Laboratory Monitoring of Nutritional Deficiencies in Children Following Restrictive Diets: A Narrative Review and Risk-Based Considerations

INTRODUCTION: Restrictive diets are increasingly encountered in pediatric practice and may be adopted voluntarily or prescribed for medical conditions. Although they can support normal growth when appropriately planned, exclusion of nutritionally important foods may increase the risk of nutrient inadequacy. This narrative review examined nutritional deficiencies and laboratory monitoring in children following plant-based, food-allergy elimination, gluten-free, ketogenic, and protein-restricted diets for inherited metabolic disorders. METHODS: Targeted searches of PubMed, Scopus, and Web of Science were conducted through 30 June 2026 using pediatric, diet-specific, nutritional-status, and biomarker terms. Because this was a narrative review, the literature was selected and synthesized qualitatively rather than through a formal systematic-screening process; no fixed study count, duplicate independent screening, or formal risk-of-bias assessment was performed. Professional guidelines and position papers were prioritized when discussing monitoring considerations, while pediatric studies were used to describe dietary intake, biochemical findings, clinically manifest deficiency, and growth outcomes. RESULTS: Nutritional risks differed according to the foods or nutrients restricted. Vitamin B12 and iron were major concerns in plant-based diets, whereas cow's milk and multiple-food elimination increased the risk of inadequate calcium, vitamin D, iodine, protein, and energy intake. Gluten-free diets were commonly associated with low fiber, iron, folate, and B-vitamin intake, particularly when refined, non-fortified products predominated. Ketogenic dietary therapy required attention to selenium, vitamin D, bone-related minerals, carnitine in selected patients, and linear growth. In phenylketonuria and related disorders, nutritional adequacy depended strongly on protein-substitute adherence and appropriate provision of essential amino acids and micronutrients. Across all dietary patterns, laboratory results required interpretation in relation to dietary intake, growth, supplementation, inflammation, medication, and the underlying condition. Across dietary patterns, inadequate intake, biochemical abnormalities, clinically manifest deficiency, and impaired growth were considered related but distinct outcomes. CONCLUSIONS: Nutritional monitoring should be individualized and based on the actual dietary restriction and clinical risk. The principal contribution of this review is a practical, risk-based framework that links the specific dietary restriction and adequacy of replacement foods with growth, symptoms, supplementation, and targeted laboratory biomarkers.