brett said:
Can someone answer what ketones burn for fuel instead when fat stores are used up, or when the body needs quick
Fuel for activity?
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Hi Brett. The average person can have up to 40,000 calories worth of energy available that is stored in fat cells. But because our modern eating habits have changed so much and we never experience periods of starvation, most western humans have never adapted to be able to tap into the bodies alternative fuel source and use fatty acids efficiently.
This 40,000 calorie number compares to approximately 2700 calories worth of calories which can be stored as glycogen in the liver. This is why many endurance athletes who are 'sugar burners' experience the bonking effect in long distance events, and have to take on extra carbohydrates during the race in order to replace the burnt glycogen - they are unable to switch between fuels because their bodies have never learned to. To become properly fat adapted you do have to eat low carb for a period of approximately four weeks. Performance in this time is definitely adversely affected as the body learns to produce ketones and turn to fatty acids for fuel. It is akin to a factory which previously made cars and that factory overnight changes to producing computers. Clearly there will be a period of transition for the workforce until the factory becomes efficient at making computers.
There then comes the argument as to what type of exercise you do. If Someone who is fat adapted stays at an aerobic level (typically less than 70% max heart rate) then they can burn fatty acids as their primary fuel source, along with some glucose. If, however, someone who is fat adapted undertakes explosive high intensity exercise then they are working at an anaerobic threshold, and their body can't burn the fatty acids quickly enough to keep up with their level of activity so their bodies burn glucose as their fuel source. Our bodies are amazing (minus the insulin deficiency thing obviously). In the absence of dietary carbohydrate, proteins are converted to glucose via the liver in a process called gluconeogenesis (however in a low carb diet, the glycogen stores never get filled properly through this process, which is why high intensity exercise suffers). This is important as our brains and other organs can run on ketones but there still remains a requirement for glucose for these organs. But, in terms of exercise, if high intensity exercise is a regular daily activity, then for an athlete to perform at their best, fatty acids won't cut it, and their will have to be a dietary intake of carbohydrate that is sufficient to fuel the activity.
As for me personally? I dip in and out of ketosis on a regular basis. I do a bit of longer distance running, mixed with football and bodyweight exercises. My distance running is at a slow and relaxed tempo and I find that my blood sugars barely drop. For instance I completed a half marathon in March and never took anything on board during the run. Granted, I wasn't very fast but it was liberating being able to achieve that without having a hypo at the front of my thoughts. I do eat slightly more carb on the days before I play football or workout in order to fill up my glycogen stores, and I then eat low carb on my rest days to minimise spikes in my blood sugar. My opinion is slightly different to others on this boards as I think the natural state of the human body is to have periods of abundance where we use glucose as our preferred fuel source, intermixed with other periods where we adapt to using fat as our primary source, during periods of scarcity. I'm not concerned with my health long term. I look, feel and perform well on a cyclical ketogenic diet and eat, in my opinion, a nutritious diet rich in vegetables, which helps balance the acidic concerns raised. Breathing also helps
