Michelle Pfeiffer admits in a new interview that vanity played a part in her decision to adopt a vegan lifestyle, but she adds that more than anything it was because of a desire to live a healthier life.

The 54-year-old actress tells Dr. Sanjay Gupta on Monday's edition of "Piers Morgan Tonight" that watching his documentary, "The Last Heart Attack," gave her plenty of food for thought.

"I was finishing up working on, I think it was 'Dark Shadows,'" she says. "And I was watching CNN, and 'The Last Heart Attack' came on."

As she was watching the documentary, which explores preventative measures for heart disease, it was former President Bill Clinton's story that really hit home.

Pfeiffer, who considers herself to also be a "foodie," watched Clinton and said, "OK, Bill Clinton loves food, so there must be something to [veganism] that's making him stick to it. And also, he's smart, so he's not going to do something unless he really thinks there's some science behind it."

After reading the book "Prevent and Reverse Heart Disease," which advocates for a plant-based, oil-free diet, Pfeiffer says her mind was made up.

"I just felt like...there was science behind it," she says. "And, you know, it was sort of irrefutable. ... I couldn't not listen to it. My father died from cancer, and the older you get, there's a lot of disease around you. And you see people struggling with chronic disease. You see people dying with terminal illnesses. And if in any way .... this is true, then you kind of have to listen to it."

As someone who loves carbs, Pfeiffer says she's enjoying the vegan diet, and has her husband of 19 years, David E. Kelley, trying to make the switch.

The older she gets, Pfeiffer says of her views on diet, the more her focus and intent is geared toward living a longer life.

"Vanity is right under there," she admits, "but I have to say that it's a close second with wanting to live long."

Watch the full interview with Michelle Pfeiffer when she stops by "Piers Morgan Tonight" at 9 p.m. ET.

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