Why does the body prefer glycolysis over ketosis when both glucose and fat are plentiful?

(Nick) #1

People sometimes talk about fat being the body’s ‘preferred’ energy source. But that’s not really true, is it? I mean, if you’re eating plenty of fat and carbohydrate, your body will choose to burn the carbs and not bother with ketosis.

What’s the evolutionary explanation for this, given all the health downsides associated with metabolising carbs? It would have made more sense to me if we’d evolved to prefer ketosis, and our bodies only ever switched to glycolysis in very low-fat situations (whether dietary or body fat).

(Erin Macfarland ) #2

Following!

(Solomom A) #3

Raised blood sugar is harmful to your body/organs. The dietary fat packaged in chylomicrons can wait without causing any immediate harm to you. Normal after meals blood sugars, if they are not lowered soon after, become harmful.

(Adam Kirby) #4

Yep, by the logic of “glucoseis the preferred fuel”, you may as well say alcohol is the REALLY preferred fuel, since that must be burned even before glucose! Fuels are burned off in reverse order of toxicity/storage capacity.

(John) #5

Path of least resistance I think is the majority of it. I don’t know that I would say it is preferred just because it would use it first if it had both. Fat has to be broken down to be used, glucose can be used more or less directly, it makes much more sense to quickly use carbs while they are ‘free’ than it does to go through the process of turning them to fat, then breaking them back out again to burn. The body is just being efficient and using it in its raw form and not storing it.

Another point is: do they prefer it? When you are fat adapted, and doing some aerobic work (link to FASTER study which has a lot of info) you can see below that people that are fat adapted have much higher fat oxidation levels.

Even the chart on the left (HCD=High Carb Diet) shows ~50% of energy coming from fat during the duration of a 3 hour endurance workout, if carbs were truly the preferred source then would we not expect near 100% carb oxidation during the first 30 minutes to 1 hour and then all fat after that? The low carb folks use ~90% fat during their 3 hour workout, that is definitely not a glucose preference.
They also did muscle biopsies after the workout and showed similar levels of glycogen remaining, so the LCD group didn’t burn it because the had to, they did because they have become adapted to using fat efficiently as a fuel source.

When you look at anaerobic workouts you see that carbohydrates are used almost 100% which is why people tire out quickly. This goes back to the earlier discussion about efficiency, during a sprint there isn’t enough time to convert fat fast enough, so it uses the glucose it already has stored in the muscle.
Edit to add one piece to this. Glycogen is stored in the liver and muscle, and is ‘preferred’ especially for high intensity activity, but if you are keto and don’t eat carbs then where does that glycogen come from? As you probably know glucose is a product of fat metabolism, so as we burn fat we slice off some fatty acids and turn the rest to glycogen to store in the muscle. So while technically the material you are using to make that muscle work in high intensity exercise is a carb, it came from metabolizing fat earlier in the day.

(KetoCowboy) #6

I think Adam nailed it. The more toxic an energy source is, the more eager the body is to burn it off.

#7

Speaking to efficiency: Why would the body break down fat to fuel with ketones and simultaneously store the glucose as fat? It would be like getting a cash payment, putting it in the bank, and withdrawing an equivalent amount of cash to spend. Why not use the cash in hand without going through an extravagant and superfluous shell game. Admittedly, not a perfect analogy.

(Siobhan) #8

“Preferred” as in “does best on” not “uses first”.
As in I spend change/pennies first over using my debit card but that is because pennies are a pain to keep around. In that sense my card is preferred because I can carry more money with me in a smaller space and I get benefits for using it. So it us preferred even though I spend change preferentially.

Fat is the preferred fuel in that all conditions being the same your body does better, and experiences less risk, wgen using it compared to glucose.

However, glucose is toxic to keep in the blood, and there is limited space to store it (muscle, water weight/glycogen, before being stored in the eyes etc) so you dont have many places to safely put it but you can’t leave it in the blood either so it is used for energy preferentially.
Fat on the other hand can be stored comparatively safely and indefinitely (to a point) so even though it is preferred (safer/cleaner/lower risk) it will be put on the back burner because it is not as dangerous to put away for later.

#9

You should ask what is fat? Fat is your bodies resource during fast or famine. It’s our stable body resource when hunting for the next meal. It has to be preserved during times of plenty or the reason for fat doesn’t make sense. Our modern world creates an unnatural constant period of plenty and our bodies are saving for a period of fast that never comes.

Quick energy is always “preferred” to ensure long term survival. It doesn’t make it better energy.