Does a breastfed baby need water in hot weather?

It is one of the first things a relative will say the minute the sun comes out. "Sure, give the baby a drop of water in this heat." It comes from a kind place, and for a young baby it is the wrong advice.

If your baby is under 6 months and exclusively breastfed, the HSE could not be clearer: they do not need any extra water, even in hot weather or when travelling somewhere hotter. Breast milk gives them everything, the food and the fluid both.

Why breast milk is enough on its own

Breast milk is mostly water to begin with, and it is cleverer than people give it credit for. The milk that comes at the start of a feed is thinner and more thirst-quenching, the milk later in the feed is richer and more filling. Your baby takes what they need from the same source.

It even adjusts to the weather. In warmer conditions breast milk tends to be more watery, which is the body quietly solving the exact problem your auntie is worried about. So the right response to a hot day is not a bottle of water, it is simply to offer the breast more often. Babies feed on demand, and on a hot day demand goes up. Let it.

The reassurance you can actually check

If you want proof they are well hydrated, it is in the nappies, not in how much water you have poured into them. A well-fed young baby has 6 or more wet nappies a day, the nappies feel heavy, and the wee is pale yellow or clear. That is the green light.

So why not just give a little water to be safe?

Because for a small baby it backfires. A baby's tummy is tiny. Water fills it up without giving any nourishment, so they then take less milk, and milk is what they actually need to grow. In very young babies, giving water in place of milk can mean they miss out on feeds. The simple rule is that milk, breast or formula, is the main drink until they are 12 months old.

One related tip if you are out and about: if you ever do give cooled boiled water to an older baby, avoid bottled water labelled as natural mineral water, because it can be higher in sodium and other minerals than is good for a baby.

What about formula-fed babies?

Same principle. First infant formula gives a baby all the nourishment and hydration they need up to age 1, so in hot weather you offer their usual feeds more often. Two things matter:

• Never water down or dilute a bottle. Make it up exactly as normal. A weaker bottle means your baby does not get the nutrients they need from that feed.
• If a bottle-fed baby seems to want a little extra in the heat, you can offer small amounts of cooled boiled water separately, between feeds, not instead of them. If you are unsure, ask your public health nurse.

And the usual safety rule when making up a bottle still stands: use freshly boiled water that is still at least 70 degrees (cooled in the kettle for no more than 30 minutes) to make up the powder, then cool the made-up bottle before giving it.

Once they are over 6 months

From around 6 months, when solids start, you can offer water in a cup at meal and snack times, alongside their milk. Handy bonus: drinking water for babies over 6 months does not need to be boiled first, ordinary tap water in a cup is fine. Milk is still the main drink for the first year.

When it is not just thirst

Knowing the warning signs takes the worry out of a hot spell. Keep an eye out for these, which the HSE lists as signs of dehydration:

The short version

• Breastfed baby under 6 months: no water needed, even in a heatwave, just feed more often
• Formula-fed: more frequent usual feeds, never a watered-down bottle
• Wet nappies (6 or more a day, pale) are your hydration check
• Water in a cup with meals from 6 months, milk still the main drink to age 1

The HSE detail is on drinks for babies 0 to 12 months, with more on hydration in our piece on keeping your baby cool and safe in hot weather.