Meghan Markle's trainer reveals 6 tips for avoiding holiday weight gain

Lagree said one of the biggest problems is that people aren't realistic or honest with themselves about their holiday plans. It's easy to say you'll work out tomorrow, but facing a snowy, possibly hungover, trek to the gym after Thanksgiving is daunting. 

"People live under false promise of 'I will do it tomorrow,' but that never happens," Lagree said. "That's why unfortunately every year, there's an accumulation of weight gain. A lot of people still haven't lost the weight they gained last year."

But going in with clear goals and a plan for sticking with them can prevent that. 

Don't hold off on eating before the holiday meal because you plan to "save" all your calories for one sitting, Lagree said. That will only make you hungrier, and more likely to overdo it once the meal is finally served. 

Instead, have a healthy, filling meal before you leave for the celebration. Something less calorie-dense but with a lot of volume and nutrition is ideal, Lagree said. Leafy greens and veggies or complex carbs like whole-grains could work. 

With buffet-style gatherings, it can be easy to overdo it, but plan to stick to reasonable portions. 

Drinking plenty of water can help give your body a break to assess how full you really are before you go back for another serving, Lagree said. 

Don't stress if you spend Thanksgiving enjoying a hearty meal with family or friends.

Lagree said that for most people with healthy lifestyle, one big day of partying or eating won't cause significant issues. 

"If it's a one-time thing, that's OK. Nothing bad is going to happen; your body will bounce back," he said. "The problem is the reptative habit." 

On a cold winter day, especially after a big meal, it can be tempting to skip the snowy trek to the gym in favor of cozying up indoors. But Lagree said it's still possible to get in a solid workout, even without equipment or much time. 

He recommended slow, isometric workouts — exercises that hold tension through the movement — like lunges, wall sits, planks, tricep dips, and crunches. 

Pick five and do each for a minute, in any order. "Move so slow you can feel gravity pulling your body to the floor, feel your body fighting gravity," Lagree said.

For an added challenge, add a "pulse" movement to static holds. In a squat, for instance, hold the position, then very slowly move up and down a tiny amount, still maintaining a low squat. For plank, lower into a partial push-up position and back into plank. 

"You don't have to do it for hours," Lagree said. "Just a few seconds will get your heart rate up."

The last mistake people make regarding post-holiday fitness is expecting to make huge changes overnight by falling for social media promises or slick marketing campaigns, Lagree said.

"If the promise seems really far-fetched, it probably is," he said. 

He recommended giving yourself the same amount of time to get back in shape as you spent getting out of shape. For instance, if you gain a few pounds during a month of holiday celebrations, a full month of consistent exercise and healthy eating is a realistic goal to burn it off.

If it's been a little longer since you've worked out, like a months or even a few years, expect is to take a proportionately longer amount of time to reach your fitness goals. 

"You want to go slow, do things in moderation, don't try anything weird, so you don't hurt yourself," Lagree said. "There's no magic pill, it's just hard work, discipline, and consistency. The slow and steady approach is best."

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