BOSTON—Plant sterols and stanols, substances that can lower cholesterol, are now being added to foods ranging from granola bars to chocolate. The catch is that you need to eat about 2 grams worth of added sterols or stanols every day to put a dent in your cholesterol, reports the Harvard Heart Letter.

When eaten, sterols and stanols gum up the body’s system for absorbing cholesterol from food. Since the liver needs cholesterol for digestion, it grabs LDL cholesterol from the bloodstream while leaving HDL cholesterol alone. Eating 2 grams per day can lower levels of LDL cholesterol by about 10%, which could translate into a 20% lower risk of having a heart attack or stroke.

If a food you eat every day is now being made with extra sterols or stanols, switching to the fortified version makes sense, says the Harvard Heart Letter. If not, adding these foods to your diet is a high-calorie way to modestly reduce cholesterol. You’ll need to eliminate the added calories somewhere else in your diet, or the resulting weight gain will counteract the cholesterol-lowering effect.

Trying to juggle a daily intake from different foods containing plant sterols and stanols could get complicated and lead to higher-than-recommended doses. Exceeding the 2-gram target doesn’t offer any extra benefit. What’s more, no one knows the long-term effects of getting too much.

The bottom line: Plant sterols and stanols can’t counteract unhealthy choices like smoking or a high-fat diet, so use them as part of a package of healthy choices.