Soy formula

OhBabyYoung

My baby girl is 4.5 weeks born at 36 weeks.  I started breastfeeding and supplementing with pumped breast milk because her suck was not stong enough being 4lbs 15 oz.   After a week she had not gained weight so the doctor assumed my breast milk was not fatty enough.  The doctor had me supplement with formula and I continued pumping for a few weeks but after a stressful and emotional decision I switched to solely formula. Over the past few weeks her spit up has been horrible. At least three large spit ups per feeding so the doctor had us switch to soy formula.  

 After looking online there seems to be some possible side effects to soy. Has anyone discussed this with their doctor before?  Also, if you started soy with your child were you able to switch back to a milk based formula as they got older?

Thanks!