Here is some info on fat metabolism during exercise, long read but good:
http://www.gssiweb.com/reflib/refs/32/d000000020000006d.cfm?pid=38&CFID=299487&CFTOKEN=48197843
Main points:
SPORTS SCIENCE EXCHANGE
FAT METABOLISM DURING EXERCISE
SSE#59, Volume 8 (1995), Number 6
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Edward F. Coyle, Ph.D.
Professor, Department of Kinesiology and Health Education
The University of Texas at Austin
Austin,Texas
Member, GSSI Sports Medicine Review Board
KEY POINTS
1. People store large amounts of body fat in the form of triglycerides within fat (adipose) tissue as well as within muscle fibers (intramuscular triglycerides).When compared to carbohydrate stored as muscle glycogen, these fat stores are mobilized and oxidized at relatively slow rates during exercise.
2. As exercise progresses from low to moderate intensity, e.g., 25-65% VO2max, the rate of fatty acid mobilization from adipose tissue into blood plasma declines, whereas the rate of total fat oxidation increases due to a relatively large use of intramuscular triglycerides. Intramuscular triglycerides also account for the characteristic increase in fat oxidation as a result of habitual endurance-training programs.
3. Dietary carbohydrate intake has a large influence on fat mobilization and oxidation during exercise; when dietary carbohydrate produces sufficient carbohydrate reserves in the body, carbohydrate becomes the preferred fuel during exercise. This is especially important during intense exercise because only carbohydrate(not fat) can be mobilized and oxidized rapidly enough to meet the energy requirements for intense muscular contractions.
INTRODUCTION
The two main sources of energy during muscular exercise are fat (triglyceride) and carbohydrate (glycogen and glucose) stored within the body, and there has been much research and practical experience over the past 30 y demonstrating the importance of muscle and liver glycogen for reducing fatigue and improving athletic performance. For example, it is well known that diets containing predominantly carbohydrate are necessary to maintain glycogen stores at high levels during bouts of intense exercise and that such diets are apparently optimal for promoting training-induced improvements in performance (Simonsen et al., 1991). The primary reason that glycogen reserves are essential is that athletes can only slowly convert their body fat stores into energy during exercise. Therefore, when muscle glycogen and blood glucose concentrations are low, the intensity of exercise must be reduced to a level that can be supported by the body's limited ability to convert body fat into energy. With endurance training, athletes can markedly increase the rate at which body fat can be oxidized, thus allowing them to exercise longer before becoming exhausted due to glycogen depletion. Of course, exercise training also increases an individual's ability to exercise more intensely, so trained athletes must continue to derive most of their energy from carbohydrate during intense training and competition because their increased ability to oxidize fat cannot meet their increased energy demands.
MOBILIZATION AND OXIDATION OF FAT DURING EXERCISE
Mobilization of Free Fatty Acids (FFA) From Adipose Tissue
The large stores of triglyceride within adipose tissue are mobilized at relatively slow rates during exercise. In this process, exercise stimulates an enzyme, hormone sensitive lipase, to dissolve the lipid or triglyceride molecule into three molecules of unbound or free fatty acids (FFA) and one glycerol molecule (Figure 1) ; this process of breaking down triglycerides is known as lipolysis. The glycerol released from this reaction is water soluble and diffuses freely into the blood. Its rate of appearance in the blood provides a direct measure of the amount of triglyceride hydrolyzed in the body. The primary factor thought to be responsible for the stimulation of adipose tissue lipolysis during exercise is the increasing plasma concentration of epinephrine, which activates betareceptors in adipocytes (Arner et al. , 1990); additional hormonal factors probably also play a role.
The fate of the three FFA molecules released from adipose tissue during lipolysis is complex (Figure 1). These fatty acids are not water soluble and thus require a protein carrier to allow them to be transported through cells and within the blood stream. At rest, about 70% of the FFA released during lipolysis are reattached to glycerol molecules to form new triglycerides within the adipocytes. However, during low-intensity exercise, this process is attenuated at the same time as the overall rate of lipolysis increases; as a result, the rate of appearance of FFA in the plasma increases by up to five fold (Klein et al., 1994; Romijn et al., 1993; Wolfe et al., 1990). Once they enter the plasma, the FFA molecules are loosely bound to albumin, a plasma protein, and transported in the circulation. Some of the fatty acids are eventually released from albumin and bound to intramuscular proteins, which in turn transport the FFA to the mitochondria for oxidation (Turcotte et al., 1991).
Figure 2. Contribution of the four major fuel substrates to energy expenditure after 30 min. of exercise at 25%, 65% and 85% of maximal oxygen uptake in fasted subjects. Reproduced with permission from Romijn et al. (1993).
Recent studies of endurance-trained men who had fasted overnight found that the rate of appearance of FFA in plasma declines as the intensity of exercise progressively increases from low (25% VO2max, comparable to a walking pace) to moderate (65% VO2max, comparable to the greatest running pace that can be sustained for 2-4 h) to high (85% VO2max, the greatest pace that can be sustained for 30-60 min) (Figure 2). The contributions of carbohydrate, i.e. muscle glycogen and blood glucose, and of fat, i.e., plasma FFA from adipose tissue plus intramuscular triglyceride, to total energy expenditure during exercise at these various intensities are shown in Figure 2. It should be noted that although the contribution of plasma FFA to the fuel supply declines as exercise intensity increases from 25% to 65% VO2max, total fat oxidation increases. Furthermore, although the use of plasma FFA for energy is reduced as intensity increases from 25% to 65% VO2max, we can't discount the possibility that at an intermediate intensity, e.g., 45% VO2max, plasma FFA might contribute more energy than at 25% VO2max.
Whole-Body Fat Oxidation During Exercise of Increasing Intensity
There is much interest in the effect of exercise intensity on fat oxidation and the sources of that fat. It is often assumed that the intensity of exercise must be kept low to burn fat optimally. However, from Figures 2 and 3 it can be seen that the rate of total fat oxidation was higher at 65% than at 25% VO2max -110 cal · kg-1 · min-1 vs. 70 cal · kg-1 · min-1. At 25% VO2max, almost all of the energy expenditure during exercise was derived from fat, but fat oxidation at 65% VO2max accounted for only 50% of the energy expenditure. However, because the total rate of energy expenditure was so much greater (2.6-fold) at 65% VO2max, the absolute rate of fat oxidation was greater, i.e., it was 50% of a much larger value (Figure 3). Therefore, expressing energy derived from fat simply as a percentage of energy expenditure without consideration of the rate of total energy expenditure is misleading. Likewise, the reduction in the rate of appearance of plasma FFA with increasing intensity of exercise does not prove that exercising at a low intensity is the best way to reduce fat stored in adipose tissue.
Figure 3. Expanded views of teh sources of fat for oxidation during exercise at 25% (walking pace), 65% (moderate running) and 85% (intense running) of maximal oxygen uptake in fasted subjects.
Both the rate of energy expenditure and the duration of exercise are critical in determining fat loss. Another consideration is the effect that exercise has on energy expenditure during the recovery periods between exercise sessions. Reductions in body fat stores as a result of long-term exercise training depend primarily on the total daily energy expenditure and not simply the actual fuel oxidized during exercise (Ballor et al., 1990).
FAT SUPPLEMENTATION DURING EXERCISE
Ingestion of Long-Chain Triglycerides
It is not possible to ingest FFA because they are too acidic and because they need a protein carrier for intestinal absorption. Thus, the only practical way of significantly raising fat in the blood is by ingesting triglycerides. Normal long-chain dietary triglycerides enter the blood 3-4 h after ingestion and are bound to chylomicrons, which are lipoprotein carriers in the plasma. The rate of breakdown of triglycerides bound to plasma chylomicrons and the rate of uptake of those triglycerides by muscles during exercise are relatively low, and these chylomicron-associated triglycerides are used primarily to replenish intramuscular triglycerides during recovery from exercise (Mackie et al., 1980; Oscai et al., 1990). Therefore, although not proven, it is unlikely that ingestion of long-chain triglycerides has much potential to provide significant fuel for muscle during exercise (Terjung et al., 1983) .
Ingestion of Medium-Chain Triglycerides
Unlike long-chain triglycerides, ingested medium-chain triglycerides (MCT) are directly absorbed into the blood and liver and are rapidly broken down to fatty acids and glycerol. They therefore provide a theoretical means of rapidly elevating plasma FFA. Another theoretical advantage of MCT is that they appear to be readily transported through cells and into the mitochondria for oxidation. Recent studies have shown that a large percentage of ingested MCT is oxidized and that the oxidation increases more rapidly when the MCT is consumed along with carbohydrate (Jeukendrup et al., 1995). However, most individuals cannot consume more than 30 g of ingested MCT without experiencing severe gastrointestinal discomfort and diarrhea. Accordingly, MCT ingestion can only contribute 3-6% of the total energy expended during exercise (Jeukendrup et al., 1995). Furthermore, when MCT is consumed with a carbohydrate feeding, the carbohydrate-stimulated insulin secretion partially inhibits the mobilization of the body's own fat stores, resulting in large reductions in fat oxidation compared to exercise when fasted.
DIETARY CARBOHYDRATE INFLUENCES FAT OXIDATION DURING EXERCISE
Eating Carbohydrate During the Hours Before Exercise
Fat oxidation during exercise is very sensitive to the interval between eating carbohydrate and the onset of exercise and to the duration of the exercise. This is due in part to the elevation in plasma insulin in response to the carbohydrate meal and the resultant inhibition of lipolysis in adipose tissues, thus reducing the mobilization of FFA into the plasma. This effect is evident for at least 4 h after eating 140 g of carbohydrate that has a high glycemic index (Montain et al., 1991). Under these conditions, the carbohydrate meal reduces both total fat oxidation and plasma FFA concentration during the first 50 min of moderate-intensity exercise. However, this suppression of fat oxidation is reversed as the duration of exercise is increased; after 100 min of exercise, the rate of fat oxidation is similar, whether or not carbohydrate was eaten before exercise. It appears that the body relies heavily on carbohydrate and less on fat when people have eaten carbohydrate during the previous few hours, and therefore carbohydrate is preferred when it is available. It is likely that insulin plays a role in regulating the mixture of carbohydrate and fat oxidized during exercise.
Plasma FFA mobilization is remarkably sensitive to even small increases in plasma insulin (Jensen et al., 1989), and it seems that lipolysis is influenced for a long time after eating carbohydrate (Montain et al., 1991). Diets that are lower in carbohydrate or that contain carbohydrates that cause less insulin secretion, probably still elicit enough of an insulin response to reduce plasma FFA mobilization. Therefore, any commercially available product or diet that claims to increase FFA mobilization and oxidation would have to almost totally eliminate the insulin response to the carbohydrate in their product, which seems unlikely. At the very least, the developers of these products must demonstrate that FFA mobilization is increased by their diets and is somehow beneficial. As discussed above, increased FFA mobilization would certainly not seem to be of any value for untrained people because their mobilization of FFA normally exceeds the ability of the muscles to oxidize FFA.
Cyp
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about this, but i have to go to bed. i am getting up early for cardio.
