It's easy to attribute feelings such as elation or depression to external factors, while forgetting that certain foods raise the brain chemicals which influence these emotional states. Brain chemicals affect how we think, feel and behave. Examples of these chemical messengers, or neurotransmitters, linked to diet are serotonin, dopamine and noradrenaline. Serotonin is associated with reducing appetite, curbing impulses, enhancing mood and promoting sleep. Low levels of the chemical can be responsible for feelings of depression, which is why many antidepressant drugs work by increasing serotonin levels. Serotonin in the brain is made from tryptophan, an amino acid found mainly in protein. Eating tryptophan-rich foods such as chicken, which has 360mg per 100-gram portion, is the first step in boosting serotonin levels. Other examples are turkey, 340mg per 100 grams; salmon, 260mg per 85 grams; rolled oats, 175mg in 85 grams; lentils, 160mg in 200 grams; and walnuts, 50mg per 25 grams. Step two involves eating carbohydrate. This increases the absorption of tryptophan by the brain. Some foods such as avocados, bananas, pineapples, plantains, plums, tomatoes, and octopus contain significant amounts of ready-made serotonin. As Amanda Geary explains in her book, The Food And Mood Handbook, because of the serotonin-enhancing effect of many carbohydrate foods, a sudden urge to eat sugary or starchy snacks can be an unconscious attempt by the body to correct low serotonin levels. To get the full benefit of the tryptophan effect, a protein meal should be followed by a high-carbohydrate food. But to avoid a roller-coaster sugar high followed by a low, you need to eat slowly absorbed, unrefined carbohydrates, such as pasta and fruit. Because carbohydrate-rich food boosts sleep-promoting serotonin, avoid rice, potatoes or pasta at lunch, unless you want to fight drowsiness all afternoon. In contrast, the brain chemicals that help us stay alert are dopamine and noradrenaline. Protein-rich foods such as meat, fish, eggs, nuts, beans and tofu will boost the levels of these neurotransmitters. Anyone who needs to stay alert after lunch should base their midday eating on protein. It follows that carbohydrate such as pasta are best eaten for evening meals as the body winds down towards sleep. Most antidepressants work by increasing the abnormally low levels of neurotransmitters common with depression. One popular drug group is selective serotonin re-uptake inhibitors. These include brands such as Prozac (fluoxetine), which stops the re-uptake of used neurotransmitters. Increased amounts of brain chemicals build up in the gaps between cells as a result, Geary explains. Levels of the body's natural opiates, the mood-altering endorphins, can also be increased by certain foods. Endorphins' effects include feelings of euphoria, high self-esteem and reduced impact of physical and emotional pain. They create the 'runner's high' that exercise produces and can result from sex, yogic breathing and meditation. Interestingly, vomiting seems to cause a stress-relieving endorphin 'high'. This is an important consideration when treating the eating disorder, bulimia nervosa. The downside of raising endorphin levels is that the good feelings often lead to an addiction to the food or activity. One example is the phenylethylamine (PEA) in chocolate. PEA is also found in red wine and some cheese, according to Geary. Our affection for chocolate may be due to the effect PEA has on the brain where not only does it enhance endorphin levels but is said to increase libido and act as a natural antidepressant. Sugar can also increase the body's natural endorphins and chocolate bars contain lots of it. No wonder so many of us are chocaholics. The writer is a specialist counsellor for eating disorders and obesity. Next week: Are you addicted to food or drink?