*Geek Box: Satiety
Satiety is a complex phenomenon, and presents a number of challenges for research. Satiety has been defined as “the feeling of fullness that persists after eating, potentially suppressing further energy intake until hunger returns” (10). However, this definition is confined to the post-meal period, when in fact satiation also occurs within a meal and – theoretically at least – should be a factor in bringing the eating episode to an end.
Thus, there are two overlapping processes: ‘intra-meal satiety’, i.e., the effect during consumption which may end the meal, and ‘inter-meal’ or ‘post-ingestive’ satiety, i.e., the effect after a meal on subsequent appetite and energy intake. Both the immediate and post-ingestive processes include gastric and intestinal signalling and physical effects [e.g., food volume and rate of gastric emptying], appetite-stimulating [orexigenic] and appetite suppressing [anorexigenic] pathways in the brain, and motivation-reward brain regions. These are just some of the neurobiological processes which go into the complex regulation of satiety in humans, not to mention the addition of psychosocial factors like mood and emotional state, attention, etc., on top.
One of the most common research designs to assess satiety has been the “preload”, where a fixed level of energy is provided [matched for variables like macronutrients, etc.] before a time interval, after which participants are presented with the test foods and allowed to eat ad libitum [i.e., as much as desired]. Food is then weighed back to determine how much energy was consumed. For example, presenting ultra-processed foods following a preload could determine either: a) the effect of the preload itself in terms of macronutrient composition or sensory properties, and/or; b) the effect of the macronutrient composition and sensory properties of the foods following the preload.
Now, these studies are often done in laboratory conditions with no other sensory inputs, i.e., participants are eating alone, with no distractions, and these conditions do not mimic the myriad psychosocial influences on appetite and energy intake in the real world. Satiety may be measured subjectively, using what are known as ‘visual analog scales’: these are 10cm length horizontal lines which are scored across from 0-10cm, i.e., 0 could be ‘not hungry at all’ while 10 could be ‘extremely hungry’. However, these scales may not always accurately predict energy intake. Objective measures of appetite hormones, for example ghrelin, leptin, or cholecystokinin, may also be taken, and ghrelin in particular correlates strongly with hunger and appetite.
Ultimately, the best measure of energy intake is energy intake itself: ad libitum studies, where participants are free to consume food to their hunger, may be useful for determine effects of different diets on total energy intake. However, these studies face their own methodological challenges in terms of personal preferences and dietary habits. As you can gather, the study of appetite, hunger, and satiety in humans is a very complex topic.