To answer this question, we first have to take a moment to consider the nature of mental illness. Mental illness is not a 'static' thing the way that physical illness is - the prevalence and the expression of mental illness is hugely dependent on the culture and surroundings a person was raised in, and the mental illnesses that we are familiar with in modern western industrialized nations often look very different than mental illnesses in non-western societies, or in pre-industrial societies. A person with untreated diabetes in modern America is going to have the same symptoms as a person with untreated diabetes in modern China or in colonial America, but a person with a mental illness is going to potentially have very different symptoms in those different settings.

Our culture and our surroundings shape who we are as people, our values and beliefs, our patterns of behaviour, and the bounds of what is 'acceptable' behaviour and what is not - when something goes wrong and we experience mental illness, it tends to happen in a way that makes sense within the greater context of our cultural surroundings. An American who experiences depression is likely to experience loss of motivation, feelings of sadness or numbness, and social withdrawal; these are symptoms that make "sense" to us culturally, and that we understand to signal depression. A Chinese person, on the other hand, is likely to experience depression as headaches, fatigue and insomnia; these symptoms are more accepted and make more sense within the bounds of this culture, and they tend to be much more prevalent among people who are experiencing what we would consider depression.

It is also possible for a mental illness to exist only within the bounds of one single culture, as a response to a pressure or stress that is unique to that culture - elderly Japanese women, for instance, experience a mental illness called 'One's Husband Being at Home Syndrome' that is common to them, but not found anywhere else in the world. The structure and power dynamics found in older Japanese couples - along with Japan's lack of social safety net - cause Japanese women to suddenly find themselves in a unique stressful situation when their husbands retire, leading to the development of a "new" mental illness that makes sense within their culture, but not anywhere outside of that particular place and population. Mental illness is flexible, and reflects the circumstances of a person's life. Eating disorders are particularly prone to cultural influence. Although they do exist all over the world, they are most prevalent in Western cultures that highly prioritize thinness as a cultural beauty standard, and we have actually watched rates of eating disorders rise as non-westerners are increasingly influenced by western cultures. We have also seen eating disorders rise in males recently, as beauty standards for men become increasingly rigid - eating disorders have a very strong tie to the culture they exist in, and are very responsive to changes in cultural ideals.

So when we think about what eating disorders - or any mental illness - was like in the past, it's important for us to consider that eating disorders may have looked very different hundreds of years ago, and that behaviours that resemble modern-day eating disorders may have had completely different origins and motivations. Since we cannot assess people who lived in Colonial America, we cannot formally diagnose them with eating disorders or assume that their unusual eating patterns were the equivalent of similar eating patterns we see today. We do have reports of behaviours in the past that outwardly resemble modern-day anorexia to some degree, but it's just not possible for us to conclusively say that we're looking at the exact same disorder - in the past, sudden and unexplained loss of appetite could have been the result of a disease, parasite or accidental poisoning. "Sally throws up after she eats" could mean anything from "Sally is allergic to something in the food and we just don't understand that" to "Sally is coming down with dysentery".

With that said, we do have historical evidence of pre-industrial people appearing to intentionally deprive themselves of food, to the point of physical harm or even death. These behaviours were often tied to religious fanaticism; early Christians prized self-sacrifice, suffering and the denial of earthly pleasures as signs of a person's purity and piousness, and many of the early accounts we have of people intentionally starving themselves to death come from people who were heavily engaged in this mindset. For people living in strict puritanical cultures, fasting was a sign of spiritual strength. "Sally doesn't eat at mealtimes" could be understood by the community and by Sally herself as "Sally is fasting to be closer to the Lord", even if there was an underlying psychopathology at play. Again though, it is not clear if this behaviour is an exact parallel for modern-day anorexia; people who deprive themselves of food to a dangerous extent for predominantly religious purposes do still exist today, and they are sometimes diagnosed with anorexia nervosa, but their stated motivations have more to do with spiritual purity than physical appearance.