Now that Dr. Atkins, the American Heart Association, and the Harvard School of Public Health are busy toppling the Food Pyramid end-over-end, it may be time to take a look at the erstwhile tiny tip and realize that not all fats are created equal.
Most people know that fats come in saturated and unsaturated forms, and that the unsaturated ones are generally the better dietary choice. It has recently been shown that trans-fats, aka hydrogenated and partially hydrogenated oils, which are altered to solidify and prolong the shelf-life of margarine and many pre-prepared foods, are unhealthy.
Many people do not realize that some kinds of fat are necessary. These are aptly called the Essential Fatty Acids, or EFAs. Biochemically speaking, there is almost a complete lack of genetic defects in the metabolism of these fats in humans. This is quite rare, since humans exhibit a range of genetic defects in the metabolism of many fats, carbohydrates, and proteins. For example, have you ever noticed the warning on diet soda, “Phenylketonurics, contains phenylalanine”? This indicates a fairly common defect of metabolism that many people have in processing the amino acid phenylalanine. But the EFAs are truly essential to human life — you cannot survive with an inability to process these fats.
The two EFAs have very similar names: linoleic (LA) acid and linolenic (LNA) acid. LA is found in plant oils such as corn, sunflower and soybean oil. LNA is mainly found in cold water fishes like salmon, although there are a few good terre strial sources like purslane, perilla and borage. The parent compound of the omega-3 fatty acids is LNA, and although it is metabolized preferentially over the omega-6 parent, LA, there is still a deficit of omega-3 in the US population. This is because the current American/Western diet is composed of a much higher ratio of LA:LNA, in some cases as high as 20:1. Since these two essential fatty acids compete for the same desaturase and elongase enzymes, the products from LNA are suppressed. That is, there is ample substrate for producing arachidonic acid and very little substrate for producing eicosapentaenoic acid. Many countries have proposed reduced dietary ratios of omega-6 to omega-3, including Japan (between 2:1 and 4:1), Sweden (5:1), as well as the WHO (between 5:1 and 10:1). Currently, the US has no formal recommendations for dietary omega-3 intake.
The ratio is important because these fatty acids are the precursors of the eicosanoids. Eicosanoids are potent, locally acting hormones present in virtually all cell membranes. Like many hormones, they can act in opposition to each other. Since the omega-6 pathway makes many more pro-inflammatory hormones than the omega-3 pathway, the imbalance of the two can lead to many serious chronic inflammatory conditions. Some diseases influenced by this balance include cancer, obesity, hyperlipidemia and hypertri glyceridemia, arthritis, arteriosclerosis, eclampsia, cystic fibrosis, Crohn’s disease, and depression. Therapies currently used to treat some of these conditions, including NSAIDS, steroids, and sulfasalazine may further shift the balance because they inhibit both types of eicosanoids.
The ratio of EFAs in the so-called Paleolithic diet was probably 1:1. What has happened since then to so radically alter this ratio? Vegetable oil-containing processed foods have increasingly been consumed instead of fish, lean meats, vegetables and fruits. The omega-3 content of these vegetable oils has also been altered by plant-breeding programs to reduce LNA and thus improve stability during storage. In addition, the fatty-acid content of the fish and livestock we raise for food has changed as a resu lt of feeding practices. Ruminant animals, such as cows, sheep and goats, are uniquely designed to ingest grass as their primary foodstuff. Grass-fed and wild animals have a significant amount of omega-3 in their tissue fatty acids and a LA:LNA ratio of 1:1. In contrast, feed-lot animals and farm-raised fish, which are fed mainly corn, have a ratio typically greater than 50:1.
To further confound the issue of good fats vs. bad fats, a new form of LA was discovered in the late 1970’s by Dr. Michael Pariza. Dubbed “conjugated linoleic acid” or CLA because of its unusual conjugated diene structure, it differs from linoleic acid only by having two double bonds, which LA lacks. Many isomers exist, but the biologically active one is formed only as a digestive by-product of ruminant fermentation by the bacterium Butyvibrio fibrisolvens. Plant-based CLA is inactive, so the only way to obtain it is from animal products, particularly dairy. Whole milk, whole-fat cheeses, lamb and beef are good sources of CLA. Again, grass-fed animal products have much higher levels of CLA than their grain-fed counterparts.
CLA is a potent antioxidant and mimics many of the anti-inflammatory properties of LNA, while LA is the only fatty acid proven to enhance carcinogenisis in experimental animals.
Physiologic concentrations of CLA can inhibit cultured cells of human malignant melanoma, colorectal cancer, and breast cancer. While this may be attributable in part to its antioxidant activity, CLA also modulates cytochrome P450 activity and suppresses the activity of ornithine decarboxylase and protein kinase C, enzymes involved in carcinogenesis. It inhibits angiogenesis in cancerous growths as well.
CLA also has a role in body fat reduction. It exerts direct dose-dependent reduction in lipoprotein lipase activity and an increase in carnitine palmitoyltransferase activity in adipocytes and skeletal muscle cells. Inhibition of fat storage in adipocytes as well as an increase of skeletal muscle mass has been demonstrated in a number of animal species, including mice, rats, pigs and humans.
So, if you choose carefully and consume in moderation, you can enjoy having a nice glass of whole milk, an extra pat of butter, or a juicy steak this holiday season!