Carbohydrate Ingestion During Exercise

The rate of utilization of glucose from stored glycogen in the body can be reduced by supplying oral CHO. For example, when food containing CHO is ingested, digested and absorbed, the digested CHO will enter the circulation as the constituent monosaccharides, mainly glucose and fructose. Accordingly, blood glucose rises after oral CHO intake. This rise reduces the need to break down liver glycogen for the maintenance of an

Figure 10 These figures give a typical representation of metabolism in a glycogen depleted state. Because of lack of glycogen in the liver, blood glucose falls, lactic acid production is decreased and fat metabolism is increased to compensate for energy deficits. The consequence is that the performance level will drop to approximately 50% of maximal capacity. Reproduced from Wagenmakers (189) with permission of the American Physiological Society

Figure 10 These figures give a typical representation of metabolism in a glycogen depleted state. Because of lack of glycogen in the liver, blood glucose falls, lactic acid production is decreased and fat metabolism is increased to compensate for energy deficits. The consequence is that the performance level will drop to approximately 50% of maximal capacity. Reproduced from Wagenmakers (189) with permission of the American Physiological Society

Figure 10 (continued)

Exercise time (min)

Figure 10 (continued)

appropriate blood glucose level. Additionally, glucose supply to and glucose uptake by the muscle will be elevated. Indeed, a large body of scientific evidence shows that oral CHO intake reduces liver glucose output (492) but increases blood glucose at a similar rate. The increased blood glucose after CHO intake will stimulate insulin release and with it glucose uptake by the muscle as well as subsequent CHO oxidation (49, 75, 82, 124, 148).

Theoretically these events will reduce the rate of muscle glycogen and protein degradation for energy production and delay the onset of fatigue/improve performance. Yaspelkis (218) observed that the ingestion of an 8.5% glucose polymer solution reduced the rate of muscle glycogen depletion during low intensity exercise in the heat, while maintaining a high rate of CHO oxidation.

Thus, in studies where CHO was ingested during exercise, total CHO utilization was found not to differ from control groups that did not ingest CHO. Since, in such studies, oral CHO was shown to be oxidized, the conclusion is that glycogen must have been spared. However, since CHO ingestion has not been found to reduce the rate of muscle glycogen degradation in active muscle, this glycogen sparing effect most probably took place in the liver and in non-active muscle (45).

The latter does not necessarily mean that muscle glycogen in active muscle cannot be modified by CHO consumption during exercise. Simply imagine that the supply of CHO during exercise is in excess of the requirement for energy production. In that case the muscle has to store the CHO as glycogen. This would reduce glycogen degradation or even lead to glycogen build-up during exercise. The question is: is that possible? The answer is yes! But, there is one prerequisite to achieving sparing or build-up of endogenous CHO pools during exercise in that the ingested CHO should be easily digested, rapidly absorbed and substantially elevate blood glucose levels.

For exercise lasting longer than 45 min it is recommended that at least 20 g, but optimally up to 60 g, be consumed, with sufficient fluid, during every following hour of exercise (39, 44). Such amounts have been shown not to delay gastric emptying to a physiologically important degree and to stimulate water absorption in the intestine. This aspect is of particular importance in endurance events in the heat, where both CHO and fluid availability may be performance limiting factors (see also Chapter 5). The CHO sources used should be rapidly digestible and absorbable. Most efficient are (soluble) CHO sources which can be ingested with fluid. The gastric emptying rate should be relatively fast and the physical form of the CHO should allow rapid digestion/enzymatic hydrolysis. This is not the case with all CHO sources. For example, the dietary fibre in which some CHO sources are 'packed' may form a physical barrier to digestive enzymes (47) and may also reduce gastric emptying rate. Normal daily meals should primarily contain foods that are rich in slowly digestible CHO and dietary fibre resulting in a low glycaemic index. Examples of such foods are whole grain products and cereals. However, foods taken shortly before and during exercise should be low in dietary fibre and have a high glycaemic index, in order to allow for a rapid gastric emptying and digestion/absorption (30, 44).

The reason for this apparent paradox is that dietary fibre may reduce gastric emptying and decrease the degree in which enzymes can reach the starch for hydrolysis. Fibre also increases gastrointestinal bulk due to water uptake and swelling. Fibre enhances transit in the gut and may be subject to bacterial fermentation causing gas production. Softening of the intestinal contents by fibre and the related improved intestinal transit are desirable in sedentary individuals but may pose a problem during intensive exercise. These factors may explain why athletes who ingest slowly digestible whole grain foods, prior to and during exercise, experience more gastrointestinal problems than athletes who ingest low fibre products (30, 156).

When dietary fibre is excluded from the CHO source, the starch will be fully accessible to enzymatic digestion. The starch and glucose polymers have been shown to be as effective in energy supply as free glucose (75). Other sources of complex CHO, such as rice, spaghetti and potato, are of particular interest for daily CHO intake, between sport sessions, but are shown to be oxidized more slowly during exercise than soluble CHO sources (76). During periods of non-intensive exercise, however, such as mountain walking, these CHO sources can be consumed satisfactorily prior to and also during the activity.

Optimal CHO sources for high intensity endurance events are processed (pre-digested) CHOs that are low in dietary fibre:

These types of CHO have the additional benefit of being easily dissolved in fluids, which is an important aspect as the requirements for CHO and fluid (see Chapter 5) are determined by the exercise intensity and duration. The types of CHO listed above have been shown to be about equally effective in increasing blood glucose levels and oxidation rates during exercise as well as in improving performance (43, 44, 82). Effects on blood insulin levels during exercise also do not appear to be different (44).

Some early studies showed that an intake of 50 - 75 g of rapidly absorbable CHO prior to exercise induces a rapid rise in blood glucose and insulin and a rebound hypoglycaemia as well as decreased performance during the subsequent exercise. However, these studies were done after an overnight fast and CHO was ingested in the resting state,

Figure 11 The difference between 'raw' carbohydrate sources and refined sources is the dietary fibre content. Dietary fibre reduces gastric emptying rate, slows down digestion and absorption and enhances the amount of intestinal bulk, which promotes normal transit n >

Supplements For Diabetics

Supplements For Diabetics

All you need is a proper diet of fresh fruits and vegetables and get plenty of exercise and you'll be fine. Ever heard those words from your doctor? If that's all heshe recommends then you're missing out an important ingredient for health that he's not telling you. Fact is that you can adhere to the strictest diet, watch everything you eat and get the exercise of amarathon runner and still come down with diabetic complications. Diet, exercise and standard drug treatments simply aren't enough to help keep your diabetes under control.

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