Medical Corner: Orange Juice, D-Limonene, and Cancer in Male Rats

If you have a male rat, one thing you shouldn't feed it is orange juice. (Or more to the point, d-limonene.) In a March 12, 1992 NPR Morning Edition report, James Swenberg (University of North Carolina) said, "It turns out that about two glasses of orange juice contains a carcinogenic amount of d-limonene for the male rat, but it has absolutely no effect on mice or on female rats, and I'm sure it has no effect on humans." It seems that d-limonene and some other chemicals bind to a naturally occurring protein (alpha2u-globulin) in the male rat's kidneys. The protein then builds up and causes cancer. The same protein is not present in female rats or most other animals of either sex, but is present in all but one of the strains of male rats Dr. Swenberg's group studied. (Male rats of the NBR strain are the only ones Dr. Swenberg found that don't have alpha2u-globulin.)

D-limonene is a naturally occurring substance found mainly in citrus oils and is used as a flavor and fragrance additive in food, cleaning products, even shampoo. It's also found in black pepper, nutmeg, and mango. In oranges, as far as I can tell, d-limonene is mostly or entirely in the orange rind and gets into the juice either during the juicing process or as an additive. People I know who have fed their rats oranges have had no problems. I can only speculate that maybe this is because they don't eat the rinds, or maybe because some domestic male rats don't have this problem. I don't know, but I'm not taking any chances on the second guess! If it happens with so many strains of lab rats, it probably happens in our rats.

The potency of carcinogens is commonly listed as TD50, the dose which will cause cancer in 50% of rats it's administered to. (It's actually slightly more complicated than this since it has to be corrected for the rate of cancer in the control group, but let's not worry about that.) The TD50 dose of d-limonene is listed by The Carcinogenic Potency Database as 204 mg d-limonene per kg (2.2 lbs.) of rat. They list the amounts found in average human doses of common foods:

FOOD	AVERAGE HUMAN SERVING	D-LIMONENE
Black Pepper	446 mg	3.57 mg
Mango (1 whole pitted fruit)	245 g	9.8 mg
Nutmeg	27.4 mg	0.466 mg
Orange Juice	6 oz.	5.49 mg

Of course you wouldn't want your rat to get anything near the TD50 dose, much smaller amounts will increase the cancer risk. On the other hand, to reach the TD50 dose of orange juice you would have to give your rat nothing but orange juice (instead of water) every day for at least a couple months. Thus, I wouldn't worry too much if you've given your rat a taste of orange juice before. I just wouldn't do it again.

For anyone who wants to ask how you can get your rat enough vitamin C: rats make their own, so they don't need nearly as much as you. Humans are rare in the mammal world in not making their own vitamin C.

Copy machine toner, unleaded auto fuels, and some bathroom deodorizers also contain such male rat kidney cancer inducing chemicals. (In fact, the cancer warning you see at the gas pump is because of this!) But I don't think you are very likely to feed those to your rats.

References:
I first heard about this from the NPR story "Rats and Mice Not Good for Testing Toxicity" by Richard Harris, Copyright 1992 National Public Radio. Additional information came from James Swenberg.

Common foods with d-limonene and TD50 values are from The Carcinogenic Potency Database (http://potency.berkeley.edu/herp.html). Note that they list some of the other chemicals that male rats are uniquely sensitive to.

Additional information came from National Toxicology Program (http://ntp-server.niehs.nih.gov/), the Florida Chemical Company, Inc. (http://www.floridachemical.com/) and probably half a dozen other toxicology reports I found when I searched for d-limonene on Google.