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Cycling Lessons

Undulating your daily carb intake can be make dieting easier and more effective

By Adam M. Gonzalez, PhD


Carbohydrate intake is subject to a lot of misconceptions and myths — one of them being the timing in which they should be consumed. Carbohydrates serve as the primary energy source for athletes training at high intensities, and levels of muscle glycogen are a major determinant of exercise capacity. Therefore, it is necessary for athletes to adopt strategies for replenishing the gas tank of carbohydrate while limiting their conversion to body fat. Carb cycling — a planned manipulation of carbohydrate intake — has become increasingly popular among physique athletes during periods of carbohydrate restriction, but is there really any advantage compared to traditional dieting? Is it possible that the timing of carbohydrates is more important than the amount of carbohydrates consumed?


Carb Back-Loading

One popular strategy, called carb back-loading, advocates eating little to no carbs in the morning hours and beginning carb intake in the evening starting with your post-workout meal. The rationale for carb back-loading is based upon the action of insulin, an anabolic hormone that facilitates the movement of glucose into cells following a meal. Insulin sensitivity is heightened following a hard training session via mechanisms that include an increase in glucose uptake transporters known as GLUT4, and an increase in the activity of glycogen synthase, the enzyme promoting glycogen storage. While it is great to appreciate the intricacies of metabolism, it’s unclear if this makes a meaningful difference at the end of the day. We need to analyze a diet over a 24-hour period, a week, and a month, rather than isolate physiological characteristics at certain time points. The case for carb back-loading comes from a study published in the Journal of Obesity, in which overweight people adhered to a calorically restricted diet for six months. These individuals were split into two groups that consumed the same amount of calories, protein, carbohydrates, and fat. The only difference was that one group ate carbs throughout the day, whereas the other group consumed the majority of their carbs (approximately 80 percent of total intake) at night. The group consuming their carbs at night lost significantly more weight and body fat than those who were free to eat carbohydrate throughout the day. Additionally, restricting carbohydrates to nighttime led to better satiety and less feelings of hunger. Overall, the study showed that limiting carb dosing to a single bolus rather than spreading them throughout the day may help limit insulin secretions and improve feelings of fullness. However, the results of this study have yet to be replicated in trained individuals.


All in all, the research on carbohydrate timing is not overly convincing to make general recommendations that we should be eating a majority of our carbs at nighttime, and carb back-loading does not appear to be the often-proclaimed magical remedy. On the other hand, constraining carbohydrate intake to a certain time frame can help limit overall carbohydrate intake during periods of restriction and provide a structured system for dietary habits. Restricting carbohydrate intake for fat loss is challenging; therefore, compartmentalizing carbohydrate intake to certain times of the day may make the psychological challenges of dieting feel easier.

Bottom line, carb back-loading does provide a viable option for limiting carbohydrates during period of restriction, but it’s not a magic solution for muscle growth and fat loss. Muscles do not have some magical ability to absorb an unlimited amount of carbs after a workout. Ultimately, the quality and quantity of carbohydrate intake appears to be more important than the timing of carbohydrate intake, so don’t use this approach as a reason to regularly stuff your face with cookies and ice cream just because you saved your carbs for nighttime. Whether you decide to eat your carbs at night or throughout the day, stick to your healthier carb choices such as grains, vegetables, and fruits, which include additional benefits such as fiber, vitamins, and minerals. Also, keep in mind that in order for weight loss to occur, a calorie deficit is needed. Maintaining a calorie deficit and assuring adequate protein in the diet is a more appropriate approach to fat loss and muscle gain, rather than indiscriminate eating within a selected time frame.


Train Low, Compete High

Another popular carb-cycling strategy among physique athletes is short-term carbohydrate restriction followed by periodic high-carb days. It is very difficult to pinpoint an optimal amount of carbohydrates to maximize both performance and body composition goals during periods of intense training because it depends on several factors, including exercise duration and intensity. However, recent evidence has demonstrated a potent effect of training with low muscle glycogen stores. While low carbohydrate availability will undoubtedly impair training intensity, recent studies have shed some light on the physiological benefits of training during short-term carbohydrate-restricted diets. For example, a study published in Medicine & Science in Sports & Exercise showed that fat oxidation was increased after training with low muscle glycogen, which may have been a result of enhanced metabolic adaptations in the exercised muscle. Another study published in the International Journal of Sport Nutrition & Exercise Metabolism demonstrated that training with low muscle glycogen stores also improved exercise capacity. Thus, deliberately employing training periods with low carbohydrate availability may help train the body to use more fat as fuel and even improve performance when carbohydrates are reintroduced during high-carb days. Therefore, the strategy of “train low, compete high” as it pertains to carbohydrate intake may help maximize performance and body composition goals of training.


The Takeaway

For decades, ensuring enough carbohydrate availability has remained a foundation of sports nutrition. While it’s true that our bodies can handle carbohydrates better at certain times of the day, such as before and after training, that does not mean we have an off switch the rest of the day. While carb back-loading does not appear to be a revolutionary approach to carbohydrate intake, it does provide a viable option for limiting carb intake and promoting long-term adherence to carbohydrate-restricted diet plans.


There is a significant need for more nutritional research on the effects of constrained eating windows and alternate day fluctuations for carbohydrate intake, which makes it difficult to make general recommendations for different types of athletes. However, following a structured system of carbohydrate intake can definitely serve some benefit by preventing unwanted snacking throughout the day. Whether this approach is right for you depends on a variety of factors and can be determined by trial and error. Try at least two to three low-carb days for every high-carb day. During the low-carb days, saving your carbohydrates for pre- and/or post-exercise may take advantage of the boost in insulin-sensitivity while keeping you on track with your daily allowance. No matter what approach to carbohydrates you take, consistency will be key.


Ultimately, the best practical advice may be to follow the concept of fueling for the demands of your daily workout. Implementing day-to-day carbohydrate periodization will help to adequately provide fuel for training while preventing overconsumption and conversion to body fat. If the goal of a training session is to compete at a high level and maximize performance, then adequate carbohydrates should be consumed for the 24-hour period leading into competition. However, it may be advantageous to purposely plan training periods on lower-carbohydrate diets to maximize the effect on body composition. IM


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