Another group of compounds present in chocolate are the alkaloid methylxanthines, most prominently caffeine (1,3,7-trimethylxanthine) and theobromine (3,7-dimethylxanthine), both of which are stimulants and, in the case of caffeine, often cause noticeable behavioral effects. Although the methylxanthines are bases, they have a very low pKa (0.5); hence, they are highly lipid soluble and are absorbed from the stomach as well as through the walls of the intestines, easily crossing both the blood-brain and placental barriers. Nehlig et al have suggested that, in the brain, the methylxanthines compete with adenosine, a presynaptic inhibitory neuromodulator, and block its receptor, thereby eliminating its inhibitory action and causing arousal
