Until she was 11 months old, Aleanie Remy-Marquez could have starred in an advertisement for breast milk. She took to nursing easily, was breast-fed exclusively for six or seven months, and ate little else even after that. She was alert and precocious and developed at astonishing speed, her mother said, sitting at four months and walking by eight months.

But once Aleanie started putting weight on her feet, her mother noticed that her legs were curving in a bow shape below the knees. Doctors diagnosed vitamin D-deficiency rickets, a softening of the bones that develops when children do not get enough of the hormone form of vitamin D  a crucial ingredient for absorbing calcium and building bone, and the one critical nutrient that breast milk often cannot provide enough of.

“I thought I was doing the best thing for her,” said Stephanie Remy-Marquez, of Hyde Park, Mass., after blood tests showed her daughter had no detectable vitamin D. X-ray images of the baby’s wrists and knees showed the edges of the bones and growth plates as blurry and fraying instead of crisp and sharp.

“Breast milk is supposed to be an entire meal, dessert and drinks included,” Ms. Remy-Marquez said. “I thought it was the ultimate cocktail.”