While the mega-popular, high-fat, low-carb ketogenic diet has led many people to severely restrict the amount of carbohydrates they eat, other health aficionados are turning to an eating strategy that embraces them, called "carb cycling."
Carb cycling involves alternating between high-carb and low-carb days over the course of a week. Unlike other diets that cut out carbs entirely, carb cycling encourages you to ramp up or cut back on your carb intake in alignment with your activity level each day.
Endurance athletes and bodybuilders use this approach to boost performance or achieve a desired physique before a competition, Torey Armul, a registered dietitian and spokesperson for the American Academy of Nutrition and Dietetics tells CNBC Make It. Fitness influencers also use this tactic to lose weight, and share photos of recipes and meal plans on Instagram and Pinterest. There are over 475,000 posts on Instagram with some version of a "#carbcycling" hashtag.
And carb cycling has been gaining traction among recreational athletes who want to reach their fitness goals, have more stamina or simply eat more carbs.
Why is this diet suddenly so popular? "Carb cycling offers the perception of a 'middle ground,' where people don't need to limit their carb intake all the time but still instate some guideposts on their carbohydrate consumption," Ali Webster, associate director of nutrition communications for the International Food Information Council Foundation tells CNBC Make It.
From a scientific standpoint, carb cycling makes a lot of sense, Armul tells CNBC Make It. "It's good practice for everyone, especially athletes, to change what you're eating based on what you're doing with workouts," she says.
And carbs are good for the body and mind. They're the body's preferred energy source, because they provide quick fuel that's easily digestible, Armul says. "They're the most obvious choice [of energy] for your body, your muscles and your brain."
The brain has a harder time utilizing other types of fuel, such as fat or protein, according to Harvard Health Publishing.
And studies have shown that depriving yourself of carbohydrates acutely impacts your thinking and cognition. In a 2008 study from Tufts University, women who ate a low-carb diet for three weeks scored worse on a memory test than those who ate a low-calorie, but balanced diet.
"The brain needs glucose for energy and diets low in carbohydrates can be detrimental to learning, memory and thinking," Holly A. Taylor, study author, said in a press release.
As for the body, when carbs aren't available it will burn fat for energy, Armul explains. And over time that could lead to weight loss (that's why people flock to the keto diet), which might be a desired effect for some. But from an energy standpoint, fat isn't as effective as carbs.
Think of it this way in terms of performance: Using carbs is like driving a sports car on a racetrack, whereas fat is like driving a tank down the highway, Armul says. Some amount of healthy fat is important in your diet, but fat isn't meant for high-speed races.
"Fat isn't as efficient as a fuel source, so it's not burned as quickly," Armul says. If performance is your goal, "ideally you're giving yourself ample glucose and carbohydrate supply for fuel."
The exact amount of carbohydrates that someone needs will vary from person to person, Webster says.
For a regular diet, the National Academies of Sciences, Engineering and Medicine advises that adults get 45% to 65% of their calories from carbohydrates. Someone eating a 2,000-calorie diet might eat between 225 and 325 grams of carbohydrates per day.
Carb cycling is an internet-popular diet, not necessarily one that's used in a clinical setting, so it's tough to say exactly how the plan would be structured. But according to the American Council on Exercise, a carb cycling diet might have someone eating 50 grams to 150 grams of carbs on a low-carb day (to put that in perspective, two slices of bread contains about 15 grams of carbohydrates), and up to 400 grams on a high-carb day. Broadly speaking, someone might plan very low-carb intake on days when they're not training, and then increase their intake on the most vigorous exercise day, Armul says.
It's also not clear how sustainable this diet would be for everyone, Webster says. Eating too few carbs could leave you feeling zapped, and the plan doesn't take into consideration other important nutrients, such as protein or fiber. "Its longevity may also depend on the schedule and extent of high- versus low-carb consumption," she adds.
If you find that you're losing weight when you don't want to be, or you have no energy, that could be a sign that you may be consuming too few carbs, or not eating them at the right time, Armul says. "Working with a dietitian or doctor can help you plan when and what to eat to maximize your performance," she adds.
Given how much tracking and restricting is required with carb cycling, it could be exhausting or frustrating for people, Armul adds. "It takes all the enjoyment of food out of it and makes it very mathematical," she says. "Some people thrive on that, but for most people it can be kind of taxing."
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