Neurotransmitters and neuromodulators are basic units of chemical communication within the nervous system. These include a variety of phenethylamines and their derivatives (dopamine, norepinephrine, epinephrine, tyramine, octopamine, and tetrahydroisoquinolines), indoleamines (serotonin, melatonin, and tryptamine), cholinergics (acetylcholine and choline), amino acids and their derivatives (glutamate, aspartate, glycine, taurine, histamine and gamma-amino butyric acid), nucleosides (adenosine and inosine), hormones (prostaglandins, corticosteroids, estrogen, testosterone, thyroid hormone and many others), and peptides (enkephalin, endorphin, substance P, cholecystokinin, somatostatin, cyclo (His-Pro), thyrotropin releasing hormone and many others). Most of these are synthesized de novo using precursors provided by the food that we eat and thus are under direct influence of the diet. As an example, I will discuss the role of dietary protein and carbohydrate in the synthesis of serotonin, a neurotransmitter ubiquitous throughout the nervous system.
A Carbohydrate-Rich/Protein-Poor Diet in the Synthesis of Serotonin
A carbohydrate-rich/protein-poor diet increases and a protein-rich diet decreases brain serotonin synthesis. The following is an explanation of the biochemical basis for the dietary control of serotonin synthesis. The synthesis of serotonin in the brain is limited by the availability of tryptophan. The large neutral amino acids (LNAA), tryptophan, valine, leucine, isoleucine, methionine, phenylalanine, and tyrosine, share the same transport carrier across the blood-brain barrier. At physiologic blood amino acid concentrations, there is competition for available carrier sites. Therefore, the transport of blood tryptophan into the brain is proportional to the ratio of its concentration to that of the sum total of other LNAA. Consumption of a protein-rich meal raises the blood level of many amino acids. Tryptophan is one of the least common amino acids in dietary protein. Therefore, a protein-rich meal contributes proportionately more competing LNAAs than tryptophan, resulting in reduced entry of tryptophan into the brain and reduced serotonin synthesis.
Conversely, a Carbohydrate-Rich in the Synthesis of Serotonin
Conversely, a carbohydrate-rich meal can also alter blood amino acid levels. This effect is mediated by the action of insulin, which promotes the uptake of most amino acids by muscle, which accounts for 45% of lean body weight. Muscle metabolizes branch-chain amino acids, thereby lowering their concentration in the blood. Plasma tryptophan levels are unaltered by carbohydrate consumption, and insulin does not promote the net uptake of tryptophan into muscle because this amino acid is largely bound to low-affinity, high-capacity sites on albumin. Normally, 75 to 85% of plasma tryptophan is bound to albumin. When insulin is secreted, the plasma levels of nonesterified fatty acids (NEFAs) fall because insulin promotes uptake of NEFAs by adipocytes. NEFAs also are adsorbed on circulating albumin, thereby increasing the number of sites available on albumin for binding tryptophan. This rise in bound tryptophan compensates for the slight fall in plasma free tryptophan caused by insulin-mediated uptake of amino acids by muscle. The total amount of tryptophan in plasma (bound plus free) determines the rate of transport of tryptophan into the brain because the affinity of the transport system for tryptophan is much greater than the affinity of albumin for tryptophan. Thus, ingestion of a high-carbohydrate/low-protein meal facilitates entry of tryptophan into the brain.
Rounding Things Up for Some Thoughts
Once in the brain, tryptophan undergoes a series of enzymatic reactions, resulting in the synthesis of the neurotransmitter serotonin. The first (and the rate-limiting) step in the conversion of tryptophan to serotonin is hydroxylation of tryptophan by the enzyme tryptophan hydroxylase, a low-affinity (Km = 0.4 mM) enzyme that is abundant in the brain. A great way to consume tryptophan is by Raw chocolate. Raw chocolate also contains tryptophan-an essential amino acid-which helps naturally produce serotonin in your body. This is great news for those who need an emotional boost. Accordingly, whenever the brain tryptophan level rises, more serotonin is produced. Rats fed or injected with tryptophan have higher brain serotonin levels than do controls. Conversely, rats fed a corn diet (tryptophan-poor) have low levels of brain serotonin.
By TTS Cofounder Botanical Chef Omid Jaffari
|
Prev
