Individuals with PKU still need protein to support growth and development. Protein is also needed and used by the body to maintain bones, muscles, blood, skin, hair and organs. The protein intake to maintain phe levels within the target range is restrictive and inadequate in total protein and other important nutrients such as iron, calcium, and folate.  

A protein substitute provides protein, without the phe. As well as phe- free protein, a protein substitute usually contains other nutrients such as carbohydrate, fat, vitamins, and minerals.  

Carbohydrate is used by the body as a source of energy for the brain, muscles, and all cells in the body. Most protein substitutes contain some carbohydrate. 

The main function of fat in the diet is to provide energy and essential fatty acids which help form cells and tissues. Fat carries the fat-soluble vitamins A, D, E, and K into the body and helps in their absorption.  Most protein substitutes contain these fat-soluble vitamins.  Many also provide the Essential Fatty Acids, and the Long Chain Polyunsaturated Fatty acids, such as DHA which the body needs. 

Vitamins A, B, C, D, E, and K are essential nutrients that are only required in very small amounts. They have many different important functions such as keeping our eyes, skin and nervous systems healthy, and supporting the immune system. Many of the protein substitutes contain vitamins. 

Like vitamins, minerals are essential nutrients that are only required in small amounts. Examples include, calcium which is crucial for building bones, and iron which is needed for healthy blood. Many of the protein substitutes contain minerals. 

As the diet for PKU is generally so restrictive, individuals are unable to get all the nutrients required by food alone. This is why a protein substitute is usually essential. 
There are many protein substitutes available and the one which best suits will change according to the person’s age, size, and requirements. Your Dietitian/Health Care Professional will advise you every step of the way which protein substitute is a good choice.