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  • Thanks to Brilliant for supporting this episode of SciShow.

  • Go to Brilliant.org/SciShow to learn how you can take today's lessons

  • even deeper with their Physics of the Everyday course!

  • [♩INTRO]

  • If you've ever strolled through a pharmacy in search of over-the-counter meds,

  • you might have noticed that lots of drugs have special children's formulas.

  • And you might think that's because smaller people need smaller doses.

  • But you'd be wrong. Because kids aren't just tiny adults.

  • In fact, children sometimes need larger doses of medicine!

  • Welcome to the weird science of allometry.

  • Allometry simply refers to the study of how physiology changes with size,

  • and it plays a big part in understanding how much medicine kids should get.

  • You might think bigger bodies would need larger doses of medicine

  • since they have more of whatever tissue or cells the drug is targeting.

  • And there are a lot of medications

  • that are prescribed based on a patient's weight

  • what's sometimes referred to as a weight-based dose.

  • For instance, an adult might be prescribed five milligrams of medicine

  • for every kilogram they weigh.

  • But what's really weird is that this dose isn't constant

  • throughout a person's lifetime.

  • You often can't take the dose per weight an adult would take

  • and simply multiply it by the child's weight to figure out how much they should get.

  • And that's largely because a person's cells use less energy as their body grows.

  • In other words, bigger people have lower mass-specific metabolic rates

  • than smaller ones.

  • Even at the cellular and subcellular level,

  • oxygen and calorie consumption is just slower in larger bodies.

  • Not only that, but it's a nonlinear relationship

  • so as bodies get smaller, metabolism increases fast.

  • And that matters when it comes to medicines

  • because it impacts how fast your body processes drugs.

  • Generally speaking, the faster the metabolism, the faster you break down a drug

  • and that means, even if you're small, you might need to take more

  • to have the amount you need stick around long enough for it to work.

  • Let's consider acetaminophen, which you might know as Tylenol.

  • It's an over the counter drug you can take to reduce pain and fevers.

  • The regular adult dose for acetaminophen is 650 milligrams.

  • So an adult weighing roughly 80 kilograms

  • the average mass for adults in North America

  • would generally take that amount every four to six hours as needed.

  • If you'd never heard of allometry, you might think a child one-quarter that weight,

  • 20 kilograms or so, would get 162.5 milligrams.

  • But that's way too little.

  • The child will metabolize it super quickly,

  • so the drug won't really have the chance to be effective.

  • With junior strength tablets, the recommended dose is actually 240 milligrams.

  • And for a 44 kilogram child, the recommended amount is 640 milligrams.

  • That's basically the same as what's recommended for adults,

  • even though the child is about half the weight!

  • And if you calculate the weight-based doses for everyone,

  • the children actually take about two times as much.

  • Other drugs can be even more extreme.

  • The adult dose of oseltamivir,

  • which is an antiviral used to treat influenza infections,

  • is 150 milligrams a day.

  • So, a little under two milligrams per kilogram per day for our 80 kilogram adult.

  • But pediatric doses range from three to six milligrams per kilogram per day!

  • As for why metabolic rate scales with mass in this strange, nonlinear way, well

  • physiologists don't know for sure.

  • But by looking at people and animals of all sizes,

  • they have come up with some good hypotheses!

  • The first and perhaps most obvious is that

  • little bodies belong to individuals that are still growing,

  • and it takes more energy to build tissues than to maintain them.

  • So, that's probably part of it.

  • But there has to be more going on,

  • because the allometric pattern holds across different sized adults, too.

  • That's where the shapes of smaller and larger bodies might be coming into play.

  • Because a smaller body has a larger surface area relative to its tiny volume,

  • it loses core heat faster.

  • That means it needs to burn more energy

  • and, therefore, run a higher metabolic rate, to stay warm.

  • But even that still doesn't fully explain the allometric scaling

  • of metabolic rate seen in nature,

  • because it only makes sense in so-calledwarm bloodedspecies like us

  • that strictly regulate their internal temperature with the heat they produce.

  • And this metabolic rate scaling occurs in reptiles

  • and other so-calledcold-bloodedspecies, as well.

  • There may be another way that overall shape impacts all this, though,

  • which helps explain that.

  • Some mathematical work suggests metabolic rates in large animals

  • are limited by their internal plumbing.

  • See, branching blood vessels have to strike a balance between their width,

  • the pressure of the blood moving against their walls,

  • and the force it takes to move that blood around.

  • In a bigger animal with a larger, longer, and branchier circulatory system,

  • blood flow ends up being slower.

  • Since it's slower and the body is larger,

  • it takes longer for blood and the oxygen it carries to get places.

  • And the animal can't beat its heart faster or harder to speed things up,

  • because if it tried, its heart would fail.

  • So as an animal grows in size,

  • its cells simply can't keep using oxygen at the same rate.

  • More work is needed to sort out how all these ideas apply to humans specifically.

  • But whatever the reason for children's elevated metabolisms,

  • the effect on drug dosing can be pretty dramatic.

  • So, ultimately, it's not that there are separate formulations

  • because kids always need less medicine.

  • If anything, it's so that a patient, or their caregiver

  • can more carefully adjust the dose they need for their body size.

  • Liquid medications in particular can be carefully tailored to a child's weight.

  • And there are other reasons to have special formulas for kids, too.

  • Like that pills can be hard to swallow or even a choking hazard,

  • especially for younger children.

  • So lots of kid's medicines avoid the issue by using chewables or liquids instead.

  • Plus, not all medications are recommended for children of all ages.

  • They might be ineffective or have more severe side effects in smaller kids.

  • Or they just haven't been thoroughly tested yet.

  • The bottom line is, if it isn't sold in a separate children's version,

  • it shouldn't be given to a child without consulting a pediatrician.

  • And you should always stick to the children's formulations when treating kids

  • because that's the best way to ensure they get the right amount

  • even though it may be more than what you'd take.

  • The allometric scaling of metabolism also helps explain

  • why we can't usually give our pets and other animals

  • weight-adjusted doses of medicine.

  • And it has a really strong impact on what they eat

  • which you can learn all about in the mammalian eating portion

  • of Brilliant's Physics of the Everyday course!

  • While physiology is complicated,

  • all that loss of heat stuff comes from fundamental physics principles.

  • Which means you can actually learn how to calculate how much heat

  • differently-sized animals lose, and from that, how much of their body mass

  • they would need to eat simply to stay warm.

  • And even though you make some entertaining assumptions,

  • like that animals are spherically shaped, the theoretical calculation

  • lines up really well with how much biologists observe animals eating.

  • Which is pretty cool, right?

  • In addition to mammal diets, the course examines the science behind

  • everything from household items to forensics.

  • So it's a fun way to learn about your world and refresh your science knowledge.

  • And right now, the first 200 people to sign up at Brilliant.org/SciShow will get

  • 20% off the annual Premium subscription.

  • Which is a great deal!

  • So if you're interested, check it out.

  • And as always, thanks for watching SciShow!

  • [♩OUTRO]

Thanks to Brilliant for supporting this episode of SciShow.

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B1 metabolic dose adult weight medicine kilogram

Why Can't Kids Just Take Smaller Doses of Adult Meds?

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    林宜悉 posted on 2020/03/30
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