Yeast extract contains glutamates, the same amino acids that are extracted and used on their own and labeled as MSG. Although they are of lower concentration in yeast extract than MSG, glutamates are an excitotoxin or neurotransmitter known to kill brain cells. They have been known to cause headaches in certain people, as well as other averse symptoms like muscle tightness, numbness, tingling, weakness and flushing. Glutamates may also induce asthma attacks in those who have asthma.