Question: I started back to work this week, and I am giving formula during the day and breastfeeding only at night. How much benefit is my baby losing?
Answer: Your baby will continue to get the benefits of breastfeeding for as long as he nurses. Any amount of breast milk is beneficial, and your baby will continue to receive the immunological advantages of your milk even if you are combining breast and bottle feeding.
The concern that I would have is whether you are going to be able to maintain your supply if you are only nursing at night. Unless your baby nurses often during the night, then your supply is likely to drop significantly unless you pump at least once or twice during the day. Breast milk production is based on supply and demand, so if you go from emptying your breasts every few hours to only emptying them a few times a day, you can expect your supply to drop. Some babies are willing to continue nursing even after your supply goes down, but others will get frustrated and start refusing to go on the breast at all.
The older your baby is when you return to work, the easier it is to manage without pumping during the day. A six month old who is eating solids and going long stretches between nursing sessions is more likely to adapt to a part-time nursing arrangement than a six week old who is used to nursing many times a day and whose diet consists totally of breast milk.
I think that it is great that you want to continue nursing after returning to work, but it can be challenging, especially if you are unable to pump while you are separated from your baby.
The articles Increasing Your Milk Supply and Returning to Work have more information you may find helpful.
Anne Smith, IBCLC