Latest: Older Siblings May Help Reduce Asthma, Improve Microbiomes

Sam Westreich, PhD

Got a younger brother or sister? Maybe you can brag about saving them from asthma
It’s black and white but don’t worry, they’re alive! Healthy, even, thanks to colonizing each other!Photo by Caroline Hernandez/Unsplash

Ugh, siblings. What are they good for? They just compete with you for food at the dinner table, tattle on you when that ball totally broke the window on its own, and demand to put their dirty, grubby paws all over your brand new video game system, even though it was your present and not theirs…

…sorry, what were we talking about? Oh yes, siblings. They may be annoying at times, but a new study suggests that children may have a powerfully beneficial effect on the gut microbiomes of their younger brothers and sisters.

The full paper, from a group at the University of Copenhagen in Denmark, is available to read online, but this article can give you the highlights.

All those times that your older brother or sister sneezed on you? They weren’t just trying to annoy you! They were trying to improve your microbiome!

What is the microbiome, and why’s it so important in kids?

Before we go any further, let’s make sure we’re clear on the science terms, and also why this is such an important feature in our bodies to understand — especially in children.

We are not alone in our bodies. Many areas of our body are literally brimming with many different kinds of bacteria, living together in an uneasy harmony/competition for survival. Each collection of many different types of bacteria is called a microbiome.

We have different microbiomes — different blends of bacteria — in different places. We have an oral microbiome in our mouth, a gut microbiome in our large intestine and colon, a skin microbiome on, well, our skin. And each of these has further degrees of variation; you’ll have different levels of bacteria living in your warm, damp armpits versus your more dry and barren forearms.

In every person, the microbiome is unique. You don’t have the same balance of microbes as I do. However, people who interact with each other, or who are in similar environments, or who do the same sorts of activities, tend to have microbiomes that are more similar to each other. Two vegans are more likely to have more similar gut microbiomes than a vegan and a meat eater, for example.

We’ve only started studying the microbiome relatively recently, in the last 20 years as our bulk bacteria-identifying technology got better, but we know that it can play a big role in health. Our microbiome interacts with our immune system, helping to train our system and teaching it the difference between a threatening disease-causing pathogen and an innocent friendly bacterium.

And that immune system training is especially important in our early years. From the moment that a baby is born and takes its first meal, its microbiome is being shaped. Different microbiomes are linked with very different health outcomes for babies, children, and adults, including:

  • risks of bowel diseases
  • risks of asthma and other autoimmune diseases
  • risks of ear infections
  • risks of eczema or other skin conditions
  • metabolic diseases

Our goal, as a scientific and medical community, should be to make sure that everyone has a microbiome that is healthy.

We just need to agree on how to do so — and what, even, should be considered a “good” microbiome.

Siblings — or perhaps their microbes — seem to contribute to “good” microbiomes

Enter the Copenhagen Prospective Studies on Asthma in Childhood 2010 (COPSAC2010) cohort, a group of 700 children who are being studied longitudinally (that is, we’re tracking them and continuing to take samples as they grow and develop). The focus is, as the name suggests, on trying to see what factors are linked with, and may contribute to, respiratory diseases.

As part of this study, researchers collected swabs of the upper respiratory tracts of the children and performed 16S rRNA sequencing on them. They also collected fecal samples, looking to get the microbiome of the gut as well as of the airway.

(I won’t go into too much detail here, but 16S rRNA sequencing is a method of identifying bacteria by scanning a specific part of a common gene, kind of like scanning barcodes on products at a supermarket.)

The big observation that is highlighted in this research paper is how the presence of an older sibling altered the microbiome of the younger child. If you had an older sibling, you showed:

  • Greater diversity of microbes in the both the airway and the gut
  • Shifts in which microbes were present (more Actinobacteria, fewer Firmicutes)
  • Lower asthma rates (although no effect on the rates of respiratory infections)

Overall, having an older sibling seemed to be a good thing, and the closer that sibling was in age to you, the better!

So what’s going on?

The most likely hypothesis is that this is another way to combat the “clean room” hygiene theory. It’s the same theory behind the encouragement to let young kids eat some dirt, or at least don’t worry about them getting dirty in the muddy outdoors.

It’s all about exposure. When we are young and our immune system is still developing, we want to expose it to a lot of different bacteria. We don’t necessarily want to expose babies to typhoid or other deadly diseases, but we want to get their immune system used to seeing many commensal microbes, those that don’t provide a big benefit but also don’t cause disease or bodily harm.

Older siblings are a great source of commensal bacteria. This is supported by further evidence suggesting that the protective effect is made stronger not by the total number of older siblings, but by how close in age the nearest sibling is to the young child. If you’re getting a steady stream of commensal bacteria from a sibling a year or two older, it’s a great training method for your immune system.

How can this pay off for better health?

Here’s the $10,000 question: now that we’ve observed this effect, how can we use it to make sure our kids are as healthy as possible from a microbiome and asthma standpoint?

Suggesting that parents plan on multiple kids is probably a bit unreasonable. It’s good to know that this protective effect exists, but it’s probably not reason enough on its own to alter any family plans.

This does suggest that we want to encourage microbial exposure in young children, and slightly older children are a great vector to do so. There might be an added advantage to older siblings, versus non-related children, given the genetic similarity. (Are siblings, because of their genetic relatedness, more likely to carry the microbes that are best for a young child?)

Still, this evidence likely supports the potential benefits of socialization for kids, even at a very young age, at regular and extended intervals. The goal is to provide a good, diverse microbiome to these kids — and that means bringing in as many microbe sources as possible!

In conclusion: kids transfer germs, and that’s good

All of us carry around microbiomes, which vary based upon a complex interplay of our genetics, our environment, our interactions with others, and the location where those microbes are growing.

In this 2022 study, building off of data collected in a longitudinal study monitoring children and parents in Copenhagen, researchers found that having an older sibling, particularly an older sibling close in age to you, was beneficial. It correlated with greater diversity of microbes and lower rates of asthma. A larger sample size might also connect it with lowered rates of other gut and autoimmune diseases.

There are limitations to the conclusions we can draw here — for instance, we don’t know exactly which microbes might be best to introduce for lowered asthma risk — but it’s another reason for us to ensure that our children are exposed to many microbe sources. Whether these are friends, family, or the outdoors, introducing microbial diversity seems to be a great strategy for producing a healthy immune system in our kids.

How do you think having older siblings might be beneficial for a child?


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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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