Can you count your heartbeats – without taking your pulse? Whether you realise it or not, you almost certainly can. We all have an "interoceptive sense", an awareness of the visceral signals that originate from inside our bodies. Nerves travel from our internal organs to the insula cortex of our brain, where a dynamic representation of our inner physiology is created – an inner dashboard, if you will.
Interoceptive awareness (IA) is the ability to perceive and process the signals from this physiological dashboard. Our personal level of interoceptive awareness, as measured by the ability to accurately count our own heartbeats, is a stable trait that varies substantially across the population. Good cardiac sensitivity has also been linked with sensitivity to other visceral organs, too.
Does it matter? It might. In recent years, several researchers have reported links between IA and our sense of "self", and with the ability to recognise and process emotions. Now, Dr Beate Herbert and colleagues have found evidence that IA is linked to body mass index, with poorer IA scores predicting a higher BMI.
Their research, published in Appetite, suggests that good interoceptive awareness is what allows "intuitive" eaters – those who eat in response to physical rather than emotional cues and as a result eat only when hungry – to keep their weight down.
Could it be that the oft-repeated advice to dieters – to forget the rules, and simply "listen to your body" – has a basis in neuroscience? Like anything involving the human brain, it's not that simple. Some of us, found Dr Herbert, can perceive our visceral signals perfectly well, but may choose to ignore them because they find them unnerving.
This discomfort with interoception could, Dr Herbert tells me, further complicate our eating patterns: "It's not enough to perceive interoceptive signals adequately. Appraising these signals as positive or negative is a separate cognitive process, which also determines eating behaviour. One needs to allow oneself to act according to these perceived signals."
Earlier this year, a team of psychologists led by Professor Manos Tsakiris at Royal Holloway, University of London, measured the interoceptive awareness of visitors to London's Science Museum. I asked him whether he thought IA influences our eating habits. "Studies have shown that interoception plays an important role in eating disorders," he told me. "It's linked to a deeper awareness of emotions in general, and anorexics – for emotional reasons – choose not to eat. But to date, we don't know whether a deficit in interoceptive awareness is a cause or an effect of anorexia."
This question of cause and effect is significant, and Prof Tsakiris's own research into interoception and body image offers a good example. It is thought that women with low IA show a stronger tendency to "self-objectify": to regard their bodies primarily as "objects", valuing appearance over function. It had been suggested that this self-objectification suppresses IA. But Tsakiris's findings suggest the opposite: that low IA is actually the cause, not the consequence, of self-objectification.
Could we be looking at a vicious circle: low interoceptive awareness not only predicts a risk of disordered eating, but also an unhealthy tendency to objectify ourselves? And if so, is there any hope for those of us who may not be particularly attuned to our inner workings?
Meditation, perhaps surprisingly since it involves focusing attention on internal states, does not appear to make a difference.
Dr Herbert offers another possibility. "Experiences of cardiac arousal [the effects of exertion, emotion etc] throughout our lifetimes could activate brain structures known to be important for processing interoceptive cardiac signals," she says. Could exposing ourselves to arousing experiences improve our overall interoceptive sensitivity - including the ability to perceive gastric signals? "At the moment, we don't have data on the development of interoceptive cardiac sensitivity, or the sensitivity for other bodily signals, during adult life."
Interestingly, though, her previous research uncovered a positive correlation between short-term fasting and interoceptive awareness. But she cautions that fasting can in itself be a trigger for disordered eating.
If we can't change our degree of interoceptive awareness, is there any value to knowing about the phenomenon? Could discovering that we have poor IA, for example, motivate us to be more conscious of our eating habits?
Dr Rebecca Park is a clinical senior lecturer specialising in eating disorders. I asked her if understanding more about interoceptive awareness could help people with these conditions. Dr Park pointed out that Dr Herbert's data was collected from a healthy population, and so the results may not apply to patients with clinically diagnosed eating disorders. Still, she believes there is potential for incorporating individual IA scores into future treatment regimens: "This mechanism could, if validated, be important in informing future interventions for eating disorders - and obesity, too. The concept of 'mindful eating', to help those with binge eating, builds on this premise."
Counting heartbeats is easy to study, using ECG, but it is only one aspect of interoception. Our physiological dashboard is a busy place. However, Dr Herbert has previously found that awareness of cardiac signals correlates well with sensitivity to gastric signals. Does this give credence to the old advice to drop the diet and simply listen to our what our bodies are trying to tell us? Well, there's a lot more research to do. But for those of us able to hear what our body is saying, it just might.
