Increased risk of vitamin B12 deficiency in patients with phenylketonuria on an unrestricted or relaxed diet

risk of vitamin B12 deficiency in patients phenylketonuria on an unrestricted or relaxed :cliet ~Iark Robinson, MBChB,MRCP,Fiona J. White, spa), g/Iaureen A. C[ea~, ~¢BChB,~ID,g/IRCP, Ed Wraith, MBChB,FRCP, Wayne K. Lain, MBChB, MRCP,and John H. Wa[te~MBChB,FRCgFRCPCH

Objective: To investigate whether dietary relaxation or cessation in patients with phenylketonuria (PKU) predisposes to vitamin B12 deficiency. S t u d y design: Patients with PKU aged 11 to 38 years underwent a neurologic examination and dietetic assessment and were divided according to their diet into 1 of 3 groups: Strict - those on a strict low phenylalanine (phe) diet with amino acid, mineral, and vitamin supplements; Relaxed those on a total protein intake of N1 g/kg/d with 50% of this from natural protein and 50% from amino acid, mineral, and vitamin supplements; Unrestricted - those on no formal protein restriction and not taking amino acid supplements. Assays of blood samples were taken for vitamin B12 and folate levels by standard assays. Results were analyzed with Student t test. Results: Vitamin B12 levels were significantly lower in the PKU groups on relaxed or unrestricted diets compared with the normal population (P < .0001 [unrestricted] and .0034 [relaxed]). Folate levels were significantly elevated in all PKU groups (<.0001). Conclusion: Patients with PKU who are no longer under strict dietary control may be at risk from vitamin B12 deficiency. We recommend that all patients should remain under medical and dietetic supervision and in particular have their vitamin B 12 status monitored. (J Pediatr 2000; 136:545-7)

To prevent neurologic disease, children with phenylketonuria must adhere to a strict low phenylalanine diet with an artificial amino acid mixture supplemented with vitamins and minerals. 1'2 In the past, this diet was followed until the end of childhood, with most centers al-

lowing some relaxation in adolescence. The rationale for this approach was that the brain was vulnerable to the toxic effects of elevated phe in the period of its maximal myelination throughout childhood. More recently, there have been concerns about the safety of discontinu-

From the Willink Biochemical Genetics Unit, zI~rancheater Children's Hoapita[a NHS Trudt, Pendlebury, Jllancbeatet; United Kingdom, and the Department of Cll)zica[ Genetics, St James University Hospital, Leeds, ~iancheste[~ Uni&d Kingdom.

Submitted for publication May 25, 1999; revision received Sept 27, 1999; accepted Oct 28, 1999. Reprint requests: J. H. Walter, MBChB, MD Willink BiochemicalGenetics Unit, Manchester Children's Hospitals NHS Trust, Hospital Rd, Pendlebury, Manchester M27 4HA, UK. Copyright © 2000 by Mosby, Inc. 0022-3476/2000/$12.00 + 0 9/22/104294 doi:10.1067/mpd.2000.104294

ing the diet, and several guidelines have been published recommending that treatment be lifelong. 3-6

The PKU diet allows only a small amount of natural protein. It is restrictive and unpalatable and requires regular monitoring of growth, dietary intake, and blood phe levels to ensure its efficacy and nutritional adequacy. Vitamin B12 must be provided either combined with the amino acid supplement or from a separate vitamin/mineral supplement. Folio acid is generally found in adequate amounts from vegetables, which are allowed freely, although patients on diet receive extra folic acid from the vitamin/mineral-fortiffed phe-free amino acid supplement. Recent reports suggest that individuals with PKU may be at risk from vitamin B12 deficiencyz-9 and consequently in danger of having neurologic problems such as peripheral neuropathy or subacute combined degeneration of the cord or psychiatric sequelae such as depression or psychosis. 10-12 Despite the recommendation for lifelong treatment, many adolescents and adults remain on an unrestricted diet with no vitamin and mineral supplements. Although a range of foods is allowed, they tend to choose foods with a low animal protein content. We hypothesized that this specific subgroup of patients with PKU is at greater risk of having vitamin B12 deficiency. We 545

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THE JOURNAL OF PEDIATRICS APRIL 2000

Table I. Blood phe values in patients with phenylketonuria (gmol/L)

Table II. Blood vitamin B I 2 values (ng/L)

Table I I I . Erythrocyte folate values (gg/L)

have tested this hypothesis in this study b y prospectively measuring vitamin B12 and folate levels in adolescents and adults with P K U on different degrees of dieta W restriction.

METHODS Adolescents and adults with classical P K U attending the W~tllink Biochemical Genetics Unit for regular follow-up had assays of vitamin B12 and red cell folate measured in venous blood by standard assays. After a dietetic assessment was performed, the patients were divided into 1 of 3 groups: S t r i c t - those on a strict low phe diet with amino acid, mineral, and vitamin supplements; Re/axed those on a total protein intake of approximately 1 g/kg/d with roughly 50% of this from natural protein and 50% from amino acid, mineral, and vitamin supplements; U n r e s t r i c t e d - those on 546

no formal protein restriction and not taking amino acid supplements. In addition to a dietetic assessment, aH patients underwent a neurologic examination. Normal population data for B12 and folate were obtained from the Office of Population Censuses and Surveys (The Dietary and Nutritional Survey of British Adults). 13 Results were compared with these data. Probability values for Student t test are reported for the differences in means between the P K U group and the normal population.

RESULTS D a t a were obtained on 83 patients with P K U (median age 2 2 years, range 11 to 58 years), of w h o m 22 were on strict, 30 on relaxed, and 31 on unrestricted diets. Mean hemoglobin values and mean cell volumes in each subgroup were not significantly different

from normal. M e a n blood phe levels, vitamin B12, and erythrocyte folate are shown in Tables I, II, and III, respectively. The number of patients tested in each group is indicated; some patients samples were insufficient for the assay of erythrocyte folate. Ten patients had vitamin B12 levels below the normal range (180 to 1132 ng/L): 6 in the unrestricted diet group, 3 in the relaxed group, and 1 in the strict diet group. In those on an unrestricted diet, the median age of the group with low B12 levels was 2 4 years compared with 20 years in those with normal levels. Only 1 of the 10 patients had an abnormally high mean cell volume. Erythrocyte folate levels were all within or above the normal range, and none of the patients had an elevated plasma free homocysteine. Further review of the patient with a low B12 level in the strict diet group showed that he was taking an amino acid supplement that did not contain vitamin B12 and was not taking separate vitamin/mineral supplements. On general examination all patients appeared healthy, with no signs of vitamin deficiency states. On further neurologic examination 34 had brisk tendon reflexes, and 10 had tremor at rest. No evidence of spasticity or loss of tendon reflexes was seen. No relationship was seen between vitamin B12 and the presence or absence of brisk reflexes or tremor.

DISCUSSION Vitamin B12 deficiency has been recognized previously in patients with PKU. Hanley et al7reported an 18-yearold female patient who had spastic paraparesis, tremor, disorientation, and a megaloblastic anemia. 7 Although prescribed dletaly treatment, she had taken amino acid supplement irregularly. Her vitamin B 12 level was markedly reduced at 65.8 pmol/L, and her symptoms responded to oral B12 supplementation. Hanley et al studied a further 37 patients

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with PKU; 6 had abnormally low levels of vitamin B12, but none had any symptoms or signs attributable to B12 deficiency. Those receiving diet were not under satisfactory biochemical control. Aung et al8 reported an adult with PKU who had progressive tiredness, forgetfulness, and poor concentration. He had discontinued a supervised diet at 14 years but had continued to deliberately avoid meat and dairy products. He had a smooth red tongue but no other clinical signs of B12 deficiency. Plasma B12 was 125 pg/mL and hemoglobin 7.3 g/dL, with mean cell volume 120.3 ft. Folate was normal, and his symptoms responded to oral B 12 supplements. Another adolescent had symptomatic B12 deficiency (spasticity, tremor, and clonus) after nitrous oxide general anesthesia at the age of 14.5 years. 9 He had received the diagnosis and been treated from the newborn period, but biochemical control was poor after the age of 8 years, and phe levels reached 2000 btmol/L. The B12 level measured after the development of neurologic disease was <110 pg/mL. Despite B12 supplementation, neurologic abnormalities have persisted. Vitamin B12 is found only in animal protein, the main sources being meat, fish, poultry, and a smaller amount in dai W products. Because these foods are forbidden for individuals with PKU, they need an adequate intake of vitamin B12 from either a vitamin/mineral-fortified phe-free amino acid supplement or from a separate vitamin/mineral supplement. The results from our study suggest that sufficient vitamin B12 is provided for patients on a strict diet. In contrast, this is not the case in adolescents and adults with P K U who are on an unrestricted diet. Such individuals often limit the amount of animal protein they ingest, either because they find such food unpalatable or because they believe it to be harmful. Because they are no longer taking an amino acid/mineral supplement, their intake of vitamin B12 is then inadequate. Patients on a relaxed diet receive approximately 50% of

their protein intake from diet and the remainder from a phe-free amino acid supplement. As with those on a strict diet, B12 and folio acid will be provided by the vitamin/mineral-fortified phe-free amino acid supplement or a separate vitamin/mineral supplement together with dietary sources. This group often shows poor compliance in taking the amino acid supplement. Their choice of natural protein intake may significantly influence intakes of vitamin B12. Folic acid is found in offal, fortified breakfast cereals, beans and lentils, and green vegetables. Because meat, cheese, fish, and eggs are not allowed in PKU, the diet is relatively rich in vegetables. E~ythrocyte folate levels were above average in the P K U groups compared with the normal population, reflecting the high content of vegetable products containing folic acid in the P K U diet. Even after dietary relaxation, individuals with P K U tend to choose a diet rich in vegetables. The results of our study indicate that in patients with PKU, the group most at risk of having vitamin B12 deficiency are those adolescents and adults who have stopped or relaxed their diet. It is likely that this group still tends to choose foods that are low in animal protein, thus restricting their B12 intake. Vitamin B 12 deficiency can cause a wide range of clinical effects including neuropathy, subacute combined degeneration of the cord, glossifis, anemia, dementia, and psychiatric states such as depression and psychoses. 10-12 It is recognized that some of these neurologic effects may occur even in the absence of anemia or macrocytosis.14 These results highlight the danger of patients with P K U following an unsupervised diet in adulthood. We therefore recommend continued dietary consultations for patients with P K U irrespective of their current diet and regular assessment of vitamin B12 status. These findings emphasize the importance of maintaining close follow-up of adolescents and adults with P K U even after dietary cessation.

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