Michael Pollan argues in In Defense of Food that we should ignore nutritional science and stick with traditional diets. He explains,
Confounding factors aside, people who eat according to the rules of a traditional food culture are generally much healthier than people eating a contemporary Western diet…I’m inclined to think any traditional diet will do; if it wasn’t a healthy regimen, the diet and the people who followed it wouldn’t still be around. (173)
This is the dietary philosophy which Daniel Engber called “nutritional Darwinism.” The idea behind the name is that people who eat unhealthy diets will be selected out (in the sense of Darwin’s natural selection) of the human population over time.
On the face of it, it might be an appealing idea, but nutritional Darwinism is reductionist in its own way. Namely, it reduces all of human health to the ability to reproduce and raise offspring who can reproduce. If we only know that a group of people has survived on a particular diet for generations, that doesn’t tell us much about how the people tend to die, how long they tend to live, or any non-fatal health problems they may experience. And as the story of beriberi shows, groups of people have survived for generations on traditional diets that had serious nutritional deficiencies.
However, to fully appreciate the weakness of this line of reasoning, consider what it might say about the “Western diet” from which Pollan urges readers to “escape.” Although Pollan contends that this diet is a relatively recent development, it’s not obvious that it can’t survive for a while. There clearly is no shortage of Americans at reproductive age. Moreover, the “Western diseases”–the conditions such as heart disease and cancer that Pollan blames on the Western diet–tend to strike later in life, so they don’t usually interfere with people’s ability to reproduce. This means that there’s no reason to believe that the Western diet will kill off its eaters any time soon. Therefore, if the principle of nutritional Darwinism is to be believed, the Western diet must be healthy!