In the face of uncertainty we often look to science to help us make sense of things.

This is particularly true in complex areas such as GMOs where adverse effects may be difficult to predict or may even be masked by other aspects of our lives and diets.

The potential link between GMOs and cancer is a good example. Do GMOs cause cancer? Many people believe they do, but cause and effect studies of this are rare.

Certainly some studies have shown a higher incidence of tumours (and tumours can be different from cancer) in animals fed GMOs and their associated herbicide glyphosate. Others have shown that the glyphosate, used widely on and absorbed by GMO crops, is an endocrine disrupter and thus can be a trigger for cancer.

Indeed the International Agency for Research on Cancer (IARC), a branch of the World Health Organization, has recently concluded that glyphosate is a 'probable human carcinogen'.

But in the real world, lots of things are known cause cancer and so it is difficult to know how much or little GMOs contribute to the rising incidence of this disease. We can't know because the tests aren't being done, for example, to find out if, for example, glyphosate might combine in food or in our bodies with other chemicals we are commonly exposed to to promote cancer, or to make it more aggressive.

It's important to remember that the absence of evidence of harm is not the same as proof of safety. For this and so many other reasons there is no scientific consensus on the safety of GMOs.

Now, with the publication of a new study, we have even more cause to question the science of GMO safety and even the wider world of toxicological testing.

'Doing' science

Before we look at the results of the study let's look at how we 'do' science.

Toxicity studies are most commonly conducted in rats. Setting aside the rights and wrongs of animal testing, many scientists believe that studying effects on rodent health has a reasonable predictive value for how a substance might affect human health.

But there are unique problems with 'lab rats' which scientists have been wrestling with for years.

The main problem is that they are so unhealthy to begin with.

Some of this is down to the fact that laboratory rats are sedentary and can develop all the same diseases that humans develop from being inactive. They can also be stressed and this too has an impact. Genetics also plays a part. The genetic manipulation used to produce strains of laboratory rats can leave them more vulnerable to disease than normal rats.

As a result, populations of laboratory rodents across the world develop high rates of so-called 'spontaneous' diseases. For instance, after two years the average incidences of mammary fibroadenomas and pituitary adenomas among certain kinds of Sprague Dawley rats can be up to 71% and 42% respectively.

When a scientist tests a toxic substance, for instance in the diet, he or she will generally divide a group of rodents into a control group, which eats a normal diet, and a test group, whose diet includes the toxic substance. Interpreting the results of the study requires that we make some assumptions about the health and 'normalcy' of the 'control group'.

Such assumptions, however, have come under fire in a new study by French scientists which looks at the toxicity of the 'normal' diet of the lab rat.

A question mark

The scientists tested 13 samples of proprietary feed. All of the samples contained significant amounts of pesticides and other contaminants.

Traces of the herbicide glyphosate (both glyphosate itself and its breakdown product AMPA) were detected in 9 of the 13 diets; and 11 of the 13 diets contained 'Roundup Ready' GMOs that are grown with large amounts of glyphosate, the main active ingredient of Monsanto's Roundup formulation.

Traces of the insecticides pirimiphos methyl, deltamethrin, chlorpyrifos methyl and ethyl, and malathion were also found in the rats' food along with significant traces of heavy metals (mostly lead and cadmium) and PCBs.

The study found that the contamination levels recorded in the food were high enough to cause serious diseases and disrupt the hormonal and nervous systems of the animals in control groups.

What this means is that instead of comparing a clean, healthy diet with a contaminated diet, scientists are comparing a contaminated diet with an even more contaminated diet.

The contamination of the 'control diet', which can make the rats sick, effectively masks the true toxic effect of the test diet - essentially making it seem less toxic than it actually is. This effect could be why we see so many studies showing 'non-significant' toxicity of some very highly toxic substances in animal trials.

This applies especially to 'endocrine disruptor' chemicals which are active at very low'trace' levels of parts per billion. Tests that examine toxic effects at such very low concentrations would produce null results if the feed is already contaminated at comparable or higher levels.

Supporting evidence

This new study is not the only to determine that rat chow can be contaminated.

Last year some of the scientists involved in the current French study analysed the rat chow used in a conventional GMO canola feeding experiment and found that it contained 18% of the Roundup tolerant maize NK603, 14.9% of MON810 (a modified Bt insecticide producing GMO) and 110 ppb of glyphosate and 200 ppb of AMPA (the breakdown product of glyphosate).

They argued that such a level of contamination invalidated the authors' conclusions about the safety of the variety studied and were a threat to sensible regulation of GMOs.

Earlier this year in the US Dr Anthony Samsel, an independent scientist and consultant, analysed the Purina diet routinely used in animal feeding experiments designed to test the safety of GMOs. His findings, showed that three of the standard Purina feeds formulated for rats, mice and other mammals contain both GMOs and glyphosate.

Scientists questioning the science

Rat chow manufacturers like Purina don't routinely test for these contaminants, and make no guarantees for the purity of their feeds in this regard. Yet these feeds are used every day, in laboratories around the world in feeding experiments.

No certification regarding the purity of test feed is required by journal editors or by food safety regulators either in the EU of the US - and this has been going on for decades.

Scientists are now beginning to speak out about these problems and their implications for the regulation of toxic substances.

GRACE (GMO Risk Assessment and Communication of Evidence) is a publicly funded EU research project. The results of its work are guiding future methods and criteria that will be used in the EU to assess the risks of genetically engineered plants for cultivation or use in feed and food.

But a recent report from the group Testbiotech demonstrated contamination of the control group feed used in the GRACE Project study of MON810 maize and called for the study to be retracted.

Heroes ... and villains

The scientists who are speaking out on this issue are heroes and should be congratulated. Instead they have been met with the full force of a well-financed sceptic army whose sole job is to detect and stamp out any criticism of GMO food and the science behind it.

But the lock-step, knee-jerk reactions of groups like the Genetic Literacy Project are looking increasingly dumb in the light of the rapidly changing landscape of GMO science.

There is no escaping that the problem of contaminated laboratory diets are a serious regulatory issue since all of our safety regulations for toxic substances are based on the results of this kind of animal testing.

It's also human health issue - possibly predictive, but certainly reflective of our own toxic diets and the 'background' damage they do to our health, as well as how hard it is to determine cause and effect when it comes to the multiple toxins we are routinely exposed to.

It is also an animal welfare issue, particularly if scientists know that they are routinely feeding contaminated food to their animals and they keep on doing it anyway.

For all these reasons we must take a much harder look at the science of GMOs and the places it can lead - but also mislead - us.

Pat Thomas is a director of the campaigning group Beyond GM, founder of GM-free Me, and a former editor of The Ecologist. This article first appeared on the GM-free Me website.