Many people rely on caffeine to get them up in the morning and sustain them throughout the day. However, high caffeine intake can increase the impact that physical and mental stress has on your body. Chronic stress can cause hormonal imbalances and other disruptions in your body that lead to an impaired immune system, raising the risk of developing infections and diseases such as cancer.
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Caffeine in Your Body
Caffeine increases activity in your brain by inhibiting the neurotransmitter adenosine, a neurotransmitter that lowers levels of other neurotransmitters, keeping them in check. Caffeine also stimulates the sympathetic nervous system that is responsible for your fight-or-flight response. During the fight-or-flight response, stress hormones such as cortisol cause your body's stores of fat and sugar to be released in preparation for physical exertion.
Stress and the Immune System
Though this stress response is important for survival, when it persists it can cause health problems, including high blood sugar, elevated cholesterol and triglycerides and suppression of the immune system. Your immune system is responsible for recognizing and destroying pathogens such as bacteria, fungi and viruses as well as removing damaged cells, both in the case of injuries and cells with abnormal behavior as well as those that produce cancer.
A 2006 study published in "Pharmacology, Biochemistry, and Behavior" found that men and women given large amounts of caffeine -- three 250-milligram doses -- experienced the release of higher levels of the stress hormone cortisol in response to physical and mental stress. Elevated cortisol levels are known to decrease the ability of your immune system to fight infections.
It is unlikely that caffeine itself will detectably decrease the function of your immune system. According to the Huffington Post, low to moderate intake of caffeine has few known health effects. Limit your intake to 200 milligrams per day, or about 2 cups of coffee. If you exceed 500 milligrams per day, you may experience the effects of an activated sympathetic nervous system including insomnia, nervousness, irritability, digestive problems, fast heartbeat and muscle tremors.