What is GMP?

Type ‘What is GMP?” into a search engine and you’ll probably get reams of information on ‘Good Manufacturing Practice’ or ‘Good Medical Practice.’ Both important, but not what we’re thinking about in relation to PKU.

GMP is GlycoMacroPeptide

That is quite a mouthful, so I’ll use the acronym. GMP is a protein which occurs naturally in cheese whey. “Great” you say, “but we can’t have cheese.” Stick with me here because when GMP is extracted from cheese whey, “it is the only known dietary protein that is naturally free of Phe.”

Sounds great, except that same paper points out that GMP has a minimal amount of Phe - 5mg per gram of protein. Or 0.5g Phe in 100g of GMP. This is border-line free for people on the PKU diet. But can have big implications for those with low phe allowances and multiple daily supplements.

This worried me enough to prevent me from trying GMP, however, my clinic were reassuring. I ran a trial, with their blessing, and that extra level of daily phe didn't seem to affect me. Please always check with a clinic or health professional before making any changes.

Benefits of GMP: taste!

All of us on the PKU diet know how hard it can be to force down an unpleasant supplement after a tasty meal. Spare a thought for those making the supplement. To be useful in any way, our medicine must contain some pretty foul tasting components.

On a PKU study day in London in 2018, the Nutricia product development team demonstrated this by asking delegates to mix their own ‘Amino acid supplements’ (pictured). The results were enough to make everyone present a little more grateful for the efforts put in by flavour developers.

All the scientific trials which I read for this article reported that the (human) participants found the improved flavour of GMP easier to consume. This is critical as the “Lifelong adherence to the PKU diet is very difficult, often resulting in poor compliance and the neuropsychological consequences”  of high phe levels. If a medicine is palatable, it is far more likely that the patient will stick to the treatment.

GMP: Hunger and heartburn

I’ve been fairly lucky with my supplements in the past, but one of my current treatments does give me gastrointestinal problems fairly frequently. This can matter just as much as flavour. When a medicine makes you feel works, one is much less inclined to keep taking it regardless of the taste.

Trials to date suggest that GMP can lead to “improved gastrointestinal symptoms and less hunger.” Anecdotal evidence from both my home testing, and from my PKU community on twitter agrees with both of these claims.

Can GMP also mimic LNAA treatment and reduce brain & blood Phe?

All of the above is sounding pretty good, for most people it’s enough that a new supplement tastes better. But it looks like there might be even more benefits to using GMP over a synthesised amino acid supplement. Of most interest is a possibility of an effect similar to that attempted in LNAA (Large Neutral Amino Acid) treatments.

The mechanism of LNAA treatment is essentially that of competition. Phe is one of nine LNAA’s which all use the same ‘transporter’ to cross the blood-brain barrier. As Phe causes problems when it crosses this barrier, the theory goes that flooding these ‘transporters’ with other LNAA’s reduces the amount of Phe which crosses the barrier. This in turn dilutes the concentration of Phe in the brain and reduces the adverse effects.

Obviously, this is a very basic outline, you can read more about LNAA here, or in my book Living with PKU. It looks like GMP supplements might also have this competition effect, another plus to these exciting new treatments, though studies are on-going.

Other benefits of GMP

Finally, but just as important, one of the studies reported that the lower cost of making GMP means that the treatment might become a more viable option in poorer countries.

It seems appropriate to end on the call of scientists across the ages – More Data Required! Meanwhile, many companies who make PKU food and supplements offer a GMP-based option. In the UK, dieticians can set up trial packs. Elsewhere, contacting the companies directly, or attending PKU events are good places to find new products.

Find more PigPen articles on GMP here: