Conditioning and Meals|
by John Pinel, University of British Columbia
© 1999 Peregrine Publishers, Inc., All Rights Reserved
Sometimes a new theoretical perspective is like turning on the lights: it can cast new light on important phenomena, making them clearly visible for the first time. Stephen Woods and Douglas Ramsay's (2000) recent theory of eating is such a theory.
Lay people and health professionals alike assume that we are motivated to eat meals by energy deficits and motivated to stop eating meals when bodily energy resources return to homeostatic levels—the levels of energy optimal for current effective functioning. Woods and Ramsay began their analysis by pointing out that this premise, despite its intuitive appeal, is inconsistent with evidence. Indeed, this premise appears to be opposite the real situation.
In almost all cases, as mealtime approaches, we are in reasonable homeostatic balance. For example, blood glucose—the body's main source of directly usable energy—is at homeostatic levels. As we start to eat, however, our circulatory systems are flooded with excess calories. These excess calories throw the internal environment out of balance. For example, blood glucose levels and liver temperature can become dangerously elevated during a large meal, especially if it is eaten rapidly. In essence, a meal disturbs the body's basic homeostasis, rather than restoring it. The body puts up with these short-term adverse consequences of meals in order to gain the long-term benefit of maintaining body stores of energy, largely in the form of body fat, well above those needed for daily functioning.
Understanding that meals are fundamentally homeostasis-disturbing, rather than homeostasis-restoring, provides major insights into many of the physiological changes that occur in the body before meal time, changes such as sudden decreases in blood glucose and liver temperature. Woods and Ramsay argue that these anticipatory physiological changes are the product of Pavlovian conditioning—not a sign that your body is running out of energy. Their theory is that the body minimizes the disruptive effects of meals (the unconditional stimuli) by learning the relationship between these effects and the stimuli that predict them, stimuli such as time of day, the kitchen table, or the smell of food (the conditional stimuli).
Woods and Ramsay have shown that stimuli that regularly precede meals come to elicit changes in the body that are opposite the disruptive effects of the meal itself. As a result, when the meal is consumed, deviations from homeostasis that are produced by the meal are minimized. For example, meal-predictive conditional stimuli trigger the pancreas to release insulin into the blood stream. The insulin reduces levels of glucose in the blood so that the subsequent increase in blood glucose associated with the consumption of a meal is minimized.
What implications does the Woods and Ramsay theory have for healthy eating? Ideally, it is best to avoid meals, particularly large ones, and instead eat several snacks each day because large meals produce much larger disruptions in homeostasis than snacks. In contrast, many people eat three substantial meals a day because of the demands of their culture. For example, your family might serve meals three times a day, and you may have little choice but to comply with this regimen. If you must eat three meals a day, two things are important to minimize the disruptive effects of the meals. First, eat slowly to reduce the level of disruption to the body. Second, try to eat regularly at the same times and in the same place to maximize the beneficial effects of Pavlovian conditioning.