Sex differences in body composition and bone mineral density in phenylketonuria: A cross-sectional study

Background: Low bone mineral density (BMD) and subsequent skeletal fragility have emerged as a long-term complication of phenylketonuria (PKU). Objective: To determine if there are differences in BMD and body composition between male and female participants with PKU. Methods: From our randomized, crossover trial [1] of participants with early-treated PKU who consumed a lowphenylalanine (Phe) diet combined with amino acid medical foods (AA-MF) or glycomacropeptide medical foods (GMP-MF), a subset of 15 participants (6 males, 9 females, aged 15-50 y, 8 classical and 7 variant PKU) completed one dual energy X-ray absorptiometry (DXA) scan and 3-day food records after each dietary treatment. Participants reported lifelong compliance with AA-MF. In a crossover design, 8 participants (4 males, 4 females, aged 16-35 y) provided a 24-h urine collection after consuming AA-MF or GMP-MF for 1-3 weeks each. Results: Male participants had significantly lower mean total body BMD Z-scores (means +/- SE, males =-0.9 +/- 0.4; females, 0.2 +/- 0.3; p= 0.01) and tended to have lower mean L1-4 spine and total femur BMD Z-scores compared to female participants. Only 50% percent of male participants had total body BMD Z-scores above -1.0 compared to 100% of females (p= 0.06). Total femur Z-scores were negatively correlated with intake of AA-MF (r =-0.58; p= 0.048). Males tended to consume more grams of protein equivalents per day from AA-MF (means +/- SE, males: 67 +/- 6 g, females: 52 +/- 4 g; p= 0.057). Males and females demonstrated similar urinary excretion of renal net acid, magnesium and sulfate; males showed a trend for higher urinary calcium excretion compared to females (means +/- SE, males: 339 +/- 75 mg/d, females: 228 +/- 69 mg/d; p = 0.13). Females had a greater percentage of total fat mass compared to males (means +/- SE, males: 24.5 +/- 4.8%, females: 36.5 +/- 2.5%; p= 0.047). Mean appendicular lean mass index was similar between males and females. Male participants had low-normal lean mass based on the appendicular lean mass index. Conclusions: Males with PKU have lower BMD compared with females with PKU that may be related to higher intake of AA-MF and greater calcium excretion.