Fat adaption is about building a flexible metabolic system which can use both Fat (Ketones) and Carbohydrate (Glycogen) for fuel. Many see Fat adaption as a path to a ketogenic or low carb (LCHF) diets. This is not necessarily true. Fat adapted athletes have the ability to utilise fat for fuel at aerobic performance, carbohydrate as the primary fuel at high intensity, allowing optimal performance in both.
Fat adaption is necessary for athletes who have been on a consistently high carbohydrate and processed food diet for many years. After extended periods of consistent processed carbohydrate supply (for many 24/7) many have a reduced efficiency to process fat as fuel thus leaving them dependent on carbohydrates at all intensities.
Carbohydrate intake should be based on the rates of glycogen depletion and the physical activity an athlete is engaged in. Low intensities involve lower carbohydrate needs. However, low intensity workouts of long duration will require a higher daily intake of carbs. High intensity workouts rely on glucose almost exclusively – there is always a high degree of glycogen depletion and therefore these kinds of workouts require a higher carbohydrate intake.
The guidelines for carbohydrate consumption during competition call for 30-60 grams hour. However it is possible that these recommendations fall short when it comes to endurance and ultra-endurance events should they be carb dependent (not fat adapted).
Examples of Carb/Fat rates for a Fat adapted and a Carb dependent athlete. This shows Calories per minute and Grams per hour at increasing intensity zones.
Fat Adapted Athlete
Carbohydrate Dependent Athlete
Many fervently defend or dispute the value of fat adaption. It is however important to understand the context presented as there is clearly a need for carbohydrates and advantages in optimising fat oxidation depending on intensity and duration of exercise. Unfortunately many articles do not clarify their context and are targeted at specific sport intensities or duration.
How do you know if you are fat adapted?
The best way is to undergo a metabolic test. This will give you an indication of what fuel system is the primary at each intensity and the cross over point. A fat adapted athlete will generally have a high cross over where carbohydrate supersedes fat as the primary fuel.
A simpler, but less reliable test is to assess if you can perform at or near to aerobic threshold (Heart rate of 180 – your age) for 2.5 hours with electrolytes only and no exogenous carbohydrates. This test should be done fasted. Should this not be achievable it is almost certain that carb utilisation is higher than fat at aerobic level.
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How to fuel for competition?
Fat Adapted: The guidelines for carbohydrate consumption during competition call for 30-60 grams hour. This recommendation is based on an elite athlete who has years on a healthy and balance diet with a good balance of aerobic and high intensity training. It would be highly probable that the athlete has the ability to oxidise fat for fuel efficiently. Fat adapted athletes should test fuelling in this range in their pre-competition phase. Referencing the chart above, the fat adapted athlete would be carb neutral by consuming 50 gram carbs per hour at 70% VO2max.
For short events much less is required as the liver and muscle already have around 450 grams carbohydrate available to draw down on. A small amount of quality carbohydrate can stimulate a “fight or flight” response enabling athletes to reach a higher level of anaerobic performance. A handful of berries, half a banana are great options.
The fat adapted bottom line: 30-60 grams per hour for events over 90 minutes. Optional low-level carbs for events under 90 minutes to support accumulated stress durability.
Carb Dependent: The carbohydrate demand for a carb dependent athlete is going to be higher, however how much is dependent of their metabolic flexibility. Although the body has approximately 450 grams carbs available, this can be utilised quickly if fat oxidation is impaired. The maximum carb absorption for many is 60 grams per hour, however with training this could be in the range of 80-100 g/hour. Referencing the chart above for the carb dependent athlete, we can see that the athlete will require over 183 grams per hour at 70% VO2max to remain carb neutral. This leaves a deficit of 93 grams per hour if consuming 90 grams. This would limit carb availability to 5 hours before the dreaded bonk! At 80% VO2max (marathon pace) the bonk would occur at around 2.5 hours!
As the carb dependent generally utilise significant carbs overnight, a carb meal would be required on the morning of competition. Fuelling during competition should be in the range of 30-60 grams per hour for events up to 90 minutes. Above 90 minutes it is really dependent of the metabolic flexibility so pre-competition testing is highly recommended.
The carb dependent bottom line: 30-60 grams for events up to 90 minutes. Fuelling for events over 90 minutes should be established through pre-competition testing.
How to fuel for training?
High Intensity: A small amount of quality carbohydrate can stimulate a “fight or flight” response enabling athletes to reach a higher level of anaerobic performance. A handful of berries, half a banana are great options. This is relevant to both fat adapted and carb dependent athletes.
Aerobic sessions up to 2.5 hours for Fat Adapted: No pre-session carbs. Electrolytes only. Replenish with real food post session with fat, protein and carbohydrate avoiding processed content (sugar and processed grain).
Aerobic sessions over 2.5 hours for Fat Adapted: No pre-session carbs. Electrolytes and 30-60 gram carbs per hour from outset. Replenish with real food post session with fat, protein and carbohydrate. Replenish with processed supplements if real food options not available within the hour.
Aerobic sessions up to 90 minutes for Carb dependent: Electrolytes and 30-60 gram carbs per hour. Replenish carbs and protein within 1 hour of completion
Aerobic sessions over 90 minutes for Carb dependent: Electrolytes and 60-90 gram carbs per hour as system allows. Replenish carbs and protein within 1 hour of completion.
Solids or Liquids?
Liquid fuel is generally easier and faster to absorb, if you are looking at efficiency and wish to reduce the volume of water and blood diverted to the gut then stay with liquid fuels if you can. For longer low intensity events like ultras or stage events, solids can be advantageous as they do ensure a sustain release of energy.
Pro’s and Con’s of Fat and Carbs
References
Fat adaptation in well-trained athletes: effects on cell metabolism: Wee Kian Yeo, Andrew L. Carey, Louise Burke, Lawrence L. Spriet, John A. Hawley: https://doi.org/10.1139/H10-089
Substrate Metabolism During Ironman Triathlon: Different Horses on the Same Courses: Maunder, E., Kilding, A.E. & Plews, D.J. Sports Med (2018). https://doi.org/10.1007/s40279-018-0938-9
