A recent study in the Journal of the American Medical Association concludes that using tub margarine instead of butter has been found to lower blood cholesterol. Researchers note that how much of a difference the spread makes varies, depending largely on genetics and body weight. But although tub margarine is clearly a healthier choice than butter, don't forget to consider the health advantages of oils as well. And the impact of any changes in the type of fat will depend on the quantity you use.

The new study, conducted at the University of Texas Southwestern Medical Center in Dallas, included families that followed diets using either butter or tub margarine as the major fat. Compared to their former levels when they used butter, the adults' LDL (bad) cholesterol dropped 11 percent after the margarine diet, and children's LDL declined 9 percent. Since the HDL ("good") cholesterol did not vary between diets, tub margarine is clearly the more "heart-healthy."

The diet using margarine lowered blood cholesterol in more than 80 percent of the people involved in the study. But cholesterol changes varied among families, suggesting that inherited traits may play an important role in people's response to changes in eating habits. Researchers also found that overweight people showed less response to changes in fat than others. Lead author of the study, Margo Denke, M.D., notes that weight loss may be more significant to lowering blood cholesterol if someone is overweight.

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How much impact a particular fat has on a given individual will also depend on the quantity used. The test diets' fat content � 35 to 37 percent of calories � was higher than the 15 to 30 percent recommended for good health. And in this study, the margarine or butter provided a higher proportion of daily fat than in most people's eating habits � for cooking and baking also, not just as a spread. If you use only small amounts of added fat or don't make your own baked goods, a change to tub margarine probably won't make as dramatic a difference in your blood cholesterol.

The bottom line is that tub margarine really is more heart-healthy than butter, a finding repeated in several studies. The choice of a tub margarine is key. Stick margarine includes more hydrogenated oils with cholesterol-raising trans fat.

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The better cholesterol readings with tub margarine are no surprise. The total cholesterol-raising fat content of the margarine diet was clearly lower than that of the butter diet. Nevertheless, many oils would score even better because they are very low in saturated fat and contain virtually no trans fat. That's why many nutrition experts recommend using a tub margarine as a spread and a modest amount of olive or canola oil for most cooking. If you follow this practice, it will probably not make any difference if you occasionally use a little butter in a particular dish.

Of course, it's your total diet that counts. If you use heart-healthy oils and tub margarine, but load up on high-fat meat, cheese and ice cream, don't expect your cholesterol to drop. And using limited amounts of "healthy" fats is just one part of overall healthful eating.

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Karen Collins, M.S., R.D.,C.D.N.