Do Probiotics Even Survive? CFU, Strains and the Delivery Problem

7 min read · Published 5 August 2026 · ⚠ Draft

A big CFU number means little if the bacteria die on the way. 90–99% of unprotected probiotics can die in stomach acid. The four things that decide if a probiotic actually arrives — CFU guaranteed at expiry, multi-strain, acid-resistant delivery, and a prebiotic to feed them.

The uncomfortable question most probiotic labels avoid

Here is a fact the probiotic aisle would rather you didn't dwell on: a large proportion of the bacteria in many probiotics never reach your intestines alive. Swallowed unprotected, they hit the stomach — a bath of acid at a pH of around 1.5 to 3.5 — and studies using simulated gastric fluid show that 90 to 99% can die within about 30 minutes. A billion-CFU number on the front of the pack means very little if most of those billions dissolve before arriving.

This is the single most important thing to understand about choosing a probiotic. It is less about the biggest number and more about how many actually survive the journey — which comes down to three things: the count that's guaranteed, the strains, and the delivery.

1. The CFU number — read it honestly

CFU ("colony-forming units") is the count of live organisms. The catch: most labels state the CFU at the time of manufacture, not at the time you actually take it. Live bacteria slowly die off over a product's shelf life, so a "50 billion at manufacture" can be considerably less by the time it reaches your spoon.

What to look for: the honest number is the CFU guaranteed at expiry, not just at manufacture. A well-made probiotic is deliberately over-filled at production so that a meaningful, stated count is still alive at the end of its shelf life — for example, a product built to guarantee tens of billions at expiry rather than only promising a big number on the day it was made.

2. Strains — variety, and the right ones

A healthy gut is a diverse one, which is why a multi-strain blend — a range of well-studied Lactobacillus and Bifidobacterium species, sometimes with others — is generally preferable to a single organism. Different strains do different jobs; a spread covers more ground than one.

3. Delivery — surviving the stomach

This is where good and mediocre probiotics truly part ways. To get past stomach acid alive, the capsule has to protect its cargo. Acid-resistant, delayed-release capsules (and similar enteric or microencapsulation technologies) are designed to hold their contents through the stomach and release them further down, where the bacteria can actually take up residence. Without that protection, a high CFU count is largely spent at the door.

[FIGURE 1 — designer, Stravian palette] A "journey of a probiotic" strip: capsule → stomach (acid) → intestine. Two paths shown: an unprotected capsule losing ~90% in the acid, and an acid-resistant/delayed-release capsule delivering its load intact to the gut.

4. The often-forgotten partner: the prebiotic

Live bacteria that arrive still need to be fed. Prebiotics — fibres such as FOS (fructo-oligosaccharides) and inulin — are food for beneficial microbes. A probiotic paired with a prebiotic (a "synbiotic") gives the newcomers something to eat as they settle in. It is a small addition that reflects how the gut actually works.

Putting it together

When you next read a probiotic label, look past the headline number and ask four questions: Is the CFU guaranteed at expiry? Is it multi-strain? Is the capsule acid-resistant/delayed-release? Does it include a prebiotic? A probiotic that can answer yes to all four is built to actually arrive — which is the entire point.