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Interview with the Expert: Dr. Denise Ney

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Whey protein has long been a favorite ingredient in most athletes' post workout recovery shake. In fact, some treat it as a matter of life and death if they don't get their whey fix shortly after a workout. As important as some athletes may feel it is for their recovery, it's even MORE important for those with the disease known as phenylketonuria (PKU). In this installment of Interview with the expert, Dr. Denise Ney explains new research on glycomacropeptide, a component of whey protein, and how it's being used to improve the quality of life for those suffering from PKU.

Dr. Denise Ney

Dr. Denise Ney, PhD, Nutritional Science. Professor at University of Wisconsin-Madison

Whey protein is a popular post workout supplement due to its highly anabolic effect on skeletal muscle tissue1. In fact, some of my friends will go into a frenzy if they don't have their post workout whey protein fix. However, whey protein serves many other purposes. In this installment of Interview with the Expert Dr. Denise Ney, PhD, is here to discuss with us her research on glycomacropeptide (GMP), a component of whey protein, and how it is being used to improve the quality of life for those with phenylketonuria (PKU).

What is PKU and how does it affect the lives of those that suffer from it if left untreated?

Phenylketonuria (PKU) is a rare genetic disease that affects approximately 1 in 10,000 births. Both parents must be carriers in order for a baby to be born with PKU. Approximately 1 in 50 individuals are carriers for PKU and most don’t know this. Individuals with PKU have a defect in the ability to convert the essential amino acid (AA) phenylalanine (phe) to tyrosine and with consumption of a normal diet they show elevated levels of phe in blood that is toxic to the brain. If treatment is not started shortly after birth, severe and permanent mental retardation occurs. Mental retardation due to the failure to diagnosis and treat PKU is no longer a problem in the U.S. with mandatory newborn screen for PKU initiated by 1970.

How is PKU currently treated?

The primary treatment for PKU is lifelong adherence to a highly restrictive, synthetic diet that is low in phe. The diet contains less than 10% of the phe consumed in a normal diet and requires ingestion of a bitter tasting, synthetic AA formula (24-32 oz/day) to meet protein needs. The diet eliminates all high protein foods such as meat, fish, poultry, milk products, eggs, nuts, beans, as well as bread, rice, pasta and some vegetables. Most of the nutrients in the diet are provided by the AA formula and this is supplemented with limited amounts of fruits and vegetables and specialty low protein products. The diet is a challenge but many families affected by PKU have learned to be creative in making low protein meals. New dietary options are needed to improve the palatability, acceptability and variety of the low-phe diet for those with PKU.

Wow, those with PKU have a restricted diet. Are many adolescents with PKU small for their age or face other physical/social problems?

The smell and taste of the AA formula can be offensive leading to poor compliance with the diet. Infants and young children generally follow the diet very well and show good control of blood phe levels before they start school. However, compliance declines as parental supervision declines and peer pressure develops and by adolescence less than half of those with PKU carefully follow their diet. If the diet is followed with good control of blood phe levels, growth and development are generally normal. However, some problems such as anxiety and difficulty with memory and planning and organizing complex tasks develop in some individuals with PKU. Recent evidence suggests that females with PKU accumulate a greater amount of fat and less lean body mass after adolescence compared to those without PKU2. Some males with PKU show poor muscle strength while others are quite physically fat. Hundreds of mutations can result in PKU and there is much that is unknown about the expression of this genetic disease.

What are the advantages of using GMP vs the traditional AA drinks?

Glycomacropeptide (GMP) is uniquely suited to the PKU diet because it is the only known dietary protein that is naturally free of phe. Thus GMP, which is abundant in sweet cheese whey, provides a physiologic alternative to synthetic AA formulas because it is an intact protein. Our clinical studies show that protein from GMP when supplemented with limiting AA is retained to a greater degree for protein synthesis than the protein equivalents provided by AA formula3. Moreover, GMP has functional properties suitable for making good-tasting, low-phe, high-protein foods as a replacement for AA formulas. Foods rated as very acceptable by those with PKU who participated in our clinical studies include: a variety of beverages either milkshake-like or clear sports drinks, pudding, puffed cereal, and a snack bar4. Cambrooke Foods of Boston is currently marketing a GMP formula for those with PKU called Bettermilk®. Our research shows that low-phe GMP foods improve the taste, variety and convenience of the PKU diet which may make it easier for those with PKU to follow their diet resulting in improved health and quality of life.

What was the initial event that sparked interest in GMP?

The Waisman Center Biochemical Genetics Clinic at the University of Wisconsin-Madison contacted me about becoming involved in investigating the use of GMP for the PKU diet in 2003. They had received positive feedback from the PKU community about the potential for using GMP to make low-phe foods in response to an article written by Professor of Food Science Mark Etzel in the National PKU News. Mark received a patent in 1999 for a new method to isolate GMP from whey. I thought GMP had real potential to improve the PKU diet and started writing grants, many of which were funded including an NIH grant which made it possible to conduct an inpatient metabolic study which demonstrated the safety and efficacy of GMP in subjects with PKU for the first time.

To my knowledge, GMP is found only in whey protein where it is the 3rd most abundant protein. There are no other known natural food sources of protein that contain minimal phe.

For those interested in finding more out about PKU, are there any websites that you recommend?

For more about the GMP story see a 10 min video developed by Fredrica Freyburg on Wisconsin Public Television

The National PKU Alliance is a nonprofit organization dedicated to improving treatments and finding a cure for PKU. They support a competitive research program which has supported my GMP research and accepts donations.

What are your other research interests?

I conduct research in gastrointestinal physiology to study how nutrients and hormones help the intestine to adapt after atrophy of the intestine due to disease or resection surgery. Areas of particular interest are short bowel syndrome and gastroschisis.

Well that’s all the questions we have for you today. Thanks for taking time out of your busy schedule to shed some light on GMP and PKU.


1 Tang JE, Moore DR, Kujbida GW, Tarnopolsky MA, Phillips SM.Ingestion of whey hydrolysate, casein, or soy protein isolate: effects on mixed muscle protein synthesis at rest and following resistance exercise in young men. J Appl Physiol. 2009 Sep;107(3):987-92. Epub 2009 Jul 9.

2 Albersen M, Bonthuis M, de Roos NM, van den Hurk DA, Carbasius Weber E, Hendriks MM, de Sain-van der Velden MG, de Koning TJ, Visser G. Whole body composition analysis by the BodPod air-displacement plethysmography method in children with phenylketonuria shows a higher body fat percentage. J Inherit Metab Dis. 2010 Jun 24. [Epub ahead of print]

3 van Calcar SC, MacLeod EL, Gleason ST, Etzel MR, Clayton MK, Wolff JA, Ney DM Improved nutritional management of phenylketonuria by using a diet containing glycomacropeptide compared with amino acids.Am J Clin Nutr. 2009 Apr;89(4):1068-77. Epub 2009 Feb 25.

4 Lim K, van Calcar SC, Nelson KL, Gleason ST, Ney DM.Acceptable low-phenylalanine foods and beverages can be made with glycomacropeptide from cheese whey for individuals with PKU. Mol Genet Metab. 2007 Sep-Oct;92(1-2):176-8. Epub 2007 Jul 23.

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Written on July 13, 2010 by Sean Casey
Last Updated: December 23, 2013

This information is not intended to take the place of medical advice.Please check with your health care providers prior to starting any new dietary or exercise program. CasePerformance is not responsible for the outcome of any decision made based off the information presented in this article.

About the Author: Sean Casey is a graduate of the University of Wisconsin-Madison with degrees in both Nutritional Science-Dietetics and Kinesiology-Exercise Physiology. Sean graduated academically as one of the top students in both the Nutritional Science and Kinesiology departments.
Field Experience: During college, Sean was active with the UW-Badgers Strength and Conditioning Department. He has also spent time as an intern physical preparation coach at the International Performance Institute in Bradenton, FL. He also spent time as an intern and later worked at Athletes Performance in Tempe, AZ. While at these locations he had the opportunity to train football, soccer, baseball, golf and tennis athletes. Sean is also active in the field of sports nutrition where he has consulted with a wide variety of organizations including both elite (NFL’s Jacksonville Jaguars) and amateur athletic teams. His nutrition consultation services are avalable by clicking on the Nutrition Consultation tab.