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Physical side of eating and hunger: neurotransmitters PHYSICAL SIDE OF EATING AND HUNGER: NEUROTRANSMITTERS
Not so long ago, biologists conceived of the brain as a kind of computer, a machine that generates and processes electrical signals. New discoveries forced us to revise that model. We now think of the brain more in terms of a chemical factory. Chemical "messengers" travel across the gap between nerve endings, and when they reach the proper receptors they trigger electrical signals.
Without the presence of a neurotransmitter, an electrical impulse comes to a biological dead end-its message can't get through. Too much or too little of a given chemical can also affect the rate and the clarity with which the signal is carried. That's why we speak of chemical imbalance as the cause of a number of mental and physical disorders.
Sitting within the skull, the brain is enmeshed in an intricate network of blood vessels. Before blood can reach the brain, it must pass through a kind of filter known as the blood-brain barrier. This barrier acts, quite literally, as a bodyguard, preventing potentially harmful molecules carried in the blood from gaining access to the brain. For example, the blood-brain barrier will not permit certain drugs to enter the brain and wreak biological havoc.
Blood contains the red cells, which transport vital supplies of oxygen to nourish the brain cells and keep them functioning, and the white cells of the immune system, responsible for protecting the body from invaders. These cells are suspended in plasma, the fluid portion of blood made up of water, minerals, glucose, fats, proteins, and other substances.
The exact composition of plasma is determined in part by the types and amounts of food you eat. Following a meal, the chemical makeup of plasma changes. For example, if you've eaten a large steak, the presence of certain amino acids-the building blocks of protein-may increase. When blood containing this particular plasma mix reaches the blood-brain barrier, the different amino acids compete with each other to squeeze through the openings in the filter. Like shoppers clawing their way to reach a K-mart blue light special, only a few can get through.
Different chemical mixtures trigger the manufacture of different neurotransmitters. Let me give you an example. (Bear in mind, however, that not all the facts are in on this intricate process.)
Eating carbohydrates stimulates the pancreas to release insulin, in turn lowering the blood levels of most amino acids. The amino acid tryptophan is unaffected by this process. More tryptophan enters the brain, since there is less competition at the blood-brain barrier from other amino acids. In the next step of the process, tryptophan is converted to a powerful neurotransmitter called serotonin. The higher the tryptophan level, the more serotonin the body can make.
Serotonin circulates in the blood, eventually reaching a certain part of the hypothalamus. When the serotonin level is high enough, it triggers a message that speeds to the other parts of the brain. The message reads something like: "Stop eating carbohydrates now and look for food with other nutrients." Thus it may be that the specific foods you have already eaten may lead to cravings for other types of foods.
Experiments have shown just how powerful these neurotransmitters can be. For example, animals who normally eat a balanced diet will turn into voracious carbohydrate-cravers, even to the point of endangering their health, if their brains are flooded with certain chemicals that stimulate carbohydrate ingestion.
There are actually three main groups of neurotransmitters involved in the regulation of appetite: the monoamines (of which serotonin is one), the amino acids, and the neuropeptides.
At this point let me introduce the term macronutrients, a word that refers to the major components in food. The three types of macronutrients are carbohydrates, proteins, and fats. Each of these supplies a different type of energy to the body. A balanced diet contains a healthy mix of all three macronutrients. As a rule, animals and most humans will seek out foods that, over the course of time (say, a day) will supply the macronutrient blend their bodies need.
Macronutrients affect the production and release of neurotransmitters. As we saw in the above example, carbohydrate consumption may lead to increased serotonin levels. Similarly, sweet or fatty foods lead to production of certain neuropeptides.
Like a biological traffic cop, the hypothalamus directs all this activity, and it's a particularly busy intersection. Signals coming from the brain affect the diet, while diet in turn affects the signals heading for the brain. Should something go wrong, a neurotransmitter disturbance might trigger an abnormal pattern of eating. This in turn might worsen the already existing neurotransmitter disturbance, causing more abnormal eating, and so on.
Such findings underscore how chemical messengers spark specific behavior. Controlling the balance of these chemicals through medications might ultimately enable us to bring abnormal eating patterns under control.
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