How often you should sterilise baby bottles, according to science

I'm forever boiling bottles. My wife and I have a new baby and we’ve been doing “combination feeding” – sometimes from the breast, sometimes from a bottle. And if you’re doing bottle-feeding, you soon learn all about the need for bottle sterilising.

The NHS states in no uncertain terms:

“It’s important to sterilise all your baby’s feeding equipment, including bottles and teats, until they are at least 12 months old.”

And that’s for every feed. The idea is that the baby’s immune system is still developing, making them extra-vulnerable to bugs that cause diarrhoea and vomiting. So you need not only to clean the bottles they’re feeding from with soap and water, but sterilise them too, to kill as many bacteria as possible.

You can do this by just boiling the bottles in water for 10 minutes, leaving them for a while in a tub of sterilising solution, or using one of many different steam-sterilising products that are on the market.

And since newborns can have eight or more feeds per day, you’ll be doing this a lot.

But how necessary is it, really? Late one night, standing in the kitchen bleary-eyed, watching the bottles spinning round in the microwave steriliser, it occurred to me to check whether there was any evidence to support what I was doing.

The first thing that occurred to me was to ask what other countries recommend. One of the most prominent things you notice during pregnancy is that countries vary wildly in their recommendations for what you do and eat: for example, sushi is a major no-no during pregnancy in the UK and the US, but is seen as completely normal—even healthy—for pregnant mothers in Japan.

It turns out it’s the same for bottle sterilising. Whereas Irish and Australian government websites give the same advice as the NHS, recommending sterilisation after every feed for twelve months, the New Zealand government says that you only need to sterilise bottles for the first six months, after which “thorough washing and rinsing are enough”. The government of the province of Alberta in Canada, conversely, says you should keep sterilising for four months.

In the US, the guidelines are different again. The website of the Centrws for Disease Control and Prevention advises that you sterilise bottles for only the first two months, or for longer if your baby has an illness that weakens its immune system. And even then, it only recommends doing so once per day, rather than after every single use. The Texas Children’s Hospital goes even further: they recommend sterilising bottles just once when you first buy them, and then not worrying about it again.

The French government’s health ministry takes things the final step, advising that “it is not necessary to sterilise feeding bottles and teats, as had been recommended in the past”.

Here’s something all these varied pieces of advice from different countries have in common: they don’t cite any scientific sources to back up their recommendations. All but one are just baldly stated – only the Canadian guidelines admit that there’s no evidence they can point to for what they’re saying.

Few authorities can point to any evidence on whether you should sterilise your baby’s bottles (Philip Toscano/PA Wire)© Provided by The i

But surely there must be some research out there? I looked, and was surprised to find very slim pickings. The last time a review of the scientific literature was done was 2008; they found “a lack of good-quality evidence” on the most effective ways to clean bottles.

There are a few studies where samples from used bottles were taken to the lab to check their levels of contamination. Probably the most comprehensive one, from 2009, found that bacteria were most likely to live in the nooks and crannies of the rim and screw cap on bottles, and that there were fewer bacteria on the bottles that had been steam-sterilised. Having said that, they also pointed out that touching the bottles on their way out of the steriliser just re-adds bacteria – and also that if the bottles weren’t washed properly to begin with, the steam sterilising can “bake” remaining bits of milk onto them, providing a particular focus for bacterial growth. You really can’t win.

The most recent relevant study, from 2019, showed that bacteria can travel from a person’s hand to the scoop they use to put powdered infant formula into the bottle: but that would imply that the really important thing is to wash your hands, just as you would before any kind of food preparation.

These studies looking at the bacteria that remain on bottles are all very well, but they don’t tell us what we really want to know, which is: how much do different cleaning techniques matter for keeping our babies healthy? On this, there’s almost no evidence.

I could find only three studies that compared the health of babies who’d had their bottle sterilised to those who hadn’t. The first was published in 1974 – it included only 26 babies and is so old it can basically be disregarded (it found no differences in illness rates). The second is from 1987, included 284 infants, and found no differences in gastroenteritis when comparing those who sterilised versus those who didn’t.

The third study, published in 2006, was based on data from England in the 90s. It found that diarrhoeal disease rates were higher in babies who’d fed from unsterilised bottles. But only a tiny number of participants in the study (39) didn’t sterilise, leading to enormous uncertainty in the results. They also found that, in babies older than 6 months, there was no longer any effect. And this wasn’t a randomised experiment, meaning that other factors—perhaps parents who were generally more careless forgot to sterilise the bottles and made their babies less healthy in other ways, like failing to wash their hands—could explain the result.

As far as I could see after quite a lot of searching, nobody has ever followed this research up – there are no other studies checking the ages at which sterilisation matters or doesn’t; and there are no studies (at least, publicly available ones) testing which of the various different sterilisation methods or devices works best.

Isn’t that remarkable? This is a routinely-made health recommendation—one that concerns the health of the most vulnerable among us, newborn babies—and scientists barely seem interested in checking it.

For want of proper studies, we could turn to an even cruder analysis: we could look at the infant mortality rates in each country. If some countries do a much worse job of looking after their babies—and if sterilising their feeding bottles in the early months of life is so important—we might see that reflected in these grim statistics. At first glance, you might think that the more lax guidelines in the US could be contributing to that country’s substantially higher infant mortality rate than ours in the UK, where we have much stricter guidelines. But France, where the recommendation is not to bother sterilising bottles at all, has basically the same rate (if anything, a little lower) as we do. There’s basically nothing we can conclude from these kinds of data either.

So, in the absence of much actual evidence, can we draw any conclusions about bottle sterilisation? It makes sense that in the very early months, when your baby’s immune system is at its weakest, you’ll want to be extra careful about cleaning bottles. Some even more cautious people might choose to go a step further and sterilise as well. But the advice from the NHS—which recommends continuing to sterilise bottles at every feed for at least a year, well after your baby is crawling around on the floor, playing with your pets, and putting who-knows-what foreign objects into their mouth—seems absurdly overblown. It’s another weight on the mind of already-stressed parents – and as we’ve seen, it has effectively zero scientific evidence to back it up.

Of course, there’s a big assumption here. The fact that I’m even asking this question—rather than just assuming that sterilisation is a good idea—is down to us living in a country with clean drinking water. It wasn’t always this way: as mentioned in the French guidelines we saw above, “in the past” the water quality wasn’t as good, so going above and beyond to sterilise bottles might’ve made more sense. Now that we can rely on water being clean, sterilisation is something extra for the more neurotic among us, rather than a vital requirement.

But in many other countries, particularly in the developing world, water quality is far from assured. You might remember the “Boycott Nestlé” campaign, which started in the late 1970s and has been running ever since. The argument was that the food company was pushing their infant formula in parts of the world with very poor water quality, discouraging breastfeeding and putting infants at risk of catching water-borne disease. However true the argument was then (or is now) about Nestlé specifically, the point about water quality is a good one: in low-income countries there is compelling evidence that improving water quality (using methods like chlorination) can reduce infant mortality.

For those of us lucky enough to live in countries with good-quality water, and where we remember to wash our hands before making up our baby’s bottle, being concerned about sterilisation after the first few months seems like a step too far. For my part,  after a couple of months I’m going to try and focus on some of the million other things to worry about as a new parent – and put the bottle-steriliser down.  

Reference: The I:Stuart-ritchie