Vitamin supplements that have niacin (nicotinic acid, also known as Vitamin B3) generally have either zinc or iron or both. B3 accelerates the absorbtion of iron and zinc into the blood stream. You do have to read the vitamin supplements but if you see niacin in the ingredients, it generally has iron and zinc added. What I am referring to or meaning to is multi-vitamins with iron. If you take it without Vitamin B3, iron and zinc will not be absorbed into the blood stream and processes out through your kidneys. You will find niacin, iron and zinc in many multi-vitamins. It will be listed in the ingredients if it has it in it. "In the form of niacin" is an incorrect statement, I should of left it to look for niacin in the ingredients and then look for iron because you need both for it to work. The best iron source though is red meat and guess what dietitians are trying to get people to avoid????? Iron supplements are recommended by my doctor when I have chronic SpO2 readings less than 90%.
Even if the supplements do not contain iron (usually in compound form) in either heme (red meat) or non-heme form, taking niacin supplements helps with the absorbtion of iron from supplements and food sources which is better if you eat red meat or other foods naturally high in iron content. That in turn helps with absorption of oxygen into the bloodstream.