The Science of Satiety
Contributed by Damian Rodriguez, DHSc, MS
At its core, weight management is a basic math equation: calories in – calories out = weight gained or lost. To lose weight and maintain low levels of body fat requires a tightrope walk along that energy deficit line. Although there are a multitude of contributing factors, ultimately the primary determinant in weight management success is managing hunger. Understanding satiety may be the primary barrier between you and your body composition goals.
Satiety, defined as the feeling of fullness after eating food, is a complex system involving several different internal processes. When you consume food, a number of internal mechanisms are initiated. As food makes its journey from your line of sight and smell, to your mouth, through the pharynx, to the esophagus, and then to the stomach, a number of different receptors induce responses and regulate communication between your gut and your brain regarding the current level of satiety.
The mechanical factors controlling satiety revolve primarily around the physical volume of food and sensory cues. The perception of fullness is largely determined by the distension of the proximal stomach. Located in parallel series on the wall of the gut reside gastric mechanoreceptors that measure elongation of the stomach. As the stomach fills, these receptors can induce hormonal secretions, modulate the rate of fat metabolism, regulate gastric emptying, and influence gut-brain communication in order to mechanically influence satiety. Eating foods that are both voluminous and low in energy density (specifically whole unrefined foods, especially those high in fiber) fills the stomach, inciting the gastric mechanoreceptors to signal that you are physically full. As common sense would suggest, the satiety response from 100 calories of kale is significantly greater than that of 1,000 calories of sugary cereal, simply because it takes up more physical space.
Sensory cues also play a role. Studies into nasogastric feeding (bypassing the mouth by inserting a tube directly into the stomach) have exposed how these cues influence feelings of fullness. When smell and taste are bypassed, as occurs in nasogastric feeding, satiety and subsequent energy intake are progressively less affected as the volume of liquid food increases (1). This is due to the effects of the cephalic phase, the stage of digestion before food enters the stomach, when sight, smell, thought, and taste communicate with the brain’s appetite centers. The longer food is exposed to these senses before entering the stomach, the greater the feeling of fullness. This is why smelling food before eating, using smaller plates, and chewing for a long time are suggested to decrease caloric intake; you are tricking your brain into thinking that more food has entered the gastrointestinal tract than the mechanoreceptors have actually measured.
Like most physiological processes, satiety involves a balancing act between opposing hormones. Satiety involves the interaction of a huge number of hormones and neurotransmitters, but it is ghrelin and leptin that are primarily responsible. Also known as the “hunger hormone,” ghrelin is a peptide hormone produced in the gastrointestinal tract that is secreted when the stomach is empty. Responding to metabolic triggers, ghrelin secretion increases when fat stores decrease so you don’t starve to death. Its counterpart leptin is often referred to as the “satiety hormone” because it regulates energy balance by obstructing hunger, mostly through neuropeptide secretion to the brain. Together, the feedback loop of ghrelin and leptin work to maintain specific levels of energy stores, in the form of adipose. In a normal state, low levels of body fat induce ghrelin secretion and suppress leptin. As your body fat levels decrease, hormonal changes make maintaining that necessary energy deficit a progressively steeper uphill climb.
While modern science has yet to discover a way to regulate the satiety hormone feedback loop, there are simple ways to feel fuller longer while on caloric-restricted diets.
1) Consume a diet based on consumption of whole foods. The foundation of almost any healthy goal begins with proper nutrition. The more a food is externally processed, the less it needs to be internally processed to be utilized by the body. Whole foods are more nutrient dense, less energy dense, contain higher amounts of fiber, and take longer to be processed. Whole foods also contain high concentrations of beneficial compounds, such as Appethyl®*, a spinach extract shown in clinical trials to reduce hunger cravings (2). Skip the bag and box aisles and stick to the perimeter of the grocery store.
2) Don’t forget your protein and fats. Protein and fat require more time to breakdown into their usable parts than do carbohydrates, increasing that cephalic phase of digestion, but their ability to increase satiation is more complex than that. A recent study in Cell Press mapped out the signals between your gut and your brain, finding that protein increases satiety through modulating mu-opioid receptors. These receptors regulate brain-gut communication. When combined with primarily carbohydrate meals, dietary fat has been shown to slow digestion in the small intestine (4).
3) Drink more water. Your gastric mechanoreceptors can’t differentiate between food and water, they only respond to the volume of the substance passing through. Water quickly passes through without need of processing, but still can temporarily trick your body into thinking it is full. If you have hunger pangs but have already met your energy needs for the day, try a glass of water first.
4) Utilize sensory cues. Smell your food before putting it in your mouth, use a smaller plate, chew your food for a while before swallowing to maximize the oral sensation—all of these small things prolong the cephalic stage and subconsciously increase food satisfaction.
5) Experiment with meal timing and frequency. Contrary to conventional belief, studies have demonstrated that skipping breakfast has no negative effects on satiety, and may result in a reduction in caloric intake (5). Intermittent fasting—cycling between extended periods of fasting and shorter periods of feeding—has been gaining popularity and there is plenty of scientific research to back up its efficacy (6). Experimenting with meal timing and frequency can help you find the meal regimen that works best for you.
*Appethyl is a registered trademark of Greenleaf Medical AB.