Effects of Meals

The commonest way in which food can affect behavior is the change in mood and arousal that occurs after eating a meal. This might sound trite, but it is not trivial: this general meal effect is probably the most reliable example of an effect of diet on behavior. Many animals, including

Table 1 Examples of nutritional variables known or suspected to affect behavior, mood, and cognition

Food restriction

Early-life undernutrition

Chronic semi-starvation

Dieting to lose weight

Short-term fasting (e.g., missing a meal)

Meal effects

Pre- to post-meal changes

Meal timing (e.g., morning, afternoon, night)

Meal size

Macronutrient composition (acute and chronic effects) Amino-acids

Neurotransmitter precursors (e.g., tryptophan, tyrosine, phenylalanine) Phenylketonuria Sugars

Sucrose (dietary intake)

Glucose (supplement, tolerance)

Micronutrients

Iodine

Iron

Selenium

B-vitamins: B1, B6, B12, folate Vitamin C Vitamin E

Diabetes

Acute effects of hypoglycemia Chronic effects

Pharmacological

Caffeine Alcohol

Nutraceuticals (e.g., plant compounds)

humans, tend to be aroused, alert, and even irritable when hungry. This encourages their search for food. However, their mental processes become distracted by this task, to the detriment of other behaviors. After eating a satiating meal, we and other animals become calm, lethargic, and may even sleep.

Nevertheless, even this seemingly straightforward phenomenon can be distorted and can vary across individuals and situations. The impact of a food or drink will depend on the person's initial state. For example, thirsty people improved their vigilance when allowed to drink water, whereas when people were asked to drink when not thirsty, their performance deteriorated. Numerous experiments have shown that manipulation of the structure of meals results in variation in postprandial changes in mood and mental function. One obvious facet of meals that has been investigated is what is eaten, i.e., nutrient composition; the other two main aspects of meal structure that have been studied are meal timing and meal size. Of course, the effect of a meal on appetite also represents a behavioral effect, but this aspect is covered elsewhere in this encyclopedia.

Besides any nutritional effects, two other influences on behavior are known to interact with attempts to measure dietary effects on behavior. First, most people are very habitual in their choice of food and in the size and timing of their meals. As a result, they have learned a set of beliefs and expectations about the impact of their habitual dietary regime. Therefore, particularly in short-term tests, these expectations may override or mitigate physiological changes. Dietary experiences that differ from a person's habitual eating pattern could lead their behavior to change through cognitive rather than (or as well as) physiological influences.

Second, there are circadian rhythms and sleep-wake cycles in arousal and performance, which complicate the interpretation of meal effects, as we discuss in the next section.

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