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Why I love German Chocolate

Coming back from a trip to Germany, I got this awesome vegan chocolate with only 0.6g of protein per serving.

Here is the great thing about German chocolate: not only does the package list the protein content with a precision of 0.1g, but it also lists the nutrition fact for 100g of chocolate. One hundred grams of chocolates is five servings, and knowing the protein content for a larger amount of chocolate helps decrease the uncertainty of the phenylalanine estimate (Phe).

Let’s do the math and you will see.

We have a 2.9g of protein for 100 g of chocolate.

One serving of chocolate is 20g, so there are 5 servings per 100 g of chocolate.

Since the protein content is rounded to the nearest 0.1g, the maximum protein content for 100g of chocolate is 2.95g, and therefore the maximum protein content for one serving of chocolate is 2.95/5=0.59g. (We just gained an extra digit!)

Similarly, the minimum protein content for 100g of chocolate is 2.85g, and therefore the minimum protein content for one serving of chocolate is 2.85/5=0.57g. (Again, we gained an extra digit!)

I am too lazy to look up the ingredients here, so I am just going to use the 20-65 multiplier rule:

  • the minimum Phe content is 0.57 x20=11.4mg
  • the maximum Phe content is 0.59 x65=38.35mg.

Picking the mid-point between these two numbers (and rounding up), my estimate is:

25mg Phe.

The maximum error of that estimate is 13.5mg (the difference between the estimate and the min/man). Good enough for me!




An easy way to increase your protein intake without changing your Phe intake

Suppose Mary and Jane both eat 100mg of Phe for lunch. Mary eats only fruits, while Jane eats a combination of vegetables and low-protein noodles. Which of the two has consumed more protein?

This answer is: Mary.

This is because the ratio of phenylalanine to protein tends to be much lower in fruits than in vegetables or other types of food (including foods made of starches). Specifically, the Phe:protein ratio for fruits varies between 20 and 39mg/g.

To put these numbers in perspective, consider that foods in general have an average Phe:protein ratio of about 42mg/g, and that for certain foods (e.g. egg white), the ratio could be as high as 65.

Therefore, one could easily double their protein intake without changing their Phe intake by following a fruit diet. Now this may be a bit extreme, and I am not sure how one’s stomach would feel eating fruits and formula at every meal. But that’s one more reason to try to prioritize eating fruit.


Our new paper is out!

Our paper suggesting a new three-step method to estimate phenylalanine in commercial foods has appeared:

J. Kim, A. Talikoti, M. Boutin, “A 3-Step Process to Estimate Phenylalanine in Commercial Foods for PKU Management“, IEEE Access, Vol. 6, 23 May 2018.

The paper shows three steps to follow to use the ingredient list and the nutrient facts written on the label in order to obtain an estimate of the Phe content of the food. The first step provides a first (rough) estimate, which is refined in the second step, and further refined in the third step.

The paper is a bit technical, mostly the third step. But the first two steps are fairly easy to understand. So if you are interested in learning how to estimate Phe from a commercial food label, go ahead and take a look. This is an Open Access publication, so you don’t need an IEEE Xplore access to be able to read the full paper.

Watch out for egg white!

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.



PKU dieting with unrestricted fruits and vegetables?

A very interesting article came out last year [1]. The paper reports on an experiment where PKU patient were allowed to eat unrestricted amounts of fruits and vegetables. More specifically, all fruits and vegetables with less than 0.75mg Phe per gram were counted as “Phe free”. Surprisingly, the blood Phe levels of the patients were “consistently good” despite the additional Phe consumed. The daily Phe tolerance of the patients thus appeared to increase by an average of 83mg per day, which is a 25% increase.

Here is the title and abstract of the paper. You can click here to read the full version.

Simplifying the diet for patients with phenylketonuria (PKU): unrestricted consumption of fruit and vegetables” By Carmen Rohde, Alena Gerlinde Thiele, Ulrike Mütze, Wieland Kiess, Skadi Beblo, Leipzig


Over a period of three years, the phenylalanine content in fruit and vegetables was not taken into account in the daily phenylalanine balance for patients with phenylketonuria, a congenital metabolic disorder. In spite of a significantly higher intake of phenylalanine, no worsening of metabolic control could be detected.

Can you imagine not having to count fruits and veggies? Now, that would make the diet so much easier to manage!


[1] Rohde C, Thiele AG, Mütze U, Kiess W, Beblo S (2014) Simplifying the diet for patients with phenylketonuria (PKU): unrestricted consumption of fruit and vegetables. Ernahrungs Umschau 61(12): 178–180

How much Phe in one gram of protein from fruit?

How much phenylalanine (Phe) does one gram of protein contain? It depends. For fruits, it could be as little as 20mg Phe, or as much as 39mg Phe. Indeed, as one can see from our data, the vast majority of fruits have between 20mg Phe and 39mg Phe per gram of protein.

Quick Fact Sheet for Phe from protein estimation

For those of you who don’t feel like doing the math every time, here is a small table that gives the minimum and maximum Phe for various protein contents (rounded to the nearest gram). There are two columns: the first one is for any food that does not contain aspartame. The second one (last one on the right) is for foods made of fruits only (or fruits plus Phe-free ingredients).  As you can see, if the Nutrition Fact Label states that the food contain 0g protein, then the food could contain as much as 32mg Phe in general (assuming no aspartame).  But if the ingredient lists contains only fruits plus Phe-free things like sugar, color, oil, etc., then the maximum Phe when there is 0g protein is only 19.5mg Phe.


The background for this Fact Sheet is in this document.


Great news! Our paper proposing new multipliers for estimating the PHE content of a food from it protein content has been accepted for publication in the Journal of Food Composition and Analysis. It will take a bit before the paper appears, but I will post a link to the article as soon as it is available. Meanwhile, Here is abstract:

 Phenylalanine (Phe) is a key nutrient in the dietary management of phenylketonuria (PKU). Since the protein content of many foods is readily available, estimating the Phe content of a food is facilitated by an understanding of the statistical distribution of the Phe:protein ratio in common foods. In particular, from the minimum and maximum Phe:protein ratio, one can obtain an upper bound and a lower bound on the Phe content of any given food from its protein content. Currently, the multipliers commonly used are 30 and 50. In this document, we present and compare the statistical distribution of the Phe:protein ratio in two databases, namely the USDA National Nutrient Database and the Danish Food Composition Databank. Based on this data, we suggest replacing the 30 –  50 multipliers by 20 –  65. When used to estimate the Phe content from the protein content, these multipliers yield estimates that are correct for more than 97% of the data analyzed (as opposed to less than 76. 3% for the multipliers 30 – 50). Furthermore, we confirm that the commonly used average of Phe:protein ratio for the foods in the categories of fruits (30) and vegetables (40) are more or less accurate.

Perhaps you have heard that multiplying the protein content of a food by 50 gives a maximum for the Phe content of the food. I personally heard this many times in the past. For example, if the food label states that one serving contains 1g or protein, then I was explained that the maximum Phe content is 1.5×50=75mg. Well, it turns out that many foods have more Phe than that, and so one should multiply by 65 to get an accurate maximum. The data to support that is in the paper.

The new multipliers have been incorporated into our PHE estimation app. The web browser version of the app is freely available at

My student Jieun also made an Android version, which you can download (for free!) from this webpage:


Estimating the Phenylalanine (Phe) content of “Sixlets” (using Phe:protein ratios)

I was recently asked to estimate how much Phe is in one serving (10 pieces) of Sixlets. Here is what I did.

1) I found the Nutrition Fact Label and ingredients lists (…/candy/p/8966903792)

2) I noted that the Nutrition Fact Label states that there is zero gram of protein. Due to rounding, this actually means that there is no more than 0.5g of protein per serving.

3) I looked at the ingredient list and noted all ingredients that contain protein. I then looked up the mg Phe per g protein ratio of each of these ingredients in this (free) food list I mentioned in my previous post.

I found that
1 g protein from whey contains 32mg Phe
1 g protein from cocoa contains 34mg Phe
1 g protein from carob contains 33mg Pge
1 g protein from cornstarch contains 50mg Phe

4) Since cornstarch has the highest phe:protein ratio at 50mg, then one serving of Sixlets contains no more than 0.5 x 50= 25 mg Phe. However, corn starch is ingredient #8, so there is probably a very small amount of cornstarch. Furthermore, cornstarch contains a very small amount of protein. So we can neglect the cornstarch and assume that all Phe comes from the next highest Phe:protein ratio (cocoa at 34mg).  Thus we get that one serving of Sixlets contains no more than 0.5 x 34= 17 mg Phe.

Final Answer: no more than 17mg Phe per serving.

A (free) Food List in PDF Format

Dear PKUers,

I want to share with you this food list, which my graduate student Jieun and I just published. It combines all the foods in the USDA Database, along with another (Danish) database. The focus of our food list is the Phe:protein ratio of the food, but it also lists the mg Phe per gram food (first column). Feel free to print it if you like. However, I find it easier to just search through the pdf on my computer.

Link to (free) Food List in PDF Format by Kim and Boutin