We generally don’t recommend using softened water with baby feed. You can use softened water for baby feeds if the sodium concentration is below the 200 mg/l limit for sodium. So, it isn’t recommended if you are unsure of your water’s sodium concentration – which is likely in most cases.
The 200 mg/l sodium level was set by the Drinking Water Inspectorate in the Drinking Water Regulations.
This limit was adopted to make sure that the total sodium levels in baby feeds, plus the water added, does not exceed 350 mg/l, considered a safe level for infants.