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The brain is so vital to life that it must be very well-protected and shielded from foreign substances like toxins and pathogens that can cause lethal damage. This protection comes in part from a highly selective membrane referred to as the blood-brain barrier (BBB). The BBB surrounds the brain and is semi-permeable, meaning it allows some materials to cross from the blood into the brain, but prevents others from gaining access.
In most regions of the body, the smallest blood vessels are lined with endothelial cells. There are small spaces between each endothelial cell so that substances can readily move across the barrier. For example, nutrients pass through gut endothelial cells in the process of absorption into the bloodstream.
In the brain, endothelial cells fit very tightly together in order to prevent a variety of substances from passing in to or out of the brain. Water, gases, and some fat soluble substances can pass through this barrier by passive diffusion, which is a process by which substances travel down a concentration gradient. Other substances, such as certain nutrients, are large molecules that cannot pass through the blood brain barrier easily.
These and other molecules require specific transporters. There are transporters for glucose (the preferred fuel source for the brain) and for amino acids that serve as important chemical messengers in the brain. Some amino acids, like glutamate, act directly as neurotransmitters, while others serve as precursors, or “building blocks,” for neurotransmitters that are assembled in the brain. In either case, it is important to closely regulate the concentration of amino acids in the brain in order to balance your brain and maintain an appropriate proportion of chemical signals that affect many, many aspects of behaviour.
There is no “unique” transporter for each individual amino acid. Rather, there are several transporters, each of which works for amino acids that share certain chemical properties, such as size or ionic charge. The names are hardly what you would call “catchy” (system L, EAAT1, ASCT2, etc.) and are sometimes identified differently, so let’s not get caught up in the specifics. Rather, it is helpful to understand conceptually why these shared transporters are so important. The requirement to share transporters makes this is a “competitive” transport system. In other words, the amino acid that is present in the largest amount or concentration in the blood has the best chance of getting carried into the brain.
Take, for example, large neutral amino acids for which there is one shared transporter. The large neutral amino acids include tryptophan, tyrosine, and the three branched-chain amino acids (BCAAs). If blood tryptophan is increased, then a greater amount of this amino acid enters the brain. Tryptophan is converted in the brain into the neurotransmitter serotonin. Serotonin is involved in many aspects of mood and behaviour. It plays a role in appetite and digestion, sleep, memory, and sexual desire and function.
Tyrosine, on the other hand, is a precursor of dopamine, an equally important neurotransmitter that is central to triggering a “reward” response that associates a behaviour like sex or physical activity with pleasure and positive feelings. Dopamine is important in learning, memory, focus, and alertness. Theoretically, if tryptophan is present in a high concentration in the blood, then more tryptophan gets into the brain, yielding more serotonin. Conversely, a high concentration of tyrosine decreases tryptophan uptake and production of dopamine will be favoured. So, we can see how specific concentrations of amino acids can greatly influence chemical signals in the brain.
Animal studies have shown that the production of neurotransmitters in the brain can be manipulated when specific amino acids are given in a large enough concentration to increase brain uptake relative to other amino acids that share the same transporter support. These types of studies can’t readily be done in humans because it is difficult to measure exactly what changes occur inside the human brain. All in all, it seems reasonable that this simplistic approach of using amino acids to control the chemicals in our brains may assist in achieving a desired mood, energy level, or whatever therapeutic effect you desire.
There are many cautions, however, to using single amino acid supplements in pharmacological doses. The body is very clever at adapting to drastic changes in its environment. For example, when anabolic steroids like testosterone are misused for bodybuilding, the testicles stop producing endogenous testosterone because the levels in the body are already too high. In the brain, constant exposure to high levels of a chemical elicits changes to minimize the responsiveness and maintain function within a normal range. This phenomenon explains tolerance to drugs and alcohol.
A reasonable approach to optimizing amino acid availability at the blood brain barrier is to provide a steady, balanced complement of all the amino acids by eating a diet rich in high-quality proteins and by utilizing mixtures of supplemental essential amino acids.
If you do want to try single amino acid therapy, you should take the supplement in the post-absorptive state (several hours after your last meal) to minimize the presence of competing amino acids. Use reasonable recommended doses and follow instructions on duration of usage.
Each individual can vary somewhat in terms of how amino acids affect them and their brain chemistry because there are so many variables involved, not just in the making of brain neurotransmitters but also in terms of how we process and react to them.
Remember, the blood brain barrier exists to protect the brain, and it does a very good job at it. The system of transporting amino acids into the brain is very sophisticated in its ability to regulate the availability of precursors for many very important neurotransmitters. Ensuring a good supply of all the essential nutrients for the brain is an excellent foundation for optimal brain function.
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