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Foods and Your Cholesterol

Elisabeth Schafer, Ph.D. Extension Nutrition Specialist
Diane Nelson. Extension Communications Specialist
Iowa State University

Your serum cholesterol is influenced by many factors, such as heredity, gender, and age, over which you have no control. However, you can control one important factor-your diet.

Eating the right foods cannot guarantee low serum cholesterol or freedom from cardiovascular disease. But choosing the right foods, not using tobacco, and exercising can help you make the most of your genetic potential.

Evaluate the evidence

Many foods and food components have been linked to serum cholesterol. The scientific evidence varies from solid to shaky. So, before changing your eating habits just because of some reported research, evaluate the quality of the evidence.

Is the evidence valid?

Do the findings reflect what is actually true? Research results can be used to prove almost anything, even things that are not true; but such research is not valid. It has faulty design, is biased, or improperly carried out. Much early research in any field contains these errors. Methods are refined as science progresses, and later work often overturns early findings.

Is the evidence reliable?

Would repeating the same study produce the same results? Can other scientists replicate the experiment and come to the same conclusion? Many research reports that reach the media are preliminary. Not until other scientists can reproduce the results consistently does one know if the findings are reliable. This issue is especially important in nutrition science. You should be cautious about making major changes in diet on the basis of only one or two research studies.

For example, some early extreme dietary advice on lowering serum cholesterol has shown, upon closer examination, to be inaccurate and even dangerous.

One of the most common beliefs is that to lower serum cholesterol you have to eliminate dietary cholesterol. Since cholesterol occurs only in foods of an animal origin. some people have given up red meats and dairy products. One result is diets low in iron, zinc, and calcium. Another serious outcome is inadequate growth in children due to restricted diets.

Food and Cholesterol Links

These foods and food components have been linked with raising serum cholesterol.

Food or               	Quality of
food component        evidence
--------------------------------
Calories              	Strong
Saturated fats        	Strong
Cholesterol           	Good
Coffee                	Good
Total fat             	Fair
-------------------------------

These foods and food components have been linked with lowering serum cholesterol.

Food or               		Quality of
food component        	evidence
---------------------------------
Polyunsaturated fat   	Strong
Niacin                		Strong
Monounsaturated fat   	Good
Fiber                 		Good
Fish oil              		Fair
Garlic                		Fair
Alcohol               		Fair
Vitamin C             		Poor
Vitamin E             		Poor
---------------------------------

What the research says

Losing weight reduces serum cholesterol.

Excess calories lead to obesity, higher serum cholesterol, higher LDL-cholesterol, and lower HDL-cholesterol. Among men and women under age 45 years, being overweight doubles the chances of having elevated serum cholesterol. One of the most effective ways to reduce serum cholesterol, especially LDL-cholesterol, is to lose weight.

Reducing dietary cholesterol reduces serum cholesterol in some people.

Several research studies showed that increasing the amount of cholesterol eaten resulted in increased serum cholesterol. For example, one study found that young men had elevated serum cholesterol after eating a diet rich in cholesterol. In another study eating six eggs a day (a very high cholesterol intake) significantly increased LDL-cholesterol.

Unfortunately, not all studies have produced such clear-cut results. For example, healthy adults with normal serum cholesterol levels were fed diets either high or low in saturated fat. Some in each diet group were also given 1,000 mg cholesterol per day. The high saturated fat diet raised total and LDL-cholesterol, but the addition of dietary cholesterol had no further effect. In another study 13 volunteers were fed either high (1,350 mg/day) or low (50 mg/day) cholesterol diets. Only 7 of the 13 showed increased serum cholesterol on the high-cholesterol diet.

These conflicting results suggest that a genetic factor may be at work. Individuals may carry a gene or group of genes that determines sensitivity to dietary cholesterol. For nonresponders, a high-cholesterol diet does not produce high serum cholesterol. In these individuals, a high-cholesterol diet signals the body to stop making cholesterol. Thus, the level of cholesterol in blood remains normal.

Responders, on the other hand, show changes in serum cholesterol depending on the amount of cholesterol in the diet. In these individuals the body does not adjust to a diet rich in cholesterol by halting internal production. The liver continues to manufacture cholesterol that, added to the amount in the diet, creates a heavy load of cholesterol in the body. This explains why some people benefit from reducing dietary cholesterol and others do not.

Studies show that restricting dietary cholesterol can lower serum cholesterol an average of 6 percent. The magnitude of effect depends on genetic factors and the degree of change from previous diet. One investigator estimated that for each 100 mg reduction in dietary cholesterol, an individual could expect a 7 mg/dL reduction in serum cholesterol.

Thus, a person eating 500 mg cholesterol per day (average U.S. intake is 450 to 500 mg/day) who reduced dietary cholesterol to 300 mg/day (the American Heart Association recommendation) could expect about a 14 mg/dL reduction in serum cholesterol. This is not much change, especially for a person whose serum cholesterol is in the high-risk range.

Reducing saturated fat reduces serum cholesterol.

There is strong scientific evidence that saturated fat raises serum cholesterol. Studies show that for every 1 percent of calories contributed by saturated fat, serum cholesterol increases by 2.7 mg/dL. A change in saturated fat intake from 7 percent of calories to 17 percent of calories could, therefore, raise blood cholesterol by 27 mg/dL.

Controlled clinical studies have repeatedly shown total serum cholesterol and LDL-cholesterol are lowered when polyunsaturated fatty acids are substituted for saturated fatty acids in the diet. Effects on HDL-cholesterol are inconsistent although diets high in polyunsaturated fat often reduce HDL-cholesterol.

Reducing saturated fat is, after loss of excess body weight, probably the single most effective dietary change for reducing serum cholesterol.

Reducing total fat is one way to reduce saturated fat.

Although the dietary guidelines offered by many organizations recommend reducing dietary fat, most studies showing a link between high fat diets and chronic disease are confounded by uncontrolled variables. Many studies have shown that reducing total dietary fat lowers serum cholesterol. But in virtually every study other dietary factors were also changed, such as type of fat, amount of dietary cholesterol, or total caloric intake.

Diets high in fat are often high in saturated fat and calories. Strong evidence exists for controlling saturated fat and calories to help reduce serum cholesterol.

To reduce saturated fat without reducing total fat implies increasing polyunsaturated and monounsaturated fat. However, diets high in polyunsaturated fat have been linked to gallstones, vitamin E deficiency. certain types of cancers, and altered immune function. The long-term safety of diets high in monounsaturated fat has not been tested.

Diets high in any kind of fat are high in calories. Fat provides 9 kilocalories per gram, more than twice as many as the 4 kilocalories per gram provided by carbohydrate and protein.

Thus, reducing total fat is important as a way to reduce calories and saturated fat without the hazards of excessively increasing polyunsaturated and monounsaturated fat.

Polyunsaturated fat can replace some saturated fat.

Evidence is strong that polyunsaturated fat, when substituted for saturated fat, reduces serum cholesterol. In one study, for example, reducing dietary cholesterol lowered serum cholesterol by 6 percent but substituting polyunsaturated for saturated fat reduced serum cholesterol by 23 percent. Repeated trials have had similar results.

The American Heart Association recommends a diet with a P/S ratio of 1.0. The P/S ratio is grams of polyunsaturated fat divided by grams of saturated fat. The average P/S ratio in the U.S. is 0.4. about twice as much saturated as polyunsaturated fat.

However, substituting polyunsaturated for saturated fat is a mixed blessing. Diets with more than 10 percent of calories as polyunsaturated fats may not be beneficial. The average U.S. diet in 1986 contained 7.6 percent of calories as poly-unsaturated fat. The goal should be to reach a balance between saturated and polyunsaturated fat in the diet.

Use monounsaturated fat in moderation.

Early research on monounsaturated fat concluded that it was neutral, neither raising nor lowering serum cholesterol. However, scientific opinion is shifting to the belief that monounsaturated fat is as effective as polyunsaturated fat when substituted for saturated fat.

Population studies show a low incidence of cardiovascular disease in the Mediterranean countries where olive oil (high in monounsaturated fatty acids) is the dominant dietary fat. Recent clinical trials feeding liquid formulas to hospitalized men showed that when the formulas contained monounsaturated fat, serum cholesterols were lower than when the liquid formulas contained saturated fat. The fact that the research used only men, in a hospital. living exclusively on liquid formula diets suggests caution in applying these results to a general population eating a varied diet.

Perhaps substitution for saturated fat, whether by polyunsaturated or monounsaturated fat, is the key. Because of the hazards of diets high in saturated or polyunsaturated fat, monounsaturated fat might be a good choice for dietary fat as long as it, too, is used in moderation.

Fiber can influence serum cholesterol.

Some kinds of dietary fiber influence serum cholesterol and triglyceride while other types are important as laxatives. Dietary fiber is a general term including a mixture of soluble and insoluble fiber types. Insoluble fiber, in wheat bran and most other whole grains, did not lower elevated serum cholesterol in most studies. Soluble fiber, in oats, fruits, vegetables, and legumes, has consistently lowered serum cholesterol in repeated studies.

How much soluble fiber do you have to eat? Scientific evidence is inadequate at this point hut many nutrition scientists recommend 25 to 35 grams total dietary fiber per day, including both soluble and insoluble.

One study with oat bran showed that 60 grams (2 ounces) a day could lower serum cholesterol 5 percent, even in people with normal, healthy blood cholesterol levels. In another study where serum cholesterol was reduced by 19 percent, the individuals ate a bowl of hot cereal and five oat bran muffins every day. In both cases the oat bran was part of a diet that was also moderate in fat, cholesterol, and calories.

There is no evidence that oat bran or other soluble fiber foods can compensate for a fatty, rich diet. A question that the research has not yet answered is whether the effect is long-lasting. The studies usually only last a couple of months.

Fish oil has pros and cons.

Drug store shelves are lined with fish oil capsules, but taking them may not be the safest or most effective way to lower elevated serum cholesterol.

Some of the research reports are impressive. For example, an Oregon study involving men and women aged 35 to 70 years reported that a diet high in salmon plus fish oil supplements for four weeks resulted in a 27 to 45 percent drop in serum cholesterol and a 64 to 79 percent drop in serum triglycerides. Many other studies using salmon, mackerel, herring, cod liver oil and sardine oil have had similarly striking results.

However, a review of the 18 best research studies using fish oil reveals that in 12 of the studies there was no effect on serum cholesterol. Effects on lipoproteins were also inconsistent.

Effects on serum triglycerides were more consistent. In the 18 studies, triglycerides dropped significantly in 15. The amount of effect depends partially on how high the cholesterol or triglycerides were before the treatment. For persons with normal or near normal serum lipids, the effects could be quite small.

Potential problems with fish oils include toxic contaminants such as mercury and pesticides, interference with blood clotting, altered immune response, increased peroxides in cells (peroxides cause cellular damage), and increased requirement for vitamin E or vitamin C.

The earliest research with fish oils focused only on effects in the cardiovascular system without much attention to effects in other parts of the body. Researchers are examining long-term safety.

The present recommendation is to obtain the suspected benefits of fish oil by eating fish once or twice a week. An additional benefit is that fish, prepared with a low-fat method, is low in saturated fat and low in calories.

Niacin is effective under physician's care.

The water-soluble B-vitamin niacin (also known as nicotinic acid or nicotinamide) is one of the most effective drugs for lowering serum cholesterol, LDL-cholesterol, and triglycerides. As an added bonus, it increases HDL-cholesterol.

The lipid-lowering effect occurs only at high doses, making niacin a drug rather than a dietary component. To get effective amounts of niacin from food alone you would have to eat, in one day, 20 pounds of beef, 5 whole chicken breasts, 20 slices of whole wheat bread, 100 oranges, and 150 cups of baked beans.

High doses of niacin have toxic side effects including itching, flushing, gastrointestinal distress, and liver malfunction. For these reasons, niacin tablets should be used only by physician prescription.

Coffee may raise serum cholesterol.

Population studies consistently report that consuming three or more cups of coffee per day is associated with increased total and LDL- cholesterol. In the Tromso Norwegian study involving 14,000 men and women, there was a positive, dose-response relation between coffee consumption and serum cholesterol and triglycerides. Whether caffeine is the effective ingredient is unclear. One study reported caffeine was the culprit. Another study found the same effect with regular and decaffeinated coffee.

Dietary changes can reduce serum cholesterol

Research studies have shown that dietary changes can reduce serum cholesterol an overall average of 44 mg/dL.

How diet affects serum cholesterol

Questionable effects

Many other food components have been promoted as effective ways to lower serum cholesterol. The following summaries reflect current research findings.

Alcohol

In the early 1980s several scientists noted a lower mortality from cardiovascular disease among light drinkers than among abstainers. Unfortunately, confounding variables make it difficult to draw clear conclusions. For example, there was no correction for smoking or for lifelong versus recent abstention. Early research reported moderate alcohol use increased HDL-cholesterol. Follow-up research found that HDL is composed of at least three subfractions that have different effects on heart disease and are influenced differently by alcohol. At this point, the whole issue is still controversial, and known hazards of alcohol use must be weighed against uncertain benefits.

Garlic

Garlic reduces total and LDL-cholesterol in chickens and rats Unfortunately the odorous compounds in garlic appear to be the effective components. Work is currently under way to isolate the odorless portions and examine their effects. Due to the odor problem and the large amounts of garlic needed for results, no controlled study has been conducted with humans.

Vitamin C

For many years, Russian physicians prescribed large doses of vitamin C to lower serum cholesterol and reduce cardiovascular disease. Although a few studies report that increasing vitamin C intake lowers serum cholesterol, the majority of studies report either no change or an increase in serum cholesterol.

Vitamin E

A 1979 study found that taking vitamin E at 50 times the RDA decreased serum cholesterol. This study and others like it often have been cited by people who want to believe in vitamin E as a cure for cardiovascular disease.

However, this and other similar studies used small numbers of people (only five individuals in the 1979 study) and were uncontrolled and poorly planned. Recent well-designed and conducted studies with adequate numbers of people (a 1988 study had 717 men) found no association between vitamin E supplementation and either total or HDL-cholesterol.

Vitamin E is an anticoagulant and may help to reduce risks of blood clots. But there is no reliable evidence that vitamin E lowers serum cholesterol or triglycerides.

Conclusion

Diet is undoubtedly one of the most significant influences on serum cholesterol, serum triglycerides and cardiovascular disease. But it is not the only one. Heredity, age, gender, exercise, and tobacco use are all vitally important.

Although dietary components may lower serum cholesterol, usually the amount needed for a significant effect is large. For example. who can eat a bowl of hot cereal, five oat-bran muffins, four ounces of fish, and 10 cloves of garlic every day? And what would be the effect on your overall nutritional status and health (not to mention social acceptance) if you did?

The conclusion from research is that many foods are valuable for good health. The benefits can be achieved through generous, but sensible, consumption of low-fat foods and cautious use of high-fat foods.

North Dakota State University
NDSU Extension Service

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