Statistically, an ageing population may be regarded as a global pandemic hiding in full view. Currently, one in nine persons alive today is aged 60 or more, and by 2050 this ratio is estimated to be one in five persons. At present, two persons are having their 60th birthdays every second, making around 58 million 60th birthday celebrations annually, and this number is accelerating every year.
There are of course serious economic and social implications of such a rapidly growing population of aged people, which includes me. Long gone are the days when I could eat a dozen hard-boiled eggs at festivals, or tuck into 60 sticks of satay. But I will not expand on the socio-economic issues here; instead, this article will be about the physiological issues concerned with our diets as we grow older and how we may age a bit more healthily, if not more gracefully.
Firstly, the tough news. There is no way to sugar-coat the fact that ageing involves radical changes in the physiological, pathological, social, and psychological conditions of a person. And this affects how we eat, what we eat, and even why we eat.
Simply poorer appetites
As an example, older people tend to have poorer appetites, which is the result of physiological changes in the digestive system and reduced olfactory sensations. Around half of people aged 65 or more appear to have demonstrable loss of smell and/or taste. This may also be caused by age-related bone structure changes in the human skull which result in the pinching of the olfactory nerve fibres leading into the brain.
Appetite is controlled mainly by sensory cells in the gastrointestinal tract (GIT). These cells sense the physical presence of food via olfactory and visual stimulation and stimulate the GIT to produce a range of hormones such as ghrelin, peptide tyrosine tyrosine (PYY), cholecystokinin (CCK), insulin, and leptin. These are released before, during, and after eating, and directly affect eating behaviours, including the amount of food consumed.
In older people, the ability to secrete the appetite-stimulating hormone, ghrelin, is reduced, hence also reducing the stomach’s secretion of digestive juices. The same reduced response applies also to insulin thereby allowing blood sugar levels to rise, which also suppresses appetite.
Conversely, levels of PYY and CCK appear to rise in older people, and they have an effect of limiting appetite via signals from the intestinal system. The production of leptin from body fat tissue is also believed to increase in overweight older people, and this hormone is an active appetite suppressant.
In short, older people appear to sense food less, and thus may enjoy eating less. However, this is clearly not true of every older person, and it may be the age-induced production of PYY, CCK and leptin which restrict the consumption of food.
A condition known as xerostomia is common among older people. This often manifests as a dry mouth due to the decrease in the secretion of saliva from the acinar cells in the tongue which are slowly depleted over time. It seems that certain common medications taken by older people can also induce xerostomia. These medications include diuretics, antihypertensives, antibiotics, bronchodilators, and certain antidepressants.
After chewing, a bolus of food is formed for transit into the stomach via the oesophagus. The path is straightforward for younger people but may be less smooth for older people. Firstly, the food bolus reaches the posterior pharyngeal wall where various muscles contract around it. This induces swallowing and the food then travels through the upper oesophageal sphincter into the oesophagus.
With age, the muscular contractions that initiate swallowing slow down significantly, increasing pharyngeal transit time. This may arise in dysphagia (swallowing difficulties) and can increase the risk of choking. Over a quarter of people aged above 75 experience dysphagia.
The action of peristalsis (pushing food through the oesophagus into the stomach) does not normally weaken too much in older people, unless they have various other co-morbidities.
In the stomach
As one would expect, the stomach of an older person cannot accommodate as much food as when the person was younger, simply due to the loss of elasticity of the stomach walls.
Once stimulated, the rate of secretion of gastric juices (mainly hydrochloric acid and pepsin) by the stomach is roughly the same between young and older people. However, up to 20% of older people appear to secrete less, resulting in a poorer digestive capability in the stomach, inferior nutrient absorption, and conditions such as chronic atrophic gastritis.
The inner walls of the stomach are protected by a layer of mucus which are produced by goblet cells on the stomach walls. These goblet cells are also affected negatively by ageing, rendering the stomach prone to conditions such as ulcers and lesions.
In older people, the stomach empties slower, so the food held remains longer, prolonging the feeling of satiation and inhibiting appetite.
The enzyme lactase is produced via the LCT gene in the small intestines and age significantly reduces its ability to create lactase. In any case, many humans are already lactose-intolerant so if one has been avoiding lactose since youth, it would be a bad idea to start consuming dairy when older.
Populations of various deleterious bacteria reside in the small intestines and can increase with age. These populations decrease the absorption of nutrients such as calcium, folic acid and iron and can lead to sensations of bloating.
Aging reduces the numbers of the neurotransmitters and neuroreceptors in the colon which affects the propulsive activity of the large intestinal tract. This is not helped by the weakened mucosa and muscle layers of the colon. The result is a delay in the colonic transit of waste which can lead to constipation and sagging of the walls of the colon.
Human gut bacteria
The human gut microbiota (HGM) can be significantly affected by ageing. In many ways, alterations to the HGM due to ageing can promote the growth of bacteria such as streptococcus, staphylococcus, enterococcus, and enterobacteriaceae, thereby creating the same conditions as the faulty HGMs associated with inflammatory bowel diseases.
The HGM is prevented from leaking from the colon into surrounding tissues or the bloodstream by a layer of epithelial cells that form the mucosal barrier. This layer is weakened by ageing and can allow pathogens to leak out from the colon, resulting in various kinds of (dangerous) infections.
Other digestive organs
The pancreas manufactures four major digestive enzymes, and with ageing, decreases in weight and some of its tissue additionally also undergoes fibrosis. Its exocrine function can be badly impaired, and the secretion of chymotrypsin and pancreatic lipase reduced, adversely affecting the ability of the small intestine to digest food.
The liver undertakes at least 114 important functions for the body. With age, the liver shrinks, the blood flow to it decreases, and therefore its functional capacity also decreases. There is a decrease in the rate of protein synthesis and of metabolism, the liver’s ability to detoxify many substances, as well as the production and flow of bile (involved in fat emulsification during digestion).
In addition, bile becomes denser, and its salt content diminishes, resulting in higher plasma concentrations of cholesterol, particularly in women. Various compounds are no longer inactivated quickly by an ageing liver and are therefore more likely to cause dose-related side-effects due to drugs. Medications therefore need to be carefully checked when used by older people.
The renal system is the most powerful regulator of the body’s internal environment. Healthy kidneys are essential to maintain homeostasis, ensuring stable conditions in which all cells can function optimally.
They perform several functions, including (a) removal of waste products such as urea, uric acid, creatinine, and breakdown of various toxic products, (b) regulation of blood volume and pressure, (c) electrolyte balance, (d) regulation of blood pH, (e) regulation of numbers of red blood cells; and (f) synthesis of vitamin D.
All the above functions are weakened with age, primarily due to lower renal blood flow resulting in kidney cell death and damaged glomeruli (filtration membranes).
Regarding diet-related damage, kidneys are the primary filtration point for by-products of food digestion called metabolites which are created during digestive processes and passed into blood plasma. Some metabolites are toxic if unfiltered. Age reduces the amount of plasma passing through the kidneys resulting in more toxins flowing through the body.
Examples of toxic metabolites are free radicals and AGEs (Advanced Glycation End-product), commonly found in fried and baked foods.
The new normal for older people
Depressing as it sounds, the reality is that eating as an older person can never be the same as eating as a young person. This is obvious from the effects of ageing. The facts suggest that an older person should chew nutrient-dense, fibre-rich foods more slowly, dividing food into smaller bolus for passing into the digestive tract. It would help to drink more water and perhaps take some vitamin and mineral supplements if advised by medical practitioners.
Regardless, a delicious, varied, even adventurous diet is always possible, with some care.
And of course, an older person should undertake some gentle exercise such as walking. Despite what we read, the 10,000 steps a day suggested for people was based on a pedometer souvenir produced for the 1964 Tokyo Olympics called in Japanese, ‘10,000 Steps Meter’.
Data suggests that 4,400 steps a day can reduce premature death by 40% in people aged 70 or more, though younger middle-aged people should aim for between 7,000 to 8,000 steps a day.
The views expressed here are entirely the writer’s own.