For a long time, it's been thought of primarily as a psychiatric disorder, but new research published this week in the journal "Nature Genetics" suggests there's an important physiological aspect as well.

This research links the disorder to genetics and metabolism, and it might change the way we understand the origins of the illness as well as potential treatments.

Anorexia nervosa is an eating disorder.

Affected people may be dangerously underweight, with a restricted intake of food, and often also have a distorted body image.

The end result is that the body is starved of sufficient nutrients, which can lead to various medical complications and even death.

In fact, according to the U.S. National Institute of Mental Health, anorexia is the most deadly mental disorder.

Even with treatment, many patients struggle to fully overcome anorexia.

The authors of this new research suggest that current treatments might be too focused on psychology, and fail to consider the physiology involved in the disorder.

This study pulled together genetic data on almost 17,000 cases of anorexia nervosa among people of European ancestry.

When they compared that dataset with a control group, they found eight genetic variants significantly associated with anorexia.

Interestingly, some of these genetic traits are known to be related to levels of physical activity, as well as to metabolism, the processes inside the body that convert sustenance into energy.

People suffering from anorexia have been known to show signs of abnormal metabolism, but this has often been thought of as a side effect of being starved of nutrients.

However, the researchers say this genetic link means that an unusual metabolism might actually be partly responsible for causing the disorder.

The researchers say we should think of anorexia nervosa as arising from a combination of both psychological and physiological factors.

They say their analysis potentially expands the list of risk factors for the disorder, that is, characteristics that increase a person's likelihood of developing it.

And that in turn opens up a whole new avenue for medical professionals looking to develop treatments for this deadly disease.

And anorexia isn't the only illness that's more complicated than it seems.

Another new study this week, published in the "Journal of the American Medical Association," found a similar duality with dementia, but the other way around.

Dementia is defined as a decline in mental ability that's severe enough to interfere with a person's daily life.

It can take many forms, but the most common is Alzheimer's disease, which affects more than five million people in the US alone, and Alzheimer's is known to be linked to genes.

There are well-known genetic risk factors that are associated with a patient's likelihood of developing Alzheimer's.

But this new study found that dementia can also be influenced by how healthy a person's lifestyle is.

Like the first study, this one looked at a wide sample of genetic data, this time from nearly 200,000 people in the UK, including more than 1,700 recorded cases of dementia.

The researchers assembled a genetic risk score for each person, but also a lifestyle health score based on each person's self-reported diet, level of physical activity, and frequency of smoking or drinking alcohol.

They found that, even among people with a high genetic risk, the incidence of dementia was significantly lower in people with a healthier lifestyle versus people living less healthy.

The difference was small but noteworthy.

Of the study participants with high genetic risk of dementia, the disease developed in 1.78% of those with unhealthy habits, but only 1.13% of those with a healthier lifestyle.

So in absolute terms, this would mean that if people at higher genetic risk improved their lifestyle, one case of dementia could be prevented for each 121 at-risk individuals every decade.

So this is certainly not a cure, but it does seem that dementia, like anorexia nervosa, is related to a combination of factors, both genetic and behavioral.

Which is exciting because as we learn more, we might discover ways for people to offset their built-in risk of dementia by adjusting their behavior and lifestyle.

But there's definitely still more to be learned.

Both of these studies looked at specific populations of people, mainly those of European ancestry.

And it will no doubt take more study in more diverse populations to tease out exact interrelationships between the various factors involved in these diseases.

But the takeaway here is that illness is complicated, especially the ones we haven't figured out how to treat yet.

Genetics can be very helpful in understanding where diseases arise, but your genes are not your destiny.

Diseases often have complex and interrelated causes, and the more we come to understand that complexity, the better chance we have in the fight against them.

Thanks for watching this episode of SciShow News.