Burn
the cookies at your own peril
Grilling and roasting make food
tastier, but could produce harmful reactions, Roger Highfield
reports
When our ancestors discovered how
to cook, they gave us a way to sterilise, to expand our diet to
include potatoes and other foods that would otherwise be indigestible,
to tenderise meats, and to enhance flavour by breaking down large
molecules into the smaller ones that we can taste.
But scientists are becoming concerned about the complex series
of chemical reactions and byproducts caused by intense heat. For
example, barbecuing or grilling food can produce small quantities
of compounds called heterocyclic amines, which can cause cancer
in animals.
Overdose: turning up the heat
is not only a disaster for the cook, burnt food could also damage
your health
Another chemical, acrylamide, is classified a potential carcinogen.
Others are linked with diabetes and heart disease. Together, they
shift the focus of food concerns away from what people eat to
how we prepare food.
Some scientists are studying advanced glycation end products
(AGEs), the toxic byproducts of spontaneous interactions of sugars,
fats, and proteins. Levels of AGEs in the body are influenced
by many factors, and can be raised by diabetes and ageing. Smoking
is another source, as is diet.
The quantity of AGEs we take in depends on the composition of
the food and the way it is cooked or prepared, notably exposure
to high temperatures. Prof Helen Vlassara and colleagues at the
Mount Sinai School of Medicine in New York have speculated that
AGEs formed in certain foods might contribute to the low-grade
inflammation seen in diabetics that leads to complications such
as heart disease.
"These are substances that are forming spontaneously in
our body from glucose reactions," she said. "The higher
the glucose is, the higher the products will be and they are highly
toxic.''
She and her team focused on the role of AGEs in 24 diabetics
in a study published in the Proceedings of the National Academy
of Sciences. Diabetic patients consuming meals with the AGE content
of a normal diet showed high levels of AGEs in their blood serum
and high levels of inflammatory markers.
In contrast, patients given similar meals cooked with reduced
heat (chicken boiled rather than roasted, or steak stewed rather
than fried) had lower concentrations of both AGEs and inflammatory
markers.
The results suggest that lowering dietary intake of AGEs reduces
the inflammation that is closely linked to the development of
heart disease. Previous studies have also demonstrated that a
low-AGE diet may protect against diabetic kidney disease and other
complications common to this condition. Most intriguingly, the
low-AGE diet was found to lessen the insulin resistance that leads
to type 2 diabetes in animals genetically prone to the disease.
Although more work must be done to firm up this hypothesis, Vlassara
believes that people at greater risk of heart disease may need
to watch not only the fat and sugar content of their food, but
also how it is cooked.
She accepts that anyone following her advice will have a blander
diet: the Maillard reaction, responsible for the flavours and
colour changes of baking and roasting, occurs at moderately high
temperatures. "What make food smell and look good are the
AGEs," she said.
Other scientists are particularly concerned by a chemical reaction
that occurs in a wide range of fried and baked foods - from crisps
and chips to bread and breakfast cereals - to create high levels
of acrylamide, a suspected cancer-causing substance. When the
worrying find was first reported by a Swedish team, the chance
discovery triggered a worldwide alert and a scramble by regulatory
authorities to assess the significance.
Prof Margareta Tornqvist and colleagues from Stockholm University
had looked for the chemical in the blood of workers and an unexposed
control group. She was initially puzzled to find high levels in
both, and then discovered elevated levels in starch-based foods
that had been cooked at high temperatures (greater than 120C).
The chemical was not found in boiled foods, which are cooked at
lower temperatures.
Her findings were confirmed in Britain by the Food Standards
Agency and in other countries. Then came the hunt for how acrylamide
is formed. The first peer-reviewed news of the link came in the
journal Nature, when Dr Richard Stadler at the Nestle Research
Centre in Lausanne, Switzerland, Prof Donald Mottram at the University
of Reading and Prof Bronek Wedzicha of Leeds, described the details.
The culprit is asparagine, a naturally occurring amino acid found
at relatively high levels in potatoes and cereals. There may be
preparation methods that are more effective at reducing acrylamide
in foods than simply reducing frying temperature.
But the jury is still out on whether it is harmful. Although
acrylamide can cause nerve damage and numbness, the levels measured
in foods are thought to be too low for that. However, the International
Agency for Research on Cancer classifies it as a probable human
carcinogen: acrylamide has been shown to cause cancer in laboratory
animals.
In the past few days, Dr Lorelei Mucci and colleagues at the
Harvard School of Public Health and the Karolinska Institute,
Sweden, have published the first study of the association between
dietary acrylamide and cancer risk, in the British Journal of
Cancer: they found that eating foods with high levels of acrylamide
appears not to increase risk for colorectal, bladder, and kidney
cancers.
They examined the diets of 987 cancer patients and 538 people
without cancer over a five-year span, searching for links between
consumption of 14 foods high in acrylamide and increased risks
of cancer.
The researchers consistently found a lack of excess risk among
those who regularly consumed foods with high (300-1,200 micrograms
per kg) or moderate (30-299 micrograms per kg) levels of acrylamide.
Those with the highest total dietary acrylamide intake were at
no greater risk of cancer than those with lower intake.
In fact, subjects in the highest quartile of acrylamide intake
had a 40 per cent lower risk of colon cancer compared with the
lowest quartile, a finding Dr Mucci said might be explained by
the presence of fibre, vitamins, or other cancer-preventive substances
in high-acrylamide foods such as breads and cereals.
The types of cancer included in the study are those scientists
believe most likely to be caused by acrylamide, because the sites
they attack are involved in metabolism and excretion of the chemical.
"Overall, the results of this
study provide some evidence that it looks as though there's much
less to worry about than was initially thought." Still, Dr
Mucci said, further studies need to examine possible association
with other forms of cancer.
26 feb
2003


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