The control of energy intake is important because it ensures that energy is available to allow work to be performed. The human body can store energy in a number of compartments, and regulation of these energy stores is pivotal in maintaining health while also providing this labile store of energy for cellular needs. Stored energy is important because humans do not graze continuously, but eat infrequently, relative to the length of the day. Regulating these stores through the control of satiety is therefore highly complex because multiple feedback signals must be analysed concomitantly to allow for an adequate intake of food without causing over nutrition. There is an underlying drive to eat, but this drive can be held in check by a number of impulses generated from both food in the gut and from stored of energy. These feedback impulses that can cause satiety and include signals from the stomach, small intestine, liver, circulating energy pool and from deposits of fat. The brain then processes the signals and regulates satiety accordingly.
The rate of stomach emptying is an important factor in satiety. The rate of stomach emptying is primarily determined by the type of food ingested. Undigested protein passing into the duodenum inhibits gastric emptying as the stomach is the primary location for the digestion of protein due to its high concentration of proteases and its low pH. High protein meals therefore induce satiety and this may explain some of their weight loss effects. The pyloric sphincter of the stomach has been described as a precise valve that empties its contents into the duodenum at a prescribed rate to prevent overnutrition. When liquified to chyme the stomach emptying rate is around 2 to 6 mL per min or roughly equivalent to 0.4 kcal per min. Addition of gums and other viscous fibres to a meal delays gastric emptying somewhat, and this may explain the beneficial glycaemic effects of legumes, a rich source of gum fibre. Of course legumes also have a high protein content and so this may further reduce their gastric emptying rates.
The intestinal phase of digestion involves food passing into the small intestine from the stomach. Here feedback mechanisms exists to limit nutrient overload to the circulation, just as the stomach limits nutrient overload to the duodenum. Distention of the duodenum leads to satiety through activation of stretch receptors in the walls of the intestine. A number of hormones released from the small intestine are also linked to the induction of satiety including cholecystokinin, bombesin and somatostatin. Administration of these hormones to animals results in a reduction in food intake because these hormones may pass to the brain and regulate satiety centrally. Once absorbed much of the energy from the small intestine passes to the liver for processing. The liver is highly sensitive to depletion of glycogen and in response can stimulate an increase in the size of a meal to compensate for low energy reserves. High liver glycogen stores therefore induce satiety and prevent overloading the liver with nutrients.
Satiety can also be induced by both the circulating energy pool and the adipose tissue. Circulating amino acids and glucose stimulate the release of insulin, and insulin can act as a primary satiety signal in the hypothalamus. Adipose tissue can regulate satiety over a longer time period because it causes the release of leptin, a peptide hormone that increases in circulation in relation to the amount of energy stored in adipose tissue. As adipose tissue accumulates, leptin levels rise and this signals the hypothalamus to down regulate energy intake in order to prevent the accumulation of too much stored fat. Insulin resistance is now thought to cause leptin resistance in the hypothalamus, and as a result the hypothalamus becomes insensitive to the signal. The hypothalamus interprets the weak signal as a situation of low levels of bodyfat and this in turn causes a paradox whereby obese individuals enter a physiological state akin to starvation. This prevents weight loss and explains the failure rate of most low calorie diets.
Dr Robert Barrington’s Nutritional Recommendation: The body has multiple mechanisms to regulate food intake. Satiety signals work to prevent overnutrition and allow maintenance of a constant body weight. However, these mechanisms are dependent on high quality nutrition including unprocessed and unrefined foods. Modern Western foods bypass many of these control mechanisms and this results in weight gain through a failure of the feedback systems that normally regulate satiety.
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