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weight (BMI<23.6 kg⁻² who lost more than 1.6 kg per year had a mortality rate of 22.6%, almost 20 times greater than the mortality rate of those with a baseline BMI of 23.6–28 kg⁻² whose weight remained stable. This interaction is concerning since the tendency for older people to lose weight is variable, with lean individuals probably most at risk. In an older person, unintentional weight loss of 5% or more over 6–12 months is associated with an increased risk of adverse effects, and a loss of 10% or more very likely means protein‐energy malnutrition.^{12, 17–19}

      There are many reasons why weight loss in older people has adverse effects. In some cases, weight loss is due to an illness, such as a malignancy. Nevertheless, weight loss and associated undernutrition are significant problems because loss of body weight after age 60 is disproportionately of lean body tissue – that is, sarcopenia – and individuals lose up to 3 kg of lean body mass per decade after age 50. Unlike loss of fat tissue, such a loss of lean tissue has adverse effects. Sarcopenia is associated with metabolic, physiological, and functional impairments and disability, including increased falls and increased risk of protein‐energy malnutrition.²⁰

      Cachexia in older people

      Weight loss and resulting adverse outcomes in older adults may be due to cachexia, malnutrition, the physiological anorexia of ageing, or some combination of these factors. Although there is often considerable overlap between them, cachexia and malnutrition are not the same. Whereas all cachectic patients are malnourished, not all malnourished patients are cachectic. Cachexia is a complex metabolic syndrome associated with underlying illness and characterized by loss of muscle with or without loss of fat mass. The prominent clinical feature of cachexia in adults is weight loss, but inflammation is a key component. Anorexia, insulin resistance, and increased muscle protein breakdown are also frequently associated with cachexia. Cachexia is distinct from starvation, age‐related loss of muscle mass, primary depression, malabsorption, and hyperthyroidism. Inflammation plays a major role in the pathogenesis of cachexia, with an absolute or relative increase in levels of inflammatory cytokines such as tumour necrosis factor alpha (TNF‐α), interleukin‐1, and interleukin‐6. Conditions that often afflict older people and that are frequently associated with cachexia include cancer, cardiac failure, chronic obstructive pulmonary disease and chronic renal failure. The European Society for Clinical Nutrition and Metabolism sub‐classified cachexia based on severity into cachexia and pre‐cachexia, the latter being present when there is (i) an underlying chronic disease, (ii) a systemic inflammatory response, (iii) anorexia, and (iv) unintentional weight loss over the previous six months of less than 5% of usual body weight.^21,22

Causes of undernutrition in older people

      Undernutrition in older adults is multifactorial, often mediated by reduced food intake, changes in physical and physiological status, changes due to chronic disease, the environment and related social determinants of health, and finally, iatrogenic causes.

      Reduced food intake

      Ageing is associated with a decline in energy (food) intake. Energy intake decreases by ~30% between ages 20 and 80.²³ Elderly people on average consume smaller meals more slowly and fewer snacks between meals,¹ and consistently report that they are less hungry than young adults.²⁴ For example, the 1989 cross‐sectional American National Health and Nutrition Examination Survey (NHANES III) study reported a decline in energy intake between the ages of 20 and 80 of 1321 cal per day in men and 629 cal per day in women.²⁵ A seven‐year New Mexico longitudinal study of 156 people age 64–91 reported a decrease of 19.3 kcal per day per year in women and 25.1 kcal per day per year in men,²⁶ and a Swedish six‐year longitudinal study of 98 people found that between the ages of 70 and 76 there was a decrease in energy intake of 610 cal per day in men and 440 cal per day in women.²⁷

      Physical factors

      Functional impairments that limit the completion of activities of daily living are also related to undernutrition. For example, immobility (e.g. stroke), tremor (e.g. Parkinson’s disease), and impaired vision may affect an older person's capacity to shop for, prepare, and consume food. Additionally, many older adults no longer have their own teeth. Poor dentition and ill‐fitting dentures may limit food type and quantity. For example, in one study, half of 260 nursing home patients, age 60–101, in Boston complained of problems with chewing, biting, and swallowing. The patients with dentures were more likely to have poor protein intake than those with their own teeth.²⁸

      Physiological changes

      The older adult experiences several physiological changes that lend to anorexia: (i) loss of homeostasis, (ii) the ageing gut, (iii) declining senses, and (iv) hormonal andneurotransmitter changes. Numerous studies have documented an age‐related decline in appetite and energy intake in healthy, ambulant, non‐institutionalized people.²³ Healthy older adults are less hungry and are more rapidly satiated after eating a standard meal than younger people. Much of this decrease in energy is likely a response to the decline in energy expenditure that also occurs during normal ageing.

      Loss of homeostasis

      Advanced age is associated with diminished homeostatic regulation of many physiological functions, including the regulation of energy intake. 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

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