The good news about brewer's yeast is that it's an excellent source of B vitamins with little or no toxicity. While it's not exactly bad news, the effectiveness of brewer's yeast's as a bug repellent is based more on anecdotal than scientific evidence. Since it has other benefits, taking brewer's yeast or adding it to a pet's food for flea control can't hurt, but don't count on it as the only bug repellent to use.
Brewer's yeast gets its common name from its use in beer-making. Saccharomyces cerevisiae is a single-cell fungus, which can also be grown for use in nutritional supplements. Brewer's yeast contains the B-complex vitamins, protein, selenium and chromium. The B-complex vitamins include thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, biotin and folic acid -- specifically vitamins B1, B2, B3, B5, B6, B7 and B9. It does not contain vitamin B12. While brewer's yeast naturally has a bitter taste, debittered varieties are available for nutritional use.
Safety and Precautions
Brewer's yeast is generally very safe, with some individuals reporting mild bouts of flatulence. People allergic to yeast should not take brewer's yeast. Those taking monoamine oxidase inhibitors should avoid brewer's yeast, as it contains tyramine, which when interacting with MAOIs may cause a rapid rise in blood pressure. Used to treat depression, MAOIs include the brand names Nardil, Parnate, Ensam, Eldepryl and Marplan. Patients taking Demerol for pain relief should avoid brewer's yeast for the same reason.
Vitamin B1, contained in brewer's yeast, as well as brewer's yeast itself is reputed to act as a mosquito repellent when taken internally. Over-the-counter nutritional supplements, many also containing garlic, are marketed for this purpose. According to North Carolina State University, some manufacturers of these tablets make claims that taking one tablet will protect a person from mosquitoes for up to 24 hours. However, NCSU states that scientific studies do not corroborate these claims for repelling mosquitoes or any other type of biting insect.
Even if its reputation as a bug repellent doesn't stand up to scientific scrutiny, brewer's yeast has many other benefits. These include aiding skin ailments such as acne, weight loss, fatigue, constipation and nervousness. Due to its chromium content, it may help diabetics manage their symptoms. Often given to pets for flea control, it improves skin and coat quality whether or not it keeps the little pests off Fido.