Many epidemiologic studies (study of factors affecting health and illness) have linked use of infant formulas to increased risk of sudden infant death syndrome (SIDS).
One review study found infants who were formula-fed were 2.11 times more likely to suffer SIDS than those who were breastfed.
Low levels of serotonin are known to be a risk for SIDS. There are two ways to cause low levels of serotonin. Reducing its precursor and increasing its receptor are two ways by which low levels of serotonin can happen.
Soy-based formulas, which are a major type of formulas, contain only 60 percent of the serotonin precursor called tryptophan found in breast milk. At least one study has showed consumption of soy-based formulas led to low levels of serotonin in mice. This could mean that soy-based formulas could be a risk for SIDS. Supplementation of tryptophan showed an increase in the serotonin concentration.
A recent study by Italian researchers revealed that when high levels of serotonin A1 receptors were produced, many mice died from some event similar to SIDS. High levels of the receptor could result in low levels of free serotonin. This is a known risk factor.
It must be mentioned SIDS is complicated and could be caused by any of many risk factors.