Food is needed for survival as it provides the chemical energy required for movement and metabolism. However, some foods are sought particularly for their pleasurable sensation and highly palatable tastes. Researchers split the two contrasting regulatory mechanism of feeding into the homeostatic and hedonic pathways because they appear to be distinct in their physiological mechanisms and purpose. While the homeostatic pathway controls the motivation to increase food consumption after energy store depletion, the hedonic pathway control the desire to eat highly palatable food during times of energy abundance. The hedonic pathway is a reward based pathway that is able to over-ride the homeostatic pathways, and is thought to function in a similar way to the reward systems activated by the ingestion of addictive drugs. The balance of these two pathways and the abundance of food has a strong influence on the energy balance of an individual.
The homeostatic pathway of food regulation is concerned with the maintenance of energy balance and is controlled primarily by leptin and ghrelin, hormones that stimulate satiety or hunger signals respectively, based on the energy reserves of the body. Leptin is synthesised by white apipocytes and plasma levels rise as the size of the energy stores increase. Leptin stimulates receptors in the hypothalamus and causes the release of pro‑opiomelanocortin and adrenocorticotropin hormone which suppresses the feeding urge. At the same time leptin inhibits neuropeptide Y (NPY) and agouti-related peptide (AgRP), which act to stimulate appetite. Grehlin is a peptide hormone synthesised in the stomach and is released in response to a negative energy balance. Ghrelin increases food intake in an effort to replenish energy reserves by stimulating receptors in the hypothalamus and causing the release of NPY and AgRP.
In contrast, highly palatable foods are thought to stimulate responses within the dopamine pathway of the brain in a similar fashion to addictive drugs. Ingestion of these foods causes a release of dopamine into the nucleus accumbens which may act to condition and arouse the individual. Much of the evidence regarding the affects of food on the dopamine reward system has been performed in animals and so care should be taken when interpreting the data. Prolonged stimulation of the limbic reward system with drugs leads to cellular changes in order to maintain homeostasis. For example, the transcription factors cyclic AMP response element binding protein (CREB) and deltaFosB may alter neuronal responsiveness to dopamine and may increase the desire to seek out rewarding chemicals. Similar changes have been describes in rodents exposed to highly palatable foods, which supports a role for the limbic system in food regulation.
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