As a young and aspiring fitness entrepreneur, you dream of spotting the next big thing. But are you looking in the right direction? The most inspired ideas may come not from the future, but from the past.

“Everything has a cycle,” says Bryan Krahn, a trainer and physique coach for more than 20 years. “Almost every trend you see right now I’ve seen at least twice already. The smart businessperson would be someone who’s bold enough to recognize these trends and catch the wave when they come back.”

Another strategy: Treat trends like stocks, and invest in the more sustainable ones. Extreme trends, like the ketogenic diet, burn fast and bright, while more moderate trends ebb and flow but never really die, Krahn says.

“You can always spot a fad by the passion of the zealots behind it,” Krahn says. “No one ever says ‘Weight training doesn’t work.’ But the more extreme things tend to invite polarizing opinions.”

We asked Krahn and other industry veterans for their take on the hottest trends right now and the history behind them.

READ ALSO: 10 Myths Your New Clients Probably Believe

1. Ketogenic diet

Nothing is buzzier right now than the keto diet.

But low-carb diets are not new. Atkins was extraordinarily popular in the 1970s, and many other incarnations have cropped up since then, like The Anabolic Diet, by Mauro Di Pasquale, and Underground Body Opus, by Dan Duchaine, in the ’90s.

“People were buying bags of gas station pork rinds as if they were health food,” recalls Selene Yeager, a fitness journalist and professional mountain bike racer.

But the method predates Atkins too. In researching his book The Lean Muscle Diet, PTDC editorial director Lou Schuler discovered that in 1863, a London undertaker named William Banting wrote about losing 50 pounds after cutting out “bread, butter, milk, sugar, beer, and potatoes.” “The diet became so popular that his name became a verb,” Schuler wrote in this history of diet books. “A person on a low-carb plan would tell friends ‘I’m Banting.’”

Heck, 2,500 years ago, the father of medicine himself, Hippocrates, developed a ketogenic-type diet as a fasting treatment for epilepsy, says Susan Kleiner, PhD, RD, author of The New Power Eating. In fact, doctors have used the treatment for almost a century, and it’s still used in some children’s hospitals today.

Starting in the late 1990s, just as low-fat diets reached their (frankly absurd) peak of popularity and credibility, lower-carb diets came back in a big way, with a resurgence of Atkins and the unexpected popularity of the macronutrient-focused Zone diet. Then came The South Beach Diet (“a kinder, gentler Atkins”) in the mid-2000s, and from there it was a race to the bottom to see which diet could kill carbs the hardest. Hence keto.

“It’s interesting,” Yeager muses. “These diets have a kernel of truth. I remember Atkins saying on TV ‘Smart Start? They should call that cereal Dumb Start!’ He wasn’t wrong. But his answer wasn’t exactly right either. We just seem so reluctant to use middle ground.”

READ ALSO: Is the Ketogenic Diet Right for Your Client?

2. Detoxes and cleanses

Chances are, you’ve had a client ask you about detox diets or juice cleanses. The latest one—celery juice—recently surpassed 40,000 hashtags on Instagram.

The trend “really exploded in lockstep with social media platforms,” says Jeff O’Connell, editor-in-chief for “A general lack of accountability on social media makes it easier than ever for celebrities and influencers to endorse products whether they work or not. Detox and cleanse products are exhibit A.”

But “cleansing” practices are actually old as dirt. People have been trying to rid their bodies of “toxins” for thousands of years, Kleiner says. Colonics and water fasts date back to ancient Egypt and biblical times. Up until fairly recently, bloodletting, enemas, and fasting were considered medical therapies. And juice has been used to treat everything from scurvy to cancer. (Even Jack LaLanne made a good living selling juicers.)

Detoxes and cleanses don’t just fall short on scientific support. They have basically none at all. So why the public obsession?

“There’s a natural psychological appeal to performing them as some sort of dietary penance,” O’Connell says. “To the uninitiated, it may seem like you should be able to flush out your body like you flush out your car’s radiator.”

Actually, there is something that does just that, O’Connell notes. It’s called your liver.

READ ALSO: How Improving Digestive Health Leads to Better Fitness

3. Low-fat diets

If low-carb is in now, what’s next? Believe it or not, low-fat might be coming back.

Krahn has noticed some bodybuilding coaches recommending it, and CrossFit (which used to recommend the Zone diet for everybody) now advises high-carb, low-fat plans for high-volume athletes. “They’re using macros that seem straight out of the ’80s,” he says.

Back then, a steep post-World War II rise in heart disease prompted researchers to target total and saturated fats in their studies. As a result, U.S. dietary guidelines recommended everyone cut back on fats.

“But the public heard ‘carbs are good, fats are bad,’” says Kleiner. “And the food industry was off to the races.”

Ironically, the lack of nuance in people’s understanding of fats or the dangers of processed carbs probably helped fuel other problems, like obesity and diabetes. Just as Atkins had been the low-carb guru, Nathan Pritikin became the low-fat guru, Kleiner says. “The low-fat diet became the darling of the age.”

But perhaps the biggest upshot of the diet wars is the dizzying array of diet methodologies we have today—Paleo, vegan, Mediterranean, Weight Watchers, raw food. Of course, the question of which is best will always come down to the individual.

READ ALSO: Focus on Weight-Loss Behaviors, Not Meals

4. High-volume training

It’s one of the great ongoing debates in bodybuilding. What’s better: high volume or high intensity? Each training style has been popular in cyclical fashion over the decades.

High volume (training lighter for longer, and more often) is how Arnold and his crew trained in the ’60s and ’70s, Krahn says, and it became popular again in the ’80s before high-intensity training reemerged in the ’90s and 2000s.

Now the tide is turning toward high volume again, Krahn says.

“I remember Dave Tate told me five years ago, ‘Oh, you wait: High volume will be the biggest thing in five years,’” Krahn recalls. “Of course he was right.”

READ ALSO: How to Help a Client Who Wants to Gain Weight

5. High-intensity training

Interestingly, in the endurance sports world, the trend is going the other way, says Selene Yeager.

“Endurance athletes used to be all about high volume, low weight. But now science is showing that is actually good for hypertrophy, which cyclists and runners don’t want,” Yeager says. “Heavy weights, lower reps are better for neuromuscular stimulation and strength. So more endurance athletes are lifting heavy.”

HIT was popularized among lifters in the 1970s by Arthur Jones, and reemerged in the ’90s thanks to Dorian Yates. It also gave rise to arguably the worst-named training system of all time—Doggcrapp (courtesy of Dante Trudel).

The cardio version of HIT—high-intensity interval training (HIIT)—has also risen to popularity and continues to be sold in a variety of new ways, like Orangetheory, metabolic circuits, Tabata, and HIIT hybrids. Not surprisingly, the promise of maximum benefits in minimal time resonates in an era when many Americans are busier than ever. But of course, runners have used the technique since the 1800s.

READ ALSO: Can Older Clients Benefit from Interval Training?

6. Steady-state cardio

Just as HIIT has become ubiquitous, now steady-state cardio is coming back. For Amy Eisinger, the special projects director at Self, it’s part of a renewed interest in moderate fitness.

“The case for moderate fitness kind of got born again,” Eisinger says. “For a long time, everything was about HIIT and pushing yourself to the extreme. But a lot of people injured themselves or just didn’t like feeling that uncomfortable. It’s tiring.”

Peloton and treadmill classes keep things interesting with climbs and moments of intensity, but the 45- to 60-minute sessions are essentially the steady-state cardio that was so popular in the ’80s and ’90s.

Krahn recalls how bodybuilders back then would start their days with 30 to 60 minutes of cardio.

“If you wanted to lose fat, you got up in the morning and did the treadmill or got on a stationary bike,” he says. “Now that’s kind of back again, whereas 10 years ago, people would make fun of you for doing cardio. ‘Cardio eats muscle,’ that’s what they’d say. And then people got a brain and realized it’s not true.”

READ ALSO: Stop Training Your Clients Like Powerlifters

7. Follow-along training

Speaking of Peloton: Live-streaming and online classes have made follow-along sessions more accessible, all from the comfort of your own home. (Especially if you’re affluent.)

“More than ever, people want to follow along with their favorite fitness personalities as they train, rather than taking their workout to the gym,” says O’Connell, whose team at did extensive research on the topic for its subscription product, All Access.

“It’s really the same approach people like Richard Simmons and Jane Fonda took decades ago,” he says. “Serious fitness people viewed them with derision, but they tapped into what people wanted. And they were ahead of their time.”

Even before that was The Jack LaLanne Show, a television exercise program hosted by the fitness icon, which ran from 1951 to 1985.

READ ALSO: How to Build an Online Following from Scratch

8. Intermittent fasting

It’s gaining traction in the research community, and has gone mainstream as a weight-loss strategy. But fasting is nothing new. Depending on your faith, it dates back to biblical times, and many religions use fasting rituals for focus, healing, and purification.

In the 5th century BCE, Hippocrates recommended fasting for his patients. “He wrote ‘To eat when you are sick, is to feed your illness,’” Kleiner says. “The ancient Greeks also used fasting for cleansing the mind and uplifting spirituality.”

Researchers began looking into fasting in the late 1800s, and in the 20th century fasting methods became increasingly sophisticated. “In my generation we called fasting ‘skipping breakfast,’” says Krahn. These days you have Ori Hofmekler’s The Warrior Diet, Brad Pilon’s Eat Stop Eat, alternate-day fasting, and who knows how many variations on those basic templates.

Studies suggest fasting is not just a healthy way to lose weight but may improve other health biomarkers as well. It appears to induce mild stress on the body, triggering adaptive changes that strengthen cells and make them better able to fight disease.

While more research is needed to understand the mechanism, Kleiner says the primary benefit is pretty straightforward: “If you don’t eat, you lose weight.”

READ ALSO: Four Ways to Use Science to Get Results for Your Clients

9. Body-weight training

When it comes to old stuff that’s new again, body-weight training may be the ultimate example. Calisthenics were staples in gym classes in the latter half of the 20th century, O’Connell says, even while the booming health club industry lured more ambitious enthusiasts into the weight room.

In the earlier part of the century, Charles Atlas made his fortune on body-weight training, selling hundreds of thousands of muscular development courses using a combination of body-weight and isometric training he called Dynamic Tension.

These days, the ability to get a good workout anywhere and anytime may suit millennial tendencies toward minimalism and staying home. But O’Connell again points to social media, particularly YouTube, for helping to bring body-weight training back to the fore.

“I think of Zuzka Light,” O’Connell says, referring to the YouTube phenom with whom he wrote 15 Minutes to Fit. “She didn’t have access to a gym, or even the desire to go to one. But she had a video camera, and herself, and that’s all she needed.”

Viewers could follow along at home, no equipment needed.

READ ALSO: How to Train a Frequent Flier

10. Sleep

People are becoming more woke about sleep—in the news, in the lab, even on Pinterest. It’s huge in fitness too.

“People are realizing that the effort they put into their sleep is almost like adding drugs to their protocol. It’s that powerful,” says Krahn. “They're limiting caffeine and sleeping in really dark rooms and scoring CPAP machines off Craigslist. And they don’t even have sleep apnea!”

Formal studies on sleep began accumulating in the latter part of the 20th century, but those ancient Greeks were on the case thousands of years ago, when Galen identified “waking and sleeping” as a main influential factor in health.

Jack LaLanne was preaching the benefits of sleep back in the ’50s. And in the ’80s, some bodybuilders took ornithine alpha-ketoglutarate (OKG) supplements in an effort to improve sleep quality and increase growth hormone, says Krahn.

In the 2000s, sleep prioritization rose in tandem with a shift toward self-care and anti-aging. “But because it’s so pedestrian, people would kind of roll their eyes. ‘I’ll sleep when I’m dead’ kind of thing,” Krahn says. Not anymore. “Now people brag about how much sleep they get,” Krahn says, “which is even more annoying.”

READ ALSO: The Sleep Tip You Should Never Give a Client (And 5 Others You Should)

11. Glute-focused workouts

Fifteen years ago, Selene Yeager pitched a booty book to her publisher. After all, the glutes had long been recognized as an important, often neglected part of the core. Seems like an easy sell, right? Wrong. The proposal died on the vine.

“Those were the days of ‘Banish your belly, butt, and thighs,’” Yeager recalls. “How things change!”

And how they stay the same: Fact is, a sculpted butt has been a covetable body part since, well, forever—but now it’s less taboo to admit it, particularly for women. Compare the mannerly Buns of Steel of the ’90s to the in-your-face Instagram posts and glute-focused workouts of today. (Thank you, Bret Contreras.) We have Best Butt Ever classes and an L.A. gym dedicated entirely to the buttocks.

“The end goal”—pun probably intended—“is the same,” says Krahn. “Just the marketing around it is different.”

Twenty years ago, only highly competitive bodybuilders sought training for bigger glutes, Krahn says. Since then, the importance of this large muscle group has gone mainstream. “Now glute training is an entire industry in itself,” Krahn says. “That’s not going anywhere.”

READ ALSO: Five Steps for Dealing with Anterior Pelvic Tilt

Final thoughts

So what does this mean for you? Or, more to the point, what does it mean for your clients? If there’s one takeaway, Krahn says, it’s that almost anything can work—for the right person, in the right circumstances. “In my coaching, I get people to tell me what they naturally gravitate to. And I just tell them to do that, but better.”

And even though it confuses the hell out of people, that includes polar opposites: low-fat and low-carb diets; fasting and small, frequent meals; steady-state cardio and HIIT; high-volume and low-volume strength training.

“What seems to matter most is whether someone believes it will work for them,” Schuler says. “Belief makes it possible to do the hard work, and hard work makes almost anything possible.”