garden vegetable. Time observed:

"The USDA and FDA alternate testing among nearly four dozen fruits and vegetables every year and do not test the same ones annually. Nor do the agencies look for the same pesticides in each round of tests. The last time kale was tested was in 2009, when it ranked eighth on the EWG's Dirty Dozen list."5

The government tests the pesticide levels on nearly 41,000 fruits and vegetables, and the EWG reports the least- and most-contaminated to consumers, but the studies that present the information don't search for specific pesticides, such as RoundUp,6 a popular but controversial chemical used to kill weeds, but that has also been detected in popular breakfast cereals and snack bars.7

Problems With Low-Growing Produce and Sporadic Testing

Nneka Leiba, director of healthy living science at EWG, admits the fact that the FDA and USDA fail to test each plant-based food for possible pesticide (and other) contamination every year is "problematic." She adds that the fact that government testing also doesn't include a full spectrum of pesticides used in fruits and vegetables is "a huge problem."8

Not only had kale not been tested since 2009, but one of the substances found at that time was imidacloprid, a nicotine substitute known to be toxic to many insects. It's "considered nontoxic to people but can cause breathing and intestinal problems if inhaled in excessive quantities."9 However, in the most recent testing, imidacloprid wasn't one of the pesticides the governmental agencies tested for.

While the USDA still contends, "Residues found in agricultural products sampled are at levels that do not pose risk to consumers' health and are safe,"10 the Organic Consumers Association had this to say about this particular pesticide:

"Imidacloprid, marketed as Merit by the original manufacturer Bayer, is well documented for its toxicity to bees, as well as birds, worms and aquatic life. Many beekeepers, environmentalists and scientists — though not all — feel that imidacloprid is the root cause of colony collapse disorder (CCD) of bees."11

The kale and spinach samples that were tested had, on average, 1.1 to 1.8 times more pesticide residue by weight than any other crop.12 One reason they (as well as strawberries) may be ranked so high on the Dirty Dozen list is because they grow so close to the ground, suggests Alexis Temkin, a toxicologist at EWG.

In fact, spinach and kale contained 10 to 80 percent more pesticide residues by weight than any of the produce samples tested. They may be sprayed more because bugs can get to them quicker than taller plants, plus there's gravity to consider.

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Organic Produce Less Toxic Compared to Conventionally Grown Produce

Incidentally, the pesticide load we all carry is due at least in part to the way farming is done on a colossal but conventional scale here in the U.S., and the percentages have grown exponentially in just the last five years. In 2015, for instance, 15 percent of all the food samples the USDA tested were free of pesticide residue,13 but just one year earlier, in 2014, 41 percent of the food samples had no detectable pesticide residues.14

It's no wonder pesticide contamination on our food is such a problem. The USDA's 2015 report explains that "465 parent pesticides, metabolites, degradates and/ or isomers, plus 23 environmental contaminants" were tested on produce. However, the mixing of "parent pesticides" means there could be any number of lethal combinations on the nonorganic kale and other plant-based foods you bring home.15

Beyond Pesticides explains that the term "pesticide" can refer to herbicides, fungicides and other substances used to control pests, weeds and more, and can include plant regulators, defoliants and desiccants. But more than the store-bought solutions labeled insecticides, fungicides or weed killers, there's more:

"The product that you buy or are exposed to is actually a pesticide formulation that contains a number of different materials, including active and inert ingredients, as well as contaminants and impurities. In addition, pesticides, when subject to various environmental conditions, break down to other materials known as metabolites, which are sometimes more toxic than the parent material."16

What's so Bad About Pesticides, Especially if the EPA Says They're Safe?

With that question comes another: Why are health-conscious consumers advised to steer clear of pesticides? According to EWG, the most frequently detected herbicide on kale is Dacthal, or DCPA, which was found on 60 percent of the kale samples tested. It has been listed since 199517 as a Group C "possible carcinogen" by the U.S. Environmental Protection Agency (EPA) and has been banned in Europe.

In a French study, published by JAMA Internal Medicine in late 2018, among nearly 69,000 study subjects, those who ate more organic food reportedly had 25 percent fewer cancers than people who did not eat organic food.18

Conversely, the American Academy of Pediatrics reported in 2012 that children are uniquely susceptible to toxins associated with pesticide residues, with exposure linked to pediatric cancers, decreased cognitive function and behavioral problems.19 The report further urged physicians to suggest that their patients seek out reliable resources for information about how to avoid consuming pesticides along with their fruits and vegetables.

Another study, conducted by colleagues from Harvard's T.H. Chan School of Public Health in Boston, "found a surprising association among study participants between the consumption of foods high in pesticide residues and fertility problems."20 Specifically, women undergoing infertility treatment who consumed more fruits and vegetables with high pesticide residues were less likely to become pregnant and have a live birth.21

Why Are Pesticides Considered Necessary?

Often applied as a soil treatment, DCPA is used "to control annual grasses and broadleaf weeds on ornamental turf and plants, strawberries, seeded and transplanted vegetables, cotton and field beans," according to the EPA.22 When it's in the soil, it can be absorbed into plants like kale, which is how produce ends up on the Dirty Dozen list.

The same EPA document estimates DCPA's carcinogenic risk through food as "generally considered to be negligible by the Agency," "practically nontoxic" to birds, small mammals and bees, "probably no more than slightly toxic to fish, slightly toxic to practically nontoxic to aquatic invertebrates" and "moderately to highly toxic to nontarget estuarine and marine organisms."23

While some agricultural entities may be convinced that pesticide use is necessary to assure healthy, abundant crops, there are signs that some governmental agencies aren't convinced, or are perhaps beginning to see the light.

Case in point: Los Angeles County recently banned the use of glyphosate, the active ingredient in Roundup weed killer, on county property over health concerns.24 Officials may be reading about the downsides, including a potential cancer risk to humans. Regarding pesticide usage on your food, EWG notes:

"Overall, the USDA found 225 different pesticides and pesticide breakdown products on popular fruits and vegetables Americans eat every day. Before testing, all produce was washed and peeled, just as people would prepare food for themselves, which shows that simple washing does not remove all pesticides."25

There are some smart ways to wash your produce, such as using baking soda to help degrade pesticides. Avocados, sweet corn and pineapples were found to have the lowest level of pesticide residues, and were placed on the EWG's "Clean 15" list, but it's still disturbing that a single sample of conventionally farmed kale could contain up to 18 different pesticides.26

The Case Is Clear: An Organic Diet Reduces Pesticide Load

A recent study published in Environmental Research27 was significant for anyone wondering about the viability of adopting an organic diet. Scientists collected urine samples from 16 people, including children, all from four racially and geographically diverse families in the U.S., before and after starting an organic diet. The urine samples were tested for individual levels of insecticides, herbicides and fungicides.

After just six days, the participants had, on average, a 60 percent reduction in the levels of synthetic pesticides, including organophosphate pesticides, neonicotinoid and pyrethroid insecticides, and the herbicide 2, 4-D in their urine, compared to when they were eating a conventional diet.

The researchers concluded, "This study adds to a growing body of literature indicating that an organic diet may reduce exposure to a range of pesticides in children and adults."28

It's clear that the problem with pesticides on our food is getting worse, not better. Rather than being able to assume you're on the receiving end of nothing but health-advantageous nutrients and phytonutrients when you bring kale and other produce home from the grocery store or farmers market, there are caveats to consider, including where and how it was grown.

The EWG said in 2006, "Consumers should opt for organic food whenever possible to reduce their exposure to pesticides,"29 but if you feel you can't afford to purchase more than a few items of organic produce, you might want to make those the fruits and vegetables with high rankings on the Dirty Dozen list.