Caffeine

AHA Recommendation
Caffeine has many metabolic effects. For example,

It stimulates the central nervous system.
It releases free fatty acids from adipose (fatty) tissue.
It affects the kidneys, increasing urination, which can lead to dehydration.

Caffeine is in coffee, tea, soft drinks, chocolate and some nuts. Whether high caffeine intake increases the risk of coronary heart disease is still under study.
Many studies have been done to see if there's a direct link between caffeine, coffee drinking and coronary heart disease. The results are conflicting. This may be due to the way the studies were done and confounding dietary factors. However, moderate coffee drinking (1–2 cups per day) doesn't seem to be harmful.
Caffeine-habituated individuals can experience "caffeine withdrawal" 12–24 hours after the last dose of caffeine. It resolves within 24–48 hours. The most prominent symptom is headache. They can also feel anxiety, fatigue, drowsiness and depression.