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Reference topic 15: The Tongue and the Nose

Loss of smell and taste: the chemical senses in a modern world

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.62 sources6 graded recommendationsTopic hub: Food

Published Updated

In December 2006, researchers at the University of California, Berkeley, described an experiment that sounds like a party game. They had soaked a 10-meter string in chocolate essence and laid it in the grass outside Barker Hall. Then they blocked volunteers' sight, hearing and touch and set them crawling on all fours to find the trail by smell.1

Two-thirds of the volunteers followed the chocolate to the end within three attempts. They zigzagged along the line "in the same way that tracking dogs follow a scent," and the faster they moved, the faster they sniffed.1 Four volunteers who then practiced over a few days roughly doubled their speed and strayed far less from the trail.12

Mismatch card

The system

Taste and smell, the chemical senses that judge food and air.

What it expects

Sugar wrapped in fiber, bitter and sour plants, and a nose used daily to find food and judge freshness.

What it gets

Sugar dissolved in drinks at all hours, debittered crops, sweeteners, polluted air, smoke, viruses and an underused nose.

What happens in the gap

Easy sugar overconsumption, lost bitter warnings, and unnoticed smell loss linked to higher mortality.

The handrail

Water first, gradual salt reduction, home cooking; smell training after loss (●●○)

Strength of evidence

●●○ Animal taste evolution and the sugar-dose problem are well established; sweetener harms, palate retraining and smell-mortality links rest on mixed or observational evidence.

What this topic covers

  • ·Five Lights on the Dashboard
  • ·Taste Follows the Menu
  • ·What the Modern Plate Says
  • ·Sweetness Without Sugar
  • ·The Nose Nobody Trusted
  • ·Five Felt-Tip Pens
  • ·Losing Smell and Getting It Back

The practical version is in the book: Chapter 5, Eat.

The handrail

Topic 40 holds the full food and drink program.

Each recommendation carries the strength of the evidence behind it. How the grades work.

  1. Make water your default drink

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    One 12-ounce cola exceeds the WHO's stricter target, under 5% of energy or roughly 25 g of free sugars a day.1859 Diet drinks can be a bridge off sugary ones (●●○), but the WHO advises against relying on sweeteners for weight control.6028 With diabetes, follow your clinician.

  2. Cut salt gradually

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    In small studies, preferred saltiness fell after two to three months of a 30-50% reduction.61 On blood-pressure or kidney medication, change salt intake only with your clinician.

  3. Do not count on a sweet tooth fading

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    In a trial of 33 adults, three months of eating much less sugar made puddings taste about 40% sweeter than they did to controls, but no more pleasant.62 In the 180-person Sweet Tooth Trial, six months of low sweet exposure changed neither sweet liking nor perceived sweetness (its authors disclose industry ties).63 Cut sugar for what it does, by changing what you buy, not by willpower.

  4. Cook from raw ingredients

    EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.

    US adults in households that cook dinner most nights eat less sugar than those that rarely cook (119 vs 135 g a day), an association only.64

  5. Train a failing nose

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Evidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials. after viral loss·Evidence grade: Emerging. Early, small, animal-only, or actively contested. for healthy noses

    Sniff four distinct odors from jars, about 10 seconds each, twice daily, for at least 12 weeks; start early and use strong odors.4950 Never put oils in your nose. Healthy noses learn too: in one small study, 10 of 20 people learned to detect an odor they had missed.65

  6. Protect your nose and plan for its failure

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Do not smoke. Avoid intranasal zinc products; the US Food and Drug Administration warned against three in 2009 (●●●).4366 Keep working smoke and gas detectors, since a failing nose may miss both.41 Sudden or persistent unexplained smell loss deserves a medical assessment: it can accompany infection or, in older adults, precede neurodegenerative disease.4439

The skeptic's corner

The case against this topic, from the research itself.

The sweetener evidence splits by study type. In the review the WHO commissioned, randomized trials linked higher sweetener intake to slightly lower body weight, under a kilogram. Cohort studies (which follow groups of people over time) linked sweetened drinks to a 23% higher risk of type 2 diabetes. Both findings were rated low certainty.5152 Cohorts carry a trap: people who are gaining weight may switch to diet drinks because of it.52

Britain's Scientific Advisory Committee on Nutrition, responding in 2025, shared the WHO's concerns about diabetes and heart disease. But it found that trials "consistently suggest NSS, compared with free sugars, reduce energy intake and therefore body weight," and that the observational data the WHO "gave greater weight" to could not support firm conclusions.52

Money complicates the trials too. Both recent trials favoring sweeteners had authors with beverage or sweetener industry ties, and one was funded by the American Beverage Association. In that year-long trial, water drinkers kept off nearly as much weight as diet-drink drinkers, a difference its authors called not clinically significant.5354 A fair summary: swapping sugary drinks for diet ones probably helps a little, water appears to do about as well, and the long-term safety of heavy use is unsettled.

The much-quoted 2014 claim that people can tell apart more than a trillion smells is an extrapolation from 26 people comparing mixtures of odor molecules.55 "This claim is wrong," one critic replied; others showed that the arithmetic supports "at most one trillion."5657 Nobody knows how many smells a person can distinguish.

Finally, because most viral smell loss clears up on its own, training studies without a comparison group flatter the method. For persistent loss after COVID, a 2022 Cochrane review of all treatments found only two small trials, neither testing smell training, and could draw no conclusions.58

Terms this topic defines

Related topics

Sources cited above

  1. 1.University of California, Berkeley (2006). Two nostrils better than one, researchers show. Press release by Robert Sanders, 18 December 2006. https://newsarchive.berkeley.edu/news/media/releases/2006/12/18_scents.shtml (also reproduced by ScienceDaily, 20 December 2006, https://www.sciencedaily.com/releases/2006/12/061219095409.htm). (32 volunteers took part; the string was 10 meters, or 33 feet, long; the two nostrils sample air about 3.5 cm apart, and when a nasal 'prism' fed both nostrils the same air, the four trained trackers were half as accurate)
  2. 2.Porter, J., Craven, B., Khan, R.M., Chang, S.J., et al. (2007). "Mechanisms of scent-tracking in humans." Nature Neuroscience, 10(1), 27-29. https://doi.org/10.1038/nn1819 (published January 2007)
  3. 51.Rios-Leyvraz, M., and Montez, J. (2022). Health effects of the use of non-sugar sweeteners: a systematic review and meta-analysis. World Health Organization. https://iris.who.int/handle/10665/353064 (figures as summarized in SACN 2025). (29 randomized trials linked higher sweetener intake to 0.71 kg lower body weight; 13 cohort studies linked sweetened drinks to a 23% higher risk of type 2 diabetes)
  4. 52.Scientific Advisory Committee on Nutrition (2025). SACN statement on the WHO guideline on non-sugar sweeteners. April 2025. https://assets.publishing.service.gov.uk/media/67ea97b3ea9f8afd8105627d/sacn-position-statement-on-non-sugar-sweeteners.pdf
  5. 53.Harrold, J.A., Hill, S., Radu, C., Thomas, P., et al. (2024). "Non-nutritive sweetened beverages versus water after a 52-week weight management programme: a randomised controlled trial." International Journal of Obesity, 48(1), 83-93. https://doi.org/10.1038/s41366-023-01393-3 (6.1 kg of weight kept off on water against 7.5 kg on diet drinks)
  6. 54.Pang, M.D., Kjølbæk, L., Bastings, J.J.A.J., Andersen, S.S.H., et al. (2025). "Effect of sweeteners and sweetness enhancers on weight management and gut microbiota composition in individuals with overweight or obesity: the SWEET study." Nature Metabolism, 7(10), 2083-2098. https://doi.org/10.1038/s42255-025-01381-z
  7. 55.Bushdid, C., Magnasco, M.O., Vosshall, L.B., and Keller, A. (2014). "Humans can discriminate more than 1 trillion olfactory stimuli." Science, 343(6177), 1370-1372. https://doi.org/10.1126/science.1249168 (26 people told apart mixtures of 10, 20 or 30 molecules drawn from a set of 128)
  8. 56.Meister, M. (2015). "On the dimensionality of odor space." eLife, 4, e07865. https://doi.org/10.7554/eLife.07865
  9. 57.Gerkin, R.C., and Castro, J.B. (2015). "The number of olfactory stimuli that humans can discriminate is still unknown." eLife, 4, e08127. https://doi.org/10.7554/eLife.08127
  10. 58.O'Byrne, L., Webster, K.E., MacKeith, S., Philpott, C., et al. (2022). "Interventions for the treatment of persistent post-COVID-19 olfactory dysfunction." Cochrane Database of Systematic Reviews, 9, CD013876. https://doi.org/10.1002/14651858.CD013876.pub3 (two trials, 30 people in all)
  11. 18.Coca-Cola Company (2026). Coca-Cola Original product page (accessed 2026). https://www.coca-cola.com/us/en/brands/coca-cola/products/original (a 12-ounce, 355 ml can)
  12. 59.World Health Organization (2015). "WHO calls on countries to reduce sugars intake among adults and children." News release, 4 March 2015. https://www.who.int/news/item/04-03-2015-who-calls-on-countries-to-reduce-sugars-intake-among-adults-and-children
  13. 60.McGlynn, N.D., Khan, T.A., Wang, L., Zhang, R., et al. (2022). "Association of low- and no-calorie sweetened beverages as a replacement for sugar-sweetened beverages with body weight and cardiometabolic risk: a systematic review and meta-analysis." JAMA Network Open, 5(3), e222092. https://doi.org/10.1001/jamanetworkopen.2022.2092
  14. 28.World Health Organization (2023). "WHO advises not to use non-sugar sweeteners for weight control in newly released guideline." News release, 15 May 2023. https://www.who.int/news/item/15-05-2023-who-advises-not-to-use-non-sugar-sweeteners-for-weight-control-in-newly-released-guideline. Guideline: World Health Organization (2023). Use of non-sugar sweeteners: WHO guideline. https://www.who.int/publications/i/item/9789240073616
  15. 61.Institute of Medicine (2010). Strategies to Reduce Sodium Intake in the United States, chapter 3. National Academies Press. https://www.ncbi.nlm.nih.gov/books/NBK50958/
  16. 62.Wise, P.M., Nattress, L., Flammer, L.J., and Beauchamp, G.K. (2016). "Reduced dietary intake of simple sugars alters perceived sweet taste intensity but not perceived pleasantness." American Journal of Clinical Nutrition, 103(1), 50-60. https://doi.org/10.3945/ajcn.115.112300
  17. 63.Čad, E.M., Mars, M., Pretorius, L., van der Kruijssen, M., et al. (2026). "The Sweet Tooth Trial: a parallel randomized controlled trial investigating the effects of a 6-month low, regular, or high dietary sweet taste exposure on sweet taste liking, and various outcomes related to food intake and weight status." American Journal of Clinical Nutrition, 123(1), 101073. https://doi.org/10.1016/j.ajcnut.2025.09.041
  18. 64.Wolfson, J.A., and Bleich, S.N. (2015). "Is cooking at home associated with better diet quality or weight-loss intention?" Public Health Nutrition, 18(8), 1397-1406. https://doi.org/10.1017/S1368980014001943
  19. 49.Hummel, T., Rissom, K., Reden, J., Hähner, A., et al. (2009). "Effects of olfactory training in patients with olfactory loss." Laryngoscope, 119(3), 496-499. https://doi.org/10.1002/lary.20101 (36 trained patients against 16 untrained; 28% of the trained group gained more than 6 points; rose, eucalyptus, lemon and clove, about 10 seconds per odor)
  20. 50.Damm, M., Pikart, L.K., Reimann, H., Burkert, S., et al. (2014). "Olfactory training is helpful in postinfectious olfactory loss: a randomized, controlled, multicenter study." Laryngoscope, 124(4), 826-831. https://doi.org/10.1002/lary.24340 (after 18 weeks, smell improved in 26% of the strong-odor group against 15% of the weak-odor group; 63% against 19% when the loss was under a year old)
  21. 65.Wysocki, C.J., Dorries, K.M., and Beauchamp, G.K. (1989). "Ability to perceive androstenone can be acquired by ostensibly anosmic people." PNAS, 86(20), 7976-7978. https://doi.org/10.1073/pnas.86.20.7976
  22. 43.Frye, R.E., Schwartz, B.S., and Doty, R.L. (1990). "Dose-related effects of cigarette smoking on olfactory function." JAMA, 263(9), 1233-1236. https://pubmed.ncbi.nlm.nih.gov/2304239/
  23. 66.US Food and Drug Administration (2009). Release advising consumers to stop using three intranasal Zicam products, reproduced by ScienceDaily, 20 June 2009. https://www.sciencedaily.com/releases/2009/06/090618130709.htm
  24. 41.Hoffman, H.J., Rawal, S., Li, C.M., and Duffy, V.B. (2016). "New chemosensory component in the U.S. National Health and Nutrition Examination Survey (NHANES): first-year results for measured olfactory dysfunction." Reviews in Endocrine and Metabolic Disorders, 17(2), 221-240. https://doi.org/10.1007/s11154-016-9364-1 (2012 survey of US adults aged 40 and over)
  25. 44.Moein, S.T., Hashemian, S.M., Mansourafshar, B., Khorram-Tousi, A., et al. (2020). "Smell dysfunction: a biomarker for COVID-19." International Forum of Allergy and Rhinology, 10(8), 944-950. https://doi.org/10.1002/alr.22587 (on a 40-odor test)
  26. 39.Ross, G.W., Petrovitch, H., Abbott, R.D., Tanner, C.M., et al. (2008). "Association of olfactory dysfunction with risk for future Parkinson's disease." Annals of Neurology, 63(2), 167-173. https://doi.org/10.1002/ana.21291 (2,267 Honolulu men aged 71 to 95; odds ratio 5.2 for the lowest quarter of odor identification against the top half)

This topic carries 62 notes in the research manuscript; the 26 above are those behind the material on this page. Full source register.