What is AFU? How Many Billions Should I Take?

Deciding to take probiotics is the easy part. The hard part is deciphering which probiotic supplement is right for you.

Virtual and brick-and-mortar shelves alike are increasingly crammed with options, each promising different benefits like better digestion, a stronger immune system, and more balanced hormones. To further complicate things, the field of probiotics is constantly evolving as researchers uncover new strains and their unique benefits.

When you’re feeling lost in the world of probiotics, it’s helpful to have a solid starting point. After all, not all supplements are created equal. In order to make an informed decision, you need to understand the basics of how probiotics are measured so you can know what to look for on the label.

And that starts by understanding AFU.

What does AFU mean in probiotics?

AFU stands for Active Fluorescent Units, the latest method for measuring viable cell counts (otherwise known as the live, active bacteria that make probiotics so potent) in probiotic supplements. It’s quickly becoming the industry standard due to its ability to detect not only viable cells necessary for colonizing the gut but also detect cells that may soon die off.

This is a big deal because it means that supplements with high AFU counts are more likely to contain the live, active bacteria you’re looking for—and those are the supplements that are going to offer you the most benefits.

Here’s how it works: Test facilitators stain fluorescent dyes on the active cultures and then measure them as they pass through a tube. This process is called flow cytometry, and it allows researchers to see how many cells are active and viable.

While the method may sound complicated, the goal is simple: to ensure that you’re getting the most effective probiotic supplement possible. After all, what good is a probiotic if it doesn’t contain any live bacteria?


AFU is still a relatively new method of measurement, which means that you’re more likely to see CFU (Colony Forming Units) on supplement labels. CFU has long been the industry standard, but it’s not as accurate as AFU.

CFU is supposed to be a count of the total number of viable cells in a probiotic supplement. And if you didn't know what to look for in probiotics, choosing the supplement with the most CFUs might sound like a good place to start.

However, current regulations only require that the total weight of cells is labeled—not the number of live, active cells. This means that a supplement with a high CFU count could contain both live and dead microorganisms. This makes it difficult to determine how many live, active cultures you're actually getting.

In other words, a high CFU count is not necessarily indicative of a high-quality probiotic.

This is where AFU comes in. As the more accurate method of measurement, AFU provides more certainty that the supplement you’re taking contains the live, active bacteria it claims to—and that those bacteria are still alive by the time you take them.

Bottom line: When you’re looking for a probiotic supplement, AFU is a more reliable marker of quality than CFU.

How many billions should I take?

The number of AFUs and CFUs varies widely in probiotics, ranging from as low as 1 billion to as high as 50 billion. But how do you know how many you need?

The answer, unfortunately, is that there’s no one-size-fits-all dosage. Probiotics are unique supplements in that what works for one person may not work for another. This is due in part to the fact that we all have different gut microbiomes—the collection of microorganisms that live in our digestive tracts.

These diverse bacterial communities play a major role in our overall health, and they can vary widely from person to person. This means that some people may need more probiotics than others to see results.

Whatever dosage you determine is right for you, it's always a good idea to start slow. Adding new bacteria to your gut, even when it's the good kind, can sometimes lead to side effects like gas, bloating, and nausea.

These uncomfortable symptoms usually subside within a few weeks to a few days. To avoid this, start with a lower dosage and then increase it gradually over time.

That being said, the most common dosage of probiotics is:

  • Between 5-10 billion CFUs/AFUs for children
  • Between 10-20 billion CFUs/AFUs for adults

To sum it up

Searching for a new probiotic supplement can be overwhelming. With so many brands, strains, and dosages to choose from, it's tough to know where to start.

When you're looking for a high-quality probiotic, make sure to look for one that uses AFU as a marker of live, active cultures. This ensures that the probiotic you're taking is actually effective.

Start slow and increase your dosage gradually to avoid any uncomfortable side effects. With a little trial and error, you'll be on your way to finding the probiotic that works best for you.


How many billion probiotics should I take for weight loss?

Don't be tempted to take more probiotics than you need in an attempt to lose weight, as more is not always better. Taking a higher dose of probiotics without first establishing a baseline may lead to side effects like gas and bloating. Start with a lower dose, such as 5-10 billion CFUs or AFUs, and increase gradually as needed.

Is 50 billion probiotics too much?

While research still needs to be done to determine an ideal probiotic dosage, it's generally recommended that adults take between 10-20 billion CFUs or AFUs per day. A higher dose may be necessary for some people, but it's always best to start slow and increase gradually as needed.

Taking 50 billion CFUs of probiotics isn't likely to make them any more effective and could lead to side effects like gas, bloating, and nausea.

How many billion probiotics should I take with antibiotics? 

The common dosage of probiotics is 10-20 billion CFUs or AFUs per day is ideal for pairing with antibiotics. Start with a lower dose, such as 5-10 billion CFUs or AFUs, and increase gradually as needed.