I have a four-week-old baby and a 21-month-old toddler. I breastfed my toddler for 11 months and am now breastfeeding my baby. However, I am finding it hard to give my toddler enough attention due to the long time it takes to breastfeed, so I have decided to mix feed. I plan to replace three feeds a day with formula – will she still get all the immune system benefits if I do?
Significant immune benefits cross over in the first few weeks, so you have done enough on that basis already. And keeping some breast feeds going will still be beneficial nutritionally, so don't worry about the mixed feeding: you are not letting your baby down. That said, however, have you thought about expressing breast milk and using it from a bottle for some or all of the other feeds? This will save time – you can express far faster than your baby can suck; your midwife or health visitor will be able to advise you on how to do it efficiently.

• This article was amended on 17 June 2011. The first sentence of the doctor's response was modified with the word "significant" to make clear that not all immune benefits transfer in the first few weeks. Although the concentration of antibodies is much higher in the early colostrum milk that is passed from mother to newborn, ongoing breastfeeding continues to help protect babies against infections.

• Do you have a question for Dr Smith? Email doctordoctor@theguardian.com