Satiety: Feeling Full
Sarah Thomas and Tim Hutton, Contributing Editors, Campden BRI
The number of the world population who are overweight or obese continues to increase year over year. In the U.S., only one state (Colorado) had a prevalence of obesity less than 20%; in 30 states, it was over 25%, and in three (Alabama, Mississippi and Tennessee), it was 30% or more (www.cdc.gov). One in five Europeans is predicted to be obese by 2010. Increasing obesity has brought with it an elevated risk of the development of type 2 diabetes, coronary heart disease, osteoarthritis, stroke, and cancers of the breast, prostate and colon. There are many reasons for the increase in obesity, both environmental and physiological. Physiological causes are related to metabolism, hormones and neurological aspects of appetite regulation. The environmental causes include a sedentary lifestyle combined with an abundance of cheap, high-calorie foods. Therefore, the food industry is being put under more pressure to provide foods that effectively can help with weight reduction and management. This overview of satiety is taken from an extensive Campden BRI review on the subject: “Review 60--A Review of Ingredients, Physiological Mechanisms and Measurements Involved in the Enhancement of Satiation” (see end of this article for more information).
In order for a food company to develop effective products, its developers need to understand appetite control and the effect of different food characteristics on appetite. Generally, an individual’s energy balance is tightly regulated over time: energy intake = energy expenditure. Relatively small but long-term excess energy intake (for example, an average of 40Kcal extra per day) can lead to obesity. In order to lose weight, energy intake must either decrease or energy expenditure increase. It may be possible to reduce an individual’s total energy intake by increasing the satiation value of foods, without increasing the energy density. Humans tend to eat in distinct episodes, either meals or snacks, until they are “full.” Satiation is characterized by the sensation of a full stomach, resulting from the consumption of food. This is the process that terminates food consumption and often limits the size of a meal. After eating, an individual will not eat for a period of time; this is defined as satiety. Therefore, satiety regulates meal frequency, which also is influenced by learned habits. The feeling of satiation is not just based on the amount of food eaten, but also on the composition of the food. Some foods are able to fill the gut but not be digested (e.g., fiber), while others will affect the release of gut hormones, influencing satiety and appetite.
How is Satiety Regulated?
Despite large differences in daily food intake, humans maintain a fairly stable body weight, as their bodies are able to match overall energy intake with expenditure over long periods of time; this is known as energy homeostasis. However, there is a slight bias in the system that favors conservation and storage of energy thought to result from the evolutionary advantage this would have provided. There are thought to be two main physiological mechanisms for satiety: one is at the brain level, while the other is at the gastrointestinal tract level. The presence of food in the stomach results in signals being transmitted to the brain via vagal afferent fibers. For example, the end of an eating episode is thought to depend on short-term signals, such as stomach distension and the release of gut hormones, such as cholecystokinin (CCK) and glucagons-like peptide (GLP 1). Sensitivity to these short-term signals is influenced by hormones that operate in the long-term. These include leptin, insulin and ghrelin. For example, when energy intake is restricted for a few days, leptin levels are seen to drop, which is thought to limit the satiating effect of CCK. Humans have a number of these satiety signals, enabling them to eat a variety of both plant and animal nutrient sources and to secrete a range of gut peptide hormones appropriate for the particular food. This system also has the ability to inform the brain precisely what has been consumed, with different signals being secreted for specific fats, carbohydrates and proteins. The brain is, therefore, able to end an eating occasion when ample calories have been consumed, but before the nutrients have entered the bloodstream in significant amounts.
Sensory characteristics also have an effect. As food is eaten, the pleasantness of the sensory characteristics and the desire to eat it declines. The pleasure of the sight and taste of food eaten to a point of fullness has been shown to decrease more than the taste and sight of foods that have not been eaten. This phenomenon is referred to as sensory-specific satiety. Along with other factors, it contributes to the termination of eating.
Satiating Effect of Foods and Macronutrients
It has long been known that different foods have different effects on satiety. In an investigation of the satiety effect of 38 different food products, including a variety of fruits, bakery products, snacks/confectionery, protein-rich foods, carbohydrate-rich foods and breakfast cereals, it was found that boiled potatoes had the highest satiety score, and croissants the lowest. Protein, fiber and water contents were seen to correlate positively with satiety index scores.
* Fiber. Dietary fiber can be described as dietary carbohydrate constituents that are resistant to hydrolysis and vary considerably in physical and chemical nature. The soluble gums and pectin have very different physical and chemical properties than that of the insoluble lignins and celluloses. These differences will not only influence their effect on the gastrointestinal tract, but also the texture, appearance and palatability of any food product into which they are incorporated. The effects these differences have on satiety may help explain why the results of satiety studies with fiber show conflicting results. The energy density of a food product affects energy intake. On average, only 40% of fiber is fermented; by not providing energy, adding fiber to a food product has the effect of diluting the energy content. Therefore, by reducing the energy density of foods by adding fiber, it is possible to reduce energy intake. As well as reducing the energy content of foods, fiber is thought to affect satiety by several other mechanisms:
* Chewing -- Foods high in fiber require greater chewing, in terms of both time and effort. This is thought to increase the flow of saliva and slow down the rate of ingestion, thereby increasing satiety.
* Gastric distension -- The extra bulk of fibrous foods and the water-absorbing properties of soluble fibers are thought to increase stomach distension, which creates post-meal satiation.
* Gastric emptying and nutrient absorption rate -- Increasing the fiber content of a food or a meal generally increases the viscosity. Increased viscosity in the stomach has been shown to slow down gastric emptying and small bowel transit time. It also impedes the mixing of digestive enzymes with foodstuffs in the stomach, which effectively decreases the nutrient absorption rate. Extending the period of time in which fat and carbohydrates are in the stomach and small intestines is thought to increase pre-absorptive satiety mechanisms.
* Decreasing palatability of food -- Bulky foods of low-energy density, containing lots of fiber, are generally less palatable and less appealing than high-energy/high-fat foods. Generally, people reach satiety more quickly with less-palatable foods than highly palatable foods.
* Protein. With the increase in popularity of the Atkins diet, much interest has been shown in the effect of protein on weight loss. Protein is thought to influence weight loss by increasing dietary thermogenesis and increasing satiety, which results in a decreased energy intake. The thermic effect can be defined as the energy required for digestion, absorption and disposal of a consumed nutrient. The typical thermic effect of protein is between 20-30%, whereas it is generally between 5-15% for carbohydrates. This is thought to be due to the fact that the body has no ability to store protein and, thus, it needs to be metabolically processed immediately. In several studies, proteins have been found to be more satiating than either carbohydrates or fats. Protein provides an increased feeling of fullness after consumption, as well as suppresses food intake during a subsequent meal. In a review of 14 studies that compared the satiety effects of a high-protein pre-load to at least one other macronutrient, 11 of the studies showed that a protein pre-load significantly increased the subjective feeling of satiety.
* Fats. Fat has been shown to be the least satiating and most energy dense (9Kcal/g) of the macronutrients; it is sensible to use it sparingly, when developing a satiating product. There is some evidence that the type of fat can affect satiety, due to its mechanism of metabolism; this may influence the choice of fat used. (For more information on fats and satiety, see this month’s cover story on page 58.)
* Carbohydrate. All digestible carbohydrates have the same calorie content of 4Kcal/g; however, how they are metabolized can influence satiety. One of the main mechanisms by which carbohydrates influence satiety is the regulation of blood sugar in the body. The consumption of slowly digestible carbohydrates causes a gentler, more moderate rise of glucose in the blood, as the carbohydrates are absorbed and broken down more slowly. In this instance, the glucose remains in the blood for a longer duration, and the release of insulin is more moderate. There have been many studies looking at the glycemic index and its role in satiety.
As the drive to develop satiety-enhanced food products continues, several food ingredient suppliers have started either exploring the satiating effect of their ingredients or developing ingredients specifically to enhance satiety.
* Novel palm and oat oil emulsion -- Consisting of fine droplets of palm oil (39.5%) and oat oil (2.5%), dispersed in water (38%).
* Hoodia gordonii -- A cactus-type plant that grows primarily in the semi-desert regions of South Africa.
* Caralluma fimbriata -- A tender succulent from the Asclepiadaceae family (the same family as Hoodia).
* Conjugated linoleic acid (CLA) -- A natural fatty acid found most abundantly in beef and dairy fats.
* Pinolenic acid -- A polyunsaturated (omega-6) fatty acid found in pine nut oil.
* Inulin and oligofructoses.
* Yerba mate -- A tea-like beverage consumed widely in South America and also used in commercial herbal preparations.
* Alpha lipoic acid (ALA), also known as thiotic acid -- A naturally occurring compound and an essential co-factor of many enzyme complexes.
* Capsaicin -- A pungent component of red pepper.
* Resistant starch.
* Hydroxycitrate (HCA) -- A derivative of citric acid found largely in Garcinia cambogia, a small, purple tamarind fruit.
Campden BRI Satiety Study
As well as the literature review on which this article is based, Campden BRI has undertaken a number of focus groups, which found that consumers had not heard of the term satiety, although they were aware of the term satiation. They were able to offer suggestions as to its meaning (e.g., “satisfied, feeling of fullness and to satiate”). Fiber was considered to be the most acceptable satiating ingredient, because it was perceived as “more natural and healthy whilst filling you up.” Protein also was deemed as acceptable; however, there was a clear preference for the use of vegetable protein rather than animal, primarily due to health scares. In contrast, fat was perceived as a “dirty” word; it was the least acceptable of the three ingredients investigated.
When participants were asked to describe the perceived characteristics of satiating foods and drinks, four components were elicited: volume of food; convenient format; carbohydrates; and sensory appeal. When asked to identify new satiating products for development, the participants demonstrated a preference for: cereal bars, dairy-based drinks, breakfast cereal or soup. Of these, the first two categories focus on the convenience and carbohydrate components, whilst the latter two concentrate more on volume, carbohydrate and sensory appeal. The next stage of the research is to conduct a feeding trial to measure satiation using fiber-enhanced and control samples. NS
Sarah Thomas, Ph.D., is the principal research officer of the Consumer and Sensory Sciences Department at Campden BRI, while Tim Hutton is the publications manager. Campden BRI is the U.K.’s largest independent, membership-based organization carrying out research and development for the food and beverages industry worldwide.
On the Web: SATIETY
* www.campden.co.uk/services/css.htm -- For more information on Campden BRI’s services
* www.campden.co.uk/publ/pubDetails.asp?pubsID=161 -- For information about Review 60; also, e-mail firstname.lastname@example.org