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