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Satiety: how does it work?

How does the body tell us we’ve eaten enough?

What is satiety?

First, a couple of definitions:

Satiation is the state of being sated and represents the cumulative effect of a variety of inhibitory sensory, cognitive, digestive, and hormonal signals that bring an eating occasion to an end; it is said to control meal size.

Satiety is the psycho-biological process that suppresses hunger after an eating occasion and prevents further eating; it is said to control snacking between meals.

Appetite is the desire to eat and encompasses the biological urge to eat as well as the complex interplay between senses, habits, past experiences, future expectations and available food.

As both satiation and satiety are processes that inhibit eating, they are of great interest in weight control. Enhancing satiety could reduce overeating, however it is not that simple. Satiety involves a complex interaction of both physiological signals and psychological, environmental and social influences. The signals that influence the duration and intensity of satiety have been conceptualised as the ‘satiety cascade’ (see figure below). To add further complexity, satiety responsiveness varies between individuals. People with low satiety responsiveness show a faster return of hunger after a meal. 

Satiety cascade

Source: Mandalari 2019

 

Gut signalling

There are several peptides known to influence appetite: ghrelin released from the stomach, also known as the hunger hormone because it stimulates food intake; and the satiety hormones cholecystokinin (CCK), glucagon-like peptide 1 (GLP-1) and peptide YY (PYY) released from the small and large intestines that inhibit food intake.

The Satiety Index

The type of food eaten - including its nutrients and physical form - can influence satiety. Australian researchers were the first to publish a Satiety Index of common foods. The highest Satiety Index (SI) measured was for boiled potato and it was seven times higher than the SI for a croissant. SI scores correlated positively with the weight of foods, protein, fibre and water content whereas fat content was negatively associated with SI scores. The superior satiating effect of potato has also been shown when compared to rice and pasta in a mixed meal.

Carbohydrates and satiety

While protein content of food is a strong determinant of satiation and short-term satiety, what happens afterwards is also important. Carbohydrates may elicit more helpful post meal sensations than protein. One study found a high carbohydrate breakfast elicited a higher hedonic response and higher ratings for satisfaction, food joy and overall wellness and fullness than a high protein breakfast. In addition, the high protein breakfast induced higher desire for sweet foods after the meal and this may have implications for subsequent food and energy intake.

Dietary fibre in food promote satiety and reduced food intake and may explain the association between fibre intake and lower body weight. Fibre is thought to enhance satiety by adding bulk and viscosity to gut contents and altering gut hormones. A good amount and variety of different dietary fibres in carbohydrate-containing plant foods can contribute to an optimal gut microbiome, and microbes can produce metabolites that affect satiety. Microbiota dysbiosis, or an imbalance in gut microbes, can stimulate food intake and contribute to weight gain and development of obesity.

Food structure

Food structure influences how easily nutrients are absorbed and digested and this can in turn influence gut hormone responses. For example, in the case of whole nuts, their fibre content reduces the digestibility and slows the absorption of the fats they contain and this is likely to enhance satiety. Conversely, high levels of food processing can make nutrients more accessible and reduce satiety. Oral processing, or chewing food more also enhances satiety and reduces food intake.

Food texture

A systematic review and meta-analysis on the influence of food texture on satiety found the hierarchy of ability to reduce hunger is:

solids -> higher viscous (thick) food -> liquid -> low viscous (thin) food. However, there was large variation between studies so results should be interpreted cautiously. This result may be due to solid foods triggering the early stages of the satiety cascade through visual and oral cues as the later processes in the cascade are not as affected by food form.

Liquid vs solid?

It is thought that liquids are consumed quickly and spend less time in the mouth, or have shorter oro-sensory exposure time, whereas foods eaten slowly spend more time in the mouth and give a higher satiety response. The reduced ability of liquids to promote satiety has been suggested as one mechanism by which sugar sweetened beverages (SSBs) promote weight gain however research has yielded mixed results. Sugar-sweetened beverages compared to milk show lower subjective fullness and satiety but no difference in subsequent energy intake. One study found no difference in satiation between SSBs and high energy (solid) snack foods.

A full stomach

The idea that food volume influences satiety is supported by the finding that gastric distention activates satiety circuitry in the brain via the vagal nerves. It does this even when the stomach is distended by a balloon and not food.  The rate of gastric emptying is also influential. Faster gastric emptying rate is correlated with higher energy intake.

Aeration (bubbles) in a beverage promotes fullness and lower hunger than non-aerated (still) beverages independent of energy content and is thought to be due to carbonated beverages having greater gastric volume.

The sensory connection

The sense of taste is an important contributor to the satiating effect of foods. Taste is a nutrient sensor for the brain telling it food is on its way. Tasting delicious food in the mouth contributes to satisfaction of appetite, while triggering activation of the gustation (taste) and reward areas of the brain, suggesting sensory stimulation has an important role in satiation. Food delivered to the stomach without oral sensory stimulation (such as during naso-gastric feeding), also activates the brain, albeit different brain areas.

The role of food pleasure

The drive to eat incorporates much more than meeting nutritional needs. Appetite is not just about hunger and satiety but includes other more wholistic, subjective evaluations such as food pleasure and how it makes us feel. The combination of taste and aroma may induce greater satiation and short-term satiety than either taste or aroma alone, suggesting that flavour quality and intensity play a role in subsequent food intake.

How does sweetness influence satiety?

Sweet foods stimulate intake but they also produce satiation as a result of their volume, nutrient and energy content. Sweet taste detection and signalling in the mouth also operate in the gut, however this appears to be based on nutrient sensing in the gut because high intensity sweeteners do not evoke the release of satiety hormones in the same way sugars do. High intensity sweeteners do not activate the brain’s reward pathways as sugar does. Non-nutritive sweeteners may uncouple the relationship between sweet taste and neuroendocrine signalling but to date the findings are unclear whether this influences food intake. In addition, there is individual variation in sweet taste function and research is ongoing to determine whether individual variation influences satiation, satiety and weight.

Sensory specific satiety

Sensory-specific satiety describes how as you eat more of a food, it’s pleasantness declines compared to that of other foods. In other words, you get tired of eating the same thing. In common parlance, ‘the dessert stomach’ describes how even though you’ve had enough dinner, you still have ‘room’ for dessert. The same might be said for the desire for the cheese plate after the dessert course.

Energy density and satiety

High energy density foods tend to be palatable and less satiating, while low energy density foods (that tend to have the most water and least fat) tend to be less palatable and more satiating. An ongoing challenge for the food industry and home-cooks alike is to produce balanced meals that are highly palatable, satisfying and not too high in energy. Research investigating the impact of preloads (specific foods or drinks given prior to offering other foods) on subsequent energy intake have found whole foods (such as fruits and vegetables), soups and salads eaten before a meal can reduce energy intake of the meal probably due to their volume, solid/semi-solid form and water content.

Satiety, interrupted

Intuitive eating is the practice of eating whatever is desired and stopping when satiated. Eating to match the body’s needs is a skill and awareness that is well calibrated in infancy and early childhood but can become blunted or distorted over time and takes practice, perseverance and sometimes health professional support to re-establish. Western food and dieting culture have been blamed for disrupting food intake regulation, as has the prevalence of hyper-palatable, energy-dense highly-processed foods. It has been suggested that humans are poorly adapted to the abundance of energy-dense foods, and highly palatable foods by-pass our drive to eat to fulfil nutritional needs. In a public health context whether to advocate a more liberal, intuitive approach or a more restrictive, risk management approach to eating remains an unresolved issue in the discourse between intuitive eating and anti-obesity advocates; success is likely to depend on the individual. The hedonic drive to eat may be even stronger for people with low-satiety responsiveness. People with a low satiety phenotype experience higher hunger levels, greater liking and wanting of high fat foods, greater tendency to snack and higher energy intake. In clinical practice, an individualised approach can help manage the risk of overconsumption and weight gain in low satiety phenotype individuals.

 

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