Today, we all know that specific foods- fruits, vegetables and the like- are good for us, while eating too many desserts and not exercising will make you fat. What is the earliest example of someone making this connection?
The Hippocratic Corpus from classical Greece (circa 5th century BCE) already contains discussions of some foods being better for you than not. For instance, On Ancient Medicine has a discussion of the effects of diet on health (in Chapter 3). The date of the work is hard to pin down more than the range of 450-400 BCE, but it's at the very beginning of Western medical literature.
It is possible that the medical literature of other civilisations may pre-date this attestation in classical Greece. It's likely that some older text will have some reference to food as healthy or not (say, pertaining to some event in a myth or epic). But the point is that as long as we've had works in the West dealing specifically with health, it has included a recognition of some foods as healthy and some as not.