There are some New-Age types designated "Breatharians" who claim they don't have to eat. Perhaps they just get by on the air they breathe, or on sunbeams. Breatharians are unlikely to be willing to be scientifically tested for this ability. I don't know of any analogous group that says they are also free from drinking water, or from breathing oxygen, but for the rest of us, taking in a bit of daily nutrition is a habit we cannot break. Since eating is something that has been on the mind of members of our species ever since we had minds, it is something we ought to know plenty of facts about, and we do. Facts in scientific style, however, have only come with difficulty over the past couple of centuries, and along with them have come a lot of fads and foolishness. All are topics within _Terrors of the Table: The Curious History of Nutrition_ (Oxford University Press) by Walter Gratzer. The author is a biophysicist and an emeritus professor at Kings College, London, who values the way we have come to some scientific understanding of nutrition, but he also enjoys telling about human folly, and it seems that eating is so essential to us that both are on generous display in this exhaustive historical survey.
Nutritional theorizing began, like everything, with the ancient Greeks. Hippocrates stressed moderation of intake and the need for exercise. Galen took up Hippocrates's theory of the bodily humors in the second century CE, and enlarged upon it, and his unquestioned (and often unwise) teaching lasted for centuries. The eventual understanding of how vitamins are essential to health forms many chapters of this book. There is a surprising adherence to a pattern of understanding for each vitamin. Doctors would have spotted a particular disease condition, but were reluctant to accept that it was due to a nutritional deficiency, explaining it instead as an infection or an intake of toxins. Different experts at diverse times would come to some understanding about what foods would clear up the problem, but the inertial resistance to change would condemn sufferers to illness for years, while authorities in the rearguard criticized (sometimes with acid vituperation) what turned out to be healthful suggestions. We groped our way toward a nutritional understanding of what was good for us, but there has never been any lack of self-appointed experts to tell us. For some reason, the United States has been the region from which the most durable food fads have sprung, with famous names like Graham, Kellogg, and Post all implicated in the silliness. The most amusing crackpot was Horace Fletcher who at the turn of the 20th century proposed that all ills could be banished by chewing food thoroughly. Just chew every bite 32 times (one for each tooth), ordered the Great Masticator, a man who had an imposing stage presence and was an accomplished liar about his qualifications. Chewing parties became a fashion among some diners, with a conductor who counted and timed the bites and authorized the swallows.
Gratzer finishes this amusing and scary survey with a chapter on current nutritional trends. It might be titled "If you're so smart, why ain't you healthy?" We know plenty about nutrition by now, we know what levels of fat, carbohydrate, proteins, minerals, and vitamins anyone ought to take in, and still we are eating badly. He reviews the questionable and complicated data that have to do with, for instance, fiber and salt intake, and finds that public understanding of such issues (fiber good, salt bad) may have little correlation with actual health results. Worry over cholesterol levels has produced millions of dollars worth of benefit for pharmaceutical companies, but beneficial effects on general health have been far harder to find. There are common additives now of which no one really knows the long-term effects, but nothing quite as yucky as the adulterants used in the past, like the snails put into watered milk to make it frothy. We eat badly, we are gaining weight, and diabetes rates are going up. Gratzer's advice: diets full of synthetic and processed foods promote hypertension and obesity. Fresh fruit and vegetables are good for you. And always: "That it is prudent to avoid excess of any type of food seems now to be the clearest message." It's been a couple of thousand years with lots of scientific experiments and papers, but Hippocrates would have agreed.