PKU & fitness - The challenge of weight loss

It's really interesting to go online and read about the various opinions people have about PKU, fitness and overall health and wellness. Opinions online generally seems to be divided - either living on a low Phenylalanine diet means that it's easy to maintain a healthy weight and be fit and healthy, or people feel like they have to choose to stick to the diet and have a healthy brain or go off diet and have a healthy body.

PKU and overall health and wellness can and should go hand in hand. Tackling weight issues (both fat loss and weight gain), the building of muscle mass and overall fitness and endurance can be a little trickier when you're living with a protein restricted diet, but it's certainly not impossible.

Let me tell you a little bit about me. I am 32 and I have classical PKU. I'm also a personal trainer, fitness instructor, Pilates teacher and sports nutritionist. My clients include a couple of people who, like me, have PKU. I wasn't always fit (after the birth of my first daughter I was 140kg/308lb), and I'm still working on fat loss, but that is for another time.

Weight management (whether you want to get bigger or smaller) on a diet can certainly be challenging, especially with a low tolerance. Any woman who has been on the preconception diet or has been pregnant will tell you that. However, it's not impossible. Likewise, building muscle isn't impossible, but we do have to be a little bit more dedicated to the scientific side of exercise physiology and also to our nutrition if we want to achieve results. Today I'm going to focus on fat loss as I meet a large number of PKU patients who have this goal in particular.

Something I tell all of my clients is this: The scales may not (and at times definitely won't) reflect your progress. Tracking your body fat percentage and your measurements is a far more accurate indication of progress than stepping on the scales. Last year doing bodyweight High Intensity Interval Training I lost almost 3 dress sizes, 8% body fat and... 1.5kg. People kept commenting how much weight I'd lost, but I hadn't lost weight. I had lost fat and gained muscle, which is awesome! I often don't even tell my clients their scale weight. There is no reason to be a slave to the scales. Similarly, it's no secret these days that BMI is a flawed system (but don't get me started on that!).

For us to achieve fat loss, several things have to happen at the same time. You need:

  • Caloric deficit
  • Adequate hydration
  • Adequate rest

However, no matter what anyone tells you, fat loss is not as simple as calories in vs calories out. The source of your calories is an important factor, as is the regularity and timing of when we take our protein substitute and what kinds of exercises we do.


Let me start by saying this: The tips in this article should in no way replace the advice given to you by your dietician. He or she is your first port of call for anything when it comes to food, but here are some tricks and tips I have learned from my training in the fitness industry and my own experiences with both myself and my clients.

  • Try to choose foods that are nutrient rich - you are going to get many more vitamins and minerals from a big plate of vegetables with some good fats than you will from a plate of plain pasta or a couple of pieces of toast.
  • Do use low protein products, but choose wisely. Biscuits, crisps, breakfast bars and other sugar laden options should be limited, and ensure you measure portion sizes. 75g of low protein pasta (dry) is one portion (this varies from brand to brand). It might not seem like much, but once you bulk this out with lots of different coloured vegetables and dress with herbs, spices and oil (or a full fat dressing), it's a healthy and filling meal.
  • Fat is not your enemy! Opt for healthy fats such as avocado, coconut oil and olive oil. Use proper butter instead of margarine (including the half fat varieties). Steer clear of products containing hydrogenated fats - these are the ones that are really bad for us.
  • Eat regularly - snacking helps to avoid hunger, just choose wisely. I encourage all of my clients - PKU and non - to follow a fairly regular meal routine that includes two to three snacks per day, along with water at every meal.
  • Lower the amount of both sugar and artificial sweeteners in your diet. Both are the catalyst for a strong hormonal response in the body which lead to fat storage and yep, you guessed it - sugar cravings.
  • There is such a thing as eating too few calories. This leads to 'starvation mode' and while it isn't great for anyone, it's particularly bad for those of us with PKU. This promotes catabolism, effectively breaking down your lean muscle mass (which we want to increase!), promoting fat storage and slowing your metabolism. To work out how many calories you need:
    • Find your Basal Metabolic Rate (see image 1)
    • Use BMI to do the Harris Benedict Formula to work out your caloric needs for weight maintenance
    • Subtract 500 kcal per day to achieve a safe, achievable weight loss of 1lb of fat per week.
    • NOTE: Do not go below a total (including formula) of 1200 calories per day. This is very low and in the fitness industry this is the absolute minimum number of calories required for daily functioning.
    • Programmes such as MyFitnessPal are great for tracking but often not very accurate. You're better off to work your calories out yourself or find an online Harris Benedict calculator.
  • Drink ALL of your protein substitute . This might sound like a bit of a catch 22, but it's important. We need all three macronutrients - protein, fat and carbohydrates - for our body to perform optimally. When we don't get enough of one of these, the balance is out, plus if you're short on protein your body can't build muscle or recover from workouts. Spread your protein substitute out through the day and ensure that you take one dose within 30 minutes post workout.
  • Drink 2 - 3L of plain water per day. If you don't like water, get an infusion bottle and flavour with chopped up fruit, cucumber and herbs. A hydrated body burns fat as an energy source more efficiently than a dehydrated body!


  • Aim to do exercise that raises your heart rate at least three to five times per week
  • 45 minutes to one hour is a perfectly sufficient workout if you train effectively - there is no need to slog it out for hours, and in fact, with PKU it can be counterproductive. 
  • Do both cardio and resistance (weight) training. If necessary, have a personal trainer work with you to write a gym or home fitness programme. It's worth the money. Focus on building muscle - ladies, you won't bulk up! It's incredibly hard for ladies to look ripped, especially on a low protein diet. The more you increase your muscle mass, the more you boost your metabolism. This means you're burning more calories, even when you're asleep!
  • Instead of running or biking for an hour straight at a steady pace for your cardio, try interval training – i.e. alternate walking with sprinting, hill climbs with bike sprints, rowing as fast as you can then complete rest etc. Experiment with the interval split, or randomise it completely (this is called Fartlek training). Your body can't adapt to this kind of training, so the changes keep happening and it is a great way of torching calories.
  • Remember that too much cardio (after about 1 - 1.5 hours) usually brings on catabolism, as after this time your body has worked its way through all its other available energy sources so it will break down muscle. It would be nice if we just kept breaking down our fat stores, but unfortunately it doesn't work that way!
  • Rest for at least one day between body parts. If you work full body, train day on, day off. 

Fat loss with PKU can be challenging, but the sense of achievement is huge! Don't be afraid to ask for help, but most importantly, don't give up. I did it, I am still doing it, and so can you! There is also a great amount of support on social media from people with PKU who are going through their own fitness journey. Make use of that.

I'm always happy to speak with other PKU patients about health, fitness and wellness, so feel free to contact me via Facebook or email

See you in the gym!

Kate Buckland