An estimated 1-5% of the world’s population suffer from an eating disorder that few people are even aware exists. Known as avoidant restrictive food intake disorder, or Arfid for short, the condition is an extreme form of restrictive eating – which, if left unchecked, can have a severe impact on a person’s life and health.
Despite how serious Arfid is, we still know very little about what causes it – making it difficult to develop effective treatments. But in the first twin study ever conducted of Arfid, our team has now revealed that genetic factors play a major role in its development.
This research brings us one step closer to understanding the causes of Arfid – and hopefully someday finding better ways of treating the condition.
Arfid was first defined as an eating disorder in 2013. People with the condition consume an extremely limited variety of food or a very low amount of food (or both). But compared with people with other eating disorders – such as anorexia nervosa – this food restriction is not driven by body dissatisfaction or the desire to lose weight.
Instead, people may restrict the foods they eat due to sensory aversion to certain tastes, smells and textures, low appetite, as well as traumatic eating experiences – such as choking on food and fear of gastrointestinal pain. This restriction is so extreme that it leads to weight loss, vitamin and mineral deficiencies, and can affect a person’s daily life.
Arfid affects as many males as females, whereas other eating disorders – such as anorexia nervosa and bulimia nervosa – occur more often in girls and women. Also, in contrast to other eating disorders, Arfid usually develops in early childhood.
Twins and genetics
We know that other eating disorders are moderately-to-highly influenced by a person’s genetics. It’s therefore highly likely that genetic factors may also partly explain why some people develop Arfid and others do not.
We wanted to find out to what extent, so we looked at something called “heritability” – a complex concept that measures the proportion by which differences in people’s characteristics are explained by differences in their genes.
To estimate the heritability of any characteristic, we can use the twin method. This makes use of the fact that identical twins share all of their genes, whereas non-identical twins on average share 50% of the genes that make people different from each other.
In addition, as twins grow up in the same environment at exactly the same time, we can rule out that influences of this shared environment cause any differences between them (as might happen with ordinary siblings of different ages).
We then compare the similarity of a certain characteristic between identical twins to its similarity between non-identical twins. The degree to which identical twins are more alike gives us an estimate of the heritability of this characteristic.
In order to estimate the heritability of Arfid, we began by looking at data from the Swedish Twin Registry, which has tracked the health of just under 34,000 twins. First, we combined data from parental questionnaires and registered clinical diagnoses to identify children with Arfid. We then applied the twin method, finding that the heritability of Arfid was 70-85%. This means that whether the children had Arfid or not was between 70-85% explained by differences in their genes.
This places Arfid among the most heritable of mental disorders. In comparison, the heritability of autism is estimated to be 79-84%, and that of ADHD is 76-89%. Moreover, the heritability of Arfid is higher than that of anorexia nervosa (48-74%), bulimia nervosa (55-61%), and binge-eating disorder (39-57%), suggesting that Arfid might be the most heritable eating disorder.
Hope for treatments
Findings like this can help reduce stigma and blame around mental disorders such as Arfid and other eating disorders. A person does not choose to develop Arfid – and similarly, a parent does not cause their child to develop it.
However, the fact that the heritability of Arfid is less than 100% also shows it’s not only the genes that make someone likely to develop the disorder. Other factors, such as choking very badly on food or severe allergic reactions, can trigger this eating disorder to develop.
Our findings also suggest that genetic studies have a huge potential to help us understand the causes of Arfid. This is what our next study will aim to do.
At the moment, treatments for Arfid using cognitive behavioural therapy and family-based therapy are being developed. However, these methods usually focus on symptoms, instead of the underlying causes of the eating disorder.
We also still don’t know how well these treatments work. But a better understanding of what causes Arfid, including its underlying biology, will help us to develop better treatments.