Last September, Sara Joy Madsen started drinking 16 ounces of fresh, pure celery juice every morning — and “it changed my life,” the 39-year-old life coach tells The Post.

In a matter of months, the Park Slope resident says she “effortlessly” lost 20 pounds, with her 5-foot-8 frame shrinking from 190 pounds to 170. She also says her skin became clearer and her moods more stable: “I’m calmer, more patient, more compassionate.” Best of all, she’s proud of herself for sticking to it. “Because I do it every day, I get like this charge, like, ‘Oh, my God, I accomplished something.’ ”

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In certain health and wellness circles, celery juice is the latest miracle elixir. Companies such as Pressed Juicery and Juice Press offer pricey bottled versions. On Instagram, more than 68,000 posts are tagged with #celeryjuice and more than 17,000 posts bear the label #celeryjuicebenefits. People post dramatic before-and-after pictures that showcase how celery juice has seemingly cured their crippling psoriasis or the digestive issues that left them uncomfortably bloated. Celebs are drinking the veggie Kool-Aid, too: Earlier this month, Debra Messing tweeted that her “New Years Resolution #1” was “upping [her] morning celery juice from 8 oz. to 20 oz.

The trend traces back to Anthony William, a Los Angeles-based health guru who calls himself the Medical Medium and the “originator of global celery juice movement.”

“It’s a powerful herbal medicine that’s killing bugs in people’s bodies,” William, who has 1.4 million Instagram followers, tells The Post.

But many question the science — or lack thereof — behind the so-called veggie panacea.

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“There’s nothing remarkable about celery juice,” Abby Langer, a registered dietitian based in Toronto, tells The Post. On her blog, she denounced the trend as “the epitome of bulls–t pseudoscience.”

William begs to differ. Although he has no conventional medical training, he believes so strongly in the juice’s benefits that he’s touted it in all four of his books, as well as in several posts written for — of course — Gwyneth Paltrow’s Goop website. (Editorial notes stress that his articles are not “a substitute for professional medical advice.”)

To reap the drink’s benefits, Williams explains, the drink must be consumed fresh, on an empty stomach and not diluted by water, ice or other juice. Its healing powers, he claims, lie in sodium cluster salts — “a subgroup of sodium that’s undiscovered,” according to him. The cluster salts, he says, “revive your stomach gland, so your hydrochloric acid is actually restored . . . and your digestion gets stronger, so protein doesn’t rot in your gut.” He claims that cluster salts kill off pathogens that can cause everything from multiple sclerosis to Lyme disease.

It’s a nice idea — but the conventional medical community says is mostly wishful thinking.

Shonali Soans, a registered dietitian with Lorraine Kearney Nutrition in the Financial District, says the “celery juice craze” is nothing more than a testament to our desire for quick fixes.

“Sure, celery is nutrient-dense and has many great health benefits,” she says, “but so do other vegetables.” Plus, she says, juicing celery strips the vegetable of one its great benefits: fiber.

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That said, she and Langer agree that sipping celery juice likely won’t do any harm. There’s some potential interference with medicines, such as statins and anti-anxiety medications, and there’s a possibility that it can promote kidney-stone growth if you’re already prone to them. But for the most part, Sloane says, it just won’t do much at all.

But the movement’s fervent followers disagree.

“I’m definitely a believer,” says Kayla Paz, a 23-year-old data entry specialist who lives in Westchester. She says her skin, digestion, irregular periods and mental clarity have improved since she started having 32 ounces of celery juice every morning for the past month. “I really like how it makes me feel.”

Langer says that when a celery juicer suddenly has clearer skin or better digestion, it’s likely just a coincidence or a result of some other lifestyle change. There’s no scientific basis for celery being a miracle drug, she says: “It’s an elaborate lie.”

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But William says the proof is in the experience that his followers have — and share on Instagram — and the conventional medical community is just behind the times.

“The only study they have is feeding mice and rats a little bit of celery. There’s no celery juice study on humans,” he says, “except for the one that’s happening now.”

This article originally appeared on the New York Post.