Jeanne M. Wallace, PhD, CNC
Following preliminary reports of thallium exposures by molecular biologist, Ernie Hubbard in Northern California, we have begun to see sensationalized reports on the internet of kale being harmful or dangerous to eat. After careful and thorough analysis, here is the viewpoint of Your Nutrition Team!
Sampling bias. One of the key missing pieces of data in this investigative report is its lack of testing of a wide spectrum of plants. We now live on a planet marinating in toxic chemical residues, air pollutants, and plumes of contaminated water. If a wide spectrum of plant and animal foods were comprehensively tested for a variety of heavy metals and other toxins, many food items available in grocery stores would have significant residues or contamination. To single out kale as the only toxic-laden vegetable is irresponsible and slanted in view.
Limits of urine sampling. The article mentions finding high levels of thallium in urine. Urine is a means for the body to eliminate harmful substances and metabolites. If a person is ingesting worrisome amounts of a heavy metal, and significant amounts of this metal show up in the urine, that would be desirable, as it signifies that the body is excreting rather than storing the toxin. A more valid assessment might be to stop eating the offending food (kale in this case) for at least 1-2 months, and then to test the urine. A second test could be taken after ingesting a chelating agent to discern levels of the metal that are being stored in the body. It would also be important to determine if the person is experiencing any of the signs/symptoms of thallium toxicity (discussed below). Most thallium leaves the body in urine and to a lesser extent in feces. It can be found in urine within 1 hour after exposure. It can be found in urine as long as 2 months after exposure [CDC ATSDR fact sheet]. About half the thallium that enters various parts of your body leaves them within 3 days.
Correlation vs causation. Hubbard points his finger at kale as the cause of his patient’s elevated thallium, but this finding is an association, not a proven causation. It’s every bit as possible that some other vector is causing the elevated thallium in his group.
Which vegetables take up thallium? The accumulation of thallium in cruciferous crops is widely known, and there are a dozen PubMed citations on the topic going back to 1986. It is not only kale, but all cruciferous crops (see box at right) that can potentially accumulate thallium from contaminated soils. So to single out kale is improper! Yet this very food group, the cruciferous vegetables, are among our best anti-cancer vegetables. The chenopod family (beet, chard spinach, lamb’s quarter, amaranth) take up moderate amounts of thallium [LaCoste et al., 2001]. Canola or rape seed is also highly contaminated (40mg/k) [Pavlíčková et al., 2006, 2005], though we know of no analyses of canola oil. Kale growing in soil contaminated with 1.4mg/kg thallium—the safe limit proposed as 1 mg/kg—contained as much as 24 mg/kg thallium (dry mass) . When soil levels are high, green bean, tomato, onion, pea and lettuce have been shown to be safe choices, with minimal thallium uptake [LaCoste et al., 2001].
Where does thallium in foods come from? While most soils have some thallium present naturally in bedrocks, this naturally occurring thallium is not the culprit. It is polluted soils that lead to high uptake of thallium in plants [Al-Najar et al., 2005]. Most of the world’s soils naturally contain only trace amounts of thallium, less than 1mg/kg. Thallium levels in unpolluted soils are not considered high enough to cause human poisoning at normal levels of intake (you’d have to eat 1-2+ kg/daily of kale and other brassica veggies to reach worrisome levels). Soils in some parts of China, France, Poland and the Czech Republic are known to be contaminated. Highly polluted areas of China, for example, have 40 to 124 mg/k, and up to 55 mg/kg have been reported in the arable soils of France (Tremel et al., 1997a). In southern Poland, where soil contamination at one site measured 43-78 mg/kg dry wt, native plants growing in the soil had thallium levels 100-1000 times that normally found in plants (0.05 mg/kg dry wt) [Wierzbicka et al., 2004]. In an area of southwestern China where mining has contaminated the soil, the average daily uptake of thallium by the villagers consuming locally grown crops has been estimated to be 1.9 mg/person [Xiao et al., 2004a, 2004b]. This is 50 times greater than the daily ingestion of individuals in thallium-free areas.
Focus blame where blame is due. If a vegetable has the potential to accumulate toxins, it’s not the vegetable that we should blame, but rather the environment rife with toxins. Rather than vilifying and avoiding kale, let’s focus awareness on heavy metal contamination of our soil and irrigation water, and mobilize efforts to remediate and heal the land. We urgently need large-scale studies on thallium contamination of soils. The issue is not that we should abstain from eating kale and cruciferous vegetables, but that we should take steps to ensure our food is not grown in contaminated soil, and that means not only thallium, but also lead, cadmium, arsenic, and other heavy metals too.
It’s ALL about soil quality. Soil is not merely a structural medium that permits plants to root themselves and stand up in the sun! The nutrient density of all foods, and their freedom from toxins, is directly dependent upon the health and diversity of the soil ecosystem. If you want to know how well your foods will work to build your health and vitality, you need to ask about the soil where those foods are raised. In the industrial world, mining of heavy metals, previously locked deep in the earth, permits their release into the atmosphere where they find their way into our water and soil. Healthy soil has protective mechanisms to bind heavy metals. Living soil has a staggering universe of soil-based organisms (SBOs) in each teaspoon. Among these are bacteria (Methylobacterium oryzae) and fungi capable of mitigating the uptake of heavy metals. Fungi, also called mycorrhizae, are the network of white threads visible in healthy soil or in decaying wood chips. Only when a colony of mycorrhizae embed themselves within a plant’s roots can the plant effectively take up minerals and other nutrients from the soil. Building soil with a robust population of metal-binding SBOs requires active steps to increase organic matter, with compost, cover crops and careful management.
Decades ago, small farmers adopting organic methods went to great lengths to build vibrant soil. Today, we see the emergence of wide-scale “Industrial Organic” agriculture, with little attention to soil quality. Currently, organic certification in the U.S. requires only that a farm withhold chemical sprays for 3 years—to allegedly “leach” the soil of pesticides and herbicides. There is no requirement for building healthy soil with a diverse ecology. There is no requirement for remediating heavy metals, or addressing residual chemicals. Organic certification also permits the spraying of “non-toxic” herbicides and pesticides, many of which can reduce populations of SBOs. Lastly, the standard practice for controlling weeds is to use black plastic barrier, which overheats the soil and reduces its oxygen levels, diminishing SBOs. So when commercially- or organically-grown produce tests positive for thallium, is worrisome not only for the heavy metals deposited there. It’s an indicator of soil so deprived of biologic diversity that it is unsuitable the growing human nourishment.
Dynamic Accumulators. In the field of permaculture, leafy green plants are well known to be “dynamic accumulators,” capable of sending down deep roots and mining minerals in the subsoil and bringing them to the surface to share with surrounding plants. In the fall, the leaves of leafy greens wilt, lay down on the soil surface to become living mulch, releasing the minerals they have accumulated to the nutrient cycle. This role of leafy green plants has been shown to bring many minerals important for health to the soil surface: calcium, selenium, potassium, magnesium and others. If there are heavy metals in the soil, these will also be taken up if there are insufficient SBOs to bind these metals and protect the plants.
Checks and balances of mineral absorption. Minerals compete with each other for absorption in the human body. Heavy metals have nutrient minerals that can compete with them for absorption. For example, uptake of mercury‚ such as from diets containing contaminated fish, is greatly reduced in persons who have adequate selenium intake. Lead can be counterbalanced by adequate calcium. Preliminary data suggests potassium may help regulate or reduce plant uptake of thallium [Renkema et al., 2015]. This would mean that soils deficient in potassium could permit plants grown in them to take up higher levels of thallium. Adding potassium to soil also reduces plant uptake of radioactive isotopes. The most potent “dynamic accumulator” of potassium is the humble dandelion [Phillips 2012:p33]. So might the tendency of farmers, both conventional and organic, to vigilantly remove dandelions, be leaving our food supply in more risk of thallium uptake? By the way, brassica family vegetables (especially kohlrabi) are also potent accumulators of potassium.
Back to soil ecology. Holistic and biodynamic farmers have long known that it’s not the absolute concentration of potassium (K) present in the soil that matters, but the presence of rhizospheric microorganisms (e.g., root zone bacteria) necessary for potassium uptake [Phillips 2012; Zörb et al., 2014; Meena et al., 2014; Bhattacharyya and Jha 2012]. Potassium is present in the soil in inorganic forms which are not directly taken up by plants. It must first be solubilized. Potassium Solubilizing Microorganisms (KSMs) are a rhizospheric microorganism that transform insoluble potassium (K) to soluble forms. K-solubilization is carried out by a large number of saprophytic bacteria (Bacillus mucilaginosus, Bacillus edaphicus, Bacillus circulans, Acidothiobacillus ferrooxidans,Paenibacillus spp.) and fungal strains (Aspergillus spp. and Aspergillus terreus). Both conventional and organic farming use soil testing results to guide their application of N-P-K fertilizers. But no amount of added potassium will boost plant uptake of potassium in the absence of KSMs. So these soil organisms, by regulating potassium, may play an essential role in displacing thallium.
Other agricultural factors influence thallium uptake too. Water seems to enable greater thallium uptake: alluvial, hydric, over-watered soils or those with a high water table, and areas with higher rainfall than arid regions. Soils with acidic pH have greater uptake than alkaline soils [Jia et al., 2013; Madejón et al., 2007].
Diversify your diet. Our culture is one that embraces the idea of a “magic bullet.” We yearn for that one pill that will reverse aging or disease. Sometimes this way of thinking transfers to foods, with people seeking miracle foods to reverse cancer. While many foods are known to be rich sources of phytonutrients—the colored pigments that help us modify gene expression and act as cancer “phyters”—only a diet that combines a wide diversity of foods can support health and wellness. I’m reminded of the “Bog People” of Northern Europe. Because their bodies were so well preserved in the tannin-rich peat bogs, archeologists have been able to analyze the contents of their stomachs…thousands of years later. Their last meals contain up to 50+ different grains, nuts, seeds and fruits! Most Americans regularly consume 10 or fewer plant foods. So instead of thinking “Gee, I need to eat kale every day” we should be nurturing the mantra “how many different, local, seasonal plant foods can I combine into my diet today?” Instead of a salad made of 2-3 vegetables, we should take the example of Steven Barstow—author of Around the World in 80 Plants—and be inspired by his 2003 world-record salad incorporating of 537 plant varieties, all picked from his suburban yard in Norway. Many food traditions include meals made from a wide variety of vegetables, from Asian stir-fries to Mediterranian pistic, a celebrated spring dish incorporating 20-30 different foraged greens.
Bioaccumulation: root, stem, seed and young vs old leaves. Kale is a spring and fall vegetable. When grown during the summer months, it can have a strong, almost bitter or metallic flavor. During summer, the plant puts down a deep tap root, enabling greater uptake of thallium and other heavy metals. In cabbage plants, 80% of the accumulated thallium is in the old leaves, with much reduced concentrations in the young, new leaves [Jia et al., 2013]. In undisturbed natural soils, the concentration of thallium (in mg/kg dry weight) across several samples of cabbage plants was 1.6-4.6 mg/kg in young leaves and 6.1-48 mg/kg in older leaves. In soil polluted by nearby mines, young leaves had 5.5-658 mg/kg while older leaves contained 58-1,503 mg/kg thalliam [Jia et al., 2013]. Many varieties of kale mature in 55-60 days, but the variety known to have the highest thallium accumulation takes 80 days to mature [Al-Najar et al., 2003]. Red Russian or Ragged Jack kale are frost-hardy choices for early season and fall/winter crops (they mature in 25-50 days), and can be picked when the leaves are 3-5” across. Micro-greens and young salad cuttings of cruciferous crops may also be a safer eating strategy! On the other hand, eating kale day in and day out, regardless of season, and juicing massive quantities of mature kale leaves? Not a good strategy, and may contribute to thallium toxicity. While thallium bioaccumulates in plant leaves and seeds, the stems and roots have shown far lower levels. This means radishes and wasabi (root) are also wise choices for cruciferous intake in those seeking anti-cancer benefits and wishing to minimize thallium exposure.
Can I test my garden soil? Midwest Labs in Nebraska [www.midwestlabs.com ] and Waters Agricultural in Georgia [http://watersag.com/service/heavy-metals] offer soil or compost testing of inorganic metals (including thallium, cadmium, mercury and lead). To find a lab in your state, search for “Agricultural and Environmental Testing Labs.”
Phytoremediation is the use of a plant, fungus or bacteria to clean the soil of toxins. Because cruciferous crops are so effective at taking up thallium from the soil, they have been proposed to be an effective means of phyto-remediation [Ning et al., 2015; Al-Najar et al., 2005, 2003]. Specifically, a plant that strongly accumulates thallium is grown as a sacrifice crop, then disposed of, leaving cleaner soil for subsequent crops. For gardeners who opt to remediate their soil, two plants known to be hyper-accumulators of thallium include: Winterbor kale and candytuft (Iberis intermedia Guers.) [Al-Najar et al., 2005, 2003; Scheckel et al., 2004]. For Iberis phytoremediation, 5 sequential croppings can reduce soil thallium levels from 1 mg/kg to 0.1 mg/kg [LaCoste et al., 2001]. By contrast rape/canola (Brassica napus) would take 9 years and green cabbage over 30 years for a similar 10-fold reduction.
What are sources of thallium pollution? It has been estimated that the average person’s daily diet contains 2 ppb (parts per billion) thallium. According to the CDC ATSDR Fact Sheet, the greatest exposure occurs from food, mostly green vegetables contaminated by thallium. Knowing your possible sources of thallium exposure can help you take steps to avoid it.
- Cement plants. Dust emitted from cement plants can release thallium into the air, which settles on the soil, and is readily taken-up by plants and domestic animals. In a neighborhood study, people living near a cement plant had significantly elevated urinary thallium levels (up to 76.5 micrograms/l) compared to a mean of 0.3mcg/L in the unexposed reference population [Dolgner et al., 1983]. Chemical analyses of home-grown vegetables and fruits showed contamination by thallium-containing atmospheric dust fall-out from nearby cement plant emissions [Brockhaus et al., 1981]. If you live near, or downwind of, a cement plant, use caution with home-grown produce, especially greens and cruciferous veggies. Commercial growers, organic farms and CSAs need to be located away from cement plants. Know where your towns cement producers are located, or check online using the “environmental watch” feature at www.usa.com.
- Mining activities. Thallium is a byproduct from refining of copper, lead, zinc and other heavy metal sulfide ores, coal burning plants, zinc and lead smelting operations, oil drilling and coal burning power plants.
- Medical waste. Medical uses of thallium include the Thallium-201 Cardiovascular Stress Test and Thallium-201 SPECT scans used in cancer imaging. Is there a medical waste incineration facility in your community?
- Commercially-raised chicken. Chicken that is not raised organically can be fed arsenic (Roxarsone) as a growth promoter and to reduce infections. A study of heavy metal content in breast of arsenic-fed chicken, compared to chickens not fed arsenic, showed elevated levels of thallium and other heavy metals [Sun and Xing 2015]. Choose only free-range, antibiotic-free organic poultry. [Update: the USDA has recently outlawed the use of arsenic in poultry feeds, but it is still be present in some animal medications.]
- Cigarette smoking is also a source of thallium. People who smoke have twice as much thallium in their bodies as do nonsmokers.
- Although rat poison containing thallium was banned in 1972, accidental poisonings from old supplies still occur, especially in children. There have been documented incidences of contaminated wheat (a crop often protected from rodents with thallium-containing rodenticides). Foods imported from outside the U.S. may be an issue.
- Hazardous waste incineration. Approximately 60–70% of thallium production is used in the electronics/computer industry.
What is a normal level on a thallium test? Thallium in human urine is normally below 1 micrograms per gram of creatinine. The normal range of thallium in human hair is approximately 5-10 ng/g [Ewers 1988].
Signs of toxicity. Thallium displaces or competes with potassium, so signs of toxicity mimic those of potassium deficiency (and toxicity may be exacerbated in persons who are potassium deficient)! Fatigue is typically the first and most pervasive symptom. Patients exposed to high doses of thallium (>1 g) may also experience vomiting, diarrhea, peripheral neuropathy and seizures, alopecia (hair loss) and renal failure. Nail dystrophy is apparently common (up to 70% of cases) and can include white bands across the nail called Mee’s Stripes, see photo, which may dissipate when pressed upon (these are also found in arsenic poisoning). Lab values may show increased alkaline phosphatase and low calcium [Saha et al., 2004].
Protective steps for those with thallium exposure. Of note, some studies have shown that kale, mustard and cabbage plants contaminated with thallium respond to protect themselves by increasing their production of antioxidants and detoxification enzymes (e.g., glutathione). Might these also protect us when we eat the plants? If your thallium level is high but you do not experience symptoms, it’s possible you may have high protective levels of glutatione. Dietary and/or supplemental potassium can help mobilize thallium from the tissue [Lameijer et al., 1979]. Magnesium deficiency seems to permit greater cell uptake of thallium Llaurado et al., 1983; Papp et al., 1969], so this should also be addressed. Selenium has been shown to counteract the toxicity of thallium and other heavy metals [Whanger 1992], perhaps because it boosts glutathione production.
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