Most of us worry about about our intake of fats or carbohydrates. What we may not realize, however, is how difficult it can be for our bodies to break down (metabolize) protein.

When protein is broken down, one of the by-products is ammonia. Because ammonia is toxic, our bodies have elaborate systems for getting rid of it. Most of the detoxifying work is done in the liver. There, ammonia travels a complex pathway featuring five enzymes that turn it into urea, which is then excreted in urine.

People with certain rare but well-understood genetic defects that interfere with the urea cycle often die in childhood. Other mutations, though, may be responsible for a set of related problems in adults, according to the May 2007 issue of the Harvard Health Letter.

One in 8,000 American children has a genetic defect that causes one or more of the enzymes involved in ammonia processing to be defective or scarce. As these children consume protein, the ammonia begins to pile up. Treatment includes eating less protein, taking medications that eliminate extra ammonia, dialysis and liver transplant. For afflicted children, the death rate is high.

Urea cycle disorders are viewed as primarily pediatric conditions, but there may be a whole range of unrecognized, genetically determined problems with protein metabolism experienced by adults. Some people may have mild genetic mutations that cause slight symptoms. This may explain why one person avoids meat while another loves nothing more than a steak meal. Problems with protein metabolism may also explain why some people have bad reactions to high-protein diets such as the Atkins diet.

The Harvard Health Letter notes that someday genetic tests may be able to routinely diagnose such metabolic disorders but for now more research is needed.