Taste is often ranked in consumer surveys as the primary reason behind people’s food choices. However, eating is a multisensory event that includes physical sensations experienced in the mouth as well as olfaction, our sense of smell that translates odors and aromatic compounds into perceived flavors.
Taste perception (sensing the five taste qualities — salty, sweet, bitter, umami and sour) and olfaction (estimated to detect more than a trillion different odors) are a sensory duo. While our ability to smell plays a dominant role in food enjoyment, quality of life and nutrition-related health outcomes, smell impairments are often undiagnosed and have negative health consequences.
Sense of smell begins with orthonasal olfaction, when the nose detects odors. Upon chewing and swallowing food, odors and aromatic compounds are sensed via retronasal olfaction in the nasal cavities, tissues and nerves that send neurological signals to the olfactory bulb in the brain.
Olfactory Impairment and Prevalence
Sinus infections or inflamed mucus membranes can inhibit or limit flavor perception without disrupting taste receptors that perceive taste qualities such as sweet and salty.
The smell disorder hyposmia, meaning a reduced sense of smell, and parosmia, which causes distorted smells, have become more prominent as a reported COVID-19 symptom along with anosmia, the complete loss of smell. Prior estimates have been obtained through the National Health and Nutrition Examination Survey, which includes self-reported, subjective assessments and objective assessments, including tests of participants’ odor perception and taste function. The 2013-2014 NHANES results indicated that more than 12% of adults 40 and older had olfactory impairment, of whom 3% had complete loss of smell or a severely reduced sense of smell.
Men may experience more smell impairments, potentially double that of women, and most studies indicate significant declines in olfactory function with age.
Olfactory impairment is commonly underrepresented when people self-report or use subjective assessments, rather than objective assessments. For example, in a systematic review and meta-analysis of 175,073 people from 10 countries, olfactory impairment was 9.5% using self-reporting compared to 29% with objective assessments.
According to Danielle Reed, PhD, associate director of Monell Chemical Senses Center in Philadelphia, “the prevalence for COVID-19-related smell loss is [as of March 2021] 46% when the sense of smell is measured by self-report and 69% when measured objectively. This estimate is based on 202 studies done on 151,389 people.”
Quality of Life and Nutrition-related Health
The smell and taste impairments caused by COVID-19 raised awareness of chemosensory disorders, which have the potential to result in significant mental and physical health consequences and present as an invisible disability. “Emotional health and well-being, socialization and physical conditions may interplay with chemosensory perceptions and affect what people think, choose and ultimately eat or drink,” says Jacqueline B. Marcus, MS, RDN, LDN, FADA, FAND, author of Aging, Nutrition and Taste (Elsevier, 2019) and founder of TasteOverTime.com.
The olfactory bulb is connected to the limbic system, which is involved in memory, emotion and learning, explaining why specific odors can evoke emotional responses, influence behavior and trigger memories of long-forgotten events of childhood pastimes, family meals and celebrations. People with anosmia and smell impairments often report feelings of isolation, anger, apathy and depression and a disconnect from people who aren’t aware of the impact of loss of smell. Marcus adds that “this ‘perfect storm’ may result in decreased interest in food consumption, adequate nutrition or compromised health. Sometimes isolation may be the turning point.”
Nutrition-related health issues can develop from smell impairment and, over time, may result in weight changes, malnutrition and nutrient deficiencies, particularly among elderly populations. The issue isn’t easily resolved by simply promoting nutrient-rich foods. “Overall, people who lose their sense of smell may avoid eating because it is not enjoyable and they may be malnourished or lose weight,” says Reed. “But some people, especially those with a partial sense of smell, may overeat to compensate for the loss of smell, for example, salty chips or sweet ice cream.”
Based on preliminary research, inadequate nutrition can also lead to further nutrient deficiencies of which minerals zinc, copper and magnesium as well as vitamins A, E, B12 and D may cause or exacerbate smell and taste impairments.
Health-related Smell Impairment
Most common: Head trauma, nasal polyps, allergic rhinitis and upper respiratory infections are leading causes of impaired olfaction. Respiratory infections from bacteria or viruses often resolve without serious threats to health; however, COVID-19 increased awareness that sensory changes can signal a potentially deadly virus and become more predictive of infection than other common COVID-19 symptoms.
Neurological: Alterations in smell also may be early symptoms of neurological diseases such as multiple sclerosis, Alzheimer’s disease and Parkinson’s disease, in which a decline in sense of smell may signal the condition several years before a diagnosis or primary symptoms develop. Multiple studies indicate 85% to 90% of patients with Alzheimer’s experience smell impairment and 45% to 96% of those with Parkinson’s disease do as well.
Diabetes: A large meta-analysis indicated olfactory impairment in people with diabetes was 1.58 times the control group and associated with both Type 1 and Type 2 diabetes. The impact of hyperglycemia and diabetic neuropathy on the olfactory nerve was proposed as a central mechanism of sensory decline. For adults 40 and older with diabetes, the 2013-2014 NHANES data indicated a significant trend in severe anosmia or severe hyposmia for people on oral and insulin treatment, but no association was observed between the duration of diabetes and the prevalence of olfactory dysfunction.
Cancer: Between 70% to 77% of cancer patients experienced taste alterations during some types of chemotherapy. In a review of 11 articles, reduced perception of sweet was associated with malnutrition and undernutrition. However, the impact of cancer treatment on olfactory function is understudied and studies of taste alterations of patients mostly rely on self-reports without pre-treatment chemosensory assessment.
Medications: Half of the top 100 prescription medications used in 2017 elicited taste or smell complaints or disorders. In a review of clinical trials, more than 350 medications were associated with taste alterations and more than 70 with olfactory effects. The incidence of adverse chemosensory effect on average was 5% across most medications but ranged as high as 66% for an insomnia drug; however, researchers noted limitations of self-reported data. A primary concern expressed in the research was the impact on reduced medication compliance for patients experiencing taste and olfactory side effects.
Assessment
Multiple studies mention the limitations of self-reported taste and olfactory symptoms and risks of undiagnosed patients. Reed believes smell and taste assessments should be part of routine clinical care and suggests practitioners “consider using new quick tests, especially for the sense of smell.”
In 2013, a continuing medical education article in the American Family Physician journal recognized that patients don’t often notice or differentiate between losses of smell or taste and recommended physicians become more familiar with symptoms, pre-clinical signs of serious conditions and consider using standardized questionnaires, office-friendly smell disorder tests and referring to otolaryngologists or smell and taste centers.
Marcus suggests that because people may be unaware of chemosensory changes, nutrition and health professionals can potentially flag nutrient and health issues by asking unobtrusive questions about basic perceptions of taste and smell.
Food Preparation and Culinary Nutrition
The sense of smell is an effective warning system for gas leaks, fire and toxic fumes, yet it fails as a reliable indicator of rotten foods; the olfactory senses cannot detect pathogenic bacteria. It’s important to teach people with chemosensory impairment to store foods properly and notice discoloration and texture changes in foods.
Chemosensory impairment often reduces appetite and interest in eating. Experiencing an appetite is key to both food enjoyment and sufficient intake of calories; however, promoting ways to build an appetite can be negatively associated with overeating. “The topic of appetite and weight is touchy,” Marcus says. “If foods and beverages look good and smell great, people may eat more, and the reverse may also be the case. Improving the appearance and texture of food and enhancing aromas may arouse appetite. But there are so many other factors that contribute to weight gain, other than the ingestion of tasty and aromatic foods and beverages.”
Depending on the level and type of chemosensory impairment, culinary skills that may help increase interest in eating include:
TASTE QUALITIES: Highlight favorite tastes that are perceived. If sour is sensed and appreciated, create sour marinades, sauces or dressings.
PHYSICAL SENSATIONS: Create textures based on preferences such as grain berries for bouncy, al dente texture, crunchy nuts or crisp vegetables for contrast and pungent spices or condiments, as desired.
HEIGHTEN FLAVORS: Use salt to enhance natural flavors or ingredients such as MSG to boost savory, umami flavors without increasing sodium.
Smell Disorder Resources
Want more info? These resources include support options and practical details for professionals:
- abScent Community Support
- Fifth Sense Community Support
- Monell Center Resources and COVID-19
- National Institutes of Health: NIDCD: Smell Disorders
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