According to a 2003 study published in the journal "Diabetes & Metabolism," omega-3 fatty acids can prevent the release of cortisol during times of anxiety. Naturopaths frequently recommend omega-3 supplementation for people experiencing chronic emotional stress.
Psychology Today's website reports that vitamin C reduces cortisol response caused by mental stress. Vitamin C is also a potent anti-oxidant and is associated with very few side effects or interactions.
All forms of the tea plant--including black, green and white teas--contain the amino acid L-theanine. After a body experiences a period of stress, L-theanine enhances the body's ability to return to normal cortisol levels.
Magnesium is very useful for reducing cortisol levels associated with intense exercise and physical stress. One 1984 study found that magnesium dramatically reduces cortisol levels in athletes with no significant side effects.
Adaptogenic herbs such as ginseng, rhodiola, eleuthero and ashwagandha may work by blunting stress-related cortisol release. Sedative herbs such as valerian, lemon balm and chamomile can mitigate anxiety response, and anti-oxidant herbs such as turmeric may prevent stress-related cellular damage.