The Omnivore's Dilemma - Michael Pollan [137]
Perhaps not surprisingly, the large quantities of beta-carotene, vitamin E, and folic acid present in green grass find their way into the flesh of the animals that eat that grass. (It’s the carotenoids that give these egg yolks their carroty color.) That flesh will also have considerably less fat in it than the flesh of animals fed exclusively on grain—also no surprise, in light of what we know about diets high in carbohydrates. (And about exercise, something pastured animals actually get.) But all fats are not created equal—polyunsaturated fats are better for us than saturated ones, and certain unsaturated fats are better than others. As it turns out, the fats created in the flesh of grass eaters are the best kind for us to eat.
This is no accident. Taking the long view of human nutrition, we evolved to eat the sort of foods available to hunter-gatherers, most of whose genes we’ve inherited and whose bodies we still (more or less) inhabit. Humans have had less than ten thousand years—an evolutionary blink—to accustom our bodies to agricultural food, and as far as our bodies are concerned, industrial agricultural food—a diet based largely on a small handful of staple grains, like corn—is still a biological novelty. Animals raised outdoors on grass have a diet much more like that of the wild animals humans have been eating at least since the Paleolithic era than that of the grain-fed animals we only recently began to eat.
So it makes evolutionary sense that pastured meats, the nutritional profile of which closely resembles that of wild game, would be better for us. Grass-fed meat, milk, and eggs contain less total fat and less saturated fats than the same foods from grain-fed animals. Pastured animals also contain conjugated linoleic acid (CLA), a fatty acid that some recent studies indicate may help reduce weight and prevent cancer, and which is absent from feedlot animals. But perhaps most important, meat, eggs, and milk from pastured animals also contain higher levels of omega-3s, essential fatty acids created in the cells of green plants and algae that play an indispensable role in human health, and especially in the growth and health of neurons—brain cells. (It’s important to note that fish contain higher levels of the most valuable omega-3s than land animals, yet grass-fed animals do offer significant amounts of such important omega-3s as alpha linolenic acid—ALA.) Much research into the role of omega-3s in the human diet remains to be done, but the preliminary findings are suggestive: Researchers report that pregnant women who receive supplements of omega-3s give birth to babies with higher IQs; children with diets low in omega-3s exhibit more behavioral and learning problems at school; and puppies eating diets high in omega-3s prove easier to train. (All these claims come from papers presented at a 2004 meeting of the International Society for the Study of Fatty Acids and Lipids.)
One of the most important yet unnoticed changes to the human diet in modern times has been in the ratio between omega-3 and omega-6, the other essential fatty acid in our food. Omega-6 is produced in the seeds of plants; omega-3 in the leaves. As the name indicates, both kinds of fat are essential, but problems arise when they fall out of balance. (In fact, there’s research to suggest that the ratio of these fats in our diet may be more important than the amounts.) Too high a ratio of omega-6 to omega-3 can contribute to heart disease, probably because omega-6 helps blood clot, while omega-3 helps it flow. (Omega-6 is an inflammatory; omega-3 an anti-inflammatory.) As our diet—and the diet of the animals we eat—shifted from one based on green plants to one based on grain (from grass to corn), the ratio of omega-6 to omega-3