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Yearly labs nearly always include a CBC (Complete Blood Count), which shows various components of red blood cells and white blood cells. One marker on this list is the RDW, or red cell distribution width. It’s a measurement of how much variability there is in the size of your red blood cells.

A low number means that the red blood cell sizes are relatively consistent. Even if it’s below the usual reference range, a low number is generally considered to be a good thing. There are some studies that dispute this, though, suggesting that RBC sizes can be too uniform; optimal is considered to be 11.4 – 12.5%.

Most problems come in when the number is high. This indicates that the body is struggling to produce red blood cells, for any number of possible reasons.

From the naturopathic paradigm, healing comes from the blood, so the implications of this are far-reaching. Blood, of course, carries oxygen (which, together with glucose, serve as the building blocks for ATP from the mitochondria), and carbon dioxide, which itself serves as the body’s primary vasodilator in a feedback system to balance oxygen delivery.

This is, presumably, why RDW is an important marker for overall health.

RDW as a Marker for Longevity and Biological Aging

Elevated RDW is strongly correlated with various aspects of biological aging, as well as a less favorable prognosis in chronic disease of all kinds. Even within the “normal” reference range (which is really just a bell curve based on population averages), in general RDW tends to increase with biological age.

RDW is non-specific, however; even when there’s no clear anemia, elevated RDW is associated with hundreds of different diseases. The general consensus is that it’s still not known whether elevated RDW is a risk factor in and of itself, or whether it’s just a sign that something is wrong elsewhere.

One thing many, if not all of the chronic diseases of aging have in common is that part of their mechanism involves oxidative stress, and inflammation. While inflammation is never the original cause of disease, it certainly can perpetuate the problem, once it exists. This study blames increased RDW on both inflammation and oxidative stress.

My suspicion, therefore, is that RDW is not a risk factor in an of itself, but a sign of another process that needs to be found and addressed (much like inflammatory markers such as CRP, hsCRP, and ESR).

Why RDW might be High

Because RDW indicates that the body is struggling to make adequate RBCs to fulfill the body’s needs, the first place to look, if there are no other obvious signs or symptoms, is to make sure there are adequate building blocks. Even if the RBC count is normal, check iron saturation, as deficiency can eventually lead to microcytic anemia (ideal is above 22%).

Folate and Vitamin B12 deficiency can lead to macrocytic anemia; check these too (and make sure you don’t take either of them in supplement form for a few days prior to the labs, as otherwise the levels will be artificially inflated, and you won’t get a real read on your levels). It’s possible that, even if you’re taking these as supplements, you might be taking the wrong form for you, and thus be unable to utilize them.

If you’ve experienced blood loss for any reason—trauma, heavy menses, blood donation, or an occult bleed, this can affect the RDW.

Even aside from its liver effects, alcoholism can cause RDW due to direct toxicity on red blood cells.

The biggest issue is likely inflammation in general though… and again, you have to find and address the source. RDW increases with various inflammatory diseases as they progress, including rheumatoid arthritis, lupus, psoriasis, Sjogren’s, sclerosis, and ankylosing spondylitis, as well as many, many other chronic diseases associated with uncontrolled inflammation.

Insomnia and sleep disorders increase RDW, too, probably because there’s an inverse relationship between hours of sleep and cortisol levels. Lack of sleep also means your body and your brain can’t repair damage incurred during waking hours very efficiently, which also increases inflammation over time.

Insomnia also leads to a higher BMI—and waist-hip ratio is positively correlated with higher RDW, too. Obesity itself, and particularly central adiposity, contributes in higher levels of systemic inflammation.

This study shows that when selenium is low, RDW tends to increase, too. My suspicion is that this is true of all of the constellation of antioxidants, though they may not all have been studied yet for that correlation: probably low levels of any one of them means the system of antioxidant regeneration won’t work as well, which means increased oxidative stress, which means increased inflammation, which presumably would mean increased RDW.

What To Do If You Don’t Know the Cause

What if you do your detective work, but you can’t find the reason for high RDW? What do you do then?

Many of the best known interventions for increased longevity are apparently beneficial for lowering RDW too. These include: