A small four-week study suggests not all low-fat diets are equally good.

Researchers found that eating a low-fat diet packed with vegetables, fruit, beans and whole grains reduces levels of LDL, or "bad," cholesterol twice as much as eating a low-fat diet heavy on processed foods.

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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 study of 120 adults appears in the May 3 issue of Annals of Internal Medicine.

Stanford University researcher Christopher Gardner said earlier studies have shown plant-based diets can lower cholesterol. But he said plant-based eaters generally consume less saturated fat and cholesterol. So, researchers set up two study groups to see what happened when the fat and cholesterol levels were the same for both diets.

The conventional diet produced, on average, a 4.6 percent LDL decrease, and the plant-based diet achieved, on average, a 9.4 percent decrease in LDL.

But researchers still don't know why levels of HDL, or "good," cholesterol and triglycerides, another fat that contributes to heart disease, were essentially the same in both groups.

"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,'" Gardner said in a news release. "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."