There are hundreds of known species of yeast. One of the best known is Saccharomyces cerevisiae, more commonly known as brewer's yeast. This single cell fungus has been used in brewing and wine making for thousands of years – we know that the ancient Egyptians were using it, for example, from evidence found in tombs.
Today, brewer's yeast is popular as a health supplement. It is very rich in chromium, which plays a crucial role in insulin signals and the balancing of blood sugar. There is evidence that brewer's yeast was taken to help control blood sugar several hundred years ago, and possibly longer, and it is still popular for this purpose today.
Brewer's yeast is also rich in selenium, which is vital for good thyroid function. Selenium also plays a part in a healthy brain, and in boosting fertility. It's a good source of B vitamins too, all of which are necessary for a healthy metabolism.
As a supplement, brewer's yeast is commonly available in tablet or capsule form. It is known to chemically interact with certain anti-depressant medications however, and so if in doubt you should check with your doctor before beginning to take brewer's yeast supplements.