Snake Oil: Don't Believe Everything You Hear About Dietary Fats


WE LOVE DIETARY fats—and the more we learn how bad they are for us, the more sophisticated we become about consuming them. Gourmets who forgo such plebian pleasures as doughnuts, french fries, and thick shakes can be found ordering croissants, premium ice cream, and pâté. Just why people seem to prefer fat over lean is still something of a mystery to food scientists. Some suggest that fat, although not necessarily flavorful in itself, serves as a carrier for the flavor molecules that give foods what we think of as their characteristic taste. Others suggest that the taste for fat evolved as a protective trait, to help primitive man stave off starvation. Dietary fat is easily stored in the body—it requires less energy to be converted into body fat than either starch or protein. This was good news for the scrawny Neanderthal but is not for the office worker with a craving for smothered pork chops.

Whatever the reason, for most Americans a true low-fat diet is a stern, unfriendly thing. The American Heart Association’s recommendation that we consume no more than 30 percent of our calories in fat, then, comes as a hard blow. Thirty percent may seem like a lot. Fats are, after all, one of three major sources of calories, and so allowing them nearly a third of our culinary space sounds reasonable. But fat has nine calories per gram—two and a quarter times the calories of protein or carbohydrates—so a little goes a long way toward tipping the nutrient balance.

Conventional wisdom has it that one can minimize feelings of deprivation while maintaining good health by substituting one fat for another, in particular vegetable fat for animal fat. This has led to some deceptive marketing practices, such as the labeling of foods that contain vegetable fat as being beneficial to cardiovascular health. The distinctions among saturated fats, unsaturated fats, and cholesterol have become clouded in the rush to promote some foods as beneficial while casting aspersions on others. There is, in fact, no proof that eating any kind of fat will reduce the risk of cancer or heart attack, and most fats seem to increase the risk of one or both of these. Perhaps the most surprising news is that dietary cholesterol, long considered the most pernicious of foods, seems to have little effect on blood cholesterol when eaten without accompanying saturated fat.

ALL FAT MOLECULES are composed of fatty acids, which are made of long chains of carbon atoms flanked with hydrogen atoms. When every carbon in the chain has a full load of hydrogen, the acid is said to be saturated. When two adjacent carbon atoms in the chain are each lacking a hydrogen atom, the acid is mono-unsaturated, and when there are several such pairs, the acid is polyunsaturated.

The consumption of most saturated fats raises the level of cholesterol in the blood, which, in turn, increases the risk of cardiovascular disease. Polyunsaturated and mono-unsaturated fats do not have this effect. But no fats that we actually eat fall neatly into one of these categories—all of them are composed of a mixture of saturated and polyunsaturated fatty acids, and some contain mono-unsaturates as well.

For example, popular belief has it that animal fat is heavily saturated, while vegetable oils are rich in polyunsaturates. But in fact beef tallow (fat) and lard contain almost as much monounsaturated as saturated fat, and palm and coconut oils are the purest known sources of saturated fat. The heart-conscious would be better off putting heavy cream in their coffee than nondairy creamer, which is usually made from coconut oil. And any vegetable fat that is chemically hydrogenated to make it solid or to keep it from becoming rancid— such as margarine, most peanut butters, or cooking fats like Crisco and Spry— is also saturated. You can generally tell how saturated a fat is by how solid it remains at room temperature. For example, corn oil is less saturated than tub margarine, which is less saturated than stick margarine. However, not every saturated fat is bad for the heart. Cocoa butter is hard at room temperature, but it is largely made up of stearic acid, a fatty acid that is not well absorbed by the body and does not significantly raise blood cholesterol.

Cholesterol is not a fat at all but an alcohol that has little in common with dietary fats except insolubility in water. Virtually every animal cell, including human cells, contains cholesterol, which is vital to the manufacture of cell membranes, sex and adrenal hormones, liver bile, and the covering of nerve cells. No one over the age of six months needs to eat other animals or drink milk to get cholesterol—a healthy liver churns out enough, about 800 milligrams a day, to cover any contingency. But the additional 500 to 600 milligrams of cholesterol we eat daily, while unnecessary, does not appear in most cases to raise the blood(or serum-) cholesterol level. This is why veal, seafood, and skinless chicken, which are moderately high in cholesterol but relatively low in saturated fat, are recommended as substitutes for red meat to people who are at risk for heart disease. Eggs, an excellent source of cheap protein, are low in saturated fat and calories, although high in cholesterol. (Cholesterol is rich in calories, but the body cannot use them as an energy source.) Studies have shown that eating as many as six eggs at a sitting does not significantly elevate blood-cholesterol levels in normal subjects who eat no saturated fat with the meal. There is some evidence, though by no means a consensus, that dietary cholesterol might increase the power of saturated fats to raise blood-cholesterol levels. And it is certain that saturated fats increase the power of cholesterol to raise blood cholesterol. Restricting saturated fat when eating high-cholesterol foods, then, is probably a good bet for those who have reason to be concerned about their blood-cholesterol levels. That means eating eggs on dry toast rather than on steak, skipping bacon and sausage, and cooking eggs in a non-stick frying pan or in polyunsaturated fat rather than in butter.

Omega-3, a family of highly unsaturated fatty acids found most commonly in fish fat, has recently been touted as a preventive of heart disease. Makers of fish-oil supplements and representatives of fishing-industry concerns point to studies showing that Greenland Eskimos, who consume enormous quantities of fatty marine animals, have very low levels of heart disease. But what is rarely mentioned is that the Eskimos studied had a very high incidence of stroke, alcoholism, and other disorders that gave them, on average, a life-span that ended before prime heart-attack age. They also led strenuous lives; exercise increases the level of high-density lipoproteins, or HDLs, which carry cholesterol out of the arteries. And, although they consumed plenty of cholesterol, they ate relatively little saturated fat. Fish oil itself has almost no effect on blood-cholesterol levels. It may, theoretically, reduce heart-attack risk by interfering with blood clotting (Greenland Eskimos have thinner blood than the rest of us), but this theory is unproved. At any rate, a similar blood-thinning effect can be had by eating half an aspirin.

Substituting fish for red meat or cheese might still be a good idea, because it will reduce one’s intake of saturated fat. But since much of the fish sold in this country is taken from polluted waters, eating fatty fish could be risky. Most carcinogens and other pollutants are fat-soluble, and concentrate in the liver and in body fat. Some nutritionists recommend that fish not be eaten more than twice a week, especially by children, and that fish-oil capsules be avoided.

Polyunsaturated fats are also widely advertised as lowering blood cholesterol, but they do not. They simply do not raise it. And polyunsaturated fats may have their own drawbacks. Most are rich in linoleic acid, which has been shown to alter blood pressure. Consuming large amounts of linoleic acid also increases the incidence of cancer in laboratory animals. It is not clear whether people who consume large amounts of polyunsaturated fat are at increased risk for cancer, and there is no evidence that the balance between polyunsaturated and saturated fats in the diet has a direct effect on life expectancy.

The good news is that mono-unsaturated fats do not provoke tumors in animals or heart attacks in human beings. The bad news is that not many foods are high in mono-unsaturates. The main sources are olive oil, almond oil, and avocado oil. Replacing saturated fats

with any of these would reduce the risk of heart disease without increasing the risk of cancer, but so would replacing them with starch or protein.

Although mono-unsaturated oils seem to pose the lowest risk, an ideal diet for a healthy person would include a small quantity of fat from each of the three categories. Naturally, for those with a history of heart disease, saturated fats should be kept to a minimum. Likewise, if cancer runs in the family, polyunsaturated fats should be minimized. The best overall strategy for everyone, though, is to minimize the total amount of fat in the diet. All things being as equal as they can be—which is not very, when it comes to diet—people who consume fewer fat calories of any sort appear to live longer and healthier lives.