Do fruits and vegetables affect satiety?
Satiety is controlled by several factors that begin when food is consumed and continues when it enters the gastrointestinal tract and is digested and absorbed. As food moves down the digestive tract, signals are transmitted to the brain and intestinal hormones are produced, which affect energy balance in various ways, including slowing gastric emptying, acting as neurotransmitters and reducing gastrointestinal secretions. This is how satiety is influenced. The term satiety is often used differently in literature and there are many methods to measure it.
In one of the most common ways of studying satiety, participants assess aspects of appetite sensations, such as feeling full or hungry at intervals, and then, after a predetermined time interval, they receive a test meal at which energy intake is measured. Long-term studies usually provide foods or drinks of a composition known to be consumed ad libitum and use measures for energy intake and/or appetite assessment as indicators of satiety. Saturation tests are often performed with liquids in which differences in macronutrient contents are easier to identify. For foods where the content of fiber, protein, fat and carbohydrates varies greatly, it is difficult to identify differences in content.
Satiety is also influenced by body weight, age, sex, regular diet, exercise and dietary restraint. Adding fiber to food decreases energy density and often palatability, both of which can affect satiety. In the 1950s, the glucostatic theory of appetite regulation was developed by Mayer (48), who hypothesized that blood sugar levels determined appetite, initiating energy intake when low and causing satiety when high. Glucose levels affect satiety and therefore carbohydrate energy intake.
Fiber includes a wide range of compounds, and although fiber generally affects satiety, not all are equally effective in influencing it. In general, whole foods that naturally contain fiber are satiating. Flood-Obbagy and Rolls compared the effect of fruit in different forms on energy intake and satiety at a meal. The results showed that an apple reduced energy consumption at lunch by 15%, compared to the control group (those who did not eat). The evaluations of the full sensation differed significantly, the whole apple consumed being the most satiating, followed by the apple juice.
Many studies have focused on the effects of fruits or vegetables on satiety and on the reaction to glucose or insulin. The authors concluded that insulin and glucose responses depend on both glucose and fruit fiber content.
Willis et al. proposed two types of breakfast, both containing 10 g of dietary fiber and 410 kcal. The difference between them was that one of them was liquid (a fiber-enhanced juice) and the other was solid (oatmeal, blueberries and apples). The gastric emptying time, measured by a Smartpill, was one hour longer for the solid breakfast. Solid mass decreased hunger more than a liquid mass with added fiber.
In another study, the ingredients of a fruit smoothie were shown to participants in a different way, half of them were shown a small portion of fruit and the other half a large portion, but both groups received the same amount of smoothie. Subsequently, the participants assessed the expected satiety of the smoothie and provided assessments of appetite before and 3 hours after consumption. The obtained satiety was significantly higher for the group that was shown the larger portion, even though both treatments were the same.
Many epidemiological studies claim that dietary fiber is linked to a reduced risk for cardiovascular disease (CVD) and probably has a role in preventing obesity. The fiber is mostly concentrated in dried fruits and cooked vegetables because the water is removed and the fiber more concentrated.
The diet plays an important role in achieving satiety. Fiber added to drinks seems to be less effective than whole fruits or vegetables. There are studies that also suggest that whole foods can slow down gastric emptying, compared to liquid fiber foods.
Whenever possible, the whole fruit or vegetable, i.e. the peel, should be consumed as such, to increase fiber intake. Fiber is certainly an active component of fruits and vegetables and a reason to further support their consumption for all its benefits.