Eat healthy? Nutrition experts have no idea

Have you ever wondered why nutrition experts so often change their minds about what constitutes a healthy diet?

In the last six months, a variety of experts and nutrition organisations have issued at least as many major dietary guidelines proclaiming the next set of instructions on what to eat to prevent cancer, whether processed foods are really food, and how to control diabetes with diet.

And the next set of dietary guidelines from the United States Department of Agriculture and the US Department of Health and Human Services, due out next year, are already creating a buzz.

These new guidelines have done little to solidify our understanding of the cause-and-effect relationship between diet and disease.

Even worse, they are likely to discredit nutritional science.

Moreover, guidelines like these tend to suggest, without proper research as proof, that people have control over whether or not they develop certain diseases.

A large part of the issue comes down to funding. Nutrition research is extremely difficult and expensive. It is also complex.

Many well-designed studies have failed simply because the subjects could not adhere to the diet: asking subjects to eat a diet lower in salt, for example, can initially make food seem tasteless.

Also, the impact of diet is subtle, and researchers may need to watch a subject for decades in order to observe any changes.

Within nutrition, and medicine as a whole, researchers rely on two broad types of studies: randomised and observational.

In a randomised study, we recruit a group of subjects with a desired set of similarities, and randomly assign them a diet.

Because the subjects are relatively similar, and the diet randomly assigned, researchers can establish cause and effect.

Observational research takes a snapshot of a population, and looks to see whether two things occur together with a high frequency - like smiling and happiness.

While less difficult and less expensive - and therefore much more popular - this type of research can only generate hypotheses about cause and effect.

Not only does observational research not establish cause and effect, but it is also often misleading and wrong, and the recommendations that come from the conclusions of these studies can lead to harm.

When two things occur together, their relationship is actually more likely due to a third, often unknown factor than what the researchers conclude.

For example, in observational studies there is an association between eating diets high in antioxidant vitamins and a lower rate of cancer. However, in proper randomised trials a diet supplemented with antioxidant vitamins has been shown to increase the risk of cancer. This result shows how far off the conclusions of observational research can be.

Because contradictory guidelines are so frequent, the public has every right to question the recommendations. But nutrition experts fail to acknowledge the weaknesses in research.

I do not believe that we should throw out all dietary recommendations. But I also know that they are not definitive - and so, intuitively, does the general public.

When we experts change our minds so frequently, we send the message that all dietary guidelines should be dismissed as the latest fad.

Until and unless the funding for very large, effective, long-term randomised studies of the effect of different diets on preventing disease becomes available, nutrition experts must educate themselves and the public about the strengths and weaknesses of the data on which their opinions are based.