Before getting into the logistics of the effect of obesity on the taste and smell, it is important to understand how the two senses are connected into what we perceive to be flavour. Olfaction (sense of smell) and gustation (sense of taste) work in a diverse interaction within several sensory systems. Both the olfactory and gustation chemosensory systems rely on receptors on the oral and nasal cavities that interact with specific molecules to generate action potentials which transmit the effects of chemical stimuli to regions of the central nervous system (CNS), the olfactory and gustatory integrative region in the CNS are reported from mapping data from both electrical and optical recordings (Sugai et al, 2005). This means both senses are integral in contribution to the overall flavour sensation. Researchers even say that 80% of flavours we taste comes from what we smell, which explains why food becomes relatively flavourless when we can’t smell properly (Binns, 2006). This recognition of the cooperative nature of the two senses is important to take into consideration when looking at the impact of obesity as an effect on smell can have an overall impact on taste.
A lot of speculation had arisen with obesity having an impact on the sensitivity of taste. In one study conducted on mice in 2013, biologists reported that severely overweight mice had an impaired detection of sweets compared to slimmer mice, this is because they were reported to have had fewer taste cells that responded to sweet stimuli and cells which did respond had a relatively weaker reaction (NewsRx Health, 2013). Taste buds are made up of around 50 – 100 cells categorised from three types sensing salt, sweet, bitter, sour and umami. In correlation to the previous study, authors of the Obesity, Fitness and Wellbeing Week magazine set out to explore changes in the taste buds in obesity by feeding mice either a normal diet of up to 14% fat or a diet higher in fat with up to 58% fat. In 8 weeks, the obese mice were reported to have a 25% decline in the amount of taste buds than the leaner counterparts (Obesity, Fitness & Wellbeing, 2018). These two studies give a relatively basic deduction of the impact of obesity on the sensitivity of taste by how it impacts the number of taste buds using a simple experiment. In more depth into why the number of taste buds declined the researchers looked closer into the death turnover of taste bud cells. Taste bud cells turn over quite quickly averaging at a 10 day lifespan. This normally arises through two processes of cell death (apoptosis) as well as new cell generation from progenitor cells. What they had observed was that while the rate of apoptosis increased in the obese mice, the number of progenitor cells declined; this altogether reducing the number of taste bud cells over time. Going back to the first study conducted by PLOS ONE, 25 mice of a normal weight compared to 25 mice in the obese range were measured in response to different tastes where they looked at calcium signalling, a process where cells are able to recognise specific tastes. This signalling involves a temporary increase in calcium levels onside the cells which the scientists were able to measure. The results showed that the taste cells of the obese mice responded weaker to sweetness and bitterness (News Rx Health, 2013).
In another study Fikentscher et al (1977) observed 30 children aged between 10-16 suffering from obesity. Olfaction was tested using blast injection while using coffee and anise oil to stimulate the olfactory nerve meanwhile lemon and peppermint oil stimulating the nerve ending of the olfactory and trigeminal nerves. An estimation was then made of odour detection thresholds as well as identification thresholds which were compared to normal ranges of non-obese children of the same age. The results showed that 23% of cases of odour detection thresholds for coffee and 10% of cases for anise oil, were at a level of mean threshold below the normal range. In conjunction, 54% of cases of odour detection for lemon oil and 41% of cases with peppermint oil also noted a range below the normal limit. To conclude the study, the olfactory thresholds were most likely linked to metabolic disturbances developed in children with obesity.
More recently, a study was conducted evaluating the relationship between taste identification ability and body mass index (BMI). The method included 30 people of normal weight, 19 healthy overweight people and 22 obese people were studied in their response to different taste stimuli which is soaked in strips of filter paper at different concentrations within four different taste qualities (salty, sour, sweet and bitter). The subjects were then asked to identify the tastes from a list of eight descriptions. The results showed a relatively lower taste sensitivity in subjects of a higher BMI (Vignini et al, 2019). In relation to the previous study, the likelihood of decreases sensitivity is due to metabolic disturbances as well as an affect on the taste receptors on the tongue in connection to the CNS.