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Home > Vitafriendspku > Your pku journey > How Much Should I Eat?

How Much Should I Eat?

The PKU Diet In Adults

If you are an adult with PKU, you may be wondering what we can tell you that you don’t already know. Although the basic principles behind the low protein diet have not changed in decades – things are constantly evolving and improving:

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  • With the number of adults with PKU increasing every year, there are more and more experts specialising in the management of PKU in adults.
  • Although control of blood phe levels is still the focus of the diet, there is now much more emphasis of having a diet that complements all aspects of a healthy lifestyle, such as weight management.
  • The range of protein substitutes and specialist low protein foods now available is much bigger than it was in the past and it continues to grow.

With everything going on in your life, keeping on top of your diet may not be a big priority for you. However, maintaining a healthy, balanced low protein diet will help you with all the other aspects of your life. Here’s a few reasons why:

  • Some people find they get ‘brain fog’ when they don’t follow their low protein diet as closely as they are supposed to. You may feel a bit muddled and your friends, colleagues or partner may notice you’re not quite yourself. That won’t help when you’ve got a packed schedule or you’re adjusting to extra responsibilities in your life, such as a recent promotion or a new addition to your family.
  • Your protein substitute, as well as helping control your phe levels, is contains all the vitamins and minerals you need to stay healthy. If you regularly skip taking your protein substitute or take less than you have been prescribed, you may not fully meet your requirements for vitamins and minerals. Short-term you can feel tired and it could also affect your skin, nails and hair. Longer-term, this may lead to serious health issues.
  • As we get older, we become more prone to weight gain but a healthy PKU diet can help you manage your weight alongside an active lifestyle.

As an adult, you may well have assumed full responsibility for your diet but that doesn’t mean you have to do it all by yourself! The specialist metabolic team are there to help you as much as they can. They’ll be more than happy to hear your thoughts and ideas on the diet (what you like and dislike, what works for you and what doesn’t) so that they can make the diet fit more around you instead of you having to fit around the diet! A supportive partner can be a big bonus too.

A suggested healthy routine is to eat 3 meals and 2-3 nutritious snacks a day to optimise your energy and get all the nutrition you need to stay fit and healthy.

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Which Foods Should I Eat And Why?

The ‘Eatwell Guide’ provides a visual image of the different types of foods and drinks that people without PKU should eat to ensure a balanced diet. The guide we have provided in this section has been adapted for a low protein diet.

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The following information outlines the 5 main food groups you should consume in your low protein diet and on average how much you should eat in a day:

  • Fruit and vegetables
  • Starchy carbohydrates
  • Protein (phe exchanges and protein substitutes)
  • Dairy and dairy free alternatives
  • Oils and spreads

See the Eatwell Guide.

PKU Eatwell Guide Pie Chart

The following sections provide information on the main food groups you should eat. We have provided average amounts as a guide only, to help give you an idea of quantities. As always, please consult your dietitian for information specific to you.

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Fruit and Vegetables
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The National Society for Phenylketonuria (NSPKU) has developed a long list of permitted fruits and vegetables that you can refer to. This can be downloaded from their website or you can contact them to request a hard copy. They are foods containing small quantities of phe, which can be used in normal quantities.

Although must fruits and vegetable are exchange free, some will need to be counted as exchanges. Please refer to the NSPKU Dietary Booklet or your dietitian for information.

Aim to eat at least five portions (80g per portion) of a wide variety of permitted fruit and vegetables each day. The Eatwell Guide shows a third of the food we eat each day comes from fruit and vegetables. This includes fresh, frozen, dried and canned permitted fruit (in juice) and vegetables.

How much is a portion?

One portion is 80g or any of the following:


  • one banana, orange, pear or apple or a similar sized fruit
  • half a grapefruit or avocado
  • a slice of large fruit such as melon or pineapple
  • two satsumas, plums or similar sized fruit
  • a handful of grapes
  • two handfuls of blueberries or raspberries
  • three heaped tablespoons of fruit salad (fresh/tinned in fruit juice)


  • three heaped tablespoons of permitted vegetables (raw, cooked, frozen or tinned)
  • one cereal bowl of salad

Dried Fruit

  • one heaped tablespoon (30g) of dried fruit, such as sultanas, currants or cranberries.

Be careful not to eat too many dried fruits, while a 30g portion of dried fruits, like apricots, dates, raisins etc. counts towards your five-a-day, once fruit is dried it also becomes a concentrated source of sugar. Limit your intake of dried fruit to no more than one portion per day.

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Starchy Carbohydrates
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The Eatwell Guide shows just over a third of the food we eat each day comes from starchy foods. This food group includes starchy foods such as breakfast cereals, pasta, rice, breads, noodles, and oats as part of your meals and snacks.

As most carbohydrate-based foods contain protein you will mostly rely on specially manufactured low protein foods such as low protein bread and flour, pasta, cereal, noodles and rice. There are a wide variety of these low protein foods available, speak to your metabolic dietitian for further information on these.

Some regular starchy carbohydrate foods may be used as exchanges. Please refer to the NSPKU dietary booklet or your dietitian for further information.

Carbohydrate-based foods provide energy and variety and at least one portion should be offered at each meal.

Example portion sizes:

  • 2-3 tablespoons low protein pasta (cooked)
  • 1 medium slice of low protein bread
  • 2-3 tablespoons of low protein rice (cooked)
  • 3 tablespoons low protein breakfast cereal

The number of portions you need will depend on how active you are. You can discuss this with your dietitian.

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Meat, fish, eggs, beans and pulses contribute to protein intake, however the phe in these foods is much too high for people with PKU and therefore must be avoided.

The protein your body needs daily will mostly come from your protein substitute. It is important you take your protein substitutes as advised by your metabolic dietitian because, not only do they contain the essential protein your body needs each day, but also vitamins and minerals that your body will not get when avoiding high protein foods.

A small amount of protein will also come from your phe exchanges, for example, from certain vegetables, starchy carbohydrate and dairy foods. These phe exchanges must be measured with scales.

The amount of protein substitute you need and the number of phe exchanges allowed will be advised by your metabolic dietitian.

What is a protein substitute?


Protein is needed for growth and building muscles. You will get most of the protein you need from your protein substitute. It is very important that you take all your recommended protein substitute every day to ensure you grow to your full potential and maintain your muscle mass. If you are thinking about a change of protein substitute, speak to your dietitian.


The amount of protein substitute you need and the number of phe exchanges allowed each day will be advised by your metabolic team.

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Dairy and Dairy Free Alternatives
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Dairy Segment

The ‘Eatwell Guide’ recommend that a healthy diet should include 3 portions of dairy or dairy alternative foods each day to provide protein, vitamins and calcium for strong bones and teeth.

Some regular diary foods may be used as phe exchanges. Please refer to the NSPKU Dietary booklet or your dietitian for information.

You will need to choose low protein alternatives that are available to help provide energy and variety to the PKU diet. There are many low protein options available. You should speak to your metabolic dietitian about the best options for you.

Example portion sizes:

  • 200ml low protein milk alternatives
  • 30g cheese alternatives
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Oils and Spread
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Oils and Spreads Section

Having some fat in your diet is an important part of a healthy diet, however, make sure you choose unsaturated fats which are healthier. Unsaturated fats are usually from plant sources and in a liquid oil rather than a solid fat, such as olive oil, vegetable oil or rapeseed oil. Remember to only use small amounts and use lower fat spreads other than butter.

Some fats in particular are necessary in your diet. Your body cannot make some omega-3 and -6 fatty acids so you must get them from your diet. You may want to take an omega-3 supplement if your protein substitute doesn’t already contain this. Your metabolic team will be able to tell you what’s appropriate for you.

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High Sugar Foods
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Low Prtein Bars Illustration

Added sugar in foods and sugar-sweetened drinks are not needed as part of a healthy diet. This means not adding sugar to drinks such as tea and coffee or onto breakfast cereal and not having foods high in sugar such as:

  • Sweets and chocolate
  • Cakes, puddings, biscuits and cereal bars
  • Soft drinks (except 'diet' or 'sugar free' ones) and fruit juices, remember to check diet drinks for aspartame.
  • Certain sugar-sweetened breakfast cereals and yogurts.
  • Sweet spreads like jam, honey and marmalade.

Avoid long periods of time without eating to help reduce the desire to eat or drink high sugar foods and drinks. Plan regular meal and snack times and what you are going to eat to help with this.

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Fluids Illustration

Aim to drink 6-8 glasses of fluid each day and have a drink after taking your protein substitute. Choose water or permitted drinks such as low protein milk and sugar-free drinks, although avoid drinks with the sweetener aspartame (E951 or E962). Always check labels as this sweetener is a source of phe and not permitted in the low protein diet. Try not to add sugar into drinks such as tea and coffee.

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To Recap:
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Alternatively, you may be wanting to make some changes but feeling a bit unsure about where to start. Perhaps just a few tweaks are required, or you might want a complete re-haul, especially if you have not been taking your protein substitute or counting your phe exchanges.

Your dietitian can work with you to develop a step-by-step process to get you back on track with your low protein diet and reduce your phe levels. Working together with the dietitian, you can build a plan that works for you.

For more information on meal planning and healthy meals, see the Live Life Well section here.

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