world cancer report - iarc

FOOD CONTAMINANTS
SUMMARY
> Food may be contaminated by natural or
man-made toxins, including substances
shown to be carcinogenic in experimental
animals and, in some cases, in
humans.
> Naturally-occurring carcinogens include
mycotoxins, particularly aflatoxins,
which contribute to causation of liver
cancer in Africa and Asia.
>Food can be contaminated by residual
pesticides. Small quantities of heterocyclic
amines, which are mutagenic and
carcinogenic in experimental animals,
can be generated during food processing
and cooking.
> Means to reduce and, in some cases,
eliminate food contamination include
storage hygiene, appropriately enforced
by regulation.
> The burden of cancer attributable to
food contamination is difficult to quantify,
except in some defined instances
(e.g. aflatoxin B1).
Differences between diets eaten by
diverse communities, in terms of amount
and relative proportion of the major food
groupings (vegetable content, fat content
etc) exert a major influence on the distribution
of cancers of the digestive tract and
some other organs (Diet and nutrition,
p62). By comparison, only a very minor
part of the worldwide burden of cancer is
attributable to contamination of foodstuffs
by toxins recognized to be chemical carcinogens.
Despite this global perspective,
the issue warrants close attention because
it may be a serious concern for particular
communities and, irrespective of demonstrated
cancer causation, food contamination
can be rectified. Removal of carcinogenic
contaminants requires that such
contaminants are identified, and that ways
are found to avoid their inclusion, or generation,
in food. Such public health aims
are amenable to regulation. Contamination
of water is not included in the present discussion,
but is considered elsewhere
(Environmental pollution, p39).
Contamination of food may occur directly
during its production, storage and preparation.
For example, grains and cereals are
subject to fungal growth and contamination
by mycotoxins. Indirect contamination
of food can occur when animals have been
given contaminated feed or been otherwise
treated with various products. The
most contentious residues occurring in
meat, milk and eggs are antibacterial
drugs, hormonal growth promoters and
certain pesticides, heavy metals and
industrial chemicals. An additional category
of contaminants comprises those generated
in the course of food preparation.
Naturally occurring contaminants
Food may be contaminated by mycotoxins,
the presence of one such agent being
indicative of the possibility that others are
also present. A single fungus can produce
several mycotoxins and food or feed can
be contaminated by several varieties of
mycotoxin-producing fungi. Only a small
number of mycotoxins have been categorized
as carcinogenic hazards.
Aflatoxins
Aflatoxins are a family of related compounds
(designated B 1, B 2, G 1, G 2 and M)
which occur as food contaminants in hot,
humid parts of the world, with particularly
high levels in traditional diets based upon
maize and groundnuts (peanuts) of sub-
Saharan Africa, South-East Asia and South
America. Aflatoxins are products of the
Aspergillus fungi and particularly accumulate
during storage of grains. In many
countries, including Europe and North
America, aflatoxin contamination is recognized
as a hazard and aflatoxin levels in
susceptible foods are subject to monitoring
and associated regulatory control.
The detection of aflatoxin adducts on
serum albumin is indicative of human
exposure and, in regions where aflatoxins
are a common food contaminant, such
adducts are detectable in up to 95% of
the population. In these regions, chronic
hepatitis virus infection (essentially
involving hepatitis B virus, HBV) occurs in
up to 20% of the population. Together,
aflatoxin exposure and HBV infection are
the main risk factors accounting for the
high incidence of hepatocellular carcinoma
in some regions of Africa, Asia and
South America [1].
Aflatoxin B 1 (the most common aflatoxin)
causes liver cancer in experimental animals.
In liver cells, aflatoxin B 1 is metabolized
to form an epoxide which binds to
the N7 position of specific guanines, leading
to the formation of G to T transversions
[2] (Carcinogen activation and DNA
repair, p89). Mutations induced by aflatoxin
B 1 are found in several genes
involved in hepatocellular carcinogenesis.
In particular, aflatoxin B 1 induces a typical
Fig. 2.27 Groundnuts (peanuts) are particularly
susceptible to contamination with aflatoxins in
some regions, such as West Africa.
Food contaminants 43