What AFU actually counts

Walk down the supplement aisle and the counts climb every year. Fifty billion. A hundred. Two hundred and fifty. The number on the front of a probiotic jar has become a competitive sport, and the sport is not measuring what you think it is.

CFU is a manufacturing number

Colony-forming units are counted by growing the organisms on a plate and counting what appears. It is a real measurement, but it is usually taken at the moment of manufacture — before the jar is boxed, shipped through a warm depot, put on a shelf and opened in your kitchen for the next thirty days.

By the time you swallow the last capsule, a count taken at manufacture can be wrong by an order of magnitude. Most labels handle this by printing the manufacturing number and saying nothing.

AFU is an arrival number

Active fluorescent units are counted by flow cytometry, which tags cells and counts them individually rather than waiting to see what grows. It catches organisms that are alive but slow to form a colony, and it can be run at the end of shelf life rather than the beginning.

We print the end-of-life figure. It is smaller than the number we could legally print, and it is the one that describes the capsule in your hand.

How to read any label

Look for the date the count refers to. If it does not say, assume manufacture. Then look for whether the dose of each strain is printed separately, or whether they are pooled into one total — a pooled total can hide a single cheap strain doing ninety per cent of the work.