Fats and Cholesterol - The Good, The Bad, and The Healthy Diet

"Eat a low-fat, low-cholesterol diet." Most of us have heard this simple recommendation so often over the past two decades that we can recite it in our sleep. Touted as a way to lose weight and prevent cancer and heart disease, it's no wonder much of the nation - and food producers - hopped on board.

Unfortunately, this simple message is now largely out of date. Detailed research -much of it done at Harvard - shows that the total amount of fat in the diet, whether high or low, isn't really linked with disease. What really matters is the type of fat in the diet. New results from the large and long Women's Health Initiative Dietary Modification Trial showed that eating a low-fat diet for 8 years did not prevent heart disease, breast cancer, or colon cancer, and didn't do much for weight loss, either.

What is becoming clearer and clearer is that bad fats, meaning saturated and trans fats, increase the risk for certain diseases while good fats, meaning monounsaturated and polyunsaturated fats, lower the risk. The key is to substitute good fats for bad fats.

And cholesterol in food? Although it is still important to limit the amount of cholesterol your eat, especially if you have diabetes, dietary cholesterol isn't nearly the villain it's been portrayed to be. Cholesterol in the bloodstream is what's most important. High blood cholesterol levels greatly increase the risk for heart disease. But the average person makes about 75% of blood cholesterol in his or her liver, while only about 25% is absorbed from food. The biggest influence on blood cholesterol level is the mix of fats in the diet.

DIETARY FATS

Monounsaturated: Olives; olive oil, canola oil, peanut oil; cashews, almonds, peanuts, and most other nuts; avocados (Lowers LDL; raises HDL)

Polyunsaturated: Corn, soybean, safflower, and cottonseed oils; fish (Lowers LDL; raises HDL)

Saturated: Whole milk, butter, cheese, and ice cream; red meat; chocolate; coconuts, coconut milk, and coconut oil (Raises both LDL and HDL)

Trans: Most margarines; vegetable shortening; partially hydrogenated vegetable oil; deep-fried chips; many fast foods; most commercial baked goods (Raises LDL)

The Cholesterol - Heart Disease Connection

Cholesterol is a wax-like substance. The liver makes it and links it to carrier proteins called lipoproteins that let it dissolve in blood and be transported to all parts of the body. Why? Cholesterol play essential roles in the formation of cell membranes, some hormones, and vitamin D.

Too much cholesterol in the blood, though, can lead to problems. In the 1960s and 70s, scientists established a link between high blood cholesterol levels and heart disease. Deposits of cholesterol can build up inside arteries. These deposits, called plaque, can narrow an artery enough to slow or block blood flow. This narrowing process, called atherosclerosis, commonly occurs in arteries that nourish the heart (the coronary arteries). When one or more sections of heart muscle fail to get enough blood, and thus the oxygen and nutrients they need, the result may be the chest pain known as angina. In addition, plaque can rupture, causing blood clots that may lead to heart attack, stroke, or sudden death. Fortunately, the buildup of cholesterol can be slowed, stopped, and even reversed.

Cholesterol-carrying lipoproteins play central roles in the development of atherosclerotic plaque and cardiovascular disease. The two main types of lipoproteins basically work in opposite directions.

Low-density lipoproteins (LDL) carry cholesterol from the liver to the rest of the body. When there is too much LDL cholesterol in the blood, it can be deposited on the walls of the coronary arteries. Because of this, LDL cholesterol is often referred to as the "bad" cholesterol.

High-density lipoproteins (HDL) carry cholesterol from the blood back to the liver, which processes the cholesterol for elimination from the body. HDL makes it less likely that excess cholesterol in the blood will be deposited in the coronary arteries, which is why HDL cholesterol is often referred to as the "good" cholesterol.

In general, the higher your LDL and the lower your HDL, the greater your risk for atherosclerosis and heart disease.

For adults age 20 years or over, the latest guidelines from the National Cholesterol Education Program recommend the following optimal levels:

• Total cholesterol less than 200 milligrams per deciliter (mg/dl)
• HDL cholesterol levels greater than 40 mg/dl
• LDL cholesterol levels less than 100 mg/dl

Dietary Fat, Dietary Cholesterol, and Blood Cholesterol Levels

One of the most important determinants of blood cholesterol level is fat in the diet - not total fat, as mentioned already, but specific types of fat. Some types of fat are clearly good for cholesterol levels and others are clearly bad for them.

Cholesterol in food

While it is well known that high blood cholesterol levels are associated with an increased risk for heart disease, scientific studies have shown that there is only a weak relationship between the amount of cholesterol a person consumes and their blood cholesterol levels or risk for heart disease. For some people with high cholesterol, reducing the amount of cholesterol in the diet has a small but helpful impact on blood cholesterol levels. For others, the amount of cholesterol eaten has little impact on the amount of cholesterol circulating in the blood.

In a study of over 80,000 female nurses, Harvard researchers actually found that increasing cholesterol intake by 200 mg for every 1000 calories in the diet (about an egg a day) did not appreciably increase the risk for heart disease.

Eggs

Long vilified by well-meaning doctors and scientists for their high cholesterol content, eggs are now making a bit of a comeback. Recent research by Harvard investigators has shown that moderate egg consumption--up to one a day--does not increase heart disease risk in healthy individuals. While it's true that egg yolks have a lot of cholesterol--and, therefore may slightly affect blood cholesterol levels--eggs also contain nutrients that may help lower the risk for heart disease, including protein, vitamins B12 and D, riboflavin, and folate.

So, when eaten in moderation, eggs can be part of a healthy diet. People with diabetes, though, should probably limit themselves to no more than two or three eggs a week, as the Nurses' Health Study found that for such individuals, an egg a day might increase the risk for heart disease. Similarly, people who have difficulty controlling their blood cholesterol may also want to be cautious about eating egg yolks and choose foods made with egg whites instead.

Dietary Fats

The Bad Fats

Some fats are bad because they tend to worsen blood cholesterol levels.

Saturated Fats

Saturated fats are mainly animal fats. They are found in meat, seafood, whole-milk dairy products (cheese, milk, and ice cream), poultry skin, and egg yolks. Some plant foods are also high in saturated fats, including coconut and coconut oil, palm oil, and palm kernel oil. Saturated fats raise total blood cholesterol levels more than dietary cholesterol because they tend to boost both good HDL and bad LDL cholesterol. The net effect is negative, meaning it's important to limit saturated fats.

Trans Fats

Trans fatty acids are fats produced by heating liquid vegetable oils in the presence of hydrogen. This process is known as hydrogenation. The more hydrogenated an oil is, the harder it will be at room temperature. For example, a spreadable tub margarine is less hydrogenated and so has fewer trans fats than a stick margarine.

Most of the trans fats in the American diet are found in commercially prepared baked goods, margarines, snack foods, and processed foods. Commercially prepared fried foods, like French fries and onion rings, also contain a good deal of trans fat.

Trans fats are even worse for cholesterol levels than saturated fats because they raise bad LDL and lower good HDL. While you should limit your intake of saturated fats, it is important to eliminate trans fats from partially hydrogenated oils from your diet.

The Good Fats

Some fats are good because they can improve blood cholesterol levels.

Unsaturated Fats--Polyunsaturated and Monounsaturated

Unsaturated fats are found in products derived from plant sources, such as vegetable oils, nuts, and seeds. There are two main categories: polyunsaturated fats (which are found in high concentrations in sunflower, corn, and soybean oils) and monounsaturated fats (which are found in high concentrations in canola, peanut, and olive oils). In studies in which polyunsaturated and monounsaturated fats were eaten in place of carbohydrates, these good fats decreased LDL levels and increased HDL levels.

Dietary Fats and Heart Disease--Beyond the "30%" Recommendation

For years, a low-fat diet was hailed as the centerpiece of a heart-healthy lifestyle, even though there was little evidence that this eating strategy prevented heart disease. The American Heart Association and others urged everyone to limit fat intake to 30% or less of daily calories. One problem with a generic low-fat diet is that it throws out fats that are good for the heart with those that are bad for it. Another problem is that many people who switch to a low-fat diet replace fats with pasta, white rice, bread, and other foods chock full of easily digested carbohydrates.

Several reports over the years have questioned the wisdom of recommending a low-fat diet for preventing or retarding heart disease. Perhaps the biggest nail in the coffin came from the Women's Health Initiative Dietary Modification Trial, published in the February 8, 2006, Journal of the American Medical Association. This eight-year trial, which included almost 49,000 women, found virtually identical rates of heart attacks, strokes, and other forms of cardiovascular disease in women who followed a low-fat diet and women who didn't.

The relation of fat intake to health is one of the areas that Harvard researchers have examined in detail over the last 20 years in two large studies. The Nurses' Health Study and the Health Professionals Follow-up Study have found no link between the overall percentage of calories from fat and any important health outcome, including cancer, heart disease, and weight gain.

What was important in these studies was the type of fat in the diet. There are clear links between the different types of dietary fats and heart disease. Logically, most of the influence that fat intake has on heart disease is due to its effect on blood cholesterol levels.

Ounce for ounce, trans fats are far worse than saturated fats when it comes to heart disease. The Nurses' Health Study found that replacing only 30 calories (7 grams) of carbohydrates every day with 30 calories (4 grams) of trans fats nearly doubled the risk for heart disease. Saturated fats increased risk as well, but not nearly as much.

For the good fats, there is consistent evidence that high intake of either monounsaturated or polyunsaturated fat lowers the risk for heart disease. In the Nurses' Health Study, replacing 80 calories of carbohydrates with 80 calories of either polyunsaturated or monounsaturated fats lowered the risk for heart disease by about 30 to 40 percent.

Fish, an important source of the polyunsaturated fat known as omega-3 fatty acid, has received much attention in the past for its potential to lower heart disease risk. And there have been some studies to back this up, although not all have shown consistent benefits. One large trial, however, found that by getting 1 gram per day of omega-3 fatty acids over a 3.5 year period, people who had survived a heart attack could lower their risk of dying from heart disease by 25 percent. The study participants got their omega-3s from a capsule - getting a gram a day from fish would mean eating a serving a day of fatty fish, such as mackerel, salmon, sardines, or swordfish.

Eating fish may help prevent heart disease in several ways. It may replace red meat or other less-healthy sources of protein. And the omega-3 fats in fish appear to protect the heart against the development of erratic and potentially deadly cardiac rhythm disturbances. The American Heart Association currently recommends that people eat at least two servings of fish a week. Although there has been some recent concern about contaminants in fish such as mercury and PCBs, the overwhelming evidence suggests that the proven health benefit of fish consumption is much greater than the potential for harm among individuals who consume fish several times per week. The main exception to this recommendation was highlighted in an FDA/EPA dietary advice statement for women who are or might become pregnant, for nursing mothers, and for young children: These groups should include fish in their diets, but should not eat fish high in mercury (swordfish, tilefish, shark, and king mackerel); they should instead two servings per week of fish that have lower mercury levels yet are good sources of omega-3 fats, such as salmon, chunk light tuna, shrimp, pollock, or catfish.

Dietary Fats and Cancer

Heart disease is not the only condition that has been linked with fat intake. Researchers once suspected an association between dietary fat and certain cancers. Here again, the type of fat - and not the total amount - seemed to be most important.

Breast Cancer

By the early 1980s, most nutrition experts believed that dietary fat was a major cause of breast cancer. This thinking was largely based on international comparisons showing higher breast cancer rates in countries with higher per capita fat intake. But such comparisons are very broad in nature. As more detailed studies were performed over the next couple of decades, the apparent link between total fat intake and breast cancer has faded. The Women's Health Initiative Dietary Modification Trial, which was specifically designed to examine the effect of a low-fat diet on the development of breast cancer, showed similar rates of breast cancer in women eating a low-fat diet and in those eating a "regular" diet.

Other studies - including those by Harvard researchers - of different types of fat have failed to find a link with breast cancer. However, some European studies have reported suggestive findings of lower breast cancer risk among women with a high intake of monounsaturated fats (mainly in the form of olive oil).

Colon Cancer

As with breast cancer, international comparisons initially suggested an association between total dietary fat intake and colon cancer risk. But later studies contradicted these earlier findings and revealed instead an association that was weak at best. As was the case with breast cancer, women in the Women's Health Initiative Dietary Modification Trial who at a low-fat diet developed colon cancer at the same rate as women who didn't. Although fat intake doesn't seem to increase colon cancer risk, high consumption of red meat still does appear to do so.

Prostate Cancer

Although the exact connection between dietary fat and prostate cancer is far from clear, there is some evidence that diets high in animal fat and saturated fat increase prostate cancer risk. However, some studies have also shown no association, while others have implicated unsaturated fats. Clearly much more research is needed to clear up the exact links between dietary fat and prostate cancer.

Other Cancers

Preliminary research has also linked the intake of certain kinds fat with other cancers, though much more research is needed to confirm these results. In the Nurses' Health Study, Harvard researchers found that a high intake of trans fats increased the risk for non-Hodgkin's lymphoma and that a high saturated fat intake increased the risk for endometrial cancer.

Dietary Fat and Obesity

It is a common belief that the more fat you eat, the more body fat you put on, and the more weight you gain. This belief has been bolstered by much of the nutrition advice given to people over the past decade, which has focused on lowering total fat intake while increasing carbohydrate intake. But it isn't completely true, and the advice has been misguided. For example, while Americans have gradually decreased the proportion of calories they get from fat over the last decade, rates of obesity have increased steeply.

Over the short term, following a low-fat diet does lead to weight loss. But so does following a high-fat, low-carbohydrate diet. Actually, almost any diet that helps you take in fewer calories works over the short term. In other words, low-fat diets appear to offer no apparent advantages over diets with fat levels close to the national average. This was demonstrated in the Women's Health Initiative Dietary Modification Trial. Women in this trial who were assigned to a low-fat diet did not lose, or gain, any more weight than women eating a "usual" diet.

Although more research is needed, a prudent recommendation for losing weight or maintain a healthy weight is to be mindful of the amount of food you eat in relation to the amount of calories you burn in a day. Exercising regularly is especially beneficial.

The Bottom Line: Recommendations for Fat Intake

Although the different types of fat have a varied - and admittedly confusing - effect on health and disease, the basic message is simple: chuck out the bad fats and replace them with good fats. Try to limit saturated fats in your diet, and try to eliminate trans fats from partially hydrogenated oils (a report from the Institute of Medicine has concluded that thtere is no safe level of trans fats in the diet). Replace saturated and trans fats with polyunsaturated and monounsaturated fats.

As of January 1, 2006, trans fat must be listed on food labels. More and more "trans-fat" free products are becoming available (there's even a trans fat-free Crisco on the market). Keep in mind, though, that according to the FDA, a product claiming to have zero trans fat can actually contain up to a half gram. (Canada set a different standard of zero as under 0.2 grams.) So you may still want to scan the ingredient list for "partially hydrogenated vegetable oil" and "vegetable shortening," and look for an alternative product without those words, especially if it's something you eat regularly.

Tips for lowering trans fat intake:

• Choose liquid vegetable oils or a soft tub margarine that is contains little or no trans fats.
• Reduce intake of commercially prepared baked goods, snack foods, and processed foods, including fast foods. To be on the safe side, assume that all such produts contain trans fats unless they are labeled otherwise.
• When foods containing hydrogenated or partially hydrogenated oils can't be avoided, choose products that list the hydrogenated oils near the end of the ingredient list.
• To avoid trans fats in restaurants, one strategy is to avoid deep-fried foods, since many restaurants continue to use partially hydrogenated oils in their fryers. You may be able to help change this cooking practice by asking your server, the chef, or manager if the establishment uses trans-free oils.

Omega-3 Fatty Acids

Fats in our food are categorized according to the predominant fatty acid that is present. Typically fats are categorized as saturated fatty acids or unsaturated fatty acids. Some examples of foods that are high in saturated fats are butter and lard. These fats are solid at room temperature. Fats that are high in unsaturated fatty acids are liquid at room temperature. Examples are vegetable oils such as canola, corn, olive or soybean. Unsaturated fatty acids may be monounsaturated or polyunsaturated.

When it comes to heart health, we are hearing more and more about the benefits of one particular type of polyunsaturated fatty acid, the omega-3 fatty acids. We can get these polyunsaturated fatty acids from the food that we eat.

The most common omega-3 fatty acids are eicosa-pentaenoic (EPA), docosahexaenoic (DHA) and alpha-linolenic (ALA) acids. EPA and DHA are found in fatty fish such as salmon, white tuna, mackerel, rainbow trout, herring, halibut, and sardines. ALA is more commonly found in soybean or canola oil, walnuts, and flaxseeds or flaxseed oil. The American Heart Association has recommended that healthy adults eat at least two servings of fish per week to boost their omega-3 fatty acid intake. Eating 2 to 4 ounces of these fish will generally provide about 1 gram of omega-3 fatty acids.

Another food source is the Omega Egg, a University of Nebraska patented product that is high in omega-3 fatty acids. The eggs are produced from hens that eat a patented diet including flaxseed. These eggs look and taste like conventional eggs but have nearly six times the omega-3 fatty acid, a third less saturated fat, and less cholesterol than conventional eggs.

Benefits of Omega-3 Fatty Acids

Research suggests that including omega-3 fatty acids in the diet may:

• reduce inflammation,
• help prevent blood from clotting and sticking to artery walls,
• help to lower the risk for blocked blood vessels and heart attacks,
• prevent hardening of the arteries,
• decrease the risk of sudden death and abnormal heart rates,
• decrease triglyceride levels,
• improve overall heart health, and
• lower blood pressure.

Omega-3 Fatty Acid Supplements

Research has demonstrated that omega-3 fatty acid supplements have some positive effects on triglycerides and HDL levels, but food is still the best source since a variety of other nutrients are provided. Fish oil supplements will not undo the effects of an otherwise high fat diet. When taking a supplement it is best to consult a physician. The Food and Drug Administration has also noted that high intakes of EPA and DHA can cause excessive bleeding in some people.

Cautions on Fish

Some types of fish may contain environmental contaminants such as methylmercury or polychlorinated biphenols that may cause a health risk. Such substances generally are highest in older, larger, and more predatory fish or marine mammals. For that reason, the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) have provided guidelines to help individuals determine their best course of action. Young children, women who may become pregnant, and pregnant or nursing women are at highest risk for exposure. Shark, swordfish, king mackerel and tilefish have the highest mercury levels and should be avoided by women and young children. For other fish and shellfish with lower mercury levels, women and young children may eat up to two regular servings per week (no more than 6 to 12 ounces). For other individuals, time-honored nutrition advice holds true: eat a variety of fish to minimize exposure and any adverse effects that may be due to such environmental contaminants.

Fat Words

Here are brief definitions of the key terms important to an understanding of the role of fat in the diet.

Cholesterol

A chemical compound manufactured in the body. It is used to build cell membranes and brain and nerve tissues. Cholesterol also helps the body make steroid hormones and bile acids.

Dietary cholesterol

Cholesterol found in animal products that are part of the human diet. Egg yolks, liver, meat, some shellfish, and whole-milk dairy products are all sources of dietary cholesterol.

Fatty acid

A molecule composed mostly of carbon and hydrogen atoms. Fatty acids are the building blocks of fats.

Fat

A chemical compound containing one or more fatty acids. Fat is one of the three main constituents of food (the others are protein and carbohydrate). It is also the principal form in which energy is stored in the body.

Hydrogenated fat

A fat that has been chemically altered by the addition of hydrogen atoms (see trans fatty acid). Vegetable shortening and margarine are hydrogenated fats.

Lipid

A chemical compound characterized by the fact that it is insoluble in water. Both fat and cholesterol are members of the lipid family.

Lipoprotein

A chemical compound made of fat and protein. Lipoproteins that have more fat than protein are called low-density lipoproteins (LDLs). Lipoproteins that have more protein than fat are called high-density lipoproteins (HDLs). Lipoproteins are found in the blood, where their main function is to carry cholesterol.

Monounsaturated fatty acid

A fatty acid that is missing one pair of hydrogen atoms in the middle of the molecule. The gap is called an "unsaturation." Monounsaturated fatty acids are found mostly in plant and sea foods. Olive oil and canola oil are high in monounsaturated fatty acids. Monounsaturated fatty acids tend to lower levels of LDL-cholesterol in the blood.

Polyunsaturated fatty acid

A fatty acid that is missing more than one pair of hydrogen atoms. Polyunsaturated fatty acids are mostly found in plant and sea foods. Safflower oil and corn oil are high in polyunsaturated fatty acids. Polyunsaturated fatty acids tend to lower levels of both HDL-cholesterol and LDL-cholesterol in the blood.

Saturated fatty acid

A fatty acid that has the maximum possible number of hydrogen atoms attached to every carbon atom. It is said to be "saturated" with hydrogen atoms. Saturated fatty acids are mostly found in animal products such as meat and whole milk. Butter and lard are high in saturated fatty acids. Saturated fatty acids tend to raise levels of LDL-cholesterol ("bad" cholesterol) in the blood. Elevated levels of LDL-cholesterol are associated with heart disease.

Trans fatty acid

A polyunsaturated fatty acid in which some of the missing hydrogen atoms have been put back in a chemical process called hydrogenation, resulting in "straighter" fatty acids that solidify at higher temperatures. Trans fatty acids are under study to determine their effects on cholesterol.

By Eleanor Mayfield, University of Nebraska-Lincoln Cooperative Extension Service, Harvard School of Public Health