Fuelling with Fat
Since the 1960s, we’ve been told that carbohydrates are a vital source of energy, and pre-endurance event carb loading is a necessity. Consequently, strategies were developed to optimize performance through the use of carbohydrates and supplements for refuelling. All this research is founded on the assumption of a ‘carbohydrate metabolism’. However, this presumption has overshadowed an alternative approach. For athletes who are ‘fat adapted,’ the conventional rules of carbohydrate consumption no longer hold true. So, how does one transition to a state of fat adaptation? The answer lies in adopting a ketogenic diet.
A ketogenic diet is a diet high in fat, moderate in protein and in which carbohydrates are restricted to below 30-50g per day. Eating this way for a period of approximately 2-4 weeks will shift the body’s metabolism from burning primarily carbohydrates (glucose) as fuel to a metabolism that relies on burning fat.
The Body's Fuel Tanks
Fuel tanks
The body has vast stores of energy available stored in two ‘fuel tanks’:
1. Glycogen (Glucose) 2,000 kCal
2. Fat >100,000 kCal
Glucose is stored in the body in the form of glycogen and has the capacity to provide approximately 2,000 calories of energy at any given time. In contrast, energy stored in the form of fat (even in non-obese individuals) is significantly larger. Even a lean individual weighing 80kg with a body fat percentage of 15% will have more than 100,000 calories of stored energy (40-50 times more than the glycogen store!).
“Newsholme and Leech (1983) have calculated that the body triacylglycerol stores have the potential to supply 30- to 40-fold more energy than the carbohydrate stores”
1. Lambert EV, Hawley JA, Goedecke J, Noakes TD, Dennis SC. Nutritional strategies for promoting fat utilization and delaying the onset of fatigue during prolonged exercise. J Sports Sci. 1997 Jun;15(3):315-24. doi: 10.1080/026404197367326. PMID: 9232557.
The size of the glycogen store remains constant, however the size of the fat store will increase with an individual’s level of adiposity.
Even in Alex’s case, as a lean individual, he still has 50 times more energy stored as fat than carbohydrates.
Adapted from source: Lambert EV, Hawley JA, Goedecke J, Noakes TD, Dennis SC. Nutritional strategies for promoting fat utilization and delaying the onset of fatigue during prolonged exercise. J Sports Sci. 1997 Jun;15(3):315-24. doi: 10.1080/026404197367326. PMID: 9232557.
What does 100k Calories of fat look like?
Body DEXA scan details exactly how much lean tissue, fat tissue and bone mass composes your body and its specific distribution to each region. Alex’s DEXA scan illustrates his body fat percentage prior to attempting the challenge.
Fat stores are coloured yellow.
It should become apparent that the vast quantity of available energy are actually contained in only a very small amount of adipose (fat) tissue.
Hitting 'The Wall' with Carbohydrates
The size of each tank is vastly different and the rate at which the body can convert this fuel into energy will depend on whether an individual has a primarily carbohydrate or fat adapted metabolism.
The regular consumption of carbohydrates essentially ‘locks the door’ to access the body’s vast fat reserves and the body then must rely on the much smaller 2000 calorie glycogen fuel tank. It explains why endurance athletes ‘hit the wall’ during an event as they have exhausted their carbohydrate dependent metabolism. In order to compensate, athletes must ‘refuel’ with carbohydrates despite having an enormous store of energy in the form of fat due to it being inaccessible.
“An endurance athlete hitting the wall is like a fuel tanker running out of gas on the highway. You have ample amount of fuel, you just can’t utilize it effectively.”
Source: Dr Stephen Phinney – ‘ The Art and Science of Low Carb Living and Performance’ 21 September 2014
Fuel Tank Depletion
During exercise, the body will convert available fuel to energy from either glucose or fat via a process referred to as oxidation. The speed at which the body can convert each fuel source will depend on the body’s dominant metabolism. This is effectively the rate of depletion of each fuel tank and is usually calculated in grams per minute.
These rates of depletion can be shown as follows:
Source [Webster CC, Noakes TD, Chacko SK, Swart J, Kohn TA, Smith JA. Gluconeogenesis during endurance exercise in cyclists habituated to a long-term low carbohydrate high-fat diet. J Physiol. 2016 Aug 1;594(15):4389-405. doi: 10.1113/JP271934. Epub 2016 Jun 8. PMID: 26918583; PMCID: PMC4967730.]
Note: the higher the rate, the faster the depletion of fuel storage
In a carbohydrate dominant metabolism, the small 2,000 kcal carb fuel tank will be exhausted rapidly due to the high rate of depletion (2.89g/min) and the conversion of the large fat stores to usable energy will be very slow (0.56g/min).
The key point to appreciate is that when fat adapted, the rate of depletion of glucose stores is much slower (1.22 vs 2.89 g/min) and the rate of energy extraction from fat is much faster (1.22 vs 0.56 g/min).
It is important to consider that regardless of whether an individual’s dominant metabolism is carbohydrate or fat, both fuel sources will be utilised. It is simply the relative contribution of each fuel source to available energy that is relevant. The fuel source contributions of both metabolisms can be viewed as follows:
Provided both fat and carbohydrate adapted metabolisms can create energy in a sufficient quantity to meet the exercise demand, a fat adapted individual will be able to perform for a significantly longer period of time at the same level of intensity [ref].
* Phinney SD, Bistrian BR, Evans WJ, Gervino E, Blackburn GL. The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism. 1983;32(8):769–776 [https://pubmed.ncbi.nlm.nih.gov/26892521/].
Converting Fuel to Energy
When it comes to energy production, ATP (adenosine triphosphate) is the molecule that carries energy in our cells. The amount of ATP generated from the oxidation of fat and glucose varies significantly. A fat molecule will yield significantly greater than the energy released from glucose.
The oxidation of 1 g of fat generates about six times as much ATP as does the oxidation of 1 g of glucose
ref: Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 8.1, Oxidation of Glucose and Fatty Acids to CO2. page 313
A fat adapted individual has the ability to not only oxidise fat at a greater rate, but the energy (ATP) derived per gram from fat will be significantly more than the energy derived from the same amount of glucose.
You can read more about this here
The paradigm of fat metabolism presents a compelling alternative to traditional carbohydrate-centric energy strategies applicable to not only athletes but to everyday life. By adopting a ketogenic diet and becoming fat adapted, individuals can unlock the potential of their extensive fat reserves, providing a vast and sustainable source of energy. In the case of athletes, this metabolic shift offers a longer-lasting and efficient energy supply, staving off the dreaded “wall” and still provides performance at equivalent intensities for extended periods. The body’s ability to convert fat into energy and the higher ATP yield per gram of fat further emphasize the advantages of this approach.
The intention of attempting to complete of 5 marathons in 5 days without any additional fuel is to demonstrate the extent of the fuel stores available for a fat adapted ketogenic individual. The extreme nature of this challenge is not expected to be repeated nor promoted. However it should illustrate that a ketogenic approach is both safe and effective in normal everyday exercise endeavours.
See here for a fantastic lecture by Dr Stephen Phinney for a deeper dive into the incredible performance of keto adapted athletes.