During prolonged endurance exercise, if one's carbohydrate stores become significantly depressed, protein becomes an active substrate, accounting for about 12-15% of oxidized fuel How much protein used differs from workout to workout and depends on

1) The length of exercise

2) The intensity of exercise

3) The frequency of training.

One could view all of these factors as what some scientists refer to as training volume.

Intense aerobic and anaerobic training have been shown to use protein as a fuel, though not to the same extent as carbohydrates or fat. Without proper nutrient intervention, this negative balance leads to the potential for decreased muscle mass and an increased injury risk.

Currently, the literature suggests that intensive and/or high­volume aerobic and weight-training exercise increases the need for specific protein/amino acids. Thus, one must attempt to ameliorate the hypoglycemia that is initially caused by low muscle glycogen concentrations but is also exacerbated by chronic overwork, overtraining, and poor nutrition,

In active individuals, protein intake above the RDA is necessary and can range from 1.2 to 2.2 g/kg of body weight, This extra protein is needed for the repair of damaged muscle fibers, the provision of additional amino acids for muscle protein accretion (vis-a-vis heavy resistance training), and as an additional fuel source during prolonged endurance exercise. Additionally, post-exercise recovery includes the following variables: normalization of blood glucose levels and restoration of skeletal muscle and liver glycogen stores,

The main substrate for muscle glycogen resynthesis is blood glucose, derived from liver glycogen breakdown as well as from exogenous carbohydrate ingestion before, during, and especially after exercise. Various studies have looked at the timing, amount, and the type of carbohydrate needed to increase muscle glycogen resynthesis following exercise. One study looked at the time of ingestion of a carbohydrate supplement on muscle glycogen resynthesis after exercise, Twelve male cyclists exercised at 68% for 70 minutes with six 2-minute intervals at 88% on two occasions. At the conclusion of the exercise test, a carbohydrate solution was ingested immediately post-exercise or 2 hours post­exercise. They found that delaying carbohydrate intake post-exercise for 2 hours reduces the rate of muscle glycogen resynthesis. Thus, the immediate consumption of carbohydrates post-exercise is critical for muscle glycogen repletion.

Another study looked at the rate of muscle glycogen resynthesis during the initial hours of recovery following prolonged lower body exercise. It was noted that muscle glycogen resynthesis occurs at a rate approximating 1-2 mmol/kg wet wt-1 if no carbohydrate is ingested. However, when carbohydrate is ingested immediately post-exercise, muscle glycogen resynthesis increases to 7-10 mmol/kg wet/wt The time required for complete muscle glycogen resynthesis after prolonged exercise is generally considered to be 24 hours provided carbohydrate intake is adequate (500-700 g); however, the first 2 hours after exercise is when muscle glycogen resynthesis is the highest, Thus, carbohydrate ingested immediately after exercise restores muscle glycogen nearly three times faster than waiting for 2 hours.

A multitude of factors affects post -exercise muscle glycogen restoration. In addition to the type of exercise performed, the type of carbohydrate consumed is critical regarding glycogen repletion. Carbohydrates that elicit a high blood-glucose response upon ingestion are considered to have a high glycemic index, whereas those carbohydrate foods evoking a small blood glucose response are considered to have a low glycemic index,Post-exercise carbohydrate intake should then consist of high glycemic foods versus low glycemic foods, thus eliciting a high blood-glucose response to increase muscle glycogen resynthesis. Thus, the ingestion of glucose or sucrose, which are simple carbohydrates with high glycemic indexes, is best used for muscle glycogen resynthesis, in which liver glycogen resynthesis is better assisted with the ingestion of a complex carbohydrate or fructose (low glycemic index).

It is well known that carbohydrate consumption post­exercise increases plasma insulin and growth hormone levels. Increasing these hormones promotes muscle protein accretion. 85 A recent publication noted that carbohydrate supplementation in excess of 1.0 g/kg body weight should be consumed immediately following intense exercise. It also noted that the best type of carbohydrate to consume was of a high glycemic index. Glucose or glucose polymers were most effective in muscle glycogen resynthesis, whereas fructose or low glycemic index carbohydrate were better used for liver glycogen resynthesis. This study not only validates the importance of carbohydrate intake after intense exercise, but also indicates that the addition of protein may increase the rate of glycogen resynthesis stimulating the secretion of insulin.

With regard to training, the maintenance of blood glucose levels is important for energy. When blood glucose levels become low; there is an increase in plasma ACTH, cortisol, and growth hormone and a decreased insulin concentration. Although this is essential for continued energy production, many immune-modulating effects occur as blood glucose concentrations are decreased, While many of these findings exist for prolonged endurance activity, similar data regarding strength and power events are not widely available.

Adequate carbohydrate intake should attenuate increases in stress hormones, and thereby diminish changes in immune function as well. In a study involving marathon runners, a 6% carbohydrate fluid given before, during, and after 2.5 hours of running attenuated the rise in both cortisol and neutrophil/lymphocyte ratios. Similar findings have also been noted in both cycling and running Furthermore, post-exercise monocytes and lymphocytes were higher in the placebo conditions, with lymphocytes falling from 1.5 to 3 hours post-exercise Further analysis of these results also show an elevated neutrophil! lymphocyte ratio in the placebo conditions for both modes of exercise as well as an increase in NK cell activity. As it may pertain to strength training, eccentric muscle activity is associated with a higher IL-6 response than concentric exercise B9 As with many factors related to exercise, these responses are related to intensity, volume, and duration of exercise. Thus, carbohydrate intake may affect immune parameters as well.

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