TasteMatters: The smell of ageing

Another year older. And, according to the bulk of research on aging and our senses of taste and smell, another year closer to permanent decline. Studies on this topic are more or less unanimous, particular with respect to our sense of smell. Across the range of different “scratch and sniff” odours evaluated by 1.5 million people in the National Geographic Smell Survey [1], both odour detection and identification ability declined noticeably after the age of 70. This finding is supported now by very many studies showing that the ability to detect, discriminate, or identify smells progressively declines once we enter our eighth decade. Those odours or flavours that we can detect or identify are, unfortunately, weaker than they once were.

This is a modern malady for a variety of reasons. Enjoyment of food is increasingly an important part of our lives. And in affluent countries, we have started to take it for granted that – barring especially unfortunate genes, particularly foolhardy behavior, or rogue buses – we will of course live well into our 70s and perhaps 80s. Moreover, those staring with most interest (and most denial) at these decades, namely the baby-boomers, expect that a life of good food and wine will continue unabated until the moment that their life-support is switched off. What none of us really anticipate is that our diets will increasingly become blander and foods less distinctive as we ‘mature’.

The food industry has, from time to time, toyed with the notion of producing particular products for elderly consumers. No doubt that this is in recognition of the importance that our sense of smell plays in food appreciation. While this might be a technologically achievable, the marketing issues would be formidable. However, attempts have been made to improve food palatability for elderly in institutionalized care such as nursing homes. Such groups are highly vulnerable to malnutrition, at least partly because as our sense of smell declines, food flavours tend to become less appetizing, and eating less enjoyable. While for many of us, weight gain is the key dietary issue, institutionalized elderly risk serious health problems from under-eating. Because declining health takes a strong psychological as well as physical toll, loss of enjoyment from food can undermine motivations to eat and lead to even poorer health [2]. Attempts to overcome this problem have focused on boosting the taste profile of food, since taste is more robust in the face of increasing age than smell. Flavour ‘amplification’ of food using enhancers such as nucleotides or glutamate has been found to improve both the enjoyment of food and nutritional status in elderly patients [3].

Attempts to understand the causes of age-related olfactory loss have had to deal with the complexity of aging. While age-related changes at all levels of the olfactory system, and indeed the brain itself, have been demonstrated, it is not clear what the causes of these changes might be. Neurological changes to the olfactory system may occur as part of the normal aging process in the same way as changes to the elasticity of our skin. In addition, however, accumulated exposure to industrial chemical or pollutants or tobacco smoke over the adult lifespan might also be a specific factor in smell loss. To further complicate matters, very many diseases are much more common as we age. As a consequence, our intake of medications – some of which are known to have an impact on the sense of smell – increases considerably. Finally, let’s consider the fact that, even in the absence of dementia, cognitive abilities such as attention and memory are also concurrently declining. This means that any test of olfactory identification, for example, must take account of a general reduction in our ability to identify anything at all.

This depressing scenario is relieved a little if you are a woman by the fact that you will not only tend to live longer, but that the decline in the sense of smell will on average be less severe. However, even for men, there’s hope. While the average decline in sense of smell after age 70 looks bad, the variability around this average starts to increase dramatically at this point (see, for example [4]). In other words, the average value itself increasingly becomes unrepresentative of any single individual. So, perhaps because of a history of exposure to something that is very detrimental to smelling, you could be very unlucky and have little ability to smell any longer. On the other hand, the amount of olfactory decline you experience might actually be quite modest.

The key issue then becomes tracking down those factors that protect or harm our sense of smell during our lifetime. Importantly, the sense of smell is not isolated from other physical and cognitive functions. Variability in smelling mirrors more general variation in functioning in aging. We have all encountered, personally or via media, examples of individuals in their 80s or 90s who seem remarkably healthy, cognitively intact and physically or intellectually productive for their age (see, for example an articulate Bertrand Russell being interviewed at the age of 87; www.youtube.com/watch?v=1bZv3pSaLtY). These individuals are often identified as having “successfully aged”. In contrast, while examples of 90-year-old smokers or heavy drinkers can always be found, it is well established that such behaviours diminish our chances of living to a ripe, or at least healthy, old age.

Recent research by Nordin and colleagues [5] raise the possibility that general health may also be crucial in maintaining a functioning olfactory system. They addressed the question of whether aging per se is responsible for olfactory loss by recruiting a group in their 70s and 80s that were medication-free, cognitively intact and absent serious health problems, industrial chemical exposure or family history of dementia. Compared with a group of university students in their 20s, the elderly group performed equivalently in a threshold task in which they were required to detect the odour of pyridine. This suggests that odour sensitivity reflects overall general physical and cognitive health.

This study does not, of course, rule out other types of olfactory deficits in similarly healthy elderly. But it does suggest the possibility that declining smell might prove to be a manifestation of those same risk factors that are important in overall health as we age. Some fortunate (probably non-smoking) baby boomers might still be enjoying their last sip of Chardonnay as they expire after all.