A flat tummy after 40 is possible — and 57-year-old trainer Jill Brown would know.

Brown, a certified master health fitness and nutrition coach, says losing weight and even getting abs won’t look the same as it did in your 20s, but if you focus less on “fancy crunches” and more on her number one priority, you’ll be on your way to seeing results.

Brown acknowledged that it’s absolutely harder to slim down in midlife — and it’s necessary to keep body changes in mind with your diet and fitness goals.

“First thing we need to realize is that we lose muscle with age. Sarcopenia [age-related muscle loss] happens to men and women starting around 30-ish,” she told The Post.

“As we lose muscle, our metabolisms slow down. So if we’re eating approximately the same amount of calories in our 40s as we did in our 20s and 30s, or exercising less, we’ll be slowly putting on weight.”

And even if your weight gain used to go to your thighs and butt, as you get older, more will settle in the middle.

“Once estrogen levels start dropping in perimenopause for women, the playing field gets leveled, and we all start to store more fat around the belly,” Brown said. “Add chronic stress to the mix, and long-term elevated cortisol will cause more fat to store around the belly.”

So how do you lose belly fat? First comes diet.

“There’s an old saying in my field that goes, ‘Muscles are made in the gym and abs (or flat abs) are made in the kitchen,’” Brown said.

Ever heard that fat loss is 80% diet and 20% exercise? Brown says there’s no scientific study to back up this ratio, but “pretty much all professionals in the fitness and nutrition worlds agree on this.”

“In order to get a flat stomach after 40, nutrition should be the number one priority. All the planks and fancy crunches in the world won’t burn a noticeable amount of fat off the belly,” she explained.

“But when you have a nutrition plan that prioritizes high-quality protein with strategies to help you achieve a calorie deficit, you will see the belly fat start to melt.

“We store extra calories as fat to use when food is scarce. Eat fewer calories and fat will be used to energy.”

You better move

That being said, Brown stressed that you still need to work out if you want abs — or any muscle definition. But strength-building exercises are the name of the game.

“No matter how much protein you eat, you can’t build muscles without exercise as a stimulus. So strength training should always be done,” she said.

“The more muscle mass we have on our bodies as we age, the better. It raises our resting metabolic rate (RMR) — not much, but every bit counts to offset the fact that metabolism slows with age primarily due to muscle loss. The higher our RMR, the more calories our body burns a day.”

Cardio can be helpful too, but only as it related to a calorie deficit — if you’re sweating on the treadmill or stationary bike, you’re burning calories.

“Traditional cardio exercises like walking, jogging, elliptical machines, etc., done at a steady, comfortable pace, don’t build muscle, but they do burn calories,” she said.

Think about your priorities

All of this may seem easier said than done, especially as people feel less energized in their 40s, 50s and 60s than they did in their 20s or even 30s.

“The rub is that as we get older we feel like taking it easier,” Brown said. “Maybe we have injuries — I have some major ones from genetics and accidents — or we may feel exhausted and have less time to prep healthy meals or fit in our workouts.”

So adjusting your mindset can make a difference.

“You need to ask yourself, how important is it to have flat abs? If you’re not genetically blessed, it takes work,” she admitted.

“So it’s a good idea to think about it like this: Do I want flat abs just to look good or do I need flatter abs for my health?”

You may have plenty of subcutaneous fat, the kind you can “pinch and see jiggle under your skin,” which sits above the muscle and below the skin.

Brown says that fat isn’t dangerous — but visceral fat, which surrounds our organs and has been linked to everything from dementia to heart disease to strokes, is dangerous and can lead to early death.

If your motivation is to blast that visceral fat and live longer, you may be more likely to eat a bit better and move a bit more — which will take care of both kinds of fat.

“My final reminder is this,” Brown concluded. “Losing the fat isn’t the hardest part. It’s keeping it off. If you want flatter abs for life, you need to make permanent lifestyle and habit changes. Not just a 60- or 90-day program where you go back to your old habits when you’re done.”