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Food Labels

Reading food labels

When you have a child with PKU it is particularly important and useful to know how to read food labels. This will allow you to check the protein content of foods and work out if the food is suitable for your child’s diet.

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This might seem confusing and overwhelming at first, but with practice this will become easier.


The protein in food is counted as phe exchanges. 1 phe exchange is the amount of food that provides 1g of protein.


1g of protein = 50mg of phe

So, 50mg of phe = 1 phe exchange


Most pre-packed foods have nutritional information on the packaging or on supermarket websites.

Here is an example of the nutritional information on a Weetabix cereal packet:

Protein Nutrients


This label provides information on the energy content shown as kilojoules (kJ) and calories (kcal). Information is also provided on the typical fat, carbohydrate and most importantly for PKU, protein content. All nutritional information is provided per 100 grams and sometimes per portion of the food. So, in this instance 2 biscuits of Weetabix contain 4.5g of protein. 

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How to read food labels

Unless a food is a specially manufactured low protein food, then it is extremely unlikely that the phe content will be provided on a food label.

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However, the phe content can be estimated from the protein content. From this, the number of phe exchanges in a certain quantity of food can be calculated.


To work out if a food needs to be counted as a phe exchange look at the protein content on the label. If the protein is 0.5g or less per 100g, then this food is classed as exchange free (please note, this does not apply to plant-based alternatives to milk, please speak to your dietitian about this). However, if it is over 0.5g per 100g, then you will need to calculate how much of that food is a phe exchange. Your dietitian will tell you how many phe exchanges your child is allowed each day.


Food labels will state the protein content but checking the ingredient list can still be useful as this may indicate that the product consists of foods which you know are exchange free. If this is the case, the food can be given without contributing towards phe exchanges.


The National Society of Phenylketonuria (NSPKU) dietary information booklet provides a comprehensive list of products which are exchange free. For further information, visit

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Calculating phe exchanges

When you know the protein content of a food per 100g, you can calculate how much of that food equals 1 phe exchange.

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For example, this yoghurt contains:

Calculating Phe Label

5.1g of protein per 100g. So, 100 ÷ 5.1 = 20g

Therefore, 20g is equal to 1 phe exchange.


When you need to calculate phe exchanges quickly, print off and use the ready reckoner below.


For example, if the protein content on the label states 0.9g per 100g, then 111g of this food is counted as 1 phe exchange.


Exchanges Table
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Phe exchanges per portion of food

Usually, it will say on the label how much protein there is in 1 portion. The table below shows how many phe exchanges are in a portion of food based on the protein content.

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If a portion of food contains 0.3g protein or less, it is exchange-free for 1 portion. However, if your child eats more than 1 portion it will need to be counted towards their daily exchange allowance as advised by their dietitian.

Small Exchanges Table


If you do not have the protein content per portion available, then you can still work out the number of phe exchanges per portion of food using the steps below:

  1. Multiply the protein content of food per 100g by the amount of the food eaten.
  2. Divide this number by 100 to give the number of phe exchanges per portion of food
Protein Nutrients Label

 If a child eats 20g of yoghurt:

5.1 (protein per 100g) x 20 (amount eaten) ÷ 100 = 1 phe exchange


Once your child is old enough, encourage them to read food labels and calculate their own phe exchanges.


Until then, at mealtimes it will be useful to get them familiar with knowing which foods contain protein or are exchange-free.


Understanding food labels will encourage independence and help to prevent your child always relying on you to do the work! Plenty of practice will increase their ability and confidence.


Always refer to the NSPKU dietary information booklet on their website for the most up-to-date nutritional information.

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