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UNDATED (CBS) -- Many parents give their children milk because it's loaded with vitamins that can keep the brain and bones healthy. But a new study shows there should be a limit.

Milk is a great source of vitamin D and calcium, but a new study in the journal Pediatrics finds too much may be a bad thing. Researchers looked at more than 1300 children ages 2 to 5 and found two cups of milk a day is enough to maintain vitamin D levels, more than that can affect iron levels.

Pediatrician Dr. Suzanne Kaseta says, "Excessive amounts of milk, which was more like three or four or more cups a day was associated with a decrease feritin level and feritin is associated with your iron stores in your body."

Getting enough iron is important for children because it plays a critical role in early brain development. Low iron can also increase your risk for anemia.

Milk isn't the only way to get vitamin D. Fatty fish like salmon is a good source and so is the sun.

The study points out some children with dark skin may need supplements because their body doesn't make enough vitamin d when exposed to sunlight.

Whole milk is recommended for children under 2-years-old because the fat in it helps brain development. After age 2, most pediatricians suggest switching to low fat milk that has all the same nutrients, without the fat.