Letter to the Editor, On the Increased Haemoglobin Concentration and Improved Oxygen Uptake After Spirulina Supplementation
Authors: Harald Engan, Alexander Patrician, Erika Schagatay
DOI / Source: https://doi.org/10.1007/s00421-021-04649-w
Date: 19 February 2021
Reading level: Beginner
Why This Matters for Freedivers
Freedivers are always looking for “safe performance edges,” and supplements are a common rabbit hole. This letter highlights a simple possibility: if a supplement boosts haemoglobin quickly (within days), it might be doing it by making the spleen contract and release extra red blood cells—not by “building new blood.” That’s important because it changes how you interpret short-term results, what to measure, and what claims to be skeptical about.
Synopsis
This is a short Letter to the Editor responding to a study that reported better oxygen uptake during arm cycling after just 7 days of Spirulina supplementation, along with an increase in haemoglobin concentration. The authors agree the results are interesting—but they challenge the most common explanation people jump to: “Spirulina is iron-rich, so it must have increased haemoglobin production.”
Their main point is about time scales. Making brand-new red blood cells is not an overnight process in healthy people with normal iron status. So if haemoglobin rises noticeably after only a week, another mechanism might be more likely.
They propose a very relevant alternative: splenic contraction. The human spleen can act like a small blood reservoir, holding concentrated red blood cells and releasing them into circulation during physiological stress (like exercise or breath-holding). When the spleen contracts, haemoglobin concentration can rise quickly—because you’ve suddenly got more red blood cells circulating, not because you’ve manufactured them.
The authors connect this to dietary nitrate (NO₃⁻) and nitric oxide (NO) biology. Spirulina is grown using nitrate as a nutrient, and the authors suggest Spirulina could contain meaningful nitrate. Nitrate can be converted in the body (via the nitrate–nitrite–NO pathway) into nitric oxide, which is linked to improved exercise efficiency. They also point out their own previous finding that dietary nitrate intake can reduce spleen volume and slightly raise haemoglobin at rest—suggesting nitrate/NO-related pathways might help explain both the performance effect and the haemoglobin increase seen in the Spirulina study.
They finish with practical “good science” criticism: short supplementation studies should control for other nitrate- and iron-containing foods, and they should measure iron status markers (like serum iron and ferritin). Otherwise, it’s hard to know whether the observed haemoglobin change is true blood-building, a spleen effect, fluid shifts, or a mix.
Abstract
This Letter to the Editor discusses a short-term Spirulina supplementation study that reported increased haemoglobin concentration and improved oxygen uptake. The authors propose that a rapid haemoglobin increase over only several days is more plausibly explained by splenic contraction and release of stored red blood cells than by new haemoglobin synthesis from iron. They suggest Spirulina’s nitrate content could increase nitric oxide availability, potentially driving both ergogenic effects and spleen-mediated haemoglobin elevation, and they call for better dietary control and relevant blood measurements in future studies.