Margarine is the heart-healthy choice
The old claim turned out to be wrong and was rejected by evidence.
TaughtButter is saturated animal fat and bad for your heart. Switch to margarine, which is made from vegetable oil.
NowThe hydrogenation that made margarine solid produced artificial trans fats, which are worse for cardiovascular health than the saturated fat they replaced. The FDA removed partially hydrogenated oils from the safe list in 2015 and the WHO has called for global elimination.
What actually happened
This one stings, because the public health system did not merely fail to warn people. It actively pushed the substitution for decades.
Trans fats raise LDL cholesterol and lower HDL cholesterol, a combination worse than saturated fat, and they were in nearly every stick margarine, shortening, and packaged baked good on the shelf. Denmark restricted them in 2003. The FDA required labeling in 2006, determined in 2015 that partially hydrogenated oils are not generally recognized as safe, and set a compliance deadline of 2018 for most uses. The WHO launched a program in 2018 to eliminate industrial trans fat from the global food supply. Modern soft tub margarines are reformulated and essentially trans-fat-free, so the advice today is not simply "go back to butter." It is that the specific product your health teacher recommended was a mistake, and the correction took thirty years.
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Who was taught this
Still standard through 2013, so anyone who finished school between 1950 and 2015 learned the earlier version.