We are obligate aerobic creatures requiring constant, good oxygenation. Allergic individuals sometimes have less than ideal oxygen delivery because of nasal obstruction or asthma.
If, in addition, they are anemic, then oxygen delivery to the tissues is reduced for a second reason.
Iron deficiency anemia is fairly common, especially in growing children and menstruating females. The usual therapy for this is oral iron supplementation. Until recently, this was recommended on a daily basis. However, recent research published in the Journal Blood is altering this advice.
It seems that within a few days of starting oral iron therapy our bodies make a peptide called hepcidin which inhibits intestinal iron absorption. And so the very act of taking iron can put on the brakes, so to speak, of absorbing it. Trying twice or thrice a day dosing only increases the hepcidin levels more.
Luckily the researchers found a simple solution: take the iron every other day. This keeps hepcidin levels low and iron absorption high.