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Metabolismo de Vitaminas e Oligoelementos

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Abstract

O termo micronutrientes designa duas classes de componentes necessários ao organismo e que estão presentes em quantidades diminu-tas nas dietas: as vitaminas e os oligoelementos. As vitaminas são nutrientes essenciais que de-vem ser providos pela dieta, por não serem sin-tetizadas ou terem taxa de síntese inadequada (por exemplo, síntese de niacina a partir do triptofano). Contudo, há exceções, como a vi-tamina D, que tem síntese endógena por ativa-ção da luz solar. As vitaminas hidrossolúveis, o ácido ascórbico e as vitaminas do complexo B são armazenados em pequena quantidade e re-querem ingestão freqüente para a manutenção dos estoques corporais. Os oligoelementos são nutrientes inorgâni-cos, o ferro, o zinco, o cobre, o manganês, o selênio, o molibdênio, o cobalto, o cromo e o iodo considerados essenciais. O boro, o silicone , o vanádio, o níquel e o arsênico podem ser essenciais para os animais, inclusive para o homem, porém as quantidades necessárias são tão pequenas em relação à quantidade presente no meio ambiente e na dieta que as manifestações de deficiência são altamente improváveis. Até há pouco tempo, os micronutrientes re-cebiam pouca atenção no contexto da nutrição. Segundo Shenkin, isso poderia ser justificado por dois motivos: 1. Como a sua quantidade no organismo é de difícil mensuração, há pouco conhecimento so-bre as necessidades, especialmente na doença. 2. Os efeitos da carência de micronutrien-tes são pequenos em comparação com os efei-tos da carência de macronutrientes, que são mais óbvios. No entanto, tem havido consciência cres-cente em relação aos efeitos das doenças sobre as necessidades de micronutrientes e sobre o papel destes na prevenção daquelas. FUNÇÕES DOS MICRONUTRIENTES Os micronutrientes têm duas funções prin-cipais: 1. Desempenham papel chave no metabolis-mo intermediário como co-fatores ou como par-te integral de enzimas. A atividade enzimática nessas duas situações pode ser modulada pela dis-ponibilidade do micronutriente. Como na doen-ça o metabolismo está aumentado, as necessida-des do micronutriente também aumentam. 2. Integram o sistema de eliminação de ra-dicais livres de oxigênio. Na doença, o metabo-lismo oxidativo está aumentado em função da resposta metabólica e o maior dano ocorre nos ácidos graxos poliinsaturados das membranas celulares e nucleares. Os eliminadores de radi-cais livres de oxigênio protegem os tecidos da lesão oxidativa. Um exemplo são o cobre e o zinco sob a forma da dismutase do superóxido. O selênio, sob a forma de glutation peroxidase, remove os hidroperóxidos; as vitaminas A, E e o betacaroteno reagem diretamente com os ra-dicais livres. As principais funções e sintomas de defici-ência dos oligoelementos e das vitaminas são mostrados nas Tabelas 15.1 e 15.2.
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