How do we use fat for energy without the glyoxylate cycle?

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Monique
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Maybe a really stupid question, but what do we do with fat besides insulation and making of membranes?

We can't metabolize fatty acids into carbohydrates for energy, since we lack the glyoxylate cycle right?

But people try to burn fat when doing exercise.. what mechanism do they use there?
 
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Yes, beta-oxidation in peroxisomes and mitochondria to produce Acetyl CoA.

A MD PhD once told me we can't use fat as a source of energy, I am trying to think why.. *thinks deeply*

I must've misremembered, I guess he said can't use fat to make proteins
 
Completely quit intaking carbs and protein and you WILL burn fat. Do cardio for 15-20 minutes and you WILL burn fat for energy. Call it what you want to call it, but you will survive and you will be fine. Of course, your fat storage will start being depleted after ketosis kicks in, but that is the goal of some dieters. Of course, protein (muscle) will be burned first - but you will also metabolize the fat.

Nautica
 
Well, not that I WANT to burn any fat

The story went like this: the prof asked us, where do you get your energy from. We said: carbohydrates. He asked: where do you get carbohydrates from. We said: fat, which was the wrong answer :)
 
I can think of only only one example where fat wouldn't give you the energy you need: Explosive movements that use up very high amounts of energy ina very short time (i.e. a few seconds). Sprinters derive their energy almost exclusively from ATP, GTP and maybe some carbs.
 
When you say ATP. Are you saying that fat can not be converted to ATP?

Nautica
 
No, of course ATP is produced both from fats and carbohydrates. But fat metabolism plays no role to speak of *during* a sprint because it's a slow process and needs time to kick in.
 
I remember seeing a rough breakdown of how your ATP supply will be used up in however many seconds/minutes of extreme exertion, and thereafter your Glycogen is broken down into glucose as your primary energy source and that will last however many minutes/hours ( I can't really remember, perhaps I should look it up), and after that it goes into your fat supplies.

But for daily energy supplies, Glycogen (essentially many glucose molecules connected in a particular branched structure) is the essential energy supplier.

I do recall one fact about fat which may be what you are getting confused with though Monique: Apparently our brain cannot use fat for energy.
 
"But for daily energy supplies, Glycogen (essentially many glucose molecules connected in a particular branched structure) is the essential energy supplier."

Depends.

Your basal metabolic rate is actually mainly (90% +) covered by fat metabolism. At night or when you just sit there doing nothing, fats provide you with most of the energy you need.

The higher your activity level, the larger the share of oxidation of glucose (citric acid cycle / Krebs cycle) becomes (glucose, in turn, is regenerated through glycogen breakdown).

When you cross the anearobic threshold, not all glucose that is needed can be oxadized by the Krebs cycle and the less efficient breakdown of glucose into lactic acid through glycolysis kicks into regenerat the depleted GTP and ATP stores.

The amount of ATP limits your capacity for muscular work at any given time; you just can't move faster or hold a resistance longer than the ATP supply permits.

To insure the best possible delivery of glucose to muscle cells, the body will shut down organ systems that are not needed, like the digestive system (which is not as badly needed in critical situations; that's why you "**** your pants" when a sudden danger arises and the body shifts into flight & escape mode). That frees more blood and nutrients for muscular work.

Also, when blood glucose levels drop low, the body converts amino acids to glucose through gluconeogenesis in addition to the breakdown of glycogen, and starts beta oxidation of fats (the latter play the most important role in endurance activities; marathon runners rely almost exclusively on it). Also, enzymes that break down muscle tissue for energy (aa's --> gluconeogenesis) are produced.

Cortisol, adrenaline and noradrenaline are the most important hormones that switch the body's functions from rest to alertness / flight / work.
 
Originally posted by Adrian
No, of course ATP is produced both from fats and carbohydrates. But fat metabolism plays no role to speak of *during* a sprint because it's a slow process and needs time to kick in.


Agreed, just making sure I understood what you said.

Nautica
 
Originally posted by Another God
I remember seeing a rough breakdown of how your ATP supply will be used up in however many seconds/minutes of extreme exertion, and thereafter your Glycogen is broken down into glucose as your primary energy source and that will last however many minutes/hours ( I can't really remember, perhaps I should look it up), and after that it goes into your fat supplies.

But for daily energy supplies, Glycogen (essentially many glucose molecules connected in a particular branched structure) is the essential energy supplier.

I do recall one fact about fat which may be what you are getting confused with though Monique: Apparently our brain cannot use fat for energy.

Like I stated earlier, it takes about 15 to 20 minutes to kick into fat burning mode (which happens after depletion of glycogen levels)

Nautica
 
"I do recall one fact about fat which may be what you are getting confused with though Monique: Apparently our brain cannot use fat for energy. "

Actually there is a possibility for the brain to use fats. Normally it doesnt, and fatty acids cannot cross the blood/brain barrier. But when the body is starving, it enters a state called ketosis. Keto bodies, which are derived from fatty acids and are basically 2 Acetyl-CoA linked with a covalent bond, accumalate in the bloodstream and are used for energy. And - these keto bodies can enter the brain.
 
Nice to have some experts on board :)

I too have seen the graph which represents the timeline in which what type of metabolism is taking place.. a while ago..

We are able to live on our glycogen supply for at least 12 hours before the supply in the liver etc run out, after which fatty acid metabolism has to take over (ofcourse during exercise this signal comes earlier), some time after that we start digesting our muscles.

Interesting fact is that there is a point of no return during starvation, where the body has digested itself (mainly the heart) irrepairably, if food was given, the person would still die :(
 
"Interesting fact is that there is a point of no return during starvation, where the body has digested itself (mainly the heart) irrepairably, if food was given, the person would still die :("

Remember that guy in London who spent several weeks w/o food in a glass cage last year? They said he had an irregular heart rate at the end, and that he would have to start eating again very slowly and carefully so that he doesn't die because of the energy expenditure for digestion...
 
Originally posted by Adrian
But when the body is starving, it enters a state called ketosis. Keto bodies, which are derived from fatty acids and are basically 2 Acetyl-CoA linked with a covalent bond, accumulate in the bloodstream and are used for energy.

Also, this is what happens when a person has type 1 diabetes and doesn't get their insulin shots. They starve to death. Glucose can't get into cells, so cells eventually start depleting fats for energy. Ketones are normal acidic products of fat breakdown. This alters the body's acid-base balance (acidosis), this imbalance disrupts brain function