Food for thought
Izelle Theunissen, MRC News
 Mention the words 'food safety' and you're guaranteed to stir up some
emotions around organic ("organic is natural and good") and
genetically modified or GM foods ('Frankenstein foods'). But people are less
knowledgeable about mycotoxins - and this can shed a different light on the food
safety debate.
Mycotoxins are naturally produced food-borne metabolites of fungi that are
natural contaminants of agricultural crops. Their toxic effects in animals have
been known for a long time, and therefore health authorities worldwide have
regulated mycotoxin levels in human food and animal feed.
One such example is the mycotoxin aflatoxin B1 (AFB1), which is produced by
the fungus Aspergillus flavus that grows on peanuts before or after
harvesting and under poor storage conditions. In 1993 the International Agency
for Research on Cancer (IARC) classified AFB1 as a carcinogen - a substance that
can cause cancer in humans.
The tolerance levels of aflatoxins in human foodstuffs are regulated
internationally and in South Africa by Government Notice No. R 313 of 16
February 1990.
Another prime example is patulin - this is a toxic secondary metabolite that
is produced by a number of fungi, most important of which is Penicillium
expansum. This fungus is a well-known post-harvest pathogen that causes
'blue mould rot' or 'soft' rot' in apples.
Patulin has been shown to possess mutagenic properties (can cause damage to
the genetic material of cells), to have adverse effects on developing rat
fetuses, and to cause immunotoxic, neurotoxic and gastro-intestinal effects in
rodents. Recommended specifications are that patulin levels should not exceed 50
parts per billion in products intended for human consumption (United Kingdom
Committees of Toxicity, 1992).
This begs the big question: what role do these and other mycotoxins play in
organic and GM foods?
Mycotoxins and organic foods
Organic agriculture is generally defined as cultivation of crops without the
use of fungicides or pesticides, implying that such produce are 'naturally good
and healthy'. This has resulted in an increasing demand for organic foods in
developed countries - and these foods cost more to produce commercially than
'conventional' crops.
Rural communities in developing countries who depend on subsistence farming
have always made use of organic farming methods, but without using the variety
of 'natural compounds' and/or biocontrol agents to try and control fungal
infection and insect manifestations as is done in commercial organic crops in
developed countries.
But either way, the fact remains that organic foods that are grown without
the use of insecticides and fungicides may be expected to be infested by insects
and infected by fungi to a larger extent than conventionally grown foods.
Infestation by insects can in many cases lead to fungal infections that produce
mycotoxins - and therefore the food is concomitantly contaminated with higher
levels of mycotoxins.
Research by the MRC's PROMEC Unit indicates that patulin levels in apple
juice made from conventionally grown apples ranged from 250-4000µg/l. But in
organically produced apple cider, a study done by other researchers has found
levels of up to 45 000µg/l.
Mycotoxins and GM foods
One example of a GM food is transgenic maize (Bt-maize). These hybrids are
genetically modified with genetic material from a naturally occurring soil-borne
bacterium, Bacillus thuringiensis, that produces a protein that is toxic
to certain insect pests, for example maize stalk borers.
These stalk borers cause damage to maize ears or kernels and this damage is
often associated with Fusarium ear rot. The fungus Fusarium verticillioides
is one of the most prevalent seed-borne fungi associated with maize intended for
human and animal consumption throughout the world. The fumonisins, a family of
food-borne carcinogenic mycotoxins, were first isolated in 1988 from cultures of
F. verticillioides at the PROMEC Unit.
Fumonisin B1 (FB1) was shown by researchers to cause equine
leuko-encephalomalacia, pulmonary oedema syndrome in pigs, and primary
hepatocellular carcinoma in rats. In addition, detailed mycological analyses of
home-grown maize samples intended for human consumption from different
oesophageal cancer (OC) rate areas in Transkei during six seasons over the
period 1976-1989 revealed a statistically highly significant correlation between
the incidence of F. verticillioides in maize and the OC rate.
The Bt-maize strains, which are resistant to insects such as maize stalk
borers, have been shown to significantly reduce the levels of insect damage -
therefore yielding maize crops with significantly lower fumonisin concentrations
compared to their non-Bt counterparts. Another plus is that the maize yields of
the Bt-strains are larger than their non-Bt counterparts. So despite the current
discussions surrounding GM foods, it appears that Bt-maize hybrids could play a
major role in lowering fumonisin levels in maize products, which should
ultimately enhance the quality and safety of maize for animal and human
consumption, particularly in an African context.
Following extensive media coverage of high levels of aflatoxins that were
allegedly found in peanut butter given to school children in the Eastern Cape
Province under the Primary Schools Nutrition Programme (PSNP), the PROMEC Unit
published a Policy Brief (No 3, June 2001) ( http://www.mrc.ac.za/policybriefs/polbrief3.htm)
to emphasise the serious health implications of supplementing the diet of school
children with peanut butter containing aflatoxin. It is further of great concern
that the liver cancer risk increases significantly if a child suffering from
hepatitis B virus (HBV) infection consumes aflatoxin-containing foodstuffs, due
to the synergism beteen AFB1 and HBV in causing liver cancer. Therefore, an
increase in liver cancer incidence in South Africa within 20-30 years from now
is possible.
For more information about mycotoxins in food, contact Prof. Walter
Marasas (tel.: (021) 938-0244 or e-mail: wally.marasas@mrc.ac.za
or Dr Hester Vismer (tel.: (021) 938-0287 or e-mail: hester.vismer@mrc.ac.za).
Useful links:
LINK Maize our Staple Food -
Should Toxins Concern us?
LINK Your hair - 'a dietary time-clock'
LINK Aflatoxin in peanut butter
|
