how donor breast milk supports the formula-fed infant microbiome

For decades, parents have been told they have two choices: breast milk or formula.

But that's not how most families actually feed.

Millions of parents combo feed—breastfeeding when they can, supplementing when they need to, and making thoughtful decisions based on real life. Yet despite this being one of the most common feeding journeys, almost no research has explored what happens when small amounts of breast milk are added alongside formula.

We're changing that.

Leche has partnered with Tiny Health, the leader in at-home microbiome testing to launch the first clinical study examining how adding a small daily amount of donor breast milk may influence the gut microbiome of formula-fed infants. With support from the leading US formula company built for modern families, Bobbie, we are helping bring not only more feeding options, but more support to families no matter what their feeding journey looks like.

why this study matters

We're learning that breast milk does much more than nourish a baby. It nourishes the microbiome.

"We're only beginning to understand the biological intelligence of breast milk. This study asks a simple but important question: if even small amounts of human milk help shape the infant microbiome, how might that change the future of infant nutrition?", -Trish Clifford, Leche Co-Founder.

Every Leche sachet contains hundreds of bioactive compounds including approximately 150–200 unique human milk oligosaccharides (HMOs) that were never designed to feed babies directly. They were designed to feed the beneficial bacteria living in their microbiome.

Those bacteria help shape:

• Immune development
• Gut health
• Digestion
• Metabolism
• Protection against pathogens
• Long-term health

Modern infant formula has made incredible progress and now includes one or two HMOs. But human breast milk naturally contains the complete ecosystem of bioactive compounds that evolved together over millions of years.

The question we've never been able to answer is:

Can even a small amount of donor breast milk meaningfully influence a formula-fed baby's microbiome?

That's what we're setting out to discover.

about the study

This prospective clinical study will follow up to 100 healthy, full-term formula-fed infants over one month.

Each participating family will:

• Complete a baseline Tiny Health gut microbiome test
• Add Leche powdered donor breast milk to daily feeds for 30 days
• Complete a second Tiny Health microbiome test

Researchers will evaluate whether introducing donor breast milk changes important markers of infant gut health, including:

• Growth of beneficial Bifidobacterium
• Improved utilization of human milk oligosaccharides (HMOs)
• Overall movement toward a more breastfed-like microbiome profile

The microbiome analyses will be performed independently by Tiny Health.

why we're doing this

At Leche, we've always believed that access matters.

Not every family exclusively breastfeeds.

Not every mother produces enough milk to meet her baby's needs.

Not every baby can receive only breast milk.

But that shouldn't mean families lose access entirely.

If breast milk provides unique biological benefits (and decades of research suggest it does) then understanding what happens when families incorporate even small amounts into combination feeding could reshape how we think about infant nutrition.

"Innovation isn't about replacing breast milk…it never has been. It's about making it more accessible. We hope this study helps move the conversation beyond 'breast versus formula' and toward giving families more ways to benefit from one of nature's most remarkable foods." -Jenney Korasick, Leche Co-Founder

This study isn't about proving one feeding method is better than another.

It's about giving families more information, more options, and more confidence.

could every drop matter?

Research has consistently shown that breast milk helps establish the infant microbiome.

What we don't yet know is how much breast milk is needed to make a measurable difference.

Could one feeding a day matter?

Could a few ounces?

Could combination feeding provide more biological benefit than we've ever measured before?

We're excited to help answer those questions.