TORONTO (Reuters) - The old adage "You are what you eat" may soon become "You are what need to eat."

Thanks to studies that have mapped humans' genetic makeup-which can outline illness we are prone to-we could, in theory at least, change our diets to include foods that combat those ailments, a Canadian study shows.

"In the future, we may choose a breakfast cereal based on our genes," said Dr. Peter Singer, director of the Joint Centre for Bioethic at the University of Toronto. "It is hypothetical, but possible, that if you have a particular gene you eat honey nut cereal to reduce your chance of heart disease, or, if you have another gene, you take the raisin bran to cut your chance of prostate cancer."

But the benefits of dining your way to genetic health-the science of nutritional genomics-or nutrigenomics-also comes with a downside, the study cautions. 

Scientists warm of the dangers of collecting and storing personal genetic information, adding nutritional genomics is in its infancy, with no evidence yet that it will be successful on a mass scale.

Since researchers mapped the human genome, or book of life, in 2001, they, along with business, have been intrigued by the idea of creating customized diet plans based on genetic road maps.

The Centre for Bioethics and the University of Guelph in Ontario led the study on the ethical questions surrounding genes and nutrition. The report does not prescribe guidelines but highlights concerns, such as privacy, confidentiality, misuse of information, discrimination based on genetic makeup and fears that employers and insurers could use genetic information to unfair advantage.

The study, which also included scientists from the United States and England, will be presented at the second annual International Nutrigenomics Conference in Amsterdam this week.

A few US, British and Canadian firms already offer tests to determine genetic risk profiles. Vancouver, British Columbia-based internet site onepersongenetics.com, for example, advertises a $379 test.

"There is general question of how strong the science actually is and how reliable are the claims [that] can be made about whether genetic tests and modification of diet will improve health," said Dr. David Castle, assistant professor of philosophy at the University of Guelph.

Nutrigenomics, unlike genetic engineering, does not involve altering foods but changing diets based on genetic makeup.

Researchers said many government health agencies have advised citizens to eat right according to various food pyramids and charts since the mid-1990s and these plans are still the best available. But they added nutrigenomics, if it can be applied on a mass scale, would have a customized plan.

Basing diets on genetic makeup can be beneficial, said Dr. Abdallah Daar, also with the University of Toronto's Joint Centre for Bioethics, because dietary patterns are linked to several of the top 10 causes of illness and death in North America, including heart disease, stroke, many types of cancer and adult-onset diabetes.

Daar, who is director of the program in applied ethics and biotechnology at the centre, said the new science is complication as more than one gene plays a role in the development of some diseases and one gene can be involved in causing many diseases.

Even though the mass application of nutrigenomics may still be decades away, Daar said its advance is "outstripping the public's ability to make informed choices about what kind of regulations should be introduced to address ethical and privacy concerns."