KINE305 Chapter 7: Fats

What is the importance of fat for athletes?

important fuel for endurance exercise

required for optimal health

provides essential fatty acids that cannot be synthesized in the body

Why not emphasize fats in sports nutrition?

they cannot be manipulated to enhance performance like carbs

the body can be better at burning fat but that does not mean it enhances performance

Why does a low-carb, high-fat diet not work for better performance outcomes?

the athletes were not able to train as hard and therefore their performance dropped

Where are fatty acids that are oxidized in the mitochondria of skeletal muscle during exercise derived from?

adipose tissue triacylglycerol (TAG) broken down by a hormone-sensitive lipase (HSL)

intramuscular triacylglycerol (IMTG) broken down by an HSL

plasma triacylglycerol broken down by endothelial lipoprotein lipase

What are limits to fat oxidation?

1. lipolysis, the breakdown of triacylglycerols to fatty acids and glycerol

2. removal of fatty acids from the fat cell

3. transport of fat by the bloodstream

4. transport of fatty acids into the muscle cell and mitochondria

5. oxidation of fatty acids in the beta-oxidation pathway and tricarboxylic acid (TCA) cycle

What is albumin?

the most abundant protein in plasma that acts as a carrier protein that transports fatty acids

it maintains osmotic pressure and carries/transports things around safely (such as FAs to muscle)

What is intramuscular triacylglycerol (IMTG)?

fat droplets in skeletal muscle, usually located adjacent to the mitochondria and is an important energy source during exercise

exercise training increases the number of IMTGs next to the mitochondria

What is carnitine?

molecule that transports FAs for their utilization

more carnitine = more transport = faster burning

the bonding between carnitine and the activated FA is the first step in the transport of the FA into the mitochondria

How do FAs get transported into the mitochondria?

1. as carnitine binds to the FA, free CoA is released

2. the newly converted fatty acyl-carnitine can be transported across the inner mitochondrial membrane

3. it is then converted back into fatty acyl-CoA where it can undergo beta oxidation

B-Oxidation of FAs in the mitochondria

1. fatty acyl-CoA undergoes b-oxidation that split a two-carbon acetyl-CoA off the FA chain

2. ATP is generated

3. acetyl-CoA is oxidized in the TCA cycle

4. complete oxidation of FAs in the mitochondria depends on the activity of enzymes of the b-oxidation pathway

Where are the maximal rates of fat oxidation observed at in endurance-trained athletes?

at 63% of VO2 max

At what point is fat oxidation inhibited?

higher intensities of exercise (>75% of VO2 max)

Why should exercise decisions be based on intensity and NOT burning fat?

burning energy means burning the carbs stored in muscle to prevent them from sitting; carb cycling is what promotes cell health

Where is most fat oxidation derived from at 25% of VO2 max?

plasma FAs, only small amounts come from IMTGs

Where is most fat oxidation derived from at 65% of VO2 max (moderate-intensity)?

the contribution of plasma FAs declines, whereas contribution of IMTGs increases

Where is most fat oxidation derived from during training?

increased contribution of IMTG to energy expenditure

How does training increase fat oxidation?

1. increased mitochondrial density

2. increase in the number of oxidative enzymes in trained muscle

3. increased capillary density, which enhances FA delivery to muscle

4. increased FABP concentrations, which increases transport of FAs

5. increased CPT concentration, facilitating transport of FAs into mitochondria

6. increased concentration of IMTG increasing its utilization

What effects does diet have on fat oxidation?

HIGH-CARB, LOW-FAT: reduces fat oxidation

HIGH-FAT, LOW-CARB: increases fat oxidation

What effects does carb feeding have on fat oxidation?

reduces fat oxidation and plasma FA availability via an insulin-mediated inhibition of lipolysis

inhibition of carnitine-dependent FA transport into the mitochondria

potentially starting the insulin-dependent and independent mechanisms at once

What is the rate of carb utilization related to?

the energy needs of the working muscle during prolonged exercise

What is the rate of fat utilization related to?

it is not as tightly regulated as carb utilization, and is mainly influenced by fat availability and rate of carb utilization

Does increased FA availability spare the limited reserves of glycogen?

evidence suggests that increases in plasma FA can cause a decrease in the rate of muscle glycogenolysis

the later you can tap into glycogen stores, the better you can perform

What is the Randle Cycle?

originally thought to explain the interaction between carb and fat metabolism in that fat metabolism regulates carb metabolism

increased b-oxidation decreases glycolysis

What is the Reverse Randle Cycle?

the theory that fat does not regulate carb metabolism but rather that carb regulates fat metabolism 

an increase in glycolysis rates decreases fat oxidation

Should long-chain triacylglycerols be ingested as a fuel source during exercise?

no, because during exercise they slow gastric emptying and enter the system circulation slowly as a minor fuel source

Should medium-chain triacylglycerols be ingested as a fuel source during exercise?

although they are rapidly emptied from the stomach and absorbed and oxidized, the larger amounts of MCTs results in gastrointestinal issues

Is fasting beneficial for exercise performance?

fasting increases lipolysis and plasma FA availability, resulting in increased oxidation of FAs at rest and during exercise

but liver glycogen stores are not maintained, so fatigue resistance and exercise performance are impaired

this decreased performance is not reversible by carb ingestion during exercise

What are the results of short high-fat diets?

diets lasting 3-5 days increase the availability of lipid substrates but reduce the storage of glycogen which results in increased fat oxidation during exercise

however, fatigue resistance and exercise performance are compromised

What is the results of chronic high-fat diets?

thought to result in adaptations that theoretically should improve exercise tolerance

increased activity of oxidative enzymes and decreased degradation of liver glycogen during exercise, resembling endurance training adaptations

despite increased fat-oxidation during exercise, performance is unaffected