Danica McKellar's simple trick to beat holiday weight gain at 50

Danica McKellar is looking fit at 50, and the Great American Family star recently divulged her secret to avoiding holiday - or any time of the year - temptations.

"My secret - one of my secrets - is that I don't eat dessert or anything that's bad for me until I'm totally full with all the healthy stuff," she revealed to Fox News Digital. "Once I'm full on the healthy stuff, then I'll have maybe a couple bites. But I don't want to be hungry when I'm eating the bad stuff."

She said if she eats everything healthy on her plate, and she's still hungry, then "I can have a little bit of something."

"The Wonder Years" alum also said she doesn't do New Year's resolutions.

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"I'm not sure why. I think because every day I try to just resolve to be the best person I can be, to do what God wants me to do, to keep listening to God for guidance," she said during the interview, which took place at Conrad Nashville. "In fact, one of the themes of my Christmas movie is listening to God."

McKellar's twelfth Christmas movie and her feature writing debut, "Have We Met This Christmas?" debuts on Great American Family on Dec. 13. It is also streaming on Great American Pure Flix. She not only wrote it, but she also stars in the film.

WATCH: DANICA MCKELLAR'S TIPS FOR AVOIDING FOOD TEMPTATIONS AND STAYING FIT AFTER 50

McKellar also just published her twelfth math book last month - a children's book called "I Love You 100: A Counting Book Full of Love."

"It's fun when you write your own Christmas movie," she said. "You get to put in the themes that you want. So, you'll see themes of listening to God and forgiveness, which is great around the holidays because of what Jesus did for us."

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McKellar stressed that she eats a "very boring diet," and her "number one secret to staying fit, healthy, and youthful" is drinking lots of water.

"I really don't drink alcohol, I don't drink coffee, I don't drink soda, certainly nothing with artificial flavors or sweeteners," she explained. "I think that stuff is evil, and it actually messes up your body more than if you just had sugar. Although sugar is really bad for you. So I just try to stay away from all that stuff."

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She added that she also doesn't eat gluten or dairy, and eats almost no processed foods.

"I cook dinner every night -  well, every other night, so we can have leftovers," she said. "I do it myself because I know what is in there. I cook even when I'm doing a movie."

On location, the 50-year-old actress said she keeps an instant pot in her hotel room, "and I make my own chicken. I bring a rice cooker, I make my own brown rice, I steam vegetables, a little salt, pepper, olive oil. It's so good, it's very tasty, and it's very healthy."

WATCH: DANICA MCKELLAR MAKES HER CHRISTMAS MOVIE WRITING DEBUT WITH NEW GREAT AMERICAN FAMILY FILM

She said that helps her "resist the urge to eat the bad stuff. And when I am resisting the urge, because, believe me, I have temptations, I think to myself - this is my other secret. I think to myself, right now is when I'm earning all the things I love about feeling good and feeling fit and healthy. Right now, I'm earning it."