I actually would be hesitant to say that eating disorders today are all about body image - I think it's got more to do with a complex relationship with control and purity and death - but leaving that aside, yes, humans have always had complicated engagements with food. The answer to your question is essentially that similar phenomena existed, but the "disorders" were different and were not necessarily something we should call a disorder, because food and the body were seen in very different ways to how they are today. In order to deal with food practices as a whole without modern bias or medicalization, I usually prefer to refer to it as "strange eating." In the culture I study (medieval Iran), strange eating was a sign of sanctity and is often mentioned in sources on saints. By strange eating, I mean any of the following: 1. refraining from food, 2. eating only very plain food, 3. eating weird food or non-food, 4. extreme gluttony, 5. any of several more complex and specific things, such as an infant saint refusing the milk of his pagan mother.

One of my favorite examples is a saint who appears in Jami's Nafahat al-Uns. This is a book that's made up of hundreds of biographical entries on information known about individual holy men/Sufis, and the entries vary enormously in length and content. One of these entries consists in its entirety of a story told about a saint who said he was hungry and was brought enough food for fifty people. People offered to cook it for him, but he refused to let them and instead ate it all, raw, in one sitting. Later that same night, he went into a room in the mosque. The story's narrator heard the sound of eating, and was like "uh... there's no food in there." When morning came the guy came out and the narrator went in and discovered that this guy had EATEN ALL THE BOOKS. End of entry. That's self-evidently a saint. The book's author feels no need to comment further. Next entry.

There's a truly wonderful book on strange eating in Europe called Holy Feast and Holy Fast by Caroline Walker Bynum. It's all about medieval women's food practices related to religion, and the religious significance of food in their lives, and describes some holy women with strange eating we would find totally wild today, such as drinking the pus of the sick, and others which we would find normal and file easily under modern anorexia. There are even some really fascinating things about breastfeeding, including one woman's dream of breastfeeding from Christ. Just like in my area, there's a huge diversity of ways to engage with food and the body and they're not all as simple as just avoiding it. And even when we're just talking about food avoidance, Bynum is always meticulous in pointing out that just because one practice resembles another on the surface, that doesn't mean the two are the same. Food avoidance for religious reasons and food avoidance for the reasons modern anorexics avoid food are very, very different things and shouldn't be conflated. To quote her on this:

To religious women food was a way of controlling as well as renouncing both self and environment. But it was more. Food was flesh, and flesh was suffering and fertility. In renouncing ordinary food and directing their being towards the food that is Christ, women moved to God not merely by abandoning their flawed physicality but also by becoming the suffering and feeding humanity of the body on the cross, the food on the altar.

She's essentially pointing out that the fact of physicality and embodiment, and the fact that our bodies need food, doesn't have to involve understandings as simple (she calls it "impoverished") as in our modern culture, where food is food and the body is the body and that's all. For most people in most places at most times, food and the body have been places to engage in profound, sometimes joyous ways with suffering, death, fertility, and the eternal.