WASHINGTON - The Food and Drug Administration said Thursday it has issued a warning letter to Iams Co. that says some of its diet pet foods contain an unapproved substance.

Eukanuba Veterinary Diets Optimum Weight Control/Canine dry, Optimum Weight Control/Feline dry, Restricted-Calorie/Canine dry and canned, and Restricted-Calorie/Feline dry and canned contain chromium tripicolinate, which is not an approved food supplement, the FDA said.

The letter was dated Jan. 8 and posted on the FDA's website Thursday.

The FDA warning follows a recall of nearly 100 brands of pet food made by Menu Foods of Mississauga, Ont. (TSX:MEW), including some Iams products made under contract by Menu Foods.

New York state's food laboratory last week identified aminopterin as the likely culprit in the Menu Foods recall, which involved "cuts and gravy" style dog and cat food sold under a variety of brand names for various pet food companies and retailers.

The FDA's warning letter to Iams urged it to remove chromium tripicolinate from the products but did not ask for a recall.

In 1996, the FDA said it would not block the use of low levels of chromium tripicolinate as a source of supplemental chromium in diets for pigs. But that did not apply to other animal food. Chromium can affect the metabolism of glucose in animals.

Iams requested that that decision concerning swine be extended to its products for overweight pets, but FDA said it denied the request. It said a 2006 letter from Iams did not contain sufficient information to address safety concerns.

Iams will remove the ingredient from its Veterinary Diets cat and dog food, spokesman Kurt Iverson said. The products are sold by prescription only for overweight pets. They have used chromium tripicolinate as a metabolism enhancer, Iverson said.

The letter is part of an "ongoing dialog" with the FDA, Iverson said.

The FDA considers chromium tripicolinate to be genotoxic, meaning it can damage DNA and cause mutations and tumors.