Whats that smell?

Introducing your senses

Our senses refer to the way in which we perceive the world around us. When we go outside on a sunny day, we can see the clear blue sky, we can smell the flowers, and we can hear the sound of the trees rustling in the breeze. If you’re eating an ice lolly to keep cool, you can taste its flavour and feel how cold it is as it touches your tongue.

These are the five basic senses: vision, smell, hearing, taste, and touch. We also have other senses. For example, if you’re standing outside on a lovely day enjoying the sunshine you might also be feeling very relaxed, and your breathing and heartrate will be calm and steady. Now if a large bee comes flying towards you, your breathing and heartrate will probably speed up! Your awareness of your breath and heartrate is part of your ability to detect your internal sensations, known as interoception. Other internal sensations include how we detect if we’re hungry, and whether or not we feel full.

Sensations are first detected by receptors that are in different locations depending on the sense. For example, our visual detectors are in our eyes, and our taste detectors are in our taste buds on our tongues. When sensations are detected by these receptors, they send information up for processing in the brain. Our brain then decides how to respond. For example, if receptors on your skin detect that you feel cold, your brain might respond by raising the hairs on your arm and giving you goosebumps to help you warm up. This is an unconscious response. Or, if you feel really cold, you might consciously respond by putting on a jumper.

Therefore, our ability to detect and process senses is really important to how we self-regulate, or how we both unconsciously and consciously manage our physical and emotional wellbeing. If these processes are disrupted this can cause problems. For example, our ability to taste and smell food is really important to how our brain manages our appetite and motivation to eat. If you can’t taste or smell anything, you might have less motivation to eat. To test this, next time you are eating pinch your nose. Can you taste what you are eating? How does this affect how much you enjoy the food?

The senses in anorexia nervosa

One of the key symptoms in anorexia nervosa is food restriction. Researchers are interested in whether food restriction in anorexia nervosa could be linked with differences in how the brain manages appetite and motivation to eat. One possibility is that people with anorexia nervosa process tastes and smells differently. Another possibility is that people with this condition might have differences in how they process interoceptive sensations, for example making it difficult for them to detect if they feel hungry or full. All of these differences would make it more difficult for the brain to self-regulate appetite and eating: it would be like a computer trying to run a complicated programme, but not getting the right input.

However, previous research on whether taste, smell or interoception is different in people with anorexia nervosa compared to people without the condition (healthy controls) is very mixed. Research where people with anorexia nervosa are asked to self-report whether they experience sensory differences suggests that they are highly sensitive to sensations, and likely to try and avoid strong sensations ¹. By contrast, different research studies which use physical tests of sensitivity (for example, giving someone different concentrations of a sweet taste and asking them to report when they can detect the taste) have found that people with anorexia nervosa might be more sensitive, less sensitive, or no different compared to healthy controls ^{2 3 4 5}.

One possible reason why these findings are very mixed is that these previous studies are not taking into account something influencing sensory processing. For example, a study on taste that does not take into account whether or not the participants smoke might end up with unusual results, as smoking decreases your taste sensitivity. One possibility is that these previous studies in anorexia nervosa have not accounted for the potential role of autism in the sensory findings. Around one third of people with anorexia nervosa present with high levels of autistic traits ⁶. Importantly, we know that autism is associated with sensory differences. 95% of autistic adults say that they experience problems with sensory processing ⁷. Therefore, difficulties with processing the senses might be particularly important for autistic people with anorexia nervosa. We talked to eating disorder clinicians, patients and carers about how we can improve treatments for autistic people with anorexia nervosa, and they told us that existing treatment approaches are not properly taking into account sensory differences ^{8 9 10}.

Our Findings

We believed that looking at the relationship between autistic traits and sensory processing in anorexia nervosa could help us understand the previous mixed research findings. Therefore, we designed a research study exploring whether people with anorexia nervosa show differences in taste, smell and interoception on physical measures. We measured taste using strips of paper placed on the tongue that tasted sweet, sour, salty or bitter, and measured smell by asking our participants to sniff smelly pens (including one that smelled like fish). We measured interoception (the ability to detect internal sensations) by exploring how well our participants could detect their own heartbeats.

If you would like to read a copy of our findings, our results have been published as open access papers here and here. We found that people with anorexia nervosa did not experience any differences on these measures compared to healthy controls. This suggests that people with anorexia nervosa do not experience differences in their ability to physically detect different sensations. The fact that people with anorexia nervosa do self-report sensory differences suggests that sensory problems could maybe relate to how people with this condition interpret sensations, rather than physical differences in processing.

We then looked at whether taste, smell and interoception in anorexia nervosa was related to self-reported autistic traits. We found no relationship between sensory measures and autistic traits. However, this area needs more research before we can draw any conclusions around sensory processing in autism and anorexia nervosa. For example, self-reporting autistic traits is not always an accurate reflection of whether or not someone is autistic, so we could have used a better measure. If we were going to do the study again, it would also be interesting to compare using these measures in people with anorexia nervosa to autistic people.