What’s carb cycling and how does it work?

In recent years, low-carbohydrate diets, such as the Atkins and the ketogenic diet, have been all the rage. Carb cycling is another variation on this carb-rationing approach, but it may be more attractive because it permits more carbs—at least on some days. 

Carb cycling has no official, standardized plan. The basic idea is to cut back on carbs most days of the week except on the day (or days) meant for serious athletic training or exercise. On those days, you consume more carbs to maintain energy and lessen fatigue. 

“Lower-carbohydrate days, when done in succession for three days, encourages the body to use up its stored carbohydrates (glycogen) and switch over to burning body fat (ketones) for fuel. Burning stored body fat will lead to weight loss (from both body fat and water),” according to the American Council on Exercise (ACE).

“People who are insulin resistant, prediabetic, have type 2 diabetes, or are weight-loss resistant can benefit from carb cycling, as the decrease in carbohydrate intake and insulin release allows the body to burn through its stores of carbohydrates and switch to using fat (in the form of ketones) for fuel,” the ACE authors wrote. 

As a short-term strategy, carb cycling is popular among bodybuilders, fitness competitors, and other athletes looking to shed fat and bulk up muscle, as well as those who want to break through weight-loss plateaus. However,  limited evidence on this regimen’s efficacy actually exists. Here’s a closer look.

Mechanics

As a group, low-carbohydrate diets boast quick weight loss (although most of this is water weight in the beginning). Once dieters become accustomed to burning ketones for fuel instead of glucose, the cravings for carbs wane. They feel more energetic and satiated longer. A low-carb diet can also steady blood sugar levels and lower insulin resistance, as well as drop blood pressure.

Carb cycling takes advantage of these effects. Eating proper levels of carbohydrates at the right time resets metabolism and triggers the production of thyroid hormones and leptin, which help with weight maintenance. Eating too many carbs, however, promotes weight gain by stimulating excess insulin release.

Although carb-cycling plans differ, the dieter on a typical carb-cycle plan eats a lower amount of carbohydrates for 2 to 3 successive days to cause the body to first utilize its glycogen stores and then its fat stores. Burning fat stores as fuel results in weight loss. After a few days of low-carb eating, the dieter increases carbohydrate intake for a day or two, in conjunction with heavy training. 

On low-carb days, dieters eat between 50 g and 150 g of non-starchy vegetables, as well as some dairy. On higher-carbohydrate days, the dieter eats between 20 g and 400 g of starchy carbs, whole grains, fruit, non-starchy carbs, and dairy. Calorie intake is held steady throughout, but fat intake is lowered on high-carb days and increased on low-carb days to give the body sufficient fuel and energy. 

An alternative approach requires no specific schedule: On heavy workout days, eat more carbs to maximize energy levels and curb fatigue, and on less-active days, eat fewer carbs to maintain or lose weight.

Evidence

From a hypothetical perspective, carb cycling seems to make sense. But the evidence supporting the diet plan is limited.

In a study published in the British Journal of Nutrition, researchers compared daily-energy restriction with two intermittent energy and carbohydrate-restriction regimens, one of which allowed for ad libitum consumption of proteins and fat. The study involved 115 overweight women, aged between 20 and 69 years, who were randomized to one of the three groups for each regimen. 

During the 3-month weight-loss phase, women on the carb-restricted regimens decreased insulin resistance and lost more weight. During the 1-month weight-maintenance phase, one day of either carb-restricted dieting approach maintained reductions in insulin resistance and weight loss. 

The researchers concluded that carb-restricted approaches outperformed daily-energy reduction in the short-term, but stressed the need for further follow-up studies. 

In a BMJ study, 164 adults between 18 and 65 years with a BMI of 25 or more were randomized to one of three test diets for 20 weeks: a high-carbohydrate diet, a moderate-carb diet, and a low-carb diet. These participants had previously lost 12% of their weight on a diet in which the researchers controlled the amounts of carbohydrates, fats, and protein to achieve target weight loss. 

After the participants completed the test diets, the researchers found that those on the moderate- and low-carb diets burned off more calories vs the high-carb group (131 kcal/d more and 278 kcal/d more, respectively). Notably, participants with the highest insulin levels pre-diet burned off even more calories on the moderate- and low-carb diets. 

“Dietary composition seems to affect energy expenditure independently of body weight. A low glycemic load, high fat diet might facilitate weight loss maintenance beyond the conventional focus on restricting energy intake and encouraging physical activity,” the researchers noted.

“If metabolic benefits of reduced glycemic load diets are confirmed, development of appropriate behavioral and environmental interventions would be necessary for optimal translation to public health. This metabolic effect may improve the success of obesity treatment, especially among those with high insulin secretion,” they concluded.

Drawbacks

When starting carb cycling, limiting carbs can result in fatigue, carb cravings, bloating, constipation, sleep disturbance, irritability, and moodiness as the body is switching from glycogen to fat for fuel. This initial stage wanes with proper hydration and adequate electrolyte intake. 

Beware that carb cycling is a bad idea for certain populations, including those with adrenal fatigue or Hashimoto thyroiditis, as this strategy can plummet thyroid hormone production and metabolism, thus countering any benefit. Carb cycling should also be avoided by those who are underweight, pregnant, lactating, or with a history of eating disorders.

Bottom line

In the short-term, active, athletic people may benefit from decreasing carb intake on some days of the week, which could result in weight loss and improvements in blood sugar and blood pressure levels. More research, however, needs to be done before this nascent diet plan can be recommended.