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Thread: Fat preferred fuel over carbohydrates - Why? (Biochemically) page

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    PCwizCube's Avatar
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    Fat preferred fuel over carbohydrates - Why? (Biochemically)

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    Hey everybody! This is my first post, and I've been enjoying reading and learning a lot on these forums.

    Recently, I read Mark's post A Metabolic Paradigm Shift, or Why Fat is the Preferred Fuel for Human Metabolism and this sparked some curiosity as in why human metabolism prefers fat, biochemically. Now don't get me wrong, I totally believe that fat is the preferred fuel, as I have been on the main site and these forums for a few months now and I am totally convinced that this is the best way to live.

    I was interested, though, when Mark said that the basic assumption that carbs is the prefferred fuel is wrong. Of course, when we think about it in evolutionary terms, it all makes sense that fat is the preferred fuel. But I want to tell this to other people too, but I don't think I could convince them very well. For a lot of people, just backing up your claim with "that's how we ate thousands of years ago" doesn't seem to be a very strong argument towards the assumption that carbs in the preferred fuel that is believed all across the world. Just stating evolution as your reason also might be insufficient to convince the more educated who would argue about the biochemics behind supporting carbs.

    After all, the assumption that carbs is the main fuel for humans makes sense, at least what I've learned at the biochemical level. In my basic High School biology class I learned that the main source of fuel for cell respiration was glucose, which would allow the cell to make ATP and then use that for fuel in the cell. Glucose is carbs, and since virtually every cell in our body can use glucose for energy, it makes a lot of sense to assume carbs is the main source of fuel for our body. So I'd like to know, for both curiosity and for being able to argue with people, why then, are fats the preferred fuel for us, and not carbs, because glucose is key to cell respiration? Is it because fats actually play a huge role to energy for cells that we've all just never heard about? Or do ketones and gluconeogenesis play a larger role? And why would the "carbs is the main source of fuel" assumption be false if our cells use glucose in cell respiration to make energy?

    If anyone could explain these things to me, or tell me something I might not know, or refer to some websites and/or books that could explain this more, it would be very appreciated. Thanks!

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    honestly i can see this post turning into a crazy but great war! LOL there is a test you can take, its called a vo2 max test. it will tell you if your body likes carbs or healthly fat for fuel. do an internet search and find a location. my body likes fat over carbs, period. yours might not!

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    Both glucose and fatty acids can be used for cellular respiration (energy production). Glucose can be used for both aerobic (oxygen-using, slow) and anaerobic (oxygen-deprived, fast) respiration, while fatty acids can only be used for aerobic respiration. Anaerobic respiration is required for high-intensity efforts such as lifting heavy weights, sprinting, etc, this means using glucose. Slow, steady-state activities such as walking, breathing, etc can be fueled by aerobic respiration and therefore either glucose or fatty acids.

    The problem with using glucose as your main fuel is that glucose is toxic in large amounts, and the body can only store so much. The liver can store about 100g (400 calories) of glycogen (animal starch) -- that's not a lot. The muscles can store about 300g more, but once in the muscle it doesn't get out so if you're not using those muscles it's "wasted". So to be fueled primarily by glucose without poisoning yourself you have to eat often, which is annoying and causes frequent blood sugar spikes. On the other hand, we have practically unlimited stores of fat in the body, and each pound of body fat contains about 3500 calories. So if the majority of your activity is aerobic (that's most of us) we can fuel it with fat for no loss of performance with the benefit of stable blood sugars and energy levels.

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    Exactly what AndreaReina said.

    I'll also add that it's an easy and understandable mistake to make thinking that glucose is the preferred fuel. After all, if you're body has both fat and glucose available in large amounts, then it will choose to use the glucose first. This makes it appear as though your body likes glucose better (prefers it in some way). In reality, your body is just trying to get rid of the sugar as fast as it can. Like Andrea said, glucose is toxic. Hyperglycemia (high blood sugar) is very dangerous. Your body isn't burning sugar first because it likes it better as fuel, your body is trying to dispose of it and get levels back down to a healthy range.

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    PCwizCube's Avatar
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    Thanks for the info guys! Makes a lot of sense

    So AndreaReina, if I understood you correctly, you said the body can undergo aerobic respiration two ways, one with glucose as the main fuel and one as fat as the main fuel? Does that mean they are two entirely separate processes? Because in my biology class, I learned the process of aerobic respiration as an "add on" to anaerobic respiration. Like in anaerobic respiration, the cell uses glucose through glycolysis to make energy, and then for aerobic respiration the mitochondria uses some of the pyruvic acid and some other stuff made during glycoysis to make more energy with the help of oxygen (via the krebs cycle and the electron transport chain). So if there are two entirely separate processes for aerobic respiration, would the one I described be the one using glucose (indirectly) as the main fuel? And I guess there would be another entirely different aerobic respiration process that I've never heard about that uses fatty acids as the main fuel?

    Sorry for being so picky with the biochemical details, but it's just very interesting. Also I'd like to prove people wrong like my Biology teacher who actually said to my class last year that humans have a hard time metabolizing fat and therefore that is why carbohydrates are the main source of fuel.

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    ciep's Avatar
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    PC, this may be useful...

    https://highered.mcgraw-hill.com/sit...ng12e_ch06.pdf

    ...scroll down to page 14 (of 18). Check out the bit titled "Fat Respiration".

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    Quote Originally Posted by PCwizCube View Post
    Thanks for the info guys! Makes a lot of sense

    Also I'd like to prove people wrong like my Biology teacher who actually said to my class last year that humans have a hard time metabolizing fat and therefore that is why carbohydrates are the main source of fuel.
    Forget it. Once you start talking about thing like "preferred fuel" the discussion will be endless, mainly because the adjective "preferred" is very differently interpreted by different people. If you want to have this argument, buy yourself a textbook and understand the details, then read loads of blogs, then read the textbook again. Then discuss with your teacher.

    Otherwise you will just be what I'd call "aggressively agreeing" - you see it on the forums all the time: people have really 95% the same opinion, but because they word it differently the argument goes on and on...

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    Quote Originally Posted by ciep View Post
    PC, this may be useful...

    https://highered.mcgraw-hill.com/sit...ng12e_ch06.pdf

    ...scroll down to page 14 (of 18). Check out the bit titled "Fat Respiration".
    Oh wow, thanks a lot! Yeah I was also wondering where acetyl coenzyme A came from, and whether fat played a big role in the the substances actually used in the Krebs cycle. So basically, the glycerol and fatty acids from the triglycerides eventually break down in to the substances used in the Krebs cycle that produces some ATP and then eventually leads to the oxidative phosphorolation in the electron transport chain. And even though the krebs cycle and ETC can only happen after glycolysis occurs, most of the ATP comes from the substances used in the Krebs cycle (from fat) and the potential energy of the electrons in the ETS, not from glucose.

    Thanks a lot for giving me the site! It answered all of my questions



    Quote Originally Posted by Thor Falk View Post
    Forget it. Once you start talking about thing like "preferred fuel" the discussion will be endless, mainly because the adjective "preferred" is very differently interpreted by different people. If you want to have this argument, buy yourself a textbook and understand the details, then read loads of blogs, then read the textbook again. Then discuss with your teacher.

    Otherwise you will just be what I'd call "aggressively agreeing" - you see it on the forums all the time: people have really 95% the same opinion, but because they word it differently the argument goes on and on...
    Yes, that's a good idea. I will actually probably keep most of this to myself until I research a lot more and have a lot more experience with primal living. Thanks for the advice!
    Last edited by PCwizCube; 05-25-2011 at 03:23 PM.

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    Gluconeogenesis

    The liver produces enough glucose for the body without the need of any dietary carbs through a process called Gluconeogenesis. Simply put, it coverts proteins and fatty acids into glucose, aka carbs.

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    Quote Originally Posted by PCwizCube View Post
    Also I'd like to prove people wrong like my Biology teacher who actually said to my class last year that humans have a hard time metabolizing fat and therefore that is why carbohydrates are the main source of fuel.
    You won't prove anyone wrong with words. Your best argument is your own body. Until you have experienced benefits I would avoid arguing about it anyway.. but if you have use that. Use it as motivation to push harder in whatever training you might do so you have physical evidence to back it up.. maybe roll off some blood test results too! Until then.. it is just something you "read on the internet".
    Last edited by statikcat; 06-12-2012 at 11:29 AM.

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