Why Can't Babies Drink Water?

Sam Westreich, PhD

Another piece of evidence that shows humans actually aren’t very good at reproducing

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Milk: good for baby. Water, not so much.Photo by Lucy Wolski

Babies are adorable. As a new parent, I can confidently say that, despite the explosive pooping, the random bouts of crying, and the inability to help with filing their tax forms, they are incredibly cute, especially in tiny footie pajamas.

But babies are also a challenge. Along with sleepless nights, stress on the bodies of the parents, increased financial costs, and the need to fill up your phone’s memory with pictures and videos, they also need special care.

Newborns need to be fed every few hours. They can do next to nothing to care for themselves, aside from screaming to warn when they need care. And for the first six months to a year, they can’t even have water.

What? Babies can’t drink water, the most basic of beverages that we need to sustain life?

That’s right. And it comes down to levels of salt.

Ignore bank account balance, focus on osmotic balance

Humans are salty. This isn’t a statement on our frustration when we lose a game; it’s a measure of the amount of dissolved ions in our tissues and our fluids. Our blood, for example, has a salinity of 9, which means that about 0.9% of our blood is dissolved salt (by weight).

When we drink water, what happens? Well, unless you’re always drinking Gatorade, that water doesn’t contain any salt. By drinking it and absorbing it, you are diluting your blood, lowering your salt levels.

Ordinarily, that’s fine. Not because it’s good for us to have diluted blood, but because we have mechanisms to correct for this. Our body is always working to keep us balanced, pulling us back from extremes towards the comfortable middle ground. This effect, known as homeostasis, is why we sweat when we get too hot but shiver when we get too cold. The goal is always to come back to the happy medium!

In the case of the water we drink, we have our kidneys to thank for maintaining homeostasis. Our kidneys do a few jobs; along with regulating our water levels, they also help filter our blood, dumping unneeded compounds into a separate stream that flows to the bladder and will become urine.

Drink too much water? Our kidneys are always monitoring for a signal hormone, called vasopressin, that serves as an indicator of whether we have enough water in our bloodstream or not:

  • If we get dehydrated, vasopressin levels rise, and our kidneys conserve more water.
  • If we are over-hydrated, vasopressin levels drop, and our kidneys dump more water to flow to the bladder.

Usually, this system works great, except in extreme cases (think: dehydration in a survival situation, or when someone forces their body to drink far too much water at once).

But if this system’s so great, what’s wrong with babies?

Babies don’t run very well (both for function, and actual track races)

The problem comes down to the fact that, when babies are born, they are still pretty underdeveloped.

Look at the baby foal from a horse, or an adorable baby giraffe. These animals are up and moving from the moment they’re born. A foal is able to stand within two hours of its birth, and can gallop within 24 hours. That’s needed to escape predators, but it also looks like an unreal achievement when we compare it to a human infant, who won’t be able to walk for an entire year (on average — no judgment for slow walkers).

Even compared to other primates, human babies don’t stack up. A newborn chimpanzee is equivalent to a nearly 2-year-old human baby.

There’s a few theories as to why babies are so useless. One theory suggests that the human head simply grows too big for our relatively narrow upright-walking hips. If the fetus developed any further inside the mother, it wouldn’t be able to fit through the pelvis.

Another, more recent theory, argues instead that the need to deliver an underdeveloped and useless baby comes down to metabolic rate. A baby needs a ton of energy, and beyond 9 months, the mother cannot supply this to the fetus. (Yes, a newborn still requires food, but at least it doesn’t need the mother to be filtering blood or keeping it warm inside the womb!)

In any case, newborns have a few issues that prevent them from efficiently handling water:

First, their tiny stomachs. Babies, especially newborns, have very small stomachs (about the size of a large grape when they’re born!). If a baby drinks water, there’s no room in the stomach for breast milk — and that means no calories. Babies need all the stomach room available in order to take in calories and nutrients via milk.

Second, baby kidneys are underdeveloped when the infant is born (sensing a theme here?). Babies have a pituitary gland that can put out vasopressin, but their kidneys simply can’t excrete enough water for them to maintain their homeostasis if they’re drinking pure water.

Just like how an overwatered plant will die, a baby that consumes water gets too hydrated, its blood becomes too dilute, and it can suffer severe symptoms, up to and including death.

No water for newborns, got it. When can they start having nature’s best drink?

Ideally, a baby doesn’t get to really taste pure water until sometime between age 1 and 3. For at least the first 12 months, ideally up to the first 24 months, an infant’s exclusive drink of choice (and they get no choice in the matter) should be breast milk.

Breast milk is pretty great stuff, both for the overall health of the baby and for its fragile but rapidly developing gut microbiome. Unlike most artificial baby formula, breast milk contains unique sugars (called oligosaccharides) that are present to explicitly feed the gut of the baby. Breast milk may even transfer certain compounds from the mother to the baby, depending on the mother’s diet.

Of course, there are also some crazy claims about breast milk, like its ability to cure practically every ailment in babies. There have been suggestions that it should be the first choice for skin rashes (actually true, it has a beneficial effect), eye infections (no) and ear infections (again, no). But we can certainly conclude that it’s both a great food for infants, and it also provides all the water that they need.

Still, not everyone has access to or can produce breast milk, and in the end, babies raised on formula still do great. Just like breast milk, formula contains the water that a baby needs — along with the appropriate nutrients and calories. Some manufacturers are even now adding some oligosaccharides, to further close the gap between formula and natural milk.

In the end, “fed is best” — breast milk if possible, but any formula will do.

Just not water, until at least they’re walking!

In summary: weak, ineffective baby kidneys mean an all-milk diet

We all drink water (or we should; most of us are probably drinking too little water and too much soda) — except for babies. Babies that are a year old or younger should not be drinking water.

Our goal is to maintain a balance, at least when it comes to our bodily function. Our kidneys help with that in regards to water, dumping the excess when we are overly hydrated and preserving water when we are dehydrated. They do a great job — but infants, who are pretty useless when they are born — don’t yet have kidneys developed enough to handle this task.

In addition, babies have tiny stomachs, and water is dead weight; it fills them up, meaning less space for calorie-rich formula or breast milk. They need all the calories and nutrients they can get to grow, and since they’re already on a liquid diet, they get all their hydration that way.

Someday, my child will get to enjoy a nice, tall glass of ice-cold water. But for now, the water is only for his exhausted parents. Water for me, milk for the baby. Just how nature intended.

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A microbiome scientist working at a tech startup in Silicon Valley, Sam Westreich provides insights into science and technology, exploring the strangest areas of biology, science, and biotechnology.

Mountain View, CA
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