by Craig Weatherby
It's the fall equinox on Sunday… that time of the year, mirrored once in the spring, when day and night are each twelve hours long.
Big changes are happening as a result of the earth's tilted orbit around the sun. In the northern hemisphere, trees are responding to the diminishing light by losing their leaves and going dormant. In the southern hemisphere, seeds from last year are beginning to germinate; the land is turning green.
This much we all know, but what scientists are just beginning to appreciate is what this seasonal shift in light means to animals that spend their days foraging for food. How it represents a simultaneous shift in the availability of the two families of essential fats, omega-3s and omega-6s, the fats animals can't make for themselves and require in their diets.
Leaves are the most abundant source of the parent omega-3 fat, alpha linolenic acid, the fat that animals turn into the better known eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), and seeds are rich in the parent omega-6 fat, linoleic acid, which animals turn into arachidonic acid.
About Susan Allport
She is the author of two other highly praised books—The Primal Feast: Food, Sex, Foraging, and Love, and A Natural History of Parenting—and has appeared on Oprah & Friends Radio and NPR's "Science Friday" and "The Splendid Table".
We're sure you'll find her contributions enlightening pleasures to read!
We're used to thinking of these polyunsaturated fats, with their long, difficult-to-pronounce names, as one big happy family. But they are two competing families (Montagues and Capulets) with very different roles to play in nature.
DHA, the longest leaf-derived, omega-3 fat, is found in highest concentrations in the cell membranes of those tissues with the fastest metabolic rates: eyes, brains, and hearts.
Similarly, alpha linolenic acid, the parent omega-3 fat, is found in highest concentrations in the chloroplasts of green leaves where it assists plants with their fastest activity, photosynthesis, the basis of all life on earth.
Concentrations of essential fats change with an animal's diet and are associated with some dramatic changes in behavior. Hibernating animals, like the yellow-bellied marmot of Colorado, do not go into hibernation when their diet is rich in omega-3s, as it is in the spring and summer when there are plenty of leaves to eat. These animals need fewer leaves and more seeds before they slow down and go into torpor.
It's a pretty neat system. Plants respond to the changing light by making or losing leaves, and animals use their changing food supply to prepare themselves for the future.
As animals reduce their intake of leaves (and/or animals that eat leaves) and increase their intake of seeds (and/or animals that eat seeds), seed fats come to outnumber leaf fats in the membranes of their cells. Metabolic rate falls and animals gain weight, which they store as fat. Come spring, as seeds germinate and form leaves (a process in which the omega-6s are turned into omega-3s by an enzyme that only plants have), an animal's new, green, faster diet prepares it for activity and reproduction.
(In the tropics, by the way, where day and night are always twelve hours long, animals experience similar shifts in the availability of the two families of essential fats. But there, these shifts result from changes in rainfall rather than daylight.)
It's a neat system for everyone but humans, whose sophisticated food processing techniques, specifically, our ability to squeeze all the oil out of seeds, enables us to eat a high omega-6 diet year round.
Per capita consumption of the parent omega-6 fat, linoleic acid, the most abundant fat in soybean, corn, safflower and sunflower oils, has been climbing steadily in the United States since the turn of the 19th century, from about seven grams a day to twenty five (almost two tablespoons), an ascent that closely parallels rises in heart disease, obesity, diabetes, depression and certain forms of cancer.
These parallels would be meaningless if not for the many mechanisms by which excessive omega-6s are known to promote (and omega-3s are known to protect against) all these conditions.
Per capita consumption of the parent omega-3 fat has remained fairly constant over this same period—about two grams a day or a half a teaspoon. But this faster fat is now being swamped, in our diet and tissues, by the many seed fats.
This is the real omega-3 story, and it makes sense of the advice to Americans to eat their twice-weekly meals of cold-water fish. It also provides a timely alternative to that advice, given the state of most of our fish stocks and the contamination of some species with mercury and PCBs ( a notable exception to both is the Alaskan wild salmon fisheries).
Because fish are one of the few animals in our diet that still eat greens and because fish live in cold, dim environments where they need more of these faster fats in their membranes, fish are, indeed, a good source of omega-3s.
But eating fish is not the only way of fixing a problem that came about from eating large amounts of high omega-6 oils.
The best way is to understand that fats, too, have their seasons and the American diet is now top heavy with the fats of seeds.