When the dinner bell rings for rats and mice in a nutrition or cancer study, they munch on an entree that has gotten as much attention paid to nutrient quality as the fare of astronauts in outer space.
Now the recipes for purified diets—in which every chemical compound is known—may be even better than commercial feed for some rodents, says ARS nutrition chemist Philip G. Reeves. He chaired the American Institute of Nutrition (AIN) committee to revise purified diets for mice and rats, and the new formulations are now the official AIN diets.
Reeves and colleagues at the Grand Forks Human Nutrition Research Center in North Dakota set out to solve the problem of calcium deposits in the kidneys of female rats and mice caused by the old purified diet. They also wanted to update the diets, based on their own experience and scattered reports of rodents' nutritional needs.
The results were better than expected. Male mice grew faster and were heavier after 12 weeks than their cousins getting commercial feed—considered the best available rodent nutrition until now.
"I guess we found a formula that works better for male mice," says Reeves, noting that the growth of female mice didn't change. But the calcium deposits disappeared. That was accomplished by reducing the amount of phosphorus in the diet by 40 percent.
"We also substituted soybean oil for corn oil because it has the right ratio of essential fatty acids," he says. "And we added just a pinch of molybdenum and several other trace elements thought to be important nutrients"—such as boron, nickel, vanadium, lithium, and fluoride.
If a test diet doesn't contain the right nutrients in the right amounts, it could confound study results. For example, the older purified diet contained too much phosphorus—a factor that could skew results when testing the effects of other nutrients.
Other problems can be caused by unknown compounds in the diets. That's why commercial feeds are not used in many cancer and toxicology studies. They contain too many unknown compounds, some of which could mimic or block a toxin or carcinogen being tested.
The National Research Council publishes nutrient requirements for laboratory animals, much like the Recommended Dietary Allowances developed for humans. Reeves was a member of the NRC committee to revise those requirements for laboratory rodents. The new ones were published earlier this year as the fourth edition of Nutrient Requirements for Laboratory Animals.
There are growth diets for young animals and maintenance diets for those past puberty, Reeves explains. "We're trying hard to make these diets nutritious for all these animals. It's probably one of the most important things we can do."
Reeves runs these lab animal diet experiments side by side with his regular work: studying the functions of zinc in humans and animals.
Even so, things can get out of hand. Around week 12 of one of the diet experiments, a rack of cages slowly began rolling across the floor when the technician turned her back.
The rack tipped as one of the wheels got caught in the floor drain, opening all the cage doors. One hundred white mice scurried in every direction, completely confusing who got what for dinner.
"It's lucky that the male mice getting the purified diet had their extra growth spurt early," Reeves says. "No important data were lost." — By Judy McBride, ARS.
"Haute Cuisine for Lab Rodents" was published in the June 1995 issue of Agricultural Research magazine.