When someone who practices good hygiene still sends off an unpleasant scent, a genetic disorder should be blamed, a recent study says.

The condition in which a generally clean and hygienic person emits body odor is known clinically as trimethylaminuria. It is caused by emitting excessive amounts of the compound trimethylamine (TMA). TMA is produced choline, a substance that reacts in the body of a person with trimethylaminuria after ingestion of saltwater fish, eggs, liver and certain legumes, such as soy and kidney beans.

Trimethylaminuria was once called the "fish-odor syndrome."

"Individuals with the metabolic disorder trimethylaminuria may sporadically produce malodors despite good hygiene," wrote study leader Paul Wise in the American Journal of Medicine.

"The psychosocial impact of trimethylaminuria may be considerable. However, trimethylaminuria is difficult to diagnose without specialized tests."

Trimethylaminuria is caused by defects in a gene known as FMO3, which hinder the body's ability to metabolize TMA and turn it into odor-free compounds. TMA itself has a strong fishy smell, but only about 10 to 15 per cent of people with trimethylaminuria have that specific malodor, which may make it tougher to get a diagnosis.

For the current study, Wise and his colleagues looked at how often trimethylaminuria was diagnosed in patients seeking help for unexplained, persistent body odor. Their studies showed about one-third of the 353 patients tested positive for trimethylaminuria.

After asking their respondents to drink a beverage with added choline, they measured TMA level in the urine and saw a direct link to the body odor.

Of the 118 patients who tested positive, only 3.5 per cent had complained of a "fishy" odor. They usually reported general body odor, bad breath and a bad taste in the mouth.

Trimethylaminuria is passed on from parent to child even when parents are unaffected carriers of the defective FM03 gene. UK studies reveal that up to 1 per cent of white people carry a defective FMO3, with some ethnic groups reporting a higher rate.

Many of the patients in the study had seen several doctors and dentists before going through Wise's testing.

Not all US laboratories can perform testing for the disorder, but to avoid trimethylaminuria, one must make diet changes, such as avoiding choline-rich foods, Wise said.