why can babies have water in formula but not actual water before 6 months?
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Doctor here, all of the answers so far are either incorrect or incomplete. Babies are tiny, and their kidneys are not as efficient at handling water, so when you give an infant more water than their kidneys can handle, the extra water spills over into your bloodstream. This will dilute the blood, making it more “watery.” The concentration of salt within the blood decreases even though the amount of salt in your blood is unchanged. This is called dilutional hyponatremia (low salt concentration in the blood).
Now, your body likes to keep everything perfectly balanced (this is called homeostasis). Your brain cells also have salt suspended in fluid, and they require a specific ratio of salt to water for optimal functioning. Your brain cells are surrounded by blood vessels. If the concentration of salt in your blood is within a narrow range that is acceptable, then the brain is happy. However, if the concentration of salt in your blood is lower than it should be, then your brain will think there is too much salt in the brain, so water will be pulled from the bloodstream into the brain in an attempt to make the concentration of salt in the brain roughly equal to the concentration of salt in the bloodstream. The salt does not actually go anywhere like some of the comments are suggesting. This will make your brain cells balloon with water. This is called cerebral edema, or swelling of the brain. This is bad for the brain because it causes damage to the brain and can increase the pressure inside the skull. The infant brain is small and still developing, so it can be more susceptible to cerebral edema.
It’s not so much about water lacking calories. Sure, this can contribute to malnourishment but you can easily fix this by giving the infant more calorie dense formula. The primary concern with giving infants water is the risk of cerebral edema as this can be lethal. Water toxicity can also happen with adults, but because healthy adult kidneys are fully developed and can handle water pretty well, we don’t really see this very often. Water toxicity is very rare in adults and is slightly more common in athletes, soldiers, patients with kidney failure, and psychiatric patients who drink ridiculously large amounts of water over a short period of time.
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