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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!





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.