Vitamin A
Vitaminas e mineraisÚltima revisão: 7 de abril de 2026. Abordagem de fontes: contexto padrão de interpretação laboratorial, material médico de referência e orientações clínicas ou de saúde pública quando relevantes.
O que é Vitamin A?
Vitamin A is a fat-soluble vitamin essential for vision, immune function, reproduction, and cellular communication. It exists in two principal forms: preformed vitamin A (retinol and retinyl esters), found in animal products such as liver, dairy, and fish, and provitamin A carotenoids (most notably beta-carotene), found in colorful fruits and vegetables. Retinol is the most biologically active form and is the primary circulating form measured in blood tests.
After absorption in the small intestine, vitamin A is stored primarily in the liver as retinyl esters—the liver stores approximately 80–90% of the body's total vitamin A. Serum retinol levels are tightly regulated by the liver and remain stable until stores are significantly depleted or excessively accumulated. This homeostatic control means that blood levels may not reflect total body stores until deficiency or toxicity is advanced. Vitamin A is critical for maintaining the integrity of epithelial tissues, supporting the differentiation of immune cells, and enabling the retinal pigment rhodopsin to function in low-light vision.
Por que isso importa
Vitamin A deficiency is the leading cause of preventable blindness in children worldwide and significantly impairs immune function, increasing susceptibility to infections and mortality from measles and diarrheal diseases. Conversely, because vitamin A is fat-soluble and stored in the liver, excessive intake—particularly from supplements or liver consumption—can cause hypervitaminosis A, leading to liver damage, bone loss, birth defects, and neurological symptoms. Maintaining appropriate vitamin A levels supports healthy vision, robust immunity, and normal fetal development.
Faixas de referência normais
| Grupo | Faixa | Unidade |
|---|---|---|
| Adults | 30–65 | mcg/dL |
| Children (1–6 years) | 20–43 | mcg/dL |
| Children (7–12 years) | 26–49 | mcg/dL |
As faixas de referência podem variar entre laboratórios. Sempre compare seus resultados com as faixas fornecidas pelo seu local de exame.
O que significam níveis altos de Vit A
Causas comuns
- Excessive vitamin A supplementation
- Chronic consumption of liver or liver products
- Accutane (isotretinoin) or other retinoid medications
- Kidney disease (impaired excretion)
- Hyperlipidemia (increased carrier proteins)
Possíveis sintomas
- Nausea, vomiting, and abdominal pain
- Headaches and blurred vision
- Dry, peeling skin and lip cracking
- Bone and joint pain
- Hair loss
- Liver enlargement and damage
- Birth defects if taken during pregnancy
O que fazer: Stop all vitamin A supplements and limit dietary sources of preformed vitamin A. Seek medical evaluation for liver function testing. Acute toxicity from a single massive dose is a medical emergency. Chronic toxicity typically resolves within weeks to months after discontinuation but may require monitoring for liver damage. Women who are pregnant or planning pregnancy should avoid retinoid medications and high-dose vitamin A supplements.
O que significam níveis baixos de Vit A
Causas comuns
- Inadequate dietary intake
- Fat malabsorption (celiac disease, Crohn's disease, cystic fibrosis)
- Liver disease (impaired storage and release)
- Zinc deficiency (zinc is needed to transport vitamin A)
- Chronic alcoholism
- Pancreatic insufficiency
Possíveis sintomas
- Night blindness (nyctalopia)—earliest symptom
- Dry eyes (xerophthalmia) and corneal damage
- Dry, rough skin and poor wound healing
- Increased susceptibility to infections
- Growth retardation in children
- Bitot's spots (white patches on the conjunctiva)
O que fazer: Mild deficiency is treated with dietary improvement—increasing intake of liver, eggs, dairy, sweet potatoes, carrots, and dark leafy greens. Moderate to severe deficiency requires supplementation under medical supervision, typically with retinyl palmitate. Underlying malabsorption issues should be addressed. In developing countries, high-dose vitamin A supplementation programs have significantly reduced childhood mortality.
Quando o exame de Vit A é recomendado?
- When night blindness or vision changes are reported
- In patients with fat malabsorption syndromes
- When liver disease is present
- If vitamin A toxicity is suspected from over-supplementation
- In malnourished populations or children with growth failure
- When taking retinoid medications
Perguntas frequentes
Biomarcadores relacionados
Referências e abordagem de revisão
As páginas do glossário de biomarcadores são explicações educativas e devem ser interpretadas junto com as faixas de referência e observações fornecidas pelo seu laboratório e pelo seu médico. Para conhecer nossos padrões editoriais e processo de revisão, veja nossa Política editorial e a nossa revisão de conteúdo.
Quer analisar seus níveis de Vit A?
Envie seus resultados de exames para receber uma análise instantânea com IA de todos os seus biomarcadores.
Enviar resultados de exames →Aviso médico: Estas informações são apenas educativas e não substituem orientação, diagnóstico ou tratamento médico profissional. As faixas de referência podem variar entre laboratórios. Sempre converse com seu profissional de saúde sobre a interpretação dos seus resultados específicos.