The simplest understanding of this phrase is that you should eat good, healthy foods to be healthy, and that this will make you healthy in body and mind.
Clearly there is some truth to this. Crazy people look crazy and often eat crazy. Even ‘normal’ people, if they eat too much are likely to become fat, lazy, and sick. There is a socio- economic effect (fat people earn less), and a physiological evidence that gut bacteria affects anxiety and depression (at least in rats). My sense here is at the diet extremes though. There is little, or no evidence to suggest you can make yourself more intelligent (or kind or good) by eating more of the right stuff, or just the right foods in just the right amounts. A better diet can make you look better, but there is a core lie at work when you extend this to imply that the real you is your body, or so tied to your body that a healthy mind can not be found in a sickly body. But most evidence is that the mind is the real you, and (following Socrates) that beautiful minds are found in sickly bodies. I’ve seen few (basically, no) healthy poets, writers, or great artists. Neither are there scientists of note (that I can recall) who lived without smoking, drinking, and any bad habits. Many creative people did drugs. George Orwell smoked cigarette, and died of TB, but wrote well to the end. There is no evidence that bad writing or thinking can be improved by health foods. Stupid is as stupid does, and many healthy people are clearly dolts.
Not that it’s always clear what constitutes good health, or what constitutes good food for health, or what constitutes a good mind. Skinny people may be admired and may earn more, but it is not clear they are healthy. Yule Gibbons, the natural food guru died young of stomach cancer. Adele Davis, another the author of “eat right to be healthy,” died of brain cancer. And Jim Fix, “the running doctor” died young of a heat attack while running. Their health foods may have killed them, and that unhealthy foods, like chocolate and coffee can be good for you. It’s likely a question of balance. While a person will feel better who dresses well, the extreme is probably no good. Very often, a person is drawn after his self-image to be the person he pretends. Show me a man who eats only vegetarian, and I’ll show you someone who sees himself as spiritual, or wants to be seen as spiritual. And that man is likely to be drawn to acting spiritual. Among the vegetarians you find Einstein, George B. Shaw, and Gandhi, people who may have been spiritual from the start, but may have been kept to spirituality from their diets. You also find Hitler: spirituality can take all sorts of forms.
Choice of diet also helps select the people you run into. If you eat vegetarian, you’re likely to associate with other vegetarians, and you will likely behave like them. If you eat Chinese, Greek, or Mexican food, you’re likely to associate with these communities and behave like them. Similarly, an orthodox Jew or Moslem is tied to his community with every dinner and every purchase from the kosher or halal store.
And now we come to the bizarre science of bio-systems. Each person is a complex bio-system, with more non-human DNA than human, and more non-human cells than human. A person has a vast army of bugs on him, and a similarly vast pool of bugs within him. Recent research suggests that what we eat affects this bio-system, and through it our mental state. For whatever the mechanism, show me someone who drinks only 30 year Scotch or 40-year-old French wine, and I’ll show you a food snob. By contrast, show me someone who eats good, cheap food, and drinks good, cheap wine or Scotch (Lauder’s or Dewar’s), and I’ll show you a decent person very much like myself, a clever man who either is a man of the people or who wants to be known as one.”Dis-moi ce que tu manges, je te dirai ce que tu es.” [Tell me what you eat and I will tell you what you are].
Robert E. Buxbaum, February, 2015. My 16-year-old daughter asked me to write on this topic. Perhaps she didn’t know what it meant, or how true I thought it was, or perhaps she liked my challenges of being 16.