Agriculture Reference
In-Depth Information
While this general topic has not yet been studied systematically in humans, conditioned taste
aversion has been documented in cancer patients receiving chemotherapy (Scalera 2002). Com-
bining a study of dietary patterns in healthy female volunteers (Cade
et
al
. 2004) with genetic
testing (Anonymous 2003) indicated nutrient/toxicant optimisation in people with haemo-
chromatosia, accumulation of excessive levels of iron due to a genetic defect in iron excretion.
Post-menopausal homozygotes for the C282Y mutation showed significantly lower intake of red
meat than other women in the same age group, even though they were not consciously aware
that high-iron foods were the cause of their symptoms.
Nevertheless, in food plants such as potatoes, toxicants regularly reach such high levels that
a single meal can cause unpleasant symptoms in sensitive persons, although serious cases
(more than a few hours of nausea and/or diarrhoea) are extremely rare, and no cases have been
reported in developed countries for more than two decades (Percival and Dixon 1997). Many
of the plant toxicants are phytoalexins, which means that they accumulate in varying amounts
in response to mechanical damage, an infection or other stress conditions, so the concentra-
tions within the same plant often vary by more than a factor of 100 depending on plant age and
sample site (Morrissey and Osbourn 1999).
In terms of organic food quality, the question is then whether high, acutely toxic concen-
trations occur in the relevant plant foods and if so, what production factors are important to
predict and preferably prevent this from happening, or if other measures can be used to allevi-
ate the harmful effects.
In temperate countries, most reports of the occurrence of toxic levels of plant toxicants refer
to furanocoumarins from parsnip or celeriac or glycoalkaloids from potatoes. Both these
compound types accumulate when the roots or tubers, respectively, are subjected to mechanical
or other damage, which can happen during or after transport to the retailer, independent of the
production system. The few existing data, mostly unpublished, indicate that organically grown,
undamaged material normally has somewhat higher average levels of these toxicants than con-
ventionally grown crops, and that these average levels are well below the established safe limits,
e.g. 200 mg/kg for glycoalkaloids in potatoes (Hajslova
et al
. 2005). However, a higher base level
could indicate a higher resilience to damage, and the study of potatoes found that despite a 20%
higher average level in the organic samples, most of the few samples near or exceeding the safety
limit were conventionally grown. A recent survey of parsnips in Sweden (J. Hajslova pers.
comm., 2005) showed equal levels in organic and conventionally grown, fresh undamaged
material, but that the increase after damage or storage was significantly lower for organic than
for conventional crops. In contrast, a UK survey of material collected in 1991-1992 found higher
levels in both undamaged and damaged organic samples (Anonymous 1996), although neither
study included information about variety, origin or storage history before purchase.
Under tropical conditions, the most prominent source of natural plant toxicants is cassava,
which contains toxic cyanogenic glycosides. In contrast to the vegetables produced in temper-
ate areas, the risk of poisoning, specifically the syndrome tropical spastic paraparesis (konzo),
is not directly linked with the plant itself but is determined by other factors, namely the
processing of the tubers and the availability of essential amino acids in the diet (since detoxifi-
cation of cyanogenic glycosides consumes sulfur-containing amino acids) (Swenne
et
al
. 1996).
Both can be jeopardised in subsistence agriculture on depleted soils, particularly under adverse
conditions such as drought (Kaiser 2002), while the content of toxicant is seen as an asset
under normal conditions (Chiwona-Karltun
et
al
. 1998). For tropical smallholders growing
food for their family's own consumption, the possible significance of organic farming methods
for prevention of cassava poisoning is thus related to the ability of these methods to preserve
soil fertility and nutritional diversity of the overall diet (Brandt and Kidmose 2002), rather
than any direct effect on the content in the plant.
