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If you were around in the 1990s, you might remember that the era’s most slammed macronutrient was fat. Fat has since made a well-deserved comeback. Fast forward a generation and carbs are the dietary villain, with many wellness-minded folks now slashing bread, fruit and other carbohydrate-rich foods in pursuit of weight loss.
Coming on the heels of carb-cutting weight-loss fads like the keto diet, carb cycling has emerged as a buzzy modified approach. Like keto, it calls for restricting carbs—but only during specific periods.
Carb cycling got its start as a nutrition plan for endurance athletes and bodybuilders hoping to optimise their energy during training, explains registered dietitian, Valerie Agyeman. It’s since gained broader traction, fuelled in part by claims that carb cycling can fire up metabolism and improve insulin levels. “Lately, non-athletes have taken up carb cycling to lose weight,” Agyeman says.
How does carb cycling work? Read on as Agyeman and other nutrition experts separate myth from fact.
What is carb cycling?
Carb cycling involves planned increases and decreases in carbohydrate intake. Sometimes the up-and-down pattern plays out over a week, sometimes it happens over a full month or a single day. Popular among bodybuilders and pro athletes, carb cycling is based on individual dietary needs, largely informed by physical activity.
“The belief is that on days in which you’re doing high-intensity exercise, like CrossFit or HIIT (High Intensity Interval Training), carbohydrate intake should be increased to fuel exercise and build muscle, whereas on resting or light-activity days, carbohydrate intake should be decreased to support the body’s use of fat for fuel instead,” says registered dietitian, Sheri Vettel. This theory echoes that behind the keto diet, which cuts way down on carbs with a goal of pushing the body into a fat-burning metabolic state known as ketosis.
But while the keto diet involves sticking to fixed percentages of macronutrients day after day, carb cycling is a more varied approach, with planned fluctuations in fat and carb intake.
Fans of the method say carb cycling can enhance physical performance, “improve” body composition and help maintain desired weight. To date, however, no research studies have determined whether carb cycling is effective for achieving any such goals or maintaining results, Agyeman says.
What's the science behind carb cycling?
The research community has little data on the efficacy of carb cycling, so much of the thinking behind this popular diet is anecdotal or theoretical, in some cases extrapolated from established information on how macronutrients are utilised in the body, notes Shoshana Pritzker, a sports nutritionist and registered dietitian.
“Carbohydrates provide energy,” Pritzker says. “Because of that, it makes sense to consume them when we need them the most, which is before, during and after intense training or long workout sessions.”
The flip side of the theory is that restricting carbs when your muscles least need them—say, when you’re planning on a rest day or a lower-intensity workout—will encourage the body to use stored fat as its fuel source instead.
Registered dietitian, Kelly Jones, stresses that the claims are unproven. “The reality is that we still have limited research on the long-term effects of low-carbohydrate diets,” she says. “And valid research studies on carb cycling are [even] more sparse.”
The truth about the health claims of carb cycling
Here’s a look at where the science stands on the practice of strategically switching up carb intake.
Can carb cycling boost energy during exercise?
It’s well-established that carb consumption in general supports exercise performance. “Adequate carbohydrates are needed to fuel intense activity and to replenish depleted glycogen stores,” Vettel says. “Athletes often time their carb intake closely according to their individual needs and predicted activity levels.” More research is needed, however, to figure out whether varying carb intake offers any advantage, she says. For now, the jury is out.
Can carb cycling help with insulin resistance?
Vettel says some research points to a correlation between carb-restricted diets and reduced circulating levels of insulin, which could bode well for health concerns such as insulin resistance. One small study of men and women following a low-carb diet, published in the American Journal of Clinical Nutrition, observed insulin reductions within six weeks. But again, this research didn’t look at carb cycling specifically. It also didn’t establish whether changes in insulin were brought on by carb restriction, the weight loss volunteers experienced, or some other factor, and only included 32 participants in the study.
Can carb cycling help me lose weight?
If intermittent carb restriction creates an overall calorie deficit, it’s possible to lose weight on a carb cycling diet—the same way a person would on any diet in which they consume fewer calories than the body uses, Agyeman says. Given there’s currently no scientific data linking carb cycling to faster-than-average weight loss, this approach is no more superior compared to other weight-loss methods.
Can carb cycling speed up metabolism?
To date, there’s been no research to support the claim that carb cycling stokes metabolism. The theory is that strategically adding high-carb days throughout the week will “shock” the metabolism to prevent it from getting sluggish, explains Pritzker. “Problem is, it’s just not true,” she says. “High-carb ‘refeed’ days do not make you burn more body fat.”
Potential health risks and disadvantages of carb cycling
Just as the potential health benefits of carb cycling haven’t been thoroughly investigated, neither have the potential risks. Here’s what researchers might be exploring in the years to come, WW’s experts say.
- Nutritional impact. Is it possible to eat a balanced diet while following a carb cycling regimen? The answer is not yet clear. Curbing carb intake could mean falling short on key vitamins and minerals over time, Agyeman says, which could raise the risk of certain health problems.
- Diabetes concerns. If good-quality studies find out the impacts on insulin described above, people who are living with type 1 or type 2 diabetes may be advised by their doctors to avoid carb cycling, Argyeman says. For now, consider steering clear just to be safe, she says.
- Unhealthy patterns of eating. “Any restrictive diet promising quick-fix weight loss puts those hoping to lose weight at risk of disordered eating,” Jones says. Vettel echoes that concern, adding that a rigid plan could promote orthorexic tendencies—a nonclinical term commonly used to describe an unhealthy preoccupation with foods perceived as healthy.
- Effect on metabolic processes. Jones says in cases where it creates a calorie deficit, carb cycling could theoretically lead to a state known as low energy availability—a shortfall of dietary energy needed for the body’s many metabolic jobs. “Even when there are days with more carbs and calories in someone’s diet, restrictive low-carb days have the ability to keep total energy intake low enough that hormones are affected,” she says. By way of example, Jones points to this small 2017 study, which found that after just five days on a calorie-restricted diet, women experienced changes in hormones that affect bone growth.
- Long-term adherence. It’s possible that some people might enjoy the experience of following a carb cycling plan. But the more likely outcome of tightly controlling carbs is a feeling of deprivation, which could undermine long-term success. Says Agyeman, “Most people don’t stick to restrictive diets for long.”
Carb cycling and weight loss
When it comes to losing weight, carb cycling and other low-carb diets are thought to work by limiting the body’s preferred energy source: carbohydrates. The idea is that this will encourage the body to burn stored fat as fuel instead, says Pritzker. But so far there’s no established link between carb cycling and higher fat burn or altered metabolism. Research suggests that low-carb diets in general aren’t any better for weight loss than higher-carb diets, Pritzker says.
For example, a small 2010 study published in the Journal of the International Society of Sports Nutrition tracked 36 previously sedentary women who were assigned to eat either a high-protein or high-carb diet—alternating weeks of higher calorie intake with weeks of lower calorie intake—while following an exercise program. After five months, volunteers in both groups lost weight and both approaches were deemed “equally effective.”
“The problem is that there is no long-term data on weight loss maintenance or information available on stress biomarkers and quality of life due to carbohydrate restriction for months at a time,” Jones says. Given the lack of research, it’s impossible to say whether carb cycling can deliver meaningful weight loss results.
How does carb cycling work?
Given the lack of good quality research on carb cycling, no standardised method for this diet exists. Many fans themselves say the optimal strategy depends on individual needs and may require time and fine tuning. Programs offered online tend to be based on proprietary opinion or anecdotal reports, Jones says, noting, “We don’t have evidence on the long-term consequences of following a plan developed by someone’s opinion of how carb cycling should look.”
Many carb cyclers opt for a set number of high, medium and low-carbohydrate days during the week or month. Pritzker says that people on carb cycling plans generally sync higher-carb days with exercise sessions. “Those days are the ones you need more energy to fuel your workouts,” she explains. “On off days or weight training days, you may not need as much immediate energy reserves, so you could limit carbs.” And because you might have a day or two a week during which you do a light workout, you might opt to have a moderate carb day, which basically functions as a “maintenance” day versus a time to boost glycogen (high-carb day) or urge the body to use fat as fuel (low-carb day).
Generally, protein intake remains consistent, while fat intake rises and falls inversely with carb intake. “On the low carbohydrate days, dietary fat intake may be increased to keep the calorie level nearly the same,” Vettel says. “In some instances, a calorie deficit may also be created, which might also help explain any weight loss achieved.” Bumping up your fat intake on low-carb days is also meant to maximise satiety, to keep you feeling fuller for longer.
Vettel offers the following big-picture breakdown of what a week of carb cycling might look like. Note that the number of calories would be dependent on a person’s lifestyle and dietary needs and carb amounts shown here are just examples, she says.
|Day||Exercise||Carb Intake||Fat Intake||Total Carbs|
Who is carb cycling for?
Carb cycling started out as a plan for bodybuilders and athletes who need to maintain their weight or physique as well as energy for intense workouts. If anyone can benefit from this protocol, it’s likely someone in that group, says Pritzker.
Ultimately, determining whether carb cycling is suitable for you is a highly individualised process. “Our bodies all work differently,” says Agyeman. Consult with your doctor or a registered dietitian before diving in, she continues: “If you’re curious about trying carb cycling, listen to your body and see how your body reacts to it. Maybe do it for a short period of time.”
As mentioned above, carb cycling is not recommended for a number of people, including those who have type 1 or type 2 diabetes, as the method may affect the body’s insulin and blood sugar levels and cause problems for people with those conditions. Always consult with your doctor before commencing any type of restrictive eating plan.
What are the best carb sources for carb cycling?
While there’s no such thing as a “good” or “bad” carbohydrate, it’s always a smart move to reach for minimally processed sources rich in fibre and other nutrients your body needs, Jones says. “Fibre helps to slow digestion—helping you feel fuller for longer—and helps maintain normal bowel function, bind to and excrete cholesterol and slowly release carbohydrates into the bloodstream for steady energy levels and better insulin responses,” she says.
Food sources of carbohydrates that check those boxes, according to Jones and Vettel include; vegetables, fruit, legumes, whole grains and potatoes.
The upshot: Does carb cycling work?
Given the lack of published research on carb cycling—and a lack of consistency in popular carb cycling methods—it’s hard to predict whether this trendy diet approach would be helpful to you in your wellness journey. While some theories behind carb cycling make sense based on related evidence, its important to note that they haven’t been well substantiated by science at this point. Claims that carb cycling can optimise metabolism and accelerate weight loss remain unproven.
It's important to chat with your doctor or a registered dietitian before starting any restrictive eating plan, including carb cycling to ensure the method is safe for you. If you decide to move ahead with it, try it for a short while and pay close attention to how you feel. Energised? Hungry? Lethargic? Then ask yourself if the method is working. For example, days of super restricted carb intake might not make sense for you if they leave you too low on energy to get off the couch.