Bone mineral density

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Osteopenia is prevalent in diet-treated persons with PKU from early life. Reduced bone mineral density and/or bone mass has been detected in up to approximately 50% of patients screened by various methods. These methods have included DEXA (dual energy X-ray absorptiometry), pQCT (peripheral quantitative computed tomography), and SPA (single photon absorptiometry). The defect seems to be characterized by a reduction in the speed of bone mineralization, especially after 8 years of age. Osteoporosis is an important cause of morbidity and mortality in older adults in the general population. Reduction in bone mass increases the risk of fracture. A reduction of one standard deviation in spine bone mass is associated with a bone fracture rate of 2.0-2.5. Some authors have reported an increased fracture rate in children over 8 years of age with PKU.

The pathogenesis of osteopenia in PKU is under study. Discrepant associations have been reported between osteopenia and blood phenylalanine levels, serum vitamin and mineral levels, protein, vitamin and mineral intakes, serum markers of bone formation and PTH, and ratio of urinary minerals, to creatinine. One theory is that impaired mineralization is a direct effect of the lifelong disease process of PKU. The total and the bone-specific fraction of alkaline phosphatase are reduced in some patients. This reduction may affect osteoblast activity and impact bone formation and turnover. High blood phenylala-nine levels have not been consistently correlated with osteopenia. High blood levels of phenylalanine and phenylalanine metabolites would result in their increased urinary excretion. Chelating of minerals with phenylalanine and phenylalanine derivatives could theoretically result in significant mineral losses.

Osteopenia may be an accumulated result of lifelong diet treatment or poor diet compliance at vulnerable stages of bone development. Compliant patients tend to have low variation in their lifelong intake of whole protein, as controlled amounts of whole protein are required to maintain good metabolic control. Compliant patients tend to have similar trends in overall intakes. Lack of adequate trace elements, whole protein, vitamins, and/or minerals may be culprits. Impaired absorption of the synthetic diet or the type of medical food used (hydro-lysate versus elemental formulation) may exert an independent effect. Inadequate intakes of calcium and phosphorous are known risk factors for the development of osteoporosis in nonaffected persons. Tailoring medical foods to specifically deliver the amounts of calcium and phosphorous recommended in the new RDIs may help to prevent osteopenia.

Trials of calcitriol (1-25 (OH)2 D) supplementation in estrogenic patients with PKU are in progress. Calcitriol has been chosen as most patients already receive expected sun exposure from participating in normal outdoor activities, and their intakes of dietary vitamin D generally meet or exceed the RDA. Clairol has been shown to be a useful treatment; treated patients require close monitoring of urinary calcium excretion and blood calcium levels.

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