Introduction

Anemia is a widespread clinical condition defined by a reduction in red blood cell (RBC) count or haemoglobin concentration, leading to impaired oxygen delivery to tissues. Among the different types of anemia, normocytic anemia presents a unique diagnostic challenge due to its subtler clinical features and the wide range of potential etiologies. Unlike microcytic or macrocytic anemia, normocytic anemia is characterised by RBCs of normal size and morphology, but insufficient quantity or haemoglobin content. Although nutritional deficiencies are classically associated with alterations in RBC size—iron deficiency with microcytosis and folate or vitamin B12 deficiency with macrocytosis—recent evidence suggests that these nutritional deficiencies can also present with normocytic indices, particularly in early or mixed stages. This review aims to explore the mechanisms by which deficiencies in iron, folate, and vitamin B12 can lead to normocytic anemia, as well as the clinical implications of such presentations.

Normocytic anemia: Definition and clinical relevance

Normocytic anemia is defined by a haemoglobin concentration below the normal threshold, accompanied by a mean corpuscular volume (MCV) between 80 and 100 femtoliters (fL). It is often observed in patients with chronic diseases, acute blood loss, bone marrow disorders, and hemolysis. However, normocytic anaemia can also result from nutritional deficiencies that have not yet altered RBC size or from a combination of multiple deficiencies that balance each other’s effects on MCV. The prevalence of normocytic anemia increases with age and comorbid conditions, particularly in hospitalised or chronically ill individuals, underscoring the need for a comprehensive diagnostic approach.

Pathophysiology of normocytic anemia

The underlying pathophysiological mechanisms of normocytic anemia are diverse and depend on the balance between RBC production, lifespan, and loss. Inadequate erythropoiesis may result from insufficient availability of essential nutrients or from suppression of bone marrow activity by systemic inflammation. Chronic inflammatory states, for instance, lead to increased levels of cytokines such as interleukin-6 and tumour necrosis factor-alpha, which impair erythropoietin production and iron mobilisation. In other cases, anemia arises from the premature destruction of RBCs or from acute bleeding, which does not immediately alter cell size. When nutritional deficiencies are involved, the presentation may be normocytic if the deficiency is in its early stages, if the hematopoietic system is compensating, or if there is a coexistence of opposing factors, such as iron and vitamin B12 deficiency.

Nutritional deficiencies as a cause of normocytic anemia

Although normocytic anemia is often attributed to non-nutritional causes, deficiencies in iron, folate, and vitamin B12 can each contribute to a normocytic phenotype under specific circumstances. These micronutrients are essential for normal erythropoiesis, and their absence leads to ineffective RBC production even in the presence of normal cell morphology.

Iron deficiency is typically associated with microcytic anemia due to impaired haemoglobin synthesis. However, in the initial stages of deficiency, iron levels may be insufficient to fully support erythropoiesis, yet not low enough to cause a reduction in cell size. Furthermore,  

in conditions of chronic inflammation, iron is sequestered in macrophages and rendered unavailable for erythropoiesis despite adequate total body stores—a state known as functional iron deficiency. This form of iron dysregulation is often normocytic and may only become microcytic as the deficiency progresses. Laboratory findings in such cases may include low serum iron and transferrin saturation, with normal or elevated ferritin due to the effects of inflammation.

Folate deficiency, while classically presenting with macrocytosis, can also manifest as normocytic anemia in its early stages or when confounded by other clinical conditions. Folate plays a crucial role in DNA synthesis during RBC production. Inadequate folate impairs nuclear maturation, leading to ineffective erythropoiesis. However, if the deficiency is mild, recent, or if concurrent conditions such as chronic inflammation or alcohol use are present, the typical macrocytic picture may not emerge. Additionally, coexisting iron deficiency can obscure macrocytosis, resulting in a normocytic appearance. Folate status is best assessed through red blood cell folate levels, which reflect tissue stores more accurately than serum folate.

Vitamin B12 deficiency is another classical cause of macrocytic anemia, but may present as normocytic in certain clinical contexts. In the early stages of deficiency, particularly before significant disruption of DNA synthesis occurs, the MCV may remain within normal limits. In patients with mixed deficiencies—such as simultaneous B12 and iron deficiency—the opposing effects on cell size may yield a normocytic MCV. Furthermore, vitamin B12 deficiency is often accompanied by neurologic symptoms, which may prompt earlier detection before hematologic abnormalities become apparent. Diagnostic markers such as elevated homocysteine and methylmalonic acid levels can aid in confirming B12 deficiency, especially when serum levels are borderline.

Clinical assessment and diagnostic considerations

Given the broad differential diagnosis of normocytic anemia, a systematic clinical evaluation is essential. The diagnostic approach begins with a thorough history and physical examination, focusing on nutritional intake, gastrointestinal symptoms, chronic illnesses, and medication use. Laboratory assessment includes a complete blood count with reticulocyte count, which provides information about bone marrow activity. Iron studies, including serum iron, ferritin, transferrin saturation, and soluble transferrin receptor, are crucial for evaluating iron status. Measurement of serum vitamin B12 and red blood cell folate is necessary to assess micronutrient levels, and markers of inflammation such as C-reactive protein and erythrocyte sedimentation rate help identify chronic inflammatory states.

In some cases, bone marrow examination may be warranted, particularly when peripheral blood tests do not yield a clear diagnosis. This is more likely in patients with pancytopenia or 

suspected marrow infiltration. It is also important to consider the possibility of mixed anemia, where the coexistence of two or more deficiencies may normalise MCV and obscure the diagnosis.

Management strategies

Management of normocytic anemia secondary to nutritional deficiencies involves targeted supplementation based on the identified deficiency. Oral iron therapy is typically the first-line treatment for iron deficiency anemia, although intravenous iron may be indicated in cases of malabsorption, intolerance, or significant anemia. Treatment response is usually seen within a few weeks, with a rise in reticulocyte count followed by improvement in haemoglobin levels.

Folate deficiency is managed with oral folic acid, often at a dose of 1 mg daily, with reassessment of levels and haemoglobin after several weeks. It is essential to rule out concomitant vitamin B12 deficiency before initiating folate therapy, as folate can mask hematologic signs of B12 deficiency while allowing neurologic damage to progress.

Vitamin B12 deficiency is commonly treated with intramuscular injections of hydroxocobalamin or cyanocobalamin, particularly in cases of malabsorption or pernicious anemia. High-dose oral supplementation may also be effective in certain individuals. Neurologic symptoms typically take longer to resolve than hematologic abnormalities, emphasising the importance of early detection and treatment.

In cases where normocytic anemia is multifactorial, such as when nutritional deficiencies coexist with chronic disease, both underlying causes must be addressed for optimal hematologic recovery. Nutritional counselling and treatment of comorbidities are important components of long-term management.

Conclusion

Normocytic anemia is a complex and frequently encountered clinical entity with a wide range of potential causes. Although it is often linked to chronic disease or acute blood loss, nutritional deficiencies in iron, folate, and vitamin B12 can also present with normocytic indices, particularly in early stages or when multiple deficiencies coexist. Recognition of these atypical presentations is essential for accurate diagnosis and timely treatment. A comprehensive approach that includes careful history taking, appropriate laboratory testing, and consideration of mixed etiologies can facilitate identification of nutritional deficiencies contributing to normocytic anemia. Correcting these deficiencies not only resolves the anemia but also prevents potential complications, especially in at-risk populations.