Carb cycling: What is it, and does it work?

You might have heard about carb cycling, especially if you’re an athlete or body builder or if you’re looking to lose some weight. The strategy — eating more carbs on active training days and fewer carbs on rest days — makes intuitive sense. And advocates tout it as a way to help lose fat while keeping muscles fueled and energy levels up. Does it live up to the hype? Here’s what you need to know.

What is carb cycling?

First developed to help athletes train and prepare for competition, carb cycling is an eating pattern that alternates between days of higher and lower carbohydrate intake. A typical carb cycling schedule might include one or two high-carb days per week timed around intense workouts, with lower carbohydrate intake on the remaining days.

The problem is, “there’s no defined term for what a carb cycling diet might look like,” says Addie Temkin, a registered dietitian at Harvard-affiliated Massachusetts General Hospital. “Some plans might recommend getting 30% of your daily calorie intake from carbohydrates on a low-carb day, while others might recommend only 10% or 20%.” That means protocols can vary widely between studies, and high-quality trials are scarce — key reasons why Temkin and other experts consider the claimed carb cycling benefits promising, but mostly unproven.

How does carb cycling work?

The theory behind carb cycling centers on how the body uses and stores carbohydrates — “the energy currency for the body,” Temkin explains. The body stores carbohydrate in muscle and liver tissue in the form of glycogen, and draws on this energy supply to fuel exercise. “On high-performance days you perform best when you have sufficient glycogen stores,” she says. “By getting an adequate amount of carbs, you’ve topped up your energy stores prior to exercise, and you’re better equipped for recovery afterward.” On lower-carb days, the body will rely more on stored fat for fuel instead.

Most research into carb cycling focuses on competitive athletes. Some preliminary findings suggest that combining carb cycling with a structured exercise routine — especially one that includes high-intensity training — may support fat loss and body composition changes better than diet alone. However, it’s not clear if those benefits actually translate to better performance.

As for carb cycling for weight loss, the evidence is even thinner, Temkin notes. Research consistently shows that low-carb diets aren’t more effective for weight loss than higher-carb diets as long as total calorie intake is equal, and the same appears to hold for cycling between the two. “Carb cycling is similar to any weight-loss diet in that it can work in the short term,” she says. “But to really work, it needs to be sustainable.”

Is carb cycling right for you?

If you’re an everyday exerciser whose “active” and “less active” days don’t vary dramatically — say, jogging for 30 minutes one day and brisk walking for an hour on another — carb cycling probably doesn’t offer any benefits over a healthy, balanced diet, says Temkin. But if you’re exercising at a higher intensity — for example, training for a race or tennis tournament — and you want to try carb cycling, here are some useful guidelines.

Don’t make a low-carb day too low. Most experts agree a healthy diet should provide around 50% to 60% of calories from carbohydrates, so Temkin recommends not going much lower than that on a low-carb day. Going too low can shortchange you on fiber — which most Americans don’t get enough of. What’s more, even on the less active days, “your body is still working to repair your muscles, and having enough carbohydrate on board is important to do that.”

Prioritize protein and fiber. Getting enough protein helps ensure your body doesn’t break down muscle tissue for energy, and it supplies the amino acids needed to rebuild and strengthen muscles. You’ll also want to make sure the carbs you do eat are fiber-rich, like whole grains, vegetables, and fruits.

Listen to your body. Watch for fatigue, brain fog, and difficulty working out, says Temkin. “If you’re feeling more tired than usual, you don’t have enough energy in the gym, or you can’t think straight at work, those are general signs that you might need to be eating more carbohydrates.”

Be realistic about the time commitment. “Carb cycling requires a lot of thinking and planning,” says Temkin. “Make sure it can fit into your life.”

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