Red Cell Distribution Width (RDW)

Red Cell Distribution Width (RDW) tells you how similar or different your red blood cells are in size. A low RDW means most cells are about the same size, while a high RDW means there is a bigger spread of small and large cells. RDW is part of a standard complete blood count (CBC) and is most useful when read alongside MCV (the average size of red blood cells). Together, they help work out common causes of anaemia such as iron, vitamin B12, or folate deficiency .

RDW is more than an anaemia clue. Higher RDW has been linked with worse outcomes across many conditions, including heart and blood vessel disease and overall survival, likely because it reflects problems with red blood cell production, nutrient supply, inflammation, and oxidative stress over time (2). Tracking RDW alongside iron studies, B12, folate, and other CBC indices helps you see trends early and act before symptoms build up.

RDW rises when red blood cell production is disrupted or when nutrient supply to the bone marrow becomes unbalanced. Common causes include:

• Iron deficiency: Increases RDW as new red cells form smaller than existing ones.

• Vitamin B12 or folate deficiency: Produces large, uneven cells, widening RDW.

• Alcohol intake: Chronic or heavy drinking can cause macrocytosis and raise RDW.

• Inflammation and oxidative stress: Low-grade inflammation disrupts red-cell production and increases variability.

• Liver or thyroid dysfunction: Can alter red-cell size distribution.

• Chronic illness or nutrient malabsorption: Impairs nutrient availability for red-cell synthesis.

• High physical training loads: Endurance training or insufficient recovery can increase iron loss and raise RDW.

Think of RDW as a “production quality” check for your red blood cells. These habits support steady, well-made cells and often improve related markers over weeks to months:

• Rebuild iron, if low. Iron deficiency commonly pushes RDW up. Food first: include heme iron (beef, lamb, sardines, mussels, chicken thighs) and plant iron (lentils, tofu, pumpkin seeds, spinach). Pair plant iron with vitamin C–rich foods in the same meal (citrus, capsicum, tomato) to boost absorption. With confirmed deficiency, iron therapy typically raises haemoglobin by about 10 g/L (1 g/dL) within ~2 weeks, and RDW usually settles as new, well-sized cells replace older ones over subsequent weeks.
  

• Restore B12 and folate. Low B12 or folate leads to large, uneven cells and a higher RDW. Regularly include B12 sources (eggs, dairy, fish, meat, or B12-fortified foods if plant-based) and folate sources (leafy greens, beans, avocado). Correcting these deficiencies normalises red cell size spread over time .
  

• Dial down alcohol. Heavy drinking can cause macrocytosis and widen RDW. The good news: macrocytosis can resolve with abstinence within a few months, even when folate alone does not fix it . If alcohol is a driver for you, scaling back consistently is one of the fastest levers.
  

• Lower whole-body inflammation. RDW tends to rise when inflammation and oxidative stress are chronically elevated. A Mediterranean-style pattern (plants, olive oil, nuts, legumes, fish), plenty of colourful produce, regular physical activity, outdoor daylight, and good sleep hygiene help quiet the background inflammation that disrupts red cell production .
  

• Build “red-cell smart” routines.

  • Space tea or coffee at least an hour away from iron-rich meals to avoid blocking plant-iron absorption .

  • If mostly plant-based, combine iron sources with vitamin C at meals most days .

  • Periodise harder training weeks with adequate recovery and iron-rich meals, as heavy training can increase iron needs .
    

As you adjust habits, keep an eye on the whole picture: RDW, MCV, haemoglobin, ferritin, B12, folate, and how you feel. Tracking these together over a long period helps confirm your changes are working.

• Optimal: 11.5–13.5%

• Borderline (mildly elevated): 13.6–14.5%

• High: >14.5%

• Low: <11.5% (usually not a concern on its own)

1. RDW Blood Test: What It Is, Procedure & Results. Cleveland Clinic, 2022. Available from: https://my.clevelandclinic.org/health/diagnostics/22980-rdw-blood-test

2. Patel KV, Ferrucci L. Red cell distribution width as a bellwether of prognosis. Progress in Cardiovascular Diseases, 2024. Available from: https://www.sciencedirect.com/science/article/pii/S1079979624000627

3. Anaemia – Iron Deficiency, Management. NICE Clinical Knowledge Summary, 2024. Available from: https://cks.nice.org.uk/topics/anaemia-iron-deficiency/management/management/

4. Macrocytosis of chronic alcoholism. The Lancet, 1974. Available from: https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(74)90482-6/fulltext

5. Vitamin C and increasing non-haem iron absorption: Scientific Opinion. European Food Safety Authority (EFSA), 2013. Available from: https://www.efsa.europa.eu/en/efsajournal/pub/3514