Ideal food should be delicious and nutritious. But sometimes, the food you eat is delicious but not nutritious, or nutritious but not delicious. To let animals know which to be delicious or nutritious is an important or valuable question. In a recent article published in Nature Neuroscience1, Friedman JM et al addressed this question and showed that the level of hormone leptin and dopamine system co-regulated the reward value of food in mice.

Leptin, a 16 kDa protein encoded by the Ob (Obese) gene, is a hormone that has a central role in regulation of fat metabolism, appetite and the balance between energy intake and consumption2. Leptin was identified and its role in fat metabolism was firstly revealed by Friedman JM et al in 1994. In mouse genetic screen study, they found that mutation in the genes encoding either leptin itself or leptin's receptor caused massively obese in mice, and this could be rescued by injections of leptin. These findings in mice coincided with the observations that humans carrying homozygous mutations in leptin gene suffered severe obesity. These results indicated that leptin signals are important in regulating fat metabolism. Later studies revealed that instead of a signal to consume the surfeit of fat, leptin is a signal informing the brain of enough nutrients in the body. Thus, when an animal is full up, its leptin level goes high that in turn sends a satiety signal to the brain and stop the animal from eating more. On the contrary, when an animal is in hunger, its leptin level drops, thus sends less satiety signal to the brain; and this drive the animal to looking for nutritious food.

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