Agriculture Reference
In-Depth Information
marketed foods to be higher in organic than in conventional products for a time (Jørgensen
et
al
. 2000), again highlighting the need to understand the causes of the differences found in
survey-type investigations.
In relation to food quality, cereals with low fungal infection levels are unsuitable for f flour
or muesli production for human consumption, due to a distinctive off-taste or inferior baking
quality. Apart from certain nuts and pulses, the risk to the consumer is very low. Infected
grains can still be used in moderate amounts for animal feed, and while mycotoxins can then
occur at detectable levels in meat, eggs or milk, the animals will show clear signs of poisoning
or even die before the levels become sufficiently high to pose a health risk to human consum-
ers. However, routine surveillance for signs of mycotoxin poisoning ('white kidneys') at abat-
toirs, and for mycotoxin residues in milk and eggs, are still important as measures to detect
and enforce violations of good practice in terms of animal welfare.
Apart from the question of how the farming system affects concentrations of mycotoxins,
additional uncertainty exists in relation to the effects of different levels on human and animal
health. The present safety regulations require that levels are a defined number of magnitudes
lower than those resulting in harmful effects in animal studies; mainly liver damage or cancer.
However, this method is very imprecise - safety limits set too low may lead to the destruction
of large volumes of food or feed, whereas limits set too high expose consumers to significant
health risks.
In a recent comparison of wheat samples produced in Italy, mycotoxin content was highest
in organic samples, although still well below the level considered safe for consumption.
However,
in
vitro
lymphocyte cultures demonstrated that the immunotoxic effect of the con-
ventional wheat was higher than that of the organic (Finamore
et
al
. 2004). The authors sug-
gested that this was as a result of toxic effects of pesticide residues or other unknown
contaminants in the conventional wheat. However, since 22 mycotoxin studies are reported to
demonstrate hormesis (beneficial effects at low exposure levels) (Calabrese and Blain 2005),
the data could equally well indicate that consumption of food with low levels of mycotoxins
may strengthen the immune system rather than harming it.
Naturally occurring toxins - plant toxins
Another class of naturally occurring toxins is the natural toxicants produced by plants as a
defence against diseases and pests, including essential oils in myrtaceous plants and glucosi-
nolates in Brassica species. From the perspective of the plant, humans are just another herbiv-
ore, and the chemical defences that protect it against animal herbivores affect humans just as
effectively. Fortunately, modern humans have inherited the physiological mechanisms to cope
with plant defences from our herbivorous primate ancestors that allow us to safely extract an
optimal amount of nutrients from plant foods such as vegetables. The best known protective
mechanism is our ability to degrade and/or excrete most naturally occurring plant chemicals
(as well as many unnatural chemicals). There are no reported cases of long-term harmful
effects on any humans of low to medium exposures to toxins from normal food plants, unless
combined with malnutrition (since some plant toxicants exert their effect by inhibiting uptake
or blocking utilisation of key nutrients). All other known cases of harm to humans were as a
result of short-term intake of plant material with very high levels of toxicants.
A less known, but probably more important, protective mechanism is 'conditioned taste
aversion' (Cross-Mellor
et
al
. 2004), which induces a rapid, profound aversion to any taste asso-
ciated with nausea or other indications of toxicosis. This study used an ingenious setup allowing
continuous control of the induction of nausea to show that conditioned taste aversion is so
precise that it allows rats to adjust their intake of toxin-containing food to obtain the optimal
provision of nutrients while avoiding harm from the toxins. Such optimisation has been dem-
onstrated in farm animals (Kyriazakis
et
al
. 1997) and is probably common for all animals.
