Body Composition and Bone Mineral Quality in Phenylketonuria: Influence of Pubertal Development

Research Article

J Endocr Disord. 2021; 7(1): 1045.

Body Composition and Bone Mineral Quality in Phenylketonuria: Influence of Pubertal Development

Tummolo A1*, Arico M2, Pesce S1, Fantasia ML3, Paterno G1, Melpignano L4, Giovanni DD1 and Faienza MF3

1Department of Metabolic Diseases, Azienda Ospedaliero Universitaria Consorziale, Italy

2Rare Diseases Unit, Azienda Ospedaliero Universitaria Consorziale, Italy

3Department of Biomedical Sciences and Human Oncology, University “A. Moro”, Italy

4Medical Direction, Azienda Ospedaliero Universitaria Consorziale, Italy

*Corresponding author: Albina Tummolo, Metabolic Diseases and Clinical Genetics and Diabetology, Children’s Hospital Giovanni XXIII, Azienda Ospedaliero-Universitaria Consorziale, Via Amendola 207, 70126, Bari, Italy

Received: March 03, 2021; Accepted: April 03, 2021; Published: April 10, 2021


Background/Objectives: Early diagnosis and a low-Phe diet significantly improved prognosis of PKU patients whose management is now-a-day mostly focused on preventing nutritional imbalances and resulting comorbidities. Puberty is a turning point for the risk to develop overweight and bone quality impairment. The present study aims to evaluate body composition and bone quality in prepubertal and pubertal PKU patients.

Subjects/Methods: This is an observational, prospective study on an historical cohort of patients with PKU aged between 4.1 and 18 years, with early diagnosis and continuous protein-restrictive diet treatment. Bioimpedance, weight measurements, height, body mass index calculation, and quantitative ultrasound were collected. Sexual maturity was evaluated by using the Tanner staging.

Results: Thirty PKU patients (14 prepubertal, 16 pubertal) were included in the study. Mean BMI was within the normal range in both groups, although fat content was higher in prepubertal, whereas lean mass was higher in the pubertal group who had also a higher total protein content. Among QUS parameters, bone quality index and broadband ultrasound were lower in prepubertal children than in adolescents, with a Z-score BQI within the osteopenia range in both of them.

Conclusions: Pubertal patients with PKU develop a higher lean mass and protein content. If on one hand the high non-Phe protein intake in the pubertal period may promote increase in bone mineral quality, on the other hand the higher protein retention, typical of this period, may increase the risk for hyperinsulinism and glucose intolerance in later life. Adjusting dietary management by modulating total protein intake in the peripubertal period could result in better management of PKU patients.

Keywords: Phenylketonuria; Body composition; Bioelectric impedance; Quantitative ultrasound; Puberty


PKU: Phenylketonuria; BIA: Bioelectric Impedance; QUS: Quantitative Ultrasound; BMI: Body Mass Index; BUA: Broadband Ultrasound Attenuation; SOS: Speed of Sound


Phenylketonuria (PKU) (OMIM: 261600) is an inborn error of metabolism caused by a deficient activity of Phenylalanine Hydroxylase (PAH), needed to convert Phenylalanine (Phe) into tyrosine [1]. Early diagnosis through neonatal screening and a low-Phe diet significantly improve prognosis, resulting in patients reaching adulthood with normal mental development and a better quality of life [2-4]. Over the years, the main target of PKU patient management progressively shifted from preventing early death and irreversible intellectual disability to preventing nutritional imbalances and resulting comorbidities [5].

Overweight and obesity in children with PKU result from unbalanced diet, low calcium intake and a sedentary life [6,7]. Relevant dietary factors include the quality and quantity of tolerated protein, and frequent excess of non-protein caloric intake [8,9]. In turn, such protein-restricted regimens may result into short and long-term nutritional risks [10]. Based on that, monitoring the patient trend in nutritional status by a more comprehensive and systemic nutritional assessment is warranted [11]. Assessing Body Composition (BC) is mandatory to increase accuracy of the nutritional status evaluation [12]. In addition to the Body Mass Index (BMI), Bioelectric Impedance (BIA) is another useful tool to assess the real tissue mass [12,13]. In particular, BIA is a 2-compartment method for body composition assessment, based on impedance measurements of biological tissues. The measured impedance is proportional to the Total Volume of Body Water (TBW). In the presence of a constant TBW to lean mass (FFM) ratio, the impedance can be converted to FFM. Fat Mass (FM) can be determined by calculating the difference between total body weight and FFM [14].

During adolescence, hormonal fluctuations cause a genderspecific increase in lean mass, fat mass and bone mass, making adolescents prone to develop insulin resistance, which in turn appears to play a reciprocal role in the changes observed in body composition [15].

A high-protein diet may lead to a further lean body mass gain, [16]; therefore, due to the high, Phefree, protein intake, PKU patients, especially in adolescence, may be more exposed to alterations in body composition [10].

An additional concern is related to bone health, which is impaired in PKU subjects, leading to stunted growth and risk of fractures [17- 19]. Thus, Bone Mineral Density (BMD) assessment is also part of the routine follow-up assessment of PKU patients [20,21]. Dual Energy X-Ray Absorptiometry (DXA) of the lumbar spine and hip is usually preferred for evaluating BMD, as it provides the most reliable measurement for predicting fracture risk and monitoring treatment. However, studies conducted so far using DXA to assess BMD have produced conflicting data in PKU subjects, and the data refer mainly to adults [22,23]. DXA provides two-dimensional (areal) values for BMD, does not distinguish cortical from trabecular bone and should be adjusted for height and weight when used in children and adolescents. Quantitative Ultrasound (QUS) is an easy, inexpensive and radiation-free alternative diagnostic tool to assess bone quality and fracture rate the in children and adolescents with bone and mineral disorders [24-27].

QUS, measuring the attenuation and velocity of ultrasound waves passing through the calcaneus bone, is now part of bone health assessment [28]. It has been reported that Broadband Ultrasound Attenuation (BUA) and Speed of Sound (SOS) exhibit distinct structural properties of bone, with BUA more related to structural parameters such as connectivity and porosity of the skeleton, and SOS more influenced by bone mineral mass and elasticity [29].

Given the lack of literature data on nutritional status and bone health in the peripubertal period, the present study aims to investigate the changes of body composition and bone quality in prepubertal and pubertal PKU patients, also exploring some predisposing factors to later life comorbidities.

Materials and Methods


We performed an observational, prospective study on an historical cohort of patients with PKU (13 males and 17 females, median age 10.8 years (range 4.1-18) diagnosed by neonatal screening and confirmed by molecular analysis. Patients with PKUindependent, concomitant conditions affecting bone mineralization and/or nutritional status and/or pubertal spurt timing (precocious or delayed puberty) were excluded. Type of PKU was assigned on the basis of neonatal pre-diet Phe levels, according to the PKU European Guidelines [11]. Written informed consent was obtained from the parents. This study was approved by local IRB. All the procedures used were in accordance with the guidelines of the Declaration of Helsinki on Human Experimentation.


Patients were weighed and measured in minimal clothing, by the same balance and altimeter. Body mass index (BMI) was calculated as the weight/height2 ratio. Normal values of BMI Z-score ranged between +1.0 and -1.0, using the World Health Organization (WHO) charts [30]. Tanner stage (prepubertal, Tanner 1; pubertal, Tanner 2 to 5), was determined by physical examination [31].

Dietary intake and biochemical measurements

All patients were treated with a protein-restrictive diet supplemented with amino acid mixtures. The same metabolic dietitian, using the Winfood Pro software (version 3.0.0, 2011, Medimatica Srl, Teramo, Italy) analyzed dietary data from each patient. Dietary protein intake, expressed in grams per kilograms per day, was compared with FAO/WHO/UNU recommended safe levels [32]. Energy intake was expressed as kcal/day and compared with FAO/WHO/UNU 1985 requirements. All patients were monitored weekly by Phe dosage on blood spot, and dietary compliance was defined as poor if mean DBS Phe level was higher than the target level, or good if mean dried blood spot Phe level was within the target Phe level, according with PKU European Guidelines [11]. Serum samples were assayed for vitamin D status (total 25(OH)D concentration), lipid panel (Total Cholesterol (TC), Triglycerides (TG), High-Density Lipoprotein Cholesterol (HDL-C), Low-Density Lipoprotein Cholesterol (LDL-C), and glucose status (fasting glucose concentration) for all participants. Risk of dyslipidemia was assessed according to reference cut-off values for fasting lipids in children [33]. Vitamin D status was also evaluated according to the current standard for Italian target population [34,35].

Body composition

A trained study assistant performed all BIA measurements. A multi-frequency (20 kHz and 100 kHz) BIA device using eight-point tactile electrode system (Inbody 230, Biospace Corp., Seoul, Korea), referred to as BIA8MF [36] was used. After the sex, age and height information introduction into the BIA8MF, subjects were asked to stand in a stable position with bare feet. Their toes and heels had to be placed in contact with the anterior and posterior electrodes of the weighting platform, respectively. The BIA8MF produces 10 impedance values to measure five segments of the body: right leg, left leg, right arm, left arm and the trunk. The measurements began when both hands grasped the grips, and feet were in the right position. Body composition parameters, calculated by manufacturer software (Lookin’Body 120, Biospace Corp., Seoul, Korea) were: Fat Mass (FM) (Kg), Percentage of Body Fat (PBF), Total Body Water (TBW) (Kg), Waist-Hip Ratio (WHR) and Lean Body Mass (LBM) (Kg).

Quantitative Ultrasound Scan (QUS)

The QUS (SONOST 3000) measurement score of the calcaneus region was used to calculate the bone mineral density (BMD) status according to the WHO criteria (normal, osteopenia and osteoporosis) [37]. The machine was calibrated daily according to the manufacturer’s instructions.

The outputs included the Bone Quality Index (BQI), the Broadband Ultrasound Attenuation (BUA, measured in dB/MHz), the speed of sound (SOS, measured in m/s). BUA reflects bone density and structure by reduction analysis of ultrasound pulse intensity through the bone; SOS expresses speed of ultrasound wave through the bone and reflects bone mineral density. SOS is related to temperature, while BUA is inversely related to temperature. These correlation coefficients (a,β) are combined with BUA and SOS to obtain the BQI (BQI = a x SOS + β x BUA) [36].

Statistical analysis

Results were expressed as mean and Standard Deviation (SD) or median and Interquartile (IQ) range, according to variable distribution. Qualitative variables were expressed as proportions or percentages. We analyzed variable distribution with Shapiro-Wilk test and compared results using the t-test, Mann Whitney, or correlation test. Associations between qualitative variables were calculated by Fisher’s exact test and p<0.05 was considered as statistically significant. Univariate linear regression was used to evaluate the relationship between each individual outcome, and the correlation coefficients were calculated, with the indication of the range at 95% confidence interval (95% CI).


Demographic and anthropometric findings

Thirty PKU patients were included in the study. Their demographic and anthropometric characteristics are summarized in Table 1. Nineteen out of 30 (63%) had a classic form of PKU (PKU guidelines), while the remaining had mild / moderate PKU. All patients were on a low-Phe diet. In three mild/moderate PKU cases, Sapropterine therapy at a dose of 10 mg/kg/day was associated. Fourteen patients (6 females) were prepubertal (Tanner stage I), while the remaining 16 (8 females) were in Tanner stage 2-5 (Table 1).