Diet-treated children and adults with PKU consume very small amounts of animal fats, including fish-derived oils and long-chain polyunsaturated fatty acids (LC PUFAs.) In infants, small amounts of cows' milk-based formula are typically used to supply phe-nylalanine requirements. The majority of fatty acids supplied are typically not longer than 18 carbons long. While many standard infant formulas in Europe and the US are now supplemented with docosohexa-noic acid (DHA) and arachadonic acid (ARA), metabolic formulas are not. Fatty acids are a structural component of all cell membranes. Alpha linolenic acid-derived compounds are essential for proper development of the central nervous system and retina. Lino-leic acid-derived compounds play a role in promoting normal growth, skin, and reproduction. Breast milk contains formed DHA and ARA, and some studies indicate that breast-fed infants have better visual and cognitive development than unsupplemented formula-
fed infants. The diets of children with PKU provide similar energy, higher carbohydrate, and lower lipid (with high unsaturated/saturated ratio) and cholesterol content than controls. Circulating plasma lipid levels of treated PKU patients contain lower concentrations of arachadonic acid (ARA), docosahexanoic acid (DHA), and eicosapentanoic acid than controls. Ery-throcyte membranes of patients contain relatively high amounts of ARA and relatively low amounts of DHA. In theory, patients receiving adequate amounts of the essential fatty acid precursors linoleic and alpha lino-lenic acids would be able to synthesize LC PUFAs via elongation and desaturation reactions. It is unclear whether the amount of LC PUFAs synthesized would be adequate for optimal tissue function. Especially in infants, DHA and ARA may be partially essential nutrients. Trials of LC PUFA supplementation in PKU patients are underway. A number of widely available PKU formulas for older children and adults do not supply fat. Patients prescribed these formulas are presently advised to regularly include good sources of linoleic and alpha-linolenic acids in their diets. Flax, canola, and walnut oils are good sources.
In theory, patients with poorly controlled phenyl-alanine levels cannot efficiently build reserves of DHA and ARA from precursors. Carnitine-dependent mitochondrial enzymes that also use a cofactor, alpha tocopherolquinone, perform elongation and desaturation reactions. Phenyllactate and phenylpyruvate, metabolic byproducts of phenylalanine, may inhibit the synthesis of the cofactor alpha tocopherolqui-none. This process may be at least partly responsible for the mental retardation and microcephaly observed in untreated PKU patients and poorly controlled maternal PKU.
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