Another interesting aspect is the influence of plasma level of melatonin and the preference for a kind of food. An experiment was performed with rats after intraperitoneal administration of four different doses of melatonin. Records of food intake show that 4 h post-injection animals present a short-term increase in total food intake due to an increase in carbohydrate-rich diets. Data reported were consistent for the different melatonin doses employed. However, authors could not find a consistent pattern for protein-rich diets (Angers et al., 2003). The hypothesis to explain obtained results considers the effect of melatonin as circadian marker. If melatonin marks the night period, rodents involved in the study will eat more. However, an opposite effect is expected in humans (eating less as night starts). Indeed, higher melatonin concentration was found in anorexic people (Arendt et al., 1992). The work by Angers and coworkers reveals the need to study the effect of melatonin on food intake as dependent of the dose and time when supplements are taken.