For example, Roberts et al. underfed 17 young and old men by 3.17 MJ per day (~750 kcal per day) for 21 days, during which time both the young and old men lost weight. After the underfeeding period, the men were allowed to again eat ad libitum. The young men ate more than at baseline (pre‐underfeeding) and quickly returned to normal weight, whereas the old men did not compensate, returned only to their baseline intake, and did not regain the weight they had lost. Older adults also have a reduced ability to detect and respond to dehydration. Consequently, after an anorectic insult (for example, major surgery), older adults are likely to take longer than young adults to regain the weight lost, remain undernourished longer, and be more susceptible to subsequent superimposed illnesses, such as infections.29
The ageing gut
Ageing is associated with cell loss in the myenteric plexus of the human oesophagus and a decline in conduction velocity within visceral neurons. The consequent reduction in sensory perception may contribute to reduced food intake by inhibiting the positive stimuli for feeding. Older adults frequently complain of increased fullness and early satiation during a meal. This may also be related to changes in gastrointestinal sensory function; ageing is associated with reduced sensitivity to gastrointestinal tract distension. Reduced sensitivity to the satiating effects of distension might be expected to increase, not decrease, the food intake in older people. However, proximal gastric distension has been found to have similar effects on food intake in healthy older and young adults, and the role of impairment of gastric sensory function in causing anorexia of ageing is unknown. Ageing is probably associated with impaired receptive relaxation of the gastric fundus. As a result, for any given gastric volume, there is more rapid antral filling and distension and earlier satiety. This impaired gastric accommodation response in the elderly may be because of altered fundic nitric oxide (NO) concentrations. Peripheral NO causes receptive and adaptive relaxation of the stomach, leading to dilation of the fundus and, ultimately, slower gastric emptying. The increase in NO with ageing may therefore contribute to the slower gastric emptying observed in the elderly.
Most, but not all, studies indicate that gastric emptying slows slightly but significantly with increasing age. Clarkston et al. found that healthy older subjects were less hungry and more satiated after a meal than young subjects and that postprandial hunger was inversely related to the rate of gastric emptying. The effects of ageing on gastric emptying rate may require ingestion of a relatively large energy content, as small meals have not been shown to have different emptying rates in old compared with young individuals. Delayed gastric emptying in older people may result, in part, from the enhanced release of small intestinal hormones such as CCK (see the following section). In contrast, it seems that age has little, if any, effect on small intestinal or colonic motor function, and orocaecal and whole‐gut transit time are not affected in the healthy elderly. Healthy older people do have slower Phase III migration velocities and more frequent ‘propagated contractions’ in the small intestine, but no differences in duration of postprandial motility or amplitude or frequency of either fasting or postprandial pressure waves.24
Declining senses
Taste and smell contribute to a pleasurable eating experience. Although there are conflicting studies, the sense of taste likely deteriorates with age, which may influence food choice. There is also strong evidence that the sense of smell declines with age, particularly after age 50. This is thought to be due to a reduction in mucus secretion, thinning of the epithelium, and a decline in the regeneration of olfactory receptor cells.30 In one study, over 60% of subjects age 65–80 and over 80% age 80 or more exhibited major reductions in their sense of smell, compared with fewer than 10% of those under 50. As these changes may influence the type of food eaten, several studies have shown a strong correlation between impaired sense of smell and reduced interest in and intake of food. Consistent with this effect of ageing on the types of food eaten is the observation that ageing is associated with a less varied, more monotonous diet.31
Sensory‐specific satiety is the normal decline in the pleasantness of the taste of a particular food after it has been consumed. Sensory‐specific satiety leads to a decrease in the consumption of a previously eaten food and a tendency to shift consumption to other food choices during a meal. This acts to promote the intake of a more varied, nutritionally balanced diet. Older adults have a reduced capacity to develop sensory‐specific satiety, perhaps because of reduced senses of smell and taste. Reduced sensory‐specific satiety may in turn favour the consumption of a less varied diet and the development of micronutrient deficiencies.32
Hormones and neurotransmitters: a selective review
Hormones that influence food intake include leptin, ghrelin, and the orexins. Additionally, the central feeding system is dependent on the stimulatory effect of neurotransmitters, including the opioids, noradrenaline, neuropeptide Y (NPY), and galanin. A recent systematic review and meta‐analysis suggest that there are increased concentrations of inhibitory hormones including insulin, leptin, CCK, and PYY circulating in older adults.33
The short‐term peripheral satiety system is largely driven by gastrointestinal mechanisms. In the longer term, factors such as leptin and cytokines become more important. Gastrointestinal sensory and motor functions are essential in the regulation of satiation. Sensory signals induced by the distension by food contribute to initial sensations of fullness during a meal. These sensations are mediated via vagal mechanisms from mechanoreceptors situated within the stomach wall. In young adult humans, gastric distension using a barostat reduces food intake by up to 30%. Distension of the distal stomach (antrum) is related to increased sensations of fullness and is likely to be more important than distension of the proximal stomach (fundus). After eating, the stomach relaxes by a process of receptive relaxation, resulting in decreased intragastric pressure and increased gastric volume. This relaxation is particularly marked in the proximal stomach and results in a proximal fundic reservoir where food is retained. Not long before it is emptied into the small intestine, food is propelled distally from the fundus into the antrum. The extent of antral filling and distension relates more closely to feelings of fullness and satiety than does proximal gastric distension.30 Studies in animals and humans have demonstrated a relationship between postprandial satiety and the rate of gastric emptying. Slowing of gastric emptying may reduce appetite and food intake by increasing and prolonging antral distension and by prolonging the effect of small intestinal satiety signals. People with gastroparesis often exhibit symptoms of early satiety, loss of appetite, nausea, and vomiting, and studies in both animals and humans have shown that there is a relationship between postprandial satiation and the rate of gastric emptying.
Once food has entered the small intestine, chemoreceptors relay signals to the hypothalamus, resulting in the cessation of food intake. These signals are mediated by the release of gastrointestinal peptide hormones including CCK, peptide YY (PYY), and glucagon‐like peptide‐1 (GLP‐1). A number of gastrointestinal and pancreatic hormones, including CCK, GLP‐1 and amylin, have feedback effects on the stomach to slow gastric emptying, an effect associated with increased fullness and reduced food intake, by increasing and prolonging gastric distension and prolonging the effect of small intestinal satiety signals. Nutrient absorption and feedback signals from peripheral fat cells via leptin and, possibly, TNF‐α contribute to satiation.
Figure 13.1 illustrates the interplay of these mechanisms involved in appetite regulation. While evidence may be limited, Figure 13.2 summarizes which hormones are thought to be stimulatory and inhibitory.
Central neurotransmitters and hormones
Monoamines
The central aminergic system affects feeding, with noradrenaline having stimulating, serotonin having inhibiting,
