Processed carbohydrate sources often have a higher glycemic index value compared to whole food counterparts because the processing of food can break it down, making it more quickly and easily digested. On the other hand, glycemic index values are lower in the presence of fat, protein, or fiber, which can slow digestion. For this reason, hummus, the processed version of chickpeas with added fats, has a lower glycemic index than chickpeas themselves. It is also important to note that eating other food sources along with a particular carbohydrate source can effectively alter its glycemic index by slowing digestion. Glycemic index values affect nutritional recommendations for body composition and performance in several ways, as detailed in the following.
Pre-Training Meal
Pre-training or pre-competition meals should ideally digest quickly enough to prevent gastrointestinal distress and be able to restock muscle glycogen in time to support the activity. In addition, a good pre-training or pre-competition meal should provide a stable stream of blood glucose throughout the activity. Types of carb sources that are ideal for pre-training depend on the timing of the meal. The further from training onset the meal is eaten, the lower its glycemic index should be. A sugary sport drink can be a great pre-training meal if it is consumed 30 minutes prior to training or competing–sports drinks have a very high glycemic index, no fat or fiber, and their liquid consistency allows them to be digested quickly. If the meal is eaten much earlier, a slower-digesting whole food meal like a turkey sandwich on whole-grain bread is a better option. The pre-training meal should always contain enough carbs for basic function in addition to carbs for training. A pre-training meal eaten further in advance of training requires more total carbs than a meal eaten immediately before training to support basic function in the hours before training along with energy for the training itself.
Intra-Training Meal
Intra-training meals should provide immediately available energy during the workout itself. Lower glycemic index carbohydrate sources increase chances of gastrointestinal discomfort and cause more blood to be allocated to the digestive tract and away from working muscles, so should be avoided entirely. Quickly digested liquids are easier to consume during training than solids, so the best recommendation for intra-workout is a high glycemic index liquid such as a sugary sports drink.
Post-Training Meal
In hours immediately following training, the body is particularly efficient in its ability to resynthesize glycogen from ingested carbohydrates. This is notable not only for single training sessions, but particularly when dealing with multiple training sessions per day, as glycogen resynthesis can be a rate-limiting step in subsequent exercise performance. For this reason, it is beneficial to bias post-training carbs toward higher glycemic index sources with minimal fiber and fat. Kids’ cereal in whey protein or fat-free milk is a great option here. Thanks to the training stimulus after a workout, your body can utilize more carbs in a shorter time span to refill glycogen stores. In this instance there is little concern about fat storage–as there might be if you were slamming Lucky Charms after spending a day sedentary on the couch. As the post-workout window stretches out, the sensitivity of the muscles to carbs declines, and previous meals have already done much of the glycogen filling. The rate of glycogen filling is no longer an immediate concern. Thus, the later it is after training, the food sources should have a lower glycemic index and the more added fiber and fats they can contain.
Meal Interval Considerations
Lower glycemic index foods and the additions of fiber, protein, and fat sources can slow digestion and better provide energy across longer intervals between meals. If you have a five-hour window between meals and need to concentrate at work, a meal of chicken, veggies, whole-grain bread, and nut butter would provide the sustained energy needed before your next meal, whereas gummy worms and whey protein shakes would likely leave you hypoglycemic after just a couple hours. Adjusting food composition for slower digestion is like creating a sustained release system for energy and should be considered when designing meals with different inter-meal intervals.
Glycemic Index Values and Real-World Eating
Popular opinion would have you believe that highly glycemic foods cause excessive insulin secretion thereby making you fatter. While there is a seed of truth to this claim, it is not consistently true, and unwanted fat gain is not always the result. Some foods with low glycemic index values cause high amounts of insulin to be secreted. A perfect example is skim milk and lean yogurt products; despite inducing significant insulin secretion, heavy consumption of these products is inversely associated with obesity. This means that people who eat lean dairy products are less likely to be obese. Carrots are another example; they have a relatively high glycemic index value, but their consumption is not generally associated with obesity. While high glycemic foods can make people want to eat more, cravings are actually more dependent on how satiating a food is, irrespective of glycemic index values. White potatoes are very glycemic but are extremely filling and so do not tend to increase cravings. Glycemic indices are not of themselves a measure of how healthy a given food item is or is not. Carb sources are generally consumed within a meal containing other foods; estimating glycemic index values for mixed meals is very difficult, and the value for any given mixed meal will be lower than any of its ingredients alone. Author Mike Israetel’s middle school physical education teacher, with the best of intentions, instructed students not to eat candy bars before sports games because the sugar would “burn up quickly and have them crashing” midway through the event. In reality, Snickers bars or any other chocolate-based candy bar have glycemic indices of around 50, similar to whole grains. The fructose in high fructose corn syrup is extremely slow digesting, and the high fat content of such candy decreases its glycemic index even more. When protein, fat, and fiber are involved in real meals people actually eat, the glycemic index value becomes just a very small factor in a bigger equation in which macro amounts, calories, and actual digestion times play a more dominant role. That being said, high glycemic carbs, eaten alone without other foods to slow their digestion, serve a valuable purpose in intra- and post-training meals, improving health and fitness outcomes in that context.
Carbohydrate Source Micronutrient Density
Along with factors affecting digestion time, the density of micronutrients within a carbohydrate source also define its compositional quality. Carbohydrate sources vary greatly in their micronutrient and phytochemical content. For example, dextrose powder can provide all needed carbs, but without any vitamins, minerals, phytochemicals, or fiber. Extreme diets that cut carbs very low for long periods (e.g., ketogenic diets) might elevate the risk of nutrient deficiencies. Getting carbs from a wide range of veggies, fruits, whole grains along with less nutrient-dense sources as needed allows you to get all the carbs you need and consume digestion speed-appropriate carbs without missing out on any nutrients.
FAT COMPOSITION
The quality of a fat source is determined based on the class of fat. Each of the four main classes of dietary fat has its own effects on body composition, performance, and general long-term health.
Monounsaturated