If you’re trying to eat a heart-healthy diet, figuring out what to believe can be overwhelming. The advice we get on everything from eggs to olive oil is often confusing and maddeningly contradictory.

Ironically, this growing confusion comes at a time when scientists who study nutrition know more than ever. Too often, though, we hear about only the latest study (which may be poorly designed) or research that’s cherry-picked to support an agenda. That’s like seeing one or two pieces of a jigsaw puzzle and trying to determine what the entire picture is.

To know what the science really shows, it pays to look at all the evidence, assigning greater weight to studies that are more rigorous. In many cases, this can give us a reliable indication of what’s really good or bad. Based on a thorough review of research, here’s what’s believable — and what’s not — regarding some familiar claims about heart health.

Nuts are good for your heart

True. Once regarded as high-fat nutritional villains to be avoided at all costs, nuts are now touted as a health food that can ward off heart disease. And perhaps rightly so. Several large cohort studies (the type in which people are asked about their dietary habits and then followed for years or decades) have consistently found lower odds of heart disease and heart-related deaths among nut eaters, regardless of sex, age, location or occupation.

These findings are bolstered by results from clinical trials demonstrating that nuts lower LDL cholesterol levels, the kind associated with an increased risk of heart disease. Nuts also appear to decrease inflammation in arteries, which may contribute to heart attacks.

So which nuts are best for you? If you listen to producers of walnuts, almonds or peanuts (which, technically, aren’t nuts but legumes), each will tell you that its nut is superior because of some ingredient it contains. The truth is that it’s impossible to say which is best because no one has done a head-to-head comparison.

All nuts are relatively high in unsaturated fats, which are thought to be good for the heart. And all nuts are relatively high in calories, so it’s important to pay attention to portion sizes. About a handful a day is enough to reap health benefits. It may even promote weight loss by helping you feel full. But going nuts and overindulging can lead to extra pounds.

Oats lower cholesterol

TRUE. Oats contain a type of soluble fiber known as beta-glucan, which is also found in barley. It’s thought to lower cholesterol by binding to bile acids and removing them from the body. Bile acids are made from cholesterol, so when the body has to deploy more of its cholesterol to help replace the eliminated bile acids, there’s less of it in the blood.

The Cochrane Collaboration, an independent group that assesses the evidence for various treatments, conducted an analysis in which it pooled results from eight randomized studies involving people with elevated cholesterol and other risk factors for heart disease. Subjects assigned to eat oat cereal every day lowered their total and LDL cholesterol levels seven or eight points more than those on a diet of refined grains. The studies lasted only four to eight weeks, so we don’t know about long-term effects.

To see a benefit, you need three grams of beta-glucan a day, which you can get from 1.5 cups of cooked oatmeal, three cups of instant oatmeal or three cups of Cheerios. Unfortunately, oatmeal cookies don’t count.

Fish oil protects your heart

True. Decades ago, scientists discovered that Greenland Eskimos rarely died from heart disease despite a diet high in fat from fish. Researchers theorized that the fish fat was somehow protective, an idea that subsequent research has largely supported.

Several cohort studies show that people who regularly eat fish are less likely to die of heart disease than those who don’t eat fish. Randomized trials involving heart attack survivors have found that subjects given fish oil supplements were less likely to die of heart disease than those who didn’t take the capsules. And in a randomized study of people with high cholesterol, participants who took fish oil had fewer heart attacks and deaths from heart disease.

The key ingredients appear to be the omega-3 fatty acids EPA and DHA, which are found in most fish but especially in oily ones such as salmon, mackerel, trout, sardines and tuna. Studies suggest that these fats may help relax blood vessels, reduce blood pressure, prevent abnormal rhythms and lower blood fats known as triglycerides.

While the evidence of benefits is strong for people who have heart disease or are at high risk for it, it’s less clear whether fish oil wards off heart attacks in those at low risk. Still, it seems reasonable to follow the American Heart Association’s recommendation and eat oily fish at least twice a week. People with heart disease are advised to get twice as much, or 1,000 milligrams per day of EPA and DHA combined.

Eggs cause heart disease

False. Researchers have conducted a number of long-term cohort studies on eggs and heart disease, which have collectively followed several hundred thousand people. In general, the research has exonerated eggs: Eating up to six a week was not associated with a higher risk of cardiovascular disease (i.e. heart attacks and strokes).

So how can this be if egg yolks are high in cholesterol? Most of our cholesterol is made by the liver, which ramps up production when we eat saturated and trans fats. But cholesterol from food appears to have little impact on most people’s cholesterol levels. And in people it does affect — so- called hyper-responders — studies show there can be an increase in good (HDL) cholesterol along with the bad kind, which helps offset any increased risk. Further, dietary cholestrerol may also result in larger LDL particles, which are thought to pose less of a threat than smaller ones.

Eggs are relatively low in saturated fat, and they contain unsaturated fats, which may be beneficial. Plus, they’re a good source of protein and several vitamins and minerals. They can be a healthful and more filling alternative to high-calorie muffins, bagels and sugary cereals.

Olive oil is the most healthful oil

False. Olive oil is often singled out as an especially heart-healthy vegetable oil because it’s high in monounsaturated fat. But it’s also lower in polyunsaturated fat than other oils. Both monounsaturated and polyunsaturated fats are considered good fats that may reduce the risk of heart disease.

Which of these fats is better for us is unclear. Some research suggests that polyunsaturated fats may have an edge when it comes to lowering LDL cholesterol, while monounsaturated fats may result in higher HDL cholesterol. One analysis called it a draw, concluding that replacing saturated fat with either monounsaturated or polyunsaturated fat has an equally beneficial effect on cholesterol levels. Another found that substituting monounsaturated for saturated fat was associated with an increased risk of heart attacks, while polyunsaturated fat was linked to lower odds.

While these results aren’t necessarily an indictment of olive oil, they poke holes in the notion that its high levels of monounsaturated fat make olive oil more healthful.

Another theory is that olive oil antioxidants known as polyphenols make it more healthful than its rivals. Research suggests that virgin and extra-virgin oils, which are high in polyphenols, may be more heart-healthy than refined olive oil. But the evidence is preliminary and doesn’t shed much light on how virgin olive oils stack up against non-olive oils. The upshot is that other oils, such as canola, may be just as healthful as olive oil, possibly more so.

Coffee is bad

False. Cohort studies, which followed tens of thousands of people for many years, have found that coffee drinkers have no greater risk of heart attacks or strokes than those who abstain; indeed, they appear to have a slightly lower risk. Though coffee can temporarily increase blood pressure, there’s little evidence that it causes hypertension. Coffee drinkers appear to live just as long as abstainers, maybe even slightly longer.

One possible reason for the apparent benefits is that coffee is rich in antioxidants. Though some studies have found that as many as six cups a day are associated with benefits, that’s more than health authorities recommend because of the potential side effects of caffeine, which include insomnia, jitters and stomach upset. For many people, the biggest health risk from coffee is weight gain. Though a cup of black coffee has only two calories, that number can rise dramatically if you add cream and sugar or drink blended beverages, which can have several hundred calories.

Margarine is better than butter

Half-true. Margarine, which is made from vegetable oils, is lower in saturated fat than butter. But the process of converting those oils into solids can result in trans fats, which may be even more hazardous to the heart than the saturated kind.

Cohort studies have found that people who eat the most margarine have a higher risk of heart disease than those who use it only rarely. In other studies, researchers had subjects eat various types of spreads and then measured the effects on cholesterol levels. Compared with butter, margarine lowered LDL cholesterol, but it also reduced HDL, the good kind. The big loser in this face-off was stick margarine, which fared worse than butter. Semiliquid margarine, on the other hand, proved to have a more beneficial effect on cholesterol levels than butter.

Manufacturers have introduced some margarines that are low in saturated fat and virtually free of trans fat. That makes them a better option than butter. Still, margarine isn’t exactly a health food. Nor is butter. Your best bet is to minimize your use of both margarine and butter, going instead with healthful vegetable oils whenever possible.

Chocolate is good for your heart

Half-true. Cocoa, a main ingredient in chocolate, is high in antioxidants known as flavanols, which are also found in red wine, tea and certain fruits. Though the evidence overall is mixed, some cohort studies have linked high flavanol intake with lower rates of heart-related deaths. Generally, dark chocolate is higher than milk chocolate in flavanols.

Small, short-term experiments — many of them funded by the chocolate industry — show that chocolate (especially the dark variety) can lower blood pressure, improve blood vessel function, reduce inflammation in arteries and make blood less likely to clot. Even though it’s relatively high in saturated fat, studies show that chocolate doesn’t raise LDL cholesterol and may even lower it. One reason may be that some of the fat is a type known as stearic acid, which doesn’t adversely affect cholesterol levels.

Several European cohort studies of elderly men, middle-aged adults and heart attack survivors have linked greater chocolate and cocoa intake to lower rates of heart attacks, strokes and premature death. But since the chocolate consumed in Europe tends to contain higher levels of cocoa than the chocolate typically eaten in the United States, it’s unclear whether the findings apply to American chocolate eaters.

Many chocolate trials have fed subjects 31 / 2 ounces a day. To get that amount, you’d need to eat two or more standard-size candy bars, which add as many as 500 calories and lots of extra pounds. That’s hardly a formula for better health. Nor is consuming the large amounts of sugar that are typically added to chocolate. Look for products that list cocoa or chocolate liquor — and not sugar — as the first ingredient.

Reprinted from “Coffee Is Good for You” by Robert J. Davis, PhD, by arrangement with Perigee, a member of Penguin Group (USA) Inc. Copyright 2012 by Robert J. Davis, PhD, MPH. Davis teaches health communications at Emory Unversity’s Rollins School of Public Health.