How do we define good carbs and bad carbs? The glycemic index system of ranking carbohydrates remains the gold standard of categorizing carbohydrate sources. Lower-glycemic index carbohydrates—including beans, sweet potatoes, brown rice, and most vegetables and fruits—are typically recommended throughout the day to increase satiety, avoid large spikes in insulin, and avoid a crash in energy levels, while higher-glycemic carbohydrates—including white rice, baked potatoes, cereals, and candy—are typically recommended post-workout to maximize the replenishment of muscle glycogen. However, while the glycemic index may be a fine system for Joe Six-Pack who is looking to be healthier, it is not a precision tool for the physique-conscious athlete who is interested in optimal satiety, body composition, and weight loss.
The Glycemic Index
At the most basic level, carbohydrates are made up of sugar molecules known as monosaccharides. The three most predominant monosaccharides include glucose, galactose, and fructose. Once absorbed by the small intestine, glucose becomes readily available in the bloodstream, while fructose and galactose are transported to the liver to be converted to glucose. Glycemic index is a ranking system that addresses how rapidly glucose appears in the bloodstream and how sustained the rise in blood glucose levels is following consumption of individual carbohydrate sources. The different carbohydrate sources are then compared and ranked based upon a standard reference such as white bread. The prevailing theory remains that high-glycemic foods cause the greatest spikes in insulin, stimulate fat synthesis, and promote insulin resistance over time. However, the glycemic index is measured using carbohydrates in isolation in subjects that are in a fasted state. This is hardly practical, because when carbohydrates are consumed with protein, fat, and fiber, the glucose and insulin response appears to be blunted.
Impact On Satiety
The feeling of being full, otherwise known as satiety, can be a key determinant in the success of any weight-loss diet. A number of studies have investigated the effect of different carbohydrate sources on the feeling of fullness and ad libitum eating—how quickly you will reach for your next snack. Studies published in the European Journal of Clinical Nutrition compared several carbohydrate-rich foods and showed that the effect of each carbohydrate source on satiety differed greatly. However, the glycemic index was not a predictor of satiety. Rather, energy density, protein, and fiber were positively associated with feelings of fullness. These studies also demonstrated that the blood-glucose response following consumption of the carbohydrate sources was not predictive of satiety. Furthermore, the glycemic index of a particular carbohydrate source appears to be highly variable not only from person to person, but also at different times of the day in the same person. How does this information relate to carbohydrate selection? Neither the glycemic index nor the subsequent blood-glucose response are predictive of how full you will feel following a meal, and taking into consideration that the glycemic index seems to be a relatively unreliable measure, the usefulness of the glycemic index in choosing a superior carbohydrate source seems questionable.
Carbs And Weight Loss
How does the glycemic index of the carbohydrates we choose to eat relate to weight-loss success? A recent study published in the journal Obesity examined the effects of four different diets on changes in body composition and resting metabolic rate. Study participants were provided a diet consisting of either 55 percent or 70 percent carbohydrates. Each of these groups was divided once again: One subgroup ate predominately low glycemic–index carbohydrates, while the other subgroup ate predominately high glycemic–index carbohydrates. To control for confounding factors, both groups were provided the same daily amount of calories, protein, and fiber. After 17 weeks of adhering to the appointed diet, the study participants showed no differences in bodyweight or fat loss. In summary, when confounding factors such as total energy intake, protein, and fiber content were controlled for, the glycemic index of the carbohydrate sources did not yield differences in weight loss, preservation of muscle, or resting metabolic rate. Another study in the International Journal of Obesity investigated the effect of a high-protein diet in conjunction with either a low- or high-glycemic index diet. Similarly, the disparity in glycemic index did not account for any differences in weight loss or weight regain following the diet. In the context of a complete diet, the glycemic index appears to have limited value for predicting weight loss or weight maintenance. Ultimately, the total amount of carbohydrates you eat seems to be much more important than the type of carbs you consume.
The Post-Workout Window
Our muscles store carbohydrates in the form of glycogen, and training will drain this gas tank of energy. Following a bout of resistance exercise, muscle glycogen may be depleted by 20 to 50 percent. High-intensity or extended aerobic training can almost completely deplete muscle glycogen. As a result, the muscles prime for glycogen re-synthesis by becoming more insulin sensitive post-workout. Additionally, the muscle expresses more glucose uptake transporters known as GLUT4, and the muscle increases the activity of the enzyme promoting glycogen storage known as glycogen synthase. These post-exercise adaptations set the stage for what many regard to be a nutritional window of opportunity for carbohydrate consumption.
A common recommendation is to consume high-glycemic index carbs post-workout to maximize blood-glucose concentrations and the insulin response. Does this theory hold true? There is evidence of a super-compensation of glycogen stores when carbohydrates are consumed immediately after training compared to delaying carbohydrate consumption by two hours. However, the need to immediately guzzle those high-glycemic carbohydrates has been challenged. A study in Medicine & Science in Sports & Exercise showed that ingestion of five high-glycemic carbohydrate meals with or without a two-hour delay following exercise yielded the same muscle glycogen levels at eight hours and 24 hours following exercise. Therefore, even following complete glycogen depletion, pre-training glycogen levels can be replenished over a 24-hour recovery period—even if the “window of opportunity” is missed. This is not to say that you shouldn’t consume carbohydrates immediately following training! Rather, it may not be offering much of an advantage in the context of a complete diet over a 24-hour period of recovery. However, if you train multiple sessions in a day or you are restricting your carb intake at other times, immediate post-workout ingestion of carbohydrates may be beneficial. IM