or several decades, experts on heart disease have promoted a low-fat, low-cholesterol diet as most effective in warding off the leading killer of Americans.
But in recent years, new evidence has brought this advice into question.
A major problem with the old advice was that it seemed to give people free rein to overindulge in simple and refined carbohydrates foods rich in sugars and white flour, which can produce unhealthy levels of blood lipids and increase the risk of becoming overweight and diabetic, raising the coronary risk.
The most significant change in thinking one, admittedly, that is still vigorously debated is that low-fat is not the answer, or at least, not the best answer.
But before you say "I told you so" and go back to eating all the steak, butter and eggs you want, a careful analysis of the evidence clearly shows that it is not a high-fat, low-carbohydrate diet that is protective.
Rather, it is a diet like the one consumed by heart-healthy people along the Mediterranean: rich in vegetables and fruits, whole grains, nuts, unsaturated vegetable oils and protein derived from fish, beans and chicken, not red meat.
"Compelling" evidence for this view was thoroughly reviewed last week in The Journal of the American Medical Association by Dr. Frank B. Hu and Dr. Walter C. Willett, nutrition and epidemiology experts at the Harvard School of Public Health, who have followed tens of thousands of Americans for decades to uncover relationships between diet, habits and health.
The original heart-saving advice was geared to lowering blood levels of cholesterol and especially low-density lipoprotein, or L.D.L., cholesterol, a waxy alcohol that in large amounts in blood serum can gum up the arteries and set the stage for life-threatening clots.
But, the scientists wrote, "the original hypothesis was overly simplistic because the effects of diet on coronary heart disease can be mediated through multiple biological pathways other than serum total cholesterol or low-density lipoprotein cholesterol."
In other words, blood levels of cholesterol, or even L.D.L.-cholesterol, do not constitute the whole story of coronary risk, a fact widely known but until recently not fully taken into account when devising dietary advice for the public.
Other important factors include blood levels of triglycerides, artery-protective high-density lipoprotein (H.D.L.) cholesterol, an amino acid called homocysteine, cardiac rhythms, clotting tendency of blood, flexibility of artery walls and levels of blood pressure and blood sugar. Each, in turn, is influenced by dietary factors beyond total fat and cholesterol.
The Harvard researchers reviewed a plethora of evidence studies conducted under carefully controlled conditions in metabolic wards, studies of many thousands of free-living people followed for many years, studies comparing the diets of people who suffered heart attacks with the diets of comparable people free of heart disease and studies involving people with heart disease and those free of it, randomly assigned to different diets to see which was most protective.
Here is what they found:
It is not total fat but rather the kinds of fats consumed that have the main influence on coronary risk. Saturated fats those that are hard at room temperature like the fats in red meats, cheese, butter and, to a lesser extent, poultry raise blood levels of harmful cholesterol and are not to be recommended as a steady diet.
Even more hazardous are the so-called trans fats that form when vegetable oils are hardened to make most margarines and the shortenings that are widely used in processed and fast foods. Check the ingredients on food labels and chances are you will find that most processed foods contain "partially hydrogenated vegetable oil," which nearly always means trans fats are present. And look for margarines free of trans fats.
In metabolic studies, even small amounts of trans fats in the diet were found to raise the harmful cholesterol and lower the protective type. Trans fats also raise blood levels of triglycerides and another harmful substance, lipoprotein-a. In addition, the trans fats may impair arterial flexibility and promote resistance to insulin, which in turn increases the risk of Type 2 diabetes, a risk factor for heart disease.
Substituting refined carbohydrates for fats is not the answer, Dr. Hu and Dr. Willett wrote, because such a change raises triglyceride levels and lowers both harmful and helpful cholesterol, resulting in no reduction in coronary risk.
Particularly troublesome, especially for people who are already insulin resistant, are carbohydrate-rich foods with high glycemic loads: lots of quickly digested sugars or starches, like sweets, white bread and potatoes.
Pasta, oatmeal and unrefined grains, on the other hand, are digested slowly and therefore have a low glycemic load and are considered heart healthy.
When oils rich in polyunsaturates and monounsaturates are substituted for saturated fats and hardened oils, harmful cholesterol levels fall and beneficial cholesterol either rises or stays the same.
And when polyunsaturates are substituted for saturated fats, insulin sensitivity may improve and the risk of diabetes may fall, the researchers noted.
Foods rich in monounsaturates include olive, canola, nut and avocado oils and the foods they come from (especially walnuts); oils rich in polyunsaturates include safflower, soybean, corn and mustard.
But the most heart healthy of fats are likely to be the omega-3 fatty acids found in fish and some plants, like flaxseed and purslane, walnuts and in canola and soybean oils and fish oil capsules.
Just two fish meals a week can significantly reduce the chance of having a heart attack. Potential benefits of omega-3 fatty acids include prevention of abnormal heart rhythms, decreased risk of clots, greater arterial flexibility and lower triglyceride levels.
Also protective are whole grains, especially those rich in soluble fiber like oats and barley, and folate, a B vitamin found in dark green leafy vegetables. It is also added to flour now.
Continuing studies of more than 120,000 health professionals showed significant cardiac benefits of a diet rich in fruits and vegetables, especially green leafy vegetables and fruits with vitamin C.
A Heart-Healthy Life
As you can see, there is no one food to eat or eliminate that will by itself protect the heart. Rather, it is an overall dietary pattern, the researchers noted. This "prudent" pattern is characterized by "higher intakes of fruits, vegetables, legumes, whole grains, poultry and fish." A typical Western pattern "characterized by higher intakes of red and processed meats, sweets and desserts, potatoes, French fries and refined grains" is associated with a higher cardiac risk.
And, of course, it is not enough just to eat right. In addition to a healthy diet, protecting one's heart should also include maintaining a healthy weight, exercising regularly for half an hour or more daily, consuming a moderate amount of alcohol and not smoking. Together, such measures could prevent 74 percent of coronary events, the Harvard researchers concluded.
They also disputed a longstanding argument that it was unwise to suggest that people could eat more than 30 percent of their calories from fat, even if it was heart-healthy fat, since so many people are already overweight. "Long-term clinical trials have provided no good evidence that reducing dietary fat per se can lead to weight loss," they wrote. Rather, it is overconsumption of calories, regardless of their source, and underexpenditure of energy, that lead to weight gain.