**7. Sensing of amino acids in the central nervous system**

Ensuring sufficient consumption of proteins is needed for growth, reproduction, and species survival [24]. Evolutionally animals had mechanisms to keep adequate protein intake. Detection of decrease or increase of single amino acids can profoundly influence feeding behavior and food preference [24–27].

Hundred years ago, the marked reduction in energy intake and growth of animals maintained on diets containing very low protein amounts or imbalanced EAA ratios was first described. Harper and colleagues showed that the anorectic response to imbalanced amino acid diets is the cause rather than the consequence of growth failure. This suggestion supports the idea that dietary amino acids are important in the regulation of food intake [28–31].

Work by two independent groups demonstrated that mice could detect rapidly dietary EAA deficiency within the APC (anterior piriform cortex) occurs via a GCN2 (general control nonderepressible 2)-dependent mechanism; this pathway is also required for rejection of for EAA-imbalanced diets in drosophila [32]. The GCN2 pathway is an evolutionarily conserved pathway identified in yeast to mediate the detection of amino acid deficiency [33, 34].

From these observations, Hooley and Blouet [35] proposed a mechanism by which animals are able to detect lack or excess of protein. In these networks, leucine and tryptophan are considered a pivotal role in the regulation of feeding behaviors.

**Figure 8** illustrates proposed protein sensing sites. APC is proposed to be a site to sense protein deficits and NTS (Nucleus tractus solitarius) is proposed to be a site to sense protein excess.

We here indicate that insulin, thus intake of carbohydrate is essential for the health of brain and peripheral organs. Especially insulin is needed for the transport of tryptophan to the brain. Tryptophan is important for feeding behavior and the production of serotonin, which is needed for emotion. Serotonin is further converted to melatonin in the pineal body. Melatonin is important for sleep. APC and NTS are sites to sense the excess and deficit of proteins. Tryptophan and leucine play important roles there.

In Section 1, we indicate that highly processed foods disturb the gut-brain circuitry so that the nutritional content in the processed foods is not accurately conveyed to the brain. It is possible that the way foods are processed and prepared

**51**

**Figure 9.**

*Gut-brain circuitry of foods contents.*

*Glucose or Sucrose Intakes and Plasma Levels of Essential and Nonessential Amino Acids*

these disturbances promote overeating and metabolic dysfunction.

affects physiology beyond the energy density or palatability of the foods, and that

Small and DiFeliceantonio [4] proposed that information of carbohydrates after oxidation of glucose is conveyed to the midbrain and further to the cortex and that information of fat through PPAR γ is conveyed through vagus nerve and transmitted to the substantia nigra, further to the cortex, where unusual amounts

*DOI: http://dx.doi.org/10.5772/intechopen.92257*

of dopamine are released, leading to overeating.

**Figure 8.** *Protein sensing sites.*

*Glucose or Sucrose Intakes and Plasma Levels of Essential and Nonessential Amino Acids DOI: http://dx.doi.org/10.5772/intechopen.92257*

affects physiology beyond the energy density or palatability of the foods, and that these disturbances promote overeating and metabolic dysfunction.

Small and DiFeliceantonio [4] proposed that information of carbohydrates after oxidation of glucose is conveyed to the midbrain and further to the cortex and that information of fat through PPAR γ is conveyed through vagus nerve and transmitted to the substantia nigra, further to the cortex, where unusual amounts of dopamine are released, leading to overeating.

**Figure 9.** *Gut-brain circuitry of foods contents.*

*New Insights into Metabolic Syndrome*

behaviors.

to sense protein excess.

play important roles there.

ate the detection of amino acid deficiency [33, 34].

Work by two independent groups demonstrated that mice could detect rapidly dietary EAA deficiency within the APC (anterior piriform cortex) occurs via a GCN2 (general control nonderepressible 2)-dependent mechanism; this pathway is also required for rejection of for EAA-imbalanced diets in drosophila [32]. The GCN2 pathway is an evolutionarily conserved pathway identified in yeast to medi-

From these observations, Hooley and Blouet [35] proposed a mechanism by which animals are able to detect lack or excess of protein. In these networks, leucine and tryptophan are considered a pivotal role in the regulation of feeding

**Figure 8** illustrates proposed protein sensing sites. APC is proposed to be a site to sense protein deficits and NTS (Nucleus tractus solitarius) is proposed to be a site

We here indicate that insulin, thus intake of carbohydrate is essential for the health of brain and peripheral organs. Especially insulin is needed for the transport of tryptophan to the brain. Tryptophan is important for feeding behavior and the production of serotonin, which is needed for emotion. Serotonin is further converted to melatonin in the pineal body. Melatonin is important for sleep. APC and NTS are sites to sense the excess and deficit of proteins. Tryptophan and leucine

In Section 1, we indicate that highly processed foods disturb the gut-brain circuitry so that the nutritional content in the processed foods is not accurately conveyed to the brain. It is possible that the way foods are processed and prepared

**50**

**Figure 8.** *Protein sensing sites.*

We propose that information of carbohydrates and fats in the processed foods is sent to the brain through vagus nerve, causing unexpected amounts of dopamine release. This may cause overeating.

**Figure 9** shows our hypothesis. We propose that information of food intake is conveyed mainly through the vagus nerve to the brain. We report this hypothesis elsewhere.
