The body has a number of ways of keeping bad things out while taking good things in. One of the things it likes to keep out are plant sterols and stanols (phytosterols), cholesterol-like molecules found in plants. The human body even has two enzymes dedicated to pumping phytosterols back into the gut as they try to diffuse across the intestinal lining: the sterolins. These enzymes actively block phytosterols from passing into the body, but allow cholesterol to enter. Still, a little bit gets through, proportional to the amount in the diet.
As a matter of fact, the body tries to keep most things out except for the essential nutrients and a few other useful molecules. Phytosterols, plant "antioxidants" like polyphenols, and just about anything else that isn't body building material gets actively excluded from circulation or rapidly broken down by the liver. And almost none of it gets past the blood-brain barrier, which protects one of our most delicate organs. It's not surprising once you understand that many of these substances are bioactive: they have drug-like effects that interfere with enzyme activity and signaling pathways. For example, the soy isoflavone genistein abnormally activates estrogen receptors. Your body does not like to hand over the steering wheel to plant chemicals, so it actively defends itself.
A number of trials have shown that large amounts of phytosterols in the diet lower total cholesterol and LDL. This has led to the (still untested) hypothesis that phytosterols lower heart attack risk. The main problem with this hypothesis is that although statin drugs do lower LDL and heart attack risk, not all interventions that lower LDL lower risk. LDL plays an important role in heart attack risk, but it's not the only factor. Statins have a number of biological effects besides lowering LDL, and some of these probably play a role in its ability to protect against heart attacks.
Lowering total cholesterol and LDL through diet and drugs other than statins does not reliably reduce mortality in controlled trials. Decades of controlled diet trials showed overall that replacing saturated fat with polyunsaturated vegetable oil lowers cholesterol, lowers LDL, but doesn't reliably reduce the risk of cardiovascular disease. Soy contains a lot of phytosterols, which is one of the reasons it's heavily promoted as a health food.
All right, let's put on our entrepreneur hats. We know phytosterols lower cholesterol. We know soy is being promoted as a healthier alternative to meat. We know butter is considered a source of artery-clogging saturated fat. I have an idea. Let's make a margarine that contains a massive dose of phytosterols and market it as heart-healthy. We'll call it Benecol, and we'll have doctors recommend it to cardiac patients.
Here are the ingredients:
Liquid Canola Oil, Water, Partially Hydrogenated Soybean Oil, Plant Stanol Esters, Salt, Emulsifiers, (Vegetable Mono- and Diglycerides, Soy Lecithin), Hydrogentated Soybean Oil, Potassium Sorbate, Citric Acid and Calcium Disodium EDTA to Preserve Freshness, Artificial Flavor, DL-alpha-Tocopheryl Acetate, Vitamin A Palmitate, Colored with Beta Carotene.Nice.
And I haven't even gotten to the best part yet. There's a little disorder called phytosterolemia that may be relevant here. These patients have a mutation in one of their sterolin genes that allows phytosterols (including stanols) to pass into their circulation more easily. They end up with 10-25 times more phytosterols in their circulation than a normal individual. What kind of health benefits do these people see? Premature atherosclerosis, an early death from heart attacks, abnormal accumulation of sterols and stanols in the tendons, and liver damage.
Despite the snappy-looking tub, margarine is just another industrial food-like substance that I am highly suspicious of. In the U.S., manufacturers can put the statement "no trans fat" on a product's label, and "0 g trans fat" on the nutrition label, if it contains less than 0.5 grams of trans fat per serving. A serving of Benecol is 14 grams. That means it could be up to 3.5 percent trans fat and still labeled "no trans fat". This stuff is being recommended to cardiac patients.
When deciding whether or not a food is healthy, the precautionary principle is in order. Margarine is a food that has not withstood the test of time. Show me a single healthy culture on this planet that eats margarine regularly. Cow juice may not be as flashy as the latest designer food, but it has sustained healthy cultures for generations. The U.S. used to belong to those ranks, when coronary heart disease was rare.