Many foods ingredients have a phe:protein ratio below 50 mg/g. This means that foods made of these ingredients can be assumed to contain less than 50mg of phenylalanine  (Phe) per 1 gram of protein. For such foods, the “50 multiplier rule” can be used to find an upper bound on the Phe content.

What is the “50 multiplier rule“? It is a simple rule of thumb to estimate the Phe content from the protein content listed on the label. Just look at the protein content listed on the label (e.g., 0g), add the rouding error (0+0.5=0.5g), and multiply the result by 50 (0.5 x 50 =25 mg Phe). The resulting number (25mg Phe) is the maximum Phe content of the food. So if the food states that the protein content is 1g, then the 50 multiplier rule yields a maximum Phe content of 1.5×50=75mg. Similarly, if the protein content is 2g, then the 50 multiplier rule yields a maximum Phe content of 2.5×50=125mg.

However, there are some ingredients that have more Phe per protein than 50, and the 50 multiplier rule should not be used to estimate the Phe content of foods containing these ingredients. An example? Eggs. For example, chicken eggs contain 53-56mg Phe per gram of protein. The yolk itself has a lower phe:protein ratio (about 43-44 mg Phe per gram protein), but the white is very high, with a 63-65mg/g ratio.

So if egg white is listed as an ingredient, the 50 multiplier rule might not be accurate. Instead, one should use the “65 multiplier rule”, replacing the multiplier 50 by the multiplier 65.

Let’s compare the 65 multiplier rule with the 50 multiplier rule. If the food contains 0g protein, then the 65 multiplier rule yields a maximum Phe content of 32.5mg (instead of 25mg, with the 50 multiplier rule). Thus, the error could be as large as 7.5 mg. This is not too bad, and one could argue that such a difference is insignificant. But the error increases with the protein content. For example, if the food contains 1g protein, then the 65 multiplier rule yields a maximum Phe content of  97.5g mg, instead of 75mg. That’s a maximum error of 22.5mg. Similarly, for a 2g protein content, the 65 multiplier rule yields a maximum Phe content of  162.5 mg, instead of 125mg for the 50 multiplier rule. Thus the error could then be as large as 37.5mg. And so on.

How to know if the food considered is an appropriate candidate for using the 50 multiplier rule? The safest way is to check that all the ingredients have a phe:protein ratio at or below 50mg/g. This can be done by searching through this this database of phe:protein ratios. The database is in pdf format, so it can be quickly searched using the search function in your pdf viewer.

From → PKU, Purdue