As it turned out, the cancer-causing mechanism involved a hormone specific to male rats that combined with the saccharin, much as the correct key fits and opens a lock. Luckily, we humans do not have the exact hormone the rats do. That difference illustrates just one among many reasons why rat results extrapolate poorly to humans. For example, rats produce a bile acid that lets them quickly eliminate cholesterol from their bodies and helps them avoid plaque build-up in their arteries. We do not.
Rats can synthesize vitamin C in their livers.
Human hearts typically beat 60 to 80 times per minute, often for seven or eight decades or more. Rats seldom develop spontaneous colon cancer, but it's the second leading cause of U.S. cancer deaths. Rats seldom suffer strokes, yet stroke is the third-leading U.S. killer, after heart disease and cancer. Nevertheless, the medical establishment inexplicably continues to use millions of rats and other animals annually in experiments of scant value. Unfortunately, the U.S. Environmental Protection Agency may soon proceed with three huge chemical testing programs that could collectively amount to the biggest animal testing endeavor yet, killing millions of animals, costing millions of dollars, resulting in years of delayed action, and returning data of little use.