Wednesday, November 17, 2010

The Decline of the Western Diet

Omega-3 molecules are a by-product of the happy meeting of sunlight, water, and carbon dioxide in the chloroplasts of terrestrial plants and marine algae. Not long ago, these fatty acids were an inescapable component of our diet. Back in the early 1900s — long before the arrival of bovine growth hormone and patented transgenic seeds — American family farms were perfect factories for producing omega-3s.

Bucolic, sun-drenched pastures supported a complex array of grasses, and cattle used their sensitive tongues to pick and choose the ripest patches of clover, millet, and sweet grass; their rumens then turned the cellulose that humans can't digest into foods that we can: milk, butter, cheese, and, eventually, beef, all of them rich in omega-3s. Cattle used to spend four to five carefree years grazing on grass, but now they are fattened on grain in feedlots and reach slaughter weight in about a year, all the while pumped full of antibiotics to fight off the diseases caused by the close quarters of factory farms.

Likewise, a few generations ago, chickens roamed those same farms, foraging on grasses, purslane, and grubs, providing humans with drumsticks, breasts, and eggs that were rich in grass-derived omega-3s. Today, most American chickens are now a single hybrid breed — the Cornish — and are raised in cages, treated with antibiotics, and stuffed full of corn.

Our animal fats were once derived from leafy greens, and now our livestock are fattened with corn, soybeans, and other seed oils. (Even the majority of the salmon, catfish, and shrimp in our supermarkets are raised on farms and fattened with soy-enriched pellets.) So not only have good fats been stricken from our diets, but these cheap, widely available seed oils are the source of another, far less healthy family of fatty acids called omega-6s, which compete with omega-3s for space in our cell membranes.

Omega-6s are essentially more rigid fatty acids that give our cells structure, while omega-3s are more fluid and help our bodies fight inflammation. Our ancestors ate a ratio of dietary omega-6s to omega-3s of approximately 1:1. The Western diet (the modern American and European eating pattern characterized by high intakes of red meat, sugar, and refined carbohydrates) has a ratio of about 20:1.

"The shift from a food chain with green plants at its base to one based on seeds may be the most far reaching of all," writes Michael Pollan in his prescriptive manifesto In Defense of Food. "From leaves to seeds: It's almost, if not quite, A Theory of Everything."

This shift began in earnest in the 1960s. Research on the links between cholesterol and saturated fats and coronary heart disease led health authorities to demonize lard, dairy products, and other animal-derived sources of fat. Meanwhile, new health guidelines lionized the polyunsaturated fats in vegetable oils and margarine (which is merely vegetable oil solidified via hydrogenation, a process that creates the dreaded trans fats).

Food processors were happy to play along: Polyunsaturated seed oils did not go rancid as quickly as omega-3s, which meant a longer shelf life for packaged foods. One form of fat in particular, omega-6-rich soybean oil, is now ubiquitous in processed foods. Soybeans, originally an import from East Asia, have become the second most valuable food crop in the United States. Genetically modified to resist pests, they are crushed to make high-protein meal for livestock, and the heavily subsidized industry has found ingenious ways of moving its product in the form of "soy isoflavones," "textured vegetable protein," "soy protein isolate," and the other novel ingredients lurking on the labels of processed foods.

Look around your kitchen and you'll find soybean oil in everything from salad dressing to Crisco, from processed cheese to granola bars. If you are eating a processed food, chances are it contains soy. Twenty percent of Americans' calories now come from soybeans; the average person eats 25 pounds of the stuff a year. Just four seed oils — soybean, corn, cottonseed, and canola oil — account for 96 percent of the vegetable oil eaten in America today.

The spread of the seed-oil-rich Western diet around the world has been tracked by a statistical rise in the so-called diseases of civilization: asthma and arthritis, depression and Alzheimer's, heart disease and cancer, as well as metabolic disorders such as diabetes and obesity. 

"There has been a thousandfold increase in the consumption of soybean oil over the past hundred years," says Joseph Hibbeln, MD, acting chief of the section on nutritional neurosciences at the National Institutes of Health in Bethesda, Maryland. The result, he states, is an unplanned experiment in brain and heart chemistry, one whose subject is the entire population of the developed world. In a series of epidemiological studies, Dr. Hibbeln showed that populations that consume high levels of omega-3s in the form of seafood are the least afflicted by the major diseases associated with the Western diet.

Living Longer on a 'Poor Man's Diet' 
How could a simple change in dietary fat have such a huge impact on so many aspects of our health? The answer lies in the nature of two specific forms of omega-3s, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), which are especially rich in seafood.

DHA has a cylindrical shape and can compress and twist like a Slinky, switching between hundreds of different shapes billions of times a second. The molecule is particularly abundant in the tails of rattlesnakes, the wings of hummingbirds, the tails of sperm, and the retinas and brain cells of people who eat fish. A neuron that is high in DHA molecules is virtually liquid, allowing for more effective reception of serotonin, dopamine, and other crucial neurotransmitters. In test subjects, this heightened neuroplasticity has been linked to better vision and eye-hand coordination, better mood, enhanced general movements, and an increased capacity for sustained attention.

EPA is no less crucial: It reduces blood clotting and dampens the inflammatory response in tissues. Such chronic inflammation is suspected to be at the root of most of the so-called diseases of civilization, from Alzheimer's and depression to heart disease and cancer.

While it's true that terrestrial plants are good sources of omega-3s, the fatty acid most present in land-based species is alpha-linolenic acid (ALA). Essential for good health, ALA can be found in fruits, vegetables, and some seeds, among them lettuce, leeks, purslane, kale, broccoli, blueberries, hemp, chia, and flaxseed. ALA is especially rich in plants that grow in intense light, and the fatty acid is thought to help the plants recover from sun damage. Though the human body is capable of turning ALA into DHA and EPA through a series of enzymatic reactions, it is not particularly good at it: Less than 1 percent of the ALA we get from vegetable sources ultimately becomes DHA and EPA. The ocean is the world's richest source of DHA and EPA, particularly from plankton-eating oily fish such as sardines, mackerel, and herring.

"You should eat vegetables and fruits, of course, and get exercise," advises Stephen Cunnane, Ph.D., researcher in brain metabolism at Quebec's University of Sherbrooke.  "But you have to eat fish. You can take fish-oil capsules, but part of the point is to enjoy the experience of eating. So buy the best fish you can afford." Seafood also has the edge on omega-3 capsules because it includes the brain-selective minerals zinc, iron, copper, iodine, and selenium, cofactors our bodies need to make optimal use of EPA and DHA.

In other words, if you are looking for a guiding principle, keep it simple and eat like your ancestors ate.

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