Q: I see trans fats listed on food labels, and I've read that the FDA may ban them. Can you remind me what trans fats are, and why they're bad for me?
A: Trans fats are a type of unsaturated fat. Once upon a time, we consumed only small amounts of naturally occurring trans fats in some meat and dairy products. But by the end of the 20th century, trans fats were everywhere.
That's because chemists discovered that they could turn liquid vegetable oil into a solid or semisolid by bubbling hydrogen gas through it (think margarine).
When hydrogen is bubbled through liquid oils, they are called "partially hydrogenated" oils, or trans fats.
Why would chemists want to create trans fats?
They don't spoil or turn rancid as readily as nonhydrogenated fats, and they respond better to repeated heating. Those characteristics made trans fats a workhorse of the food industry. By the late 1990s, nearly all prepared cookies and crackers contained trans fats.
Restaurant frying oils were also rich in trans fats. At first, doctors and nutrition scientists thought that trans fat in food might be a healthy substitute for saturated fat, which was known to increase blood cholesterol levels and the risk of heart disease.
So people hoped that substituting trans fats for saturated fat would reduce the risk of heart disease. For example, my mother stopped using butter and started using stick margarine. That's what I did, and that's what I recommended to my patients.
I can't recall any medical colleague of mine challenging the practice of promoting stick margarine over butter. In fact, the only person I knew who disagreed was my very shrewd sister. She stuck with butter. She said she knew you should go light on the butter, but she just suspected that stick margarines were even worse. Not for the first time, I should have listened to her.
In the 1990s, nutrition scientists -- led by my Harvard colleague Dr. Walter Willett -- discovered that trans fats were at least as heart-unhealthy as saturated fats. Eating trans fats boosts LDL (bad) cholesterol and lowers protective HDL cholesterol.
Trans fats also have unhealthy effects on triglycerides. They increase the risk of blood clots and they feed inflammation, which plays a key role in heart disease, stroke and diabetes.
And yet, for years the U.S. Food and Drug Administration (FDA) labeled trans fats as "generally recognized as safe" (GRAS). That allowed them to be used without testing or approval.
In November, the FDA proposed removing trans fats from the GRAS list. That means companies will have to prove that trans fats are safe if they want to continue to put them in their products.
The FDA's proposal, if finalized, should eliminate artificial trans fats from our food supply. Food companies have already found healthier alternatives, so your taste buds are unlikely to even notice the change.
But your heart and the rest of your body most certainly will.
•Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com.