It’s natural to compare the growth of babies, especially twins, so when Becky Jackson noticed her son outgrowing her daughter, it made her wonder.

“We kept comparing the two of them, (saying) maybe she’s going to be little and he’s going to be big,” Jackson said.

[Video: Infant hospitalized after Vitamin D overdose]

But looking back at photos, there were other signs that her infant daughter was sick.

“You can see how sunken her eyes were, and her skin was a weird color,” Jackson said.

Jackson took the baby to the pediatrician when she started having feeding problems at around 6 months of age. Elizabeth was losing weight and was listless.

“They labeled her as ‘failure to thrive,’” said Jackson, a term used to describe babies who are headed toward malnourishment.

“I was scared. I didn’t know what was going to happen,” Jackson said.

Blood tests revealed high calcium levels in her blood caused by a toxic Vitamin D overdose. Jackson was told to take the baby to the hospital immediately.

“It can cause kidneys to have calcium deposits and kidney failure and it can cause irregular heartbeat,” said Dr. Monina Cabrera, a pediatric endocrinologist at Omaha’s Children’s Hospital and Medical Center.

Cabrera hospitalized Elizabeth for 10 days, putting her on IV fluids and working on feedings to help the infant stabilize. Then she started investigating the source of the toxic levels of vitamins. Jackson realized quickly what went wrong, telling the doctors who were making rounds.

“I did this. I did this. I had no idea there could be such a negative effect,” Jackson said.

With twin babies to feed, Jackson said she had only enough breast milk for one baby. So she decided to feed the breast milk to her son, Joe, who had a fussy tummy. For Elizabeth, she found a recipe for formula.

“I was supplementing her with a very organic homemade formula,” said Jackson, who bought items like powdered goat’s milk, coconut milk and a variety of liquid vitamins.

Jackson followed a recipe that she found online, with a little something extra.

“I was adding too much Vitamin D. I wasn’t following the recipe as it said. I thought more must be good,” Jackson said.

The recipe called for a drop of Vitamin D for every 32 ounces of formula. Instead, Jackson added an entire dropper of Vitamin D to each batch, later estimating that the child received 8,000 times the amount she needed.

“Just be careful. Just be careful,” Cabrera said, referring to Internet recipes for infant formula.

She said that after reviewing the ingredients, she determined that the recipe itself was fine, but directions must be followed. She encouraged parents to always share an infant formula recipe with a pediatrician and a dietitian before feeding it to a child. Cabrera said Vitamins D, A and E are fat soluble, which means they accumulate in the body’s fat and don’t flush away easily.

In Elizabeth’s case, too much Vitamin D was toxic. Two months later, the baby’s levels are still high, but she’s back on the growth charts and eating a calcium-free, store-bought, prepared formula.

And her parents, grateful for no lingering health issues, are sharing their story, hoping that others learn from it.

“I think it’s just important to realize the impact that vitamins can have, both negative and positive, and not to think it’s OK., that it’s just a vitamin. It’s no big deal,” Jackson said.