Fat is ideal fuel for sustained efforts. The reliance on fat as a source of energy during sub maximal exercise is greater after endurance training.The importance of a shift to a greater reliance on fat as a fuel for muscular exercise can be related to 2 aspects-

1. Fat has several characteristics-
2. Fat contains twice as energy per unit weight than CHO
3. It is not hydrated when stored in body.
4. Body stores. Since fat constitutes the single largest available energy pool in the body as can be seen below-

The body has enormous capacity to store fat-ADIPOSE TISSUE AND SUBCUTANEOUS TISSUE being main sites of fat storage.

	MAN	WOMAN
BODY WEIGHT	80 KG	60 KG
BODY FAT %	15-25	25-35
KCAL@7KCAL/G ADIPOSE TISSUE FAT	1, 50, 000	1, 65, 000
KCAL FROM CHO	2500-4500 Hence fat calories is 30-60 fold more as compared to CHO

 

DIETARY FAT FOR ATHLETES

15-20% CALORIES SHOULD BE coming from diet. In one research diets supplying 20% en from fat vs 40% en from fat was compared in muscular endurance sports-

20% en from fat	40% en from fat
Increased glycogen stores	Not healthy
Increased power output
Faster speeds

 

DIFFERENCES WITH RESPECT TO CHO AND FAT IN ENERGY PRODUCTION

Lactate levels of 5 mEq/L DECREASES ffa liberation and at 10 mEq/L Lipolysis reduced by 70%,

 

Well trained athletes	Untrained athletes
More fat utilized at higher intensity. Hence glycogen stores are saved .Indicating higher aerobic capacity
ENERGY can be aerobically obtained even at 85% VO2 MAX	At exercise levels of 70-75% VO2 MAX lactate accumulates which inhibites lipolysis
EFA AND ketone bodies utilized for energy production
Ketone bodies used for energy	Ketone bodies cannot be used

 

TRAINING AND FAT UTILIZATION DURING EXERCISE

A number of important adaptations occur in the body esp in trained skeletal muscle

1. Increase in concentration of enzymes for
2. Citric acid cycle
3. Fatty acid oxidation and
4. Electron transport chain.

This results in increase concentration of mitochondria protein per unit weight of muscle.

2. Increase in capillarization of muscle, with both greater number of capillaries per muscle fiber and a decrease in the area supplied by a single capillary.

Increased capillarization provides for greater surface area for interaction of blood borne substrates with the muscle fibers. Since transport of ffa from adipose tissue in to the blood requires that they pass through several barriers as well as cellular membranes. At level of individual muscle fiber the greater the area the capillary that can be exposed to the muscle the greater is the potential for exchange of intra cellular and extra cellular materials. The reasons for increased capillary density to increase fat utilization are

1. More capillaries are open
2. Capillaries may be longer
3. Have greater diameter after training compared to before training.

This adaptation becomes important when the same blood volume flows through more capillaries. The time the blood spends in the capillaries increases.

 

3. KETONE BODIES

During exercise there is a sharp rise in total ketone body concentration in non-athletes, whereas there is no change in athletes. This is an adaptive response which enables more ketone bodies to be used during exercise.

TRAINING INDUCED ADAPTATION AND PERFORMANCE

Training can lead to 5 fold increase in the mitochondrial content of skeletal muscle of sedentary human subjects. Also in elite athletes the mitochondrial content may vary through the year as a function of training intensity.

 

Cyclists in the Tour de France which covers A DISTANCE of 3000km over period of 3 weeks, have only about half the mitochondrial content in the winter season (reduced frequency and intensity of training) that they have in the middle of summer when they are competing daily in cycling races covering 200-300 km and often including the ascent of several mountain passes. Their VO2MAX is only 2%-5% lower in winter than in summer because it takes much longer for cardiovascular system to adapt. Their endurance performance, however, is much better in summer due to the higher mitochondrial content of skeletal muscle.

 

ADVANTAGES OF INCREASED MITOCHONDRIAL CONTENT

The major advantage of the increased number of mitochondrial muscle of trained subjects

1. Decrease in muscle ATP AND Creatine Phosphate content.
2. Increases in free ADP, free AMP, and inorganic phosphate are smaller in trained muscle with a high mitochondrial content.
3. Untrained muscle will produce more lactate than the trained muscle.

Therefore elite runners with very high mitochondrial content are able to run at very high exercise intensities without lactate production, whereas recreational runners and sedentary subjects start to produce lactate at more energy than glycolysis alone (38 ATPs per glucose molecule), elite runners are able to run a much greater distance using same amount of muscle glycogen.

A second advantage of the higher mitochondrial content is that more fat can be oxidized at high intensities. Trained muscle extract a greater percentage of the plasma free fatty acids delivered to muscle than do untrained muscles.

For this reason, elite marathon runners (in contrast to recreational runners) are still able to oxidize fat to considerable rates when working at 80% to 90% of maximal aerobic power.

The major metabolic consequences of high mitochondrial content in elite marathon runners are

• Slower utilization of muscle glycogen and blood glucose
• A greater reliance on fat oxidation
• Less lactate production during exercise of moderate to high intensity.

For all these reasons elite marathoners can run at a speed of 20km/hr whereas most recreational runners are exhausted within minutes at such a speed.

 

QUESTIONS

1. Compare fat metabolism/ utilization between trained and untrained athletes.
2. Discuss skeletal muscle adaptation to training.
3. Why is fat a preferred fuel for endurance training?