This video will examine the nutritional considerations and needs for various types of activities and competitions. Not surprisingly, the nutritional needs for optimal training and performance will depend upon the sport or activity in which you are participating. As you would imagine, the nutritional requirements for a football lineman will differ greatly from that of a marathon runner or a triathlete. There are many facets to the nutritional requirements for exercise and performance. I will focus on three main factors. How much to eat, what to eat, and when to eat. First, let's begin with how much to eat. Endurance athletes do not wish to carry any excess body weight. Therefore, they generally wish to remain weight stable. This brings us to the energy balance equation shown here. If total calories consumed through the diet equals the total calories expended throughout the day, then the individual will be in energy balance, and thus, weight stable. As shown here, the daily energy expenditure can vary greatly depending on the type of activity and training involved. Competitive athletes generally have a good idea what their caloric intake should be to remain weight stable. When increasing training intensity and/or volume, care should be taken to avoid entering into negative energy balance, which may interfere with one's ability to compete and contribute to chronic fatigue. What to eat for endurance athletes revolves around maintaining or increasing carbohydrate stores in muscle and liver. Given the limited amount of carbohydrates stored in the body, coupled with the impact that its depletion has on endurance performance both during training and in competition, it is recommended that 55% to 60% of calories consumed should come from carbohydrate. Shown here is a study that highlights this point. When athletes perform strenuous exercise on three consecutive days, leg muscles glycogen levels decrease continually over time when consuming a low carbohydrate diet of 40%. When the same athletes perform these tests while on a 70% high carbohydrate diet, muscle glycogen levels recovered more completely between exercise trials. Notice that even when on the high carbohydrate diet, by the end of day three, muscle glycogen levels were still somewhat lower than those measured prior to the first exercise session. This emphasizes the need to maintain a high carbohydrate diet when involved in intense training. This leads me to when to eat carbohydrates. It can take up to 24 hours to completely replenish muscle glycogen stores after a strenuous bout of exercise. To optimize the replenishment of your carbohydrate stores after a competition or training session, the ideal time to consume carbohydrates is during the first one to two hours immediately post-exercise. Shown here is the dramatic three-fold increase in glycogen synthesis when carbohydrates were given immediately after exercise as opposed to two hours post-exercise. The primary explanation for this response relates to the increase in insulin sensitivity observed up to two hours post-exercise. As you remember from our video on the endocrine system, insulin promotes glucose uptake by muscle and liver. An increase in insulin sensitivity, coupled with elevated blood glucose levels following the ingestion of carbohydrates immediately after exercise, will significantly enhance one's ability to replenish carbohydrate stores during this one-to-two-hour window. Shown here is one study demonstrating that all ten subjects showed an increase in insulin sensitivity following a single bout of exercise. I will revisit this topic when discussing the role of exercise in the prevention and treatment of diabetes in module four. Regarding the precompetition meal, there are several important guidelines to consider. The meal should be consumed 2-3 hours prior to competition to allow for complete digestion. This will reduce the need of the digestive system for blood flow, thus allowing for greater flow to the working muscles. Also, this will allow time for insulin levels to return to normal after the meal. An elevation in insulin levels immediately before the onset of exercise would promote glucose uptake in all insulin-dependent tissues, including adipose and non-active muscles, which will limit blood glucose availability for the working muscles. The precompetition meal should be light, between 300 and 500 calories, and be high in carbohydrates and low in fats and fiber that take longer to digest. Additionally, I remind you of other nutritional considerations previously mentioned in this course. First, having more glycogen stored in muscle prior to a competition will slow its depletion, delaying the onset of fatigue, resulting in an improvement in performance or time to exhaustion. By adjusting one's diet in advance, the amount of glycogen stored in muscle can be manipulated. This is known as carbohydrate loading. The classic regimen, the carbohydrate load, involves using a bout of strenuous, exhaustive exercise approximately seven days before competition to deplete glycogen in the muscles recruited. This is followed by three-day diet, high in fat and protein, and low in carbohydrate, to keep the muscle glycogen stores low. After being starved from carbohydrates for this prolonged period, the athlete then switches to a very high carbohydrate diet for the remaining three days. This results in an over or super compensation in muscle glycogen stores just before competition. While effective in increasing glycogen stores, the classic regimen is not always well tolerated by athletes as it can affect mood, training, and possible susceptibility to illness. Thus, a modified regiment was proposed whereby the athlete reduces their training intensity, or tapers during the week prior to competition, while increasing their carbohydrate intake up to 70% of total calories days before the event. This modified regimen appears to be effective in carbohydrate loading while being less traumatic on the athlete. Finally, as discussed in the carbohydrate video, carbohydrate feedings during exercise can help to maintain blood glucose levels, thereby delaying the onset of fatigue. As indicated here, when elite cyclists exercised at 70% to 75% of their max VO2 to exhaustion without carbohydrate feeding, they fatigued on average at the three-hour time point. When a dilute carbohydrate drink, an approximate 4% to 8% solution, is given every 30 minutes after the onset of exercise, notice that these same subjects could exercise an additional hour before fatiguing. Thus, the ingestion allowed for an additional source of carbohydrate, thereby sparing liver glycogen. Endurance athletes need to take proper precautions to avoid dehydration during prolonged bouts of exercise. The increase in sweating, coupled with fluid shifts in the body, can significantly reduce one's plasma volume. This will result in a reduced venous return to the heart causing a decrease in stroke volume and cardiac output. Heart rate will increase in an attempt to maintain cardiac output, but this will contribute to early fatigue. Shown here is the relationship between the degree of dehydration and the effect it has on impairing performance. Thus it is critical, particularly when exercising in hot environments, for the individual to keep well hydrated. Nutritional considerations for individuals wishing to increase muscle mass and strength are very different than those for the endurance athlete. In order to increase muscle mass, the rate of muscle protein synthesis must be greater than the rate of muscle protein breakdown. This can be accomplished by combining heavy resistance training with a high protein intake. The protein intake should be approximately double that of the recommended daily allowance, approaching 1.6 grams of protein per kilogram of body weight per day. This can be achieved through a sound diet consisting of meats and dairy protein. Frequently advertised protein supplements are not necessary, and ultimately lead to excessive urine as excess nitrogen is excreted. As stated in the protein metabolism video, the rates of protein synthesis are elevated immediately post-exercise. This is likely due to high levels of growth hormone as well as an increase in insulin sensitivity discussed above. Thus, ingestion of protein within the first hour following training would elicit an optimal response in protein synthesis. Whey and casein protein, which are digested more rapidly, are ideal for this effect. Shown here is just one of many studies demonstrating this response. Following an intense bout of resistance training, the rate of muscle protein synthesis is the greatest during the first few hours into the recovery period. Further, the rate of protein synthesis is increased by an additional 30% when fed protein during this period. Finally, some power and Olympic weightlifters, as well as NFL linemen, consume massive amounts of food to ensure they're getting adequate protein as well as to increase overall body mass. This puts them in a positive energy balance resulting in weight gain that is in the form of both muscle and fat. This practice is not recommended and can lead to health issues later in life such as heart disease and diabetes. In summary, attention to carbohydrate metabolism and stores before, during and after exercise is essential for endurance athletes. It is important for endurance athletes to stay hydrated during training sessions and in competition. Individuals wishing to increase muscle mass should combine heavy resistance training with a diet high in protein.
