Skip to content

How to estimate the Phe content of a food from its protein content: Part 1

May 10, 2013

This blog entry is the first in a series of four posts that introduce this simple web app which estimates the phenylalanine (Phe) content of a food from its protein content.

https://engineering.purdue.edu/brl/PKU/method_0.html

—-
Phenylalanine (Phe) is an amino acid that is present in all proteins we consume. Generally speaking, the more protein a food contains, the more Phe it contains. But exactly how much Phe does one gram of protein contain? Actually, it depends on the food. For example, if you look up “coconut milk” in the USDA database, you will see that one gram of coconut milk contains 0.02g of protein and 1mg of Phe. This gives a Phe to protein ratio equal to 1/0.02=50mg Phe per gram protein. On the other hand, an apple contains 0.0026g protein and 0.06mg Phe per gram of apple. This gives a Phe to protein ratio equal to 0.06/0.0026, which is approximately 23 mg Phe per gram protein. As you can see, the proteins in an apple contain much less Phe (less than half!) than the proteins in coconut milk.

Therefore, it is impossible to know exactly how much Phe a food contains solely from its protein content. However, it is possible to find an estimate for the Phe content. More specifically, it is possible to find a range of Phe values that contains the true Phe value. The idea is to multiply the protein content of the food by the maximum possible Phe per protein ratio, and by the minimum possible Phe per protein ratio, to obtain the maximum and the minimum possible Phe values, respectively. Let me explain.

Suppose that some food contains 1.2g of protein. First make sure that the food does not contain any aspartame. (As we all know, aspartame is a big “no no” for people with PKU.)

Step 1: Multiply the protein content in grams by 64.5. Here we have 1.2 x 64.5= 78.48. The result, 78.48 in this case, is the maximum Phe content of the food. In other words, the Phe content of the food cannot be higher than 78.48mg.

Step 2: Multiply the protein content in grams by 20. Here we have 1.2 x 20= 24. The result, 24 in this case, is the minimum Phe content of the food. In other word, the Phe content of the food cannot be lower than 24mg.

Where do the multipliers 64.5 and 20 come from? My graduate student Jieun Kim and I chose them because, according to our study, the vast majority of foods without aspartame (more than 97%) have a Phe:protein ratio above 20 and below 64.5.

Some of you may have been told by their dietician to use the multipliers 30 and 50 instead of 20 and 64.5. Indeed, the “traditional multipliers”, which have been used for decades by dieticians, are 30-50. However, we found that these traditional multipliers are only accurate for about 67% of all foods and ingredients without aspartame. Thus, we prefer the more accurate 20-64.5 multipliers.

To be continued in Part 2.

Advertisements

From → PKU

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: