All very interesting. Is the "fat adaptation" thing thought to be an on/off phenomenon, or continuous? Tim Noakes, for instance, writes about 25-30g carbs/day being necessary to ensure fat burning and ketogenesis, but without necessarily varying that figure according to body mass (whether ideal or actual).
In other words, can one be "fat adapted" at 29g carbs/day and "carb adapted" at (say) 50g/day? Surely it's continuous? I know of few physiological processes that really are on/off (mast cell degranulation being the only one I can think of off-hand).
You body needs ATP to be made in order to provide energy to our cells. It can make ATP from fat, glucose/glycogen, protein, lactate etc.
Your blood carries all those energy sources around, the glucose/glycogen as is and the fat/protein aboard lipoproteins.
Fat adaptation is just swinging the balance is favour of fat; it doesn't stop your body using carbs as energy, just limits it. Diet and fasted training are the two primary methods of forcing the adaptation. A vegan on a really low fat diet will still be able to burn fat just as an eskimo still has the ability to use glucose. It isn't an either/or situation rather a changing of the proportion; for audax cycling shifting the balance to provide the vast majority of energy with fat is the goal of many here.
Australian Olympic coach Dr Louise Bourke has a good podcast on scienceofultra.com.
My own view is that a ketogenic diet is too impractical in the modern world to comply with, along with the down regulation in glucose use and subsequent power loss to make it ideal. Also in the real world of a multi day audax, are you really going to ask a volunteer at 4am in a shelter that the balance of your meal isn't quite right for your diet....
Also many don't do it properly and are often just on a low fat diet anyhow.
The last thing most people should be doing is limiting plants in their diet; a half arsed attempt at a ketogenic diet can do that.
Good podcast here by a non zealot, but with an eye on performance and metabolic flexibility. (skip the first 5 mins where they tell each other how great they are..)
http://sigmanutrition.com/episode86/This is taken from optimumnutrition4sport.com and is just a basic description of how body makes ATP
Energy Systems
Now for the science bit. I don’t want to go into too much detail, but understanding how your body functions is the key to understanding how food works. So very briefly, here are the different ways your body can produce ATP (i.e. energy)
PhosphoCreatine System: ADP +Cr P –> ATP
This system is used to produce the first 10-15secs of energy that we need for sprints. Obviously, this is not a system that ultra endurance athletes use very often. It doesn’t require glucose or oxygen and it is fuelled through the use of creatine, a natural molecule produced in the body consisting of 3 amino acids.
Anaerobic System (Glycolysis): Glucose –> Pyruvate + ATP + H+
This is a system more familiar to people which burns glucose quickly and coverts it to lactic acid. The result is quick energy, the type needed for high intensity exercise like interval training or hill climbs. This process doesn’t require any oxygen but the trade-off is a large production of hydrogen ions which decrease the pH resulting in muscle fatigue.
Cori-Cycle (The Lactic Acid Cycle): Lactate + ATP –> Glucose
Despite what people think, lactate is not the bad guy. The acid builds up and muscle fatigue/pain is mainly caused by an increase in hydrogen ions (which lowers the pH). The lactate produced in the muscle can be recycled in the liver and converted back to glucose. This glucose can then be shuttled to the muscle and used again to produce energy.
Aerobic System (The Krebs Cycle/Citric Acid Cycle): Glucose + O2 –> CO2 + H2O + ATP
This is the main system that our cells use to produce energy. It’s a series of enzyme controlled chemical reactions that use oxygen to breakdown glucose. The first few steps of this reaction involve converting glucose into a smaller carbon chain intermediate. What’s important to note here is that both fats and proteins can be broken down and converted into this exact same intermediate. In other words, as well as pure glucose, our cells can use fats and proteins to fuel the aerobic system. This is a complicated pathway but here is a very simple diagram to illustrate what I mean
Lipolysis/Beta-Oxidation
Lipolysis is the conversion of triglycerides into glycerol and free fatty acids. These free fatty acids can then be transferred to muscle where they are further broken down through beta-oxidation to prepare them for the Krebs Cycle. The advantage of using fatty acids is 1. The amount we can store is far greater than carbs and 2. They provide roughly twice the amount of energy per gram. So we have lots more of this type of fuel and it produces more energy. The only disadvantage is that because they are much bigger molecules than glucose, they require more oxygen for their combustion. Therefore, fatty acids from the plasma and adipose tissue are oxidised at a higher rate when the intensity is low (i.e. when you can take in more oxygen, i.e. breath more)
ITMG (Intramuscular Triglycerides) Fat Oxidation
So when fats are used to produce energy, they can come from three different locations. 1. Adipose Tissue, where the majority of itis stored 2. Muscle and 3. Blood Plasma. The fat stored in muscle is called Intramuscular Triglycerides. It is this which gives meat its marbled appearance. As ITMG’s are already present in the muscle, transport and delivery is not an issue. Therefore, the ability to use them is increased especially as exercise intensity increases.
Glycolytic Proteins/Protein Oxidation
Amino acids (such as Leucine, Isoleuncine and Valine) can also be converted into Acetly-CoA (the intermediate that both glucose and fatty acids are converted to) and then used in the Krebs Cycle. It is estimated that 5-10% of energy can come from the oxidation of proteins. However, this is not ideal if amino acids are not plentiful as it means that the source of amino acids will be from muscle tissue. This can be reduced by supplying amino acids and increasing the rate of fat oxidation to spare the use of protein.
Glucose Alanine Cycle
Just like the Cori Cycle, where a waste product is converted back into glucose, the same can be done with amino acids. The amino acids Alanine and Glutamine can be used to convert Pyruvate back into Glucose.
