Sally Brown's Research Commentary August 2012
I’ve heard people say that they have passed up beautiful chanterelles at the Pack Forest Research Facility because they are concerned that the mushrooms will be so high in heavy metals from biosolids applications as to be dangerous. This month’s library goes through the process of determining whether these concerns have any basis in science.
The way to start on this is to look at the literature to see if mushrooms take up significant amounts of heavy metals. Not all edible plants do- with highest uptake normally seen in leafy green vegetables. Other crops such as tomatoes and corn have very low metal uptake. For mushrooms the library starts with an article on mushroom metal uptake for specimens collected near a lead smelter in Czechoslovakia. The smelter had been operational for about 200 years. Here the authors found that metal concentrations in mushrooms varied both by the type of mushroom and the distance from the smelter. For example, high concentrations of lead were observed in Boletus (50-110 mg kg dry weight) collected from within 1 km of the smelter. These fell off rapidly outside of the 1 km radius (2-20 mg kg dry weight) for 1 - 5 km from the smelter. Mean concentrations for different types of Boletus in Bohemia ranged from 0.9 to 2.3 mg kg (Kalac et al., 1991).
From this we know that mushrooms can take up metals from contaminated soils. Next thing to consider is whether mushrooms take up metals from biosolids-amended soils. Here we can turn to a study that actually looked at metal concentrations in mushrooms collected from the Northwest (2nd article in the library). The Pack forest sites had all received biosolids in the early 1980s, long before both the EPA 503 regulations. The cadmium content of the biosolids here ranged from 38-50 mg kg-1. Uptake of cadmium in mushrooms varied widely on the species collected from the biosolids amended sites with values ranging from below detection to 64 mg kg. (Zabowski et al., 1990)
If we look at a recent study, however, you get a very different answer. For the 3rd article in the library, mushrooms were collected from forests in the South of France that had been amended with different types of biosolids, all at an agronomic application rate of 6 Mg ha. The biosolids used for application had cadmium concentrations ranging from <1 to 2.3 mg kg. Total lead in the biosolids ranged from <15 to 100 mg kg. For this effort, the researchers collected only edible species of mushrooms and combined all samples collected from within each plot for analysis. They found that there was a wide range in metal concentration for mushrooms collected from control sites. For the sites with biosolids, only for some of the sites and some of the biosolids, cadmium and lead concentrations were ‘slightly increased’. Here lead in the mushrooms ranged from about <0.5 to 3 mg kg and cadmium ranged from about 1.5-4.5 mg kg (Benbrahim et al., 2006).
The 4th article in the library shows trends in biosolids metal concentrations in PA over a 20 year period (1978-1998). While nutrient content has stayed the same or slightly increased, metal concentrations in biosolids have plummeted. Median lead concentrations have dropped from 300 to about 10 mg kg. Cadmium concentrations have fallen from 7.5 to about 2 mg kg. Based on the US National Sewage Sludge Survey, the results from PA reflect improvement in biosolids quality across the country. This says that if you are foraging for mushrooms at a site where biosolids have been applied at some point over the last 20+ years, the mushrooms are not likely to have high metal concentrations.
The library closes with an article on how we access the potential for risk associated with eating foods that are high in metals- cadmium in particular. The first point here is that there have been no cases where people have suffered from lead poisoning as a result of eating foods with high lead concentrations. In contrast, there have been cases where people have been poisoned by eating foods that are high in cadmium. Excess cadmium in your diet can cause harm over time. Time measured in decades rather than months or years. In Japan, a group of people that ate milled rice grown in paddies that were contaminated by cadmium and zinc from mine wastes, developed weak bones and kidney failure after decades of exposure. This cadmium poisoning has resulted in extensive research on how dietary cadmium can cause harm, what the indicators are of excess cadmium in a diet, how other elements interact with cadmium in the human digestive system, and how total plant cadmium concentrations relate to the fraction that can actually cause harm.
The main take home message of this work is that a balanced diet is the best way to eliminate any danger associated with eating high cadmium foodstuffs. The population in Japan that was poisoned by excess cadmium in their diets was poorly nourished, with milled rice as the staple grain. Milled rice is notably low in important nutrients. Scientists have looked at a range of other populations with high cadmium in their diets. For all populations where people ate sufficient iron, calcium and zinc, excess cadmium in the diet was not a problem. Nutritional status protects us from taking up excess cadmium, even when the cadmium is present in high concentrations in our food.
So the answer is- if you eat a balanced diet and primarily forage for mushrooms on sites amended with modern day biosolids, you have no cause for concern. Even if you venture to the high metal historic sites in Pack Forest you should be OK. A sample calculation of how many mushrooms you would have to eat using international limits on Cd intake is given below .
Using 25 ug of cadmium per kg body weight a month, it is informative to figure out how many mushrooms that would be per month.
· Mushrooms are about 90% water.
· If you assume a total body weight of 60 kg and allowable cadmium of 0.025 mg kg body weight, that means you can eat 1.5 mg cadmium per month and be fine
· For a mushroom that has 64 mg kg Cd (highest seen for the one specific type of mushroom in the historic biosolids grown on the research plots) that would mean that you would have to limit yourself to about 250 g of fresh mushrooms from that plot per month to reach your recommended maximum cadmium allowance (for simplicity here, cadmium concentrations of other foods are not considered in this example). It is questionable whether you could find enough of that one type of mushroom grown on historic biosolids amended sites to support this level of consumption for any amount of time.
· If you got your mushrooms from the newer biosolids plots (here we are talking about the average cadmium concentration across all types of mushrooms including chanterelles and porcinis), using the value of 2 mg kg, you would have to eat 7.5 kg of fresh mushrooms each month to reach your limit.
That is a lot of mushrooms.
Here are a variety of papers which don't fall into a specific category. Some of the work is on greenhouse gas emissions, public perception or biosolids management strategies.
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