All low-fat diets are not equal, Stanford study shows
STANFORD - A low-fat diet rich in vegetables, fruits, whole grains and beans has twice the cholesterol-lowering power of a conventional low-fat diet, according to a new study from the Stanford University School of Medicine.
In other words, a meal of spinach salad, egg and oatmeal-carrot cookies is healthier for your heart than stir-fried lean beef and asparagus and low-fat chocolate chip cookies - even when both meals contain the same amount of saturated fat and cholesterol.
The finding, published in the May 3 issue of the Annals of Internal Medicine, comes from a meticulous comparison of two low-fat diets. One, the conventional diet, focused solely on avoiding harmful saturated fat and cholesterol. Diners ate such foods as frozen waffles and turkey bologna sandwiches. The second diet included the same proportions of fat and cholesterol, plus lots of plant-based foods in accordance with American Heart Association guidelines. Those diners ate such foods as hot grain cereals and vegetable soups.
Both diets lowered total and low-density lipoprotein (LDL, or "bad") cholesterol over the course of the four-week study. The conventional diet produced, on average, a 4.6 percent LDL decrease. But the plant-based diet beat that hands-down: It achieved, on average, a 9.4 percent decrease in LDL. Researchers found no significant differences in changes in triglycerides or high-density lipoprotein ("good") cholesterol.
"The effect of diet on lowering cholesterol has been really minimized and undermined by a lot of clinicians and researchers saying, ‘Yes, it has an effect but it's really trivial: It would be better to put you on drugs to control your cholesterol,'" said Christopher Gardner, PhD, assistant professor (research) of medicine at the Stanford Prevention Research Center and lead author on the National Institutes of Health-funded study. "But we think part of the reason was that we weren't really giving diet a fair shake. We were so focused on the negative - just what to avoid - and not what to include."
The bottom line? Mother knows best: Do eat your veggies - and other nutrient-dense foods. It's not enough to simply steer clear of saturated fat and cholesterol.
"We would really hope that people would appreciate the new American Heart Association Guidelines," said Gardner, who decorates his office with splashy posters of squashes and peas. "Include more whole grains and vegetables and beans and colors - not iceberg lettuce, but red bell peppers and carrots and broccoli and red cabbage and the really colorful foods. Those are all really low in saturated fat and cholesterol, and they're really high in other nutrients and phytochemicals that are good for you."
A "plant-based" diet is not necessarily a vegetarian diet. It simply includes a foundation of whole grains, vegetables, beans, nuts, seeds and fruits. The 2000 AHA guidelines recommend at least five daily servings of vegetables and fruits and at least six daily servings of grains with an emphasis on whole grains.
Previous studies, Gardner said, have shown plant-based diets to be effective in lowering cholesterol. But plant-based eaters also tend to consume less saturated fat and cholesterol than conventional low-fat eaters.
The Stanford study breaks new ground by comparing two patient groups eating different foods but identical amounts of total and saturated fat, protein, carbohydrate and cholesterol. So the two groups' different levels of blood cholesterol change are attributable to the different foods - dark green salads and bean burritos, for example, versus iceberg lettuce and frozen pizza - and not differences in saturated fat and cholesterol intake.
The randomized clinical trial included 120 adults, ages 30 to 65. All were members of the target population for food-based approaches to lowering cholesterol: They had moderately high LDL levels, between 130 and 190 mg/dL. Researchers randomly divided the outpatients into two diners' groups: 61 ate the conventional diet, while 59 ate the plant-based diet. Each weekday for a month, they visited a research dining hall for a specially prepared, carefully weighed, chemically analyzed lunch or dinner.
The study required that participants maintain a constant weight so that any changes in blood cholesterol would be attributable to the diets themselves - not to any changes in weight brought on by the diets. When changes in weight were observed, the participants' calories were changed accordingly to help them stay stable. In general researchers tended to add calories to the meal plan over the course of the study as participants were observed to be more likely to lose weight than to gain weight on both diets.
The scientists also requested that no one change exercise habits, saying: "If you are a marathon runner, keep running marathons. But if you're a couch potato, we need you to stay a couch potato."
Gardner, a 20-year vegetarian who specializes in nutrition and preventive medicine, expects a plant-based diet combined with weight loss and exercise to achieve even more impressive cholesterol-lowering results. Plus, he said, people can eat even less saturated fat and cholesterol than the study participants did.
Gardner's research collaborators include Ann Coulston; Lorraine Chatterjee; Alison Rigby, PhD, MPH; Gene Spiller, PhD, and John Farquhar, MD.
Stanford Medicine integrates research, medical education and health care at its three institutions - Stanford University School of Medicine, Stanford Health Care (formerly Stanford Hospital & Clinics), and Lucile Packard Children's Hospital Stanford. For more information, please visit the Office of Communication & Public Affairs site at http://mednews.stanford.edu.