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Fat preferred fuel over carbohydrates - Why? (Biochemically)

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  • Fat preferred fuel over carbohydrates - Why? (Biochemically)

    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!

  • #2
    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!

    i went primal in feb 2010
    200lbs
    37% body fat
    38-40 waist

    1 year to the day
    140lbs
    7.8% body fat
    29 waist

    currently
    137lbs
    7% body fat
    29 waist

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    • #3
      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.

      Comment


      • #4
        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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        • #5
          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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          • #6
            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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            • #7
              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...
              http://thorfalk.wordpress.com

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              • #8
                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



                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, 03:23 PM.

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                • #9
                  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.

                  Comment


                  • #10
                    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, 11:29 AM.

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                    • #11
                      I prefer to think of the macronutrient ratios of hunter-gatherers for inspiration. The body really can do fine on a wide variety of staple foods. There are pathways for both carbohydrate metabolism and fat metabolism and I think it prudent to train both pathways for efficiency. People eat everything they can find. Always have. Just stick to what they've been finding in the wild - and you're miles ahead of the rat race.

                      I'm a high-fat guy, but I'm also of European and Native American descent, with tendencies toward the autism spectrum - so ketosis is a happy place for me. That might not be true for everyone, so to assert "the body" has a "preferred" source is preposterous.
                      Crohn's, doing SCD

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                      • #12
                        Mitochondria prefer to burn fat. Most cells have mitochondria. There are no good glucose disposal pathways so overeating it causes health issues. There are only a couple things that are glucose or ketone dependent (brain, red blood cell...). So, eat enough glucose for those cells that need it plus however much more for your particular level of high intensity exercise and get the rest of energy from fat to prevent the problems that arise from too much glucose. Thats the short story.

                        Oh, and since you asked for links, websites, and books. The above is the predominant strategy that the "Perfect Health Diet" and "Primal" tend to agree on. The PHD is probably a bit higher in carb since they only count startches and fruits, while Primal is all inclusive of vegetables and fiber and such.
                        Last edited by Neckhammer; 06-12-2012, 01:41 PM.

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                        • #13
                          I don't think the body has a preferred fuel. I don't think the body cares what it runs on assuming it's real, whole foods. I don't think animal fat is necessarily a healthier fuel than carbohydrate from sweet potatoes. I can tell you with absolute certainty that I STRONGLY believe carbs from fruits and tubers are FAR healthier than soybean and/or canola oil! With anything, it comes down to the food source, and every study seems to show that indigenous cultures, regardless of macronutrient breakdown, seem to be relatively free of modern disease. Kitavans that eat huge amounts of carbs and very little fat and protein don't seem to be any less healthy than Inuits eating almost nothing but fat and protein with nary a vegetable in sight.

                          The real killers seem to be grains, sugars, legumes, PUFA-oils and the like - you know, fake frankenfood crap.

                          Mark seems to prefer fat for fuel because of the sedentary nature of the modern human. You know what, if you're sitting on your butt all day at your desk, you're probably better off eating a fattier/lower carb diet because it's a little more stressful to process carbs than animal-based fats. Using that logic, you may age less on a low carb diet if you're very sedentary. For someone active, I believe the opposite to be true. Active people tend to suffer and have elevated cortisol on needlessly low-carb diets. I'm a modest weight lifter, and if I go low-carb too long I feel lousy, stressed and tired. Carbs make me feel better and less stressed, so I eat them regularly and in large amounts when I get the urge. The amazing thing is we naturally seem to cry out for the fuel we want. Sedentary people generally feel tired and "crash" eating high carb diets while gym rats and athletes tend to feel like crap eating low-carb.

                          The real key is eating what makes YOU feel best. Pair your food to your energy levels and that's the key to success. Don't look at it in terms of macronutrients. If you're feeling sluggish, add some carbs. Reach for an apple, some berries, a banana, a sweet potato - whatever. If you're crashing after lunch at work, dial down the carbohydrate and eat fattier meat. Don't stress over health - a sweet potato is just as good as a steak in terms of health. Eat the one that makes you feel best. The only thing I stress is to make sure you're getting adequate protein. 1 pound of meat on average is ~100g of protein. IMO, most of us should be aiming for at least that, with smaller sedentary women being able to get away with the 75-100g range.

                          Remember - humans are omnivores. We eat meat, vegetables, fruits and tubers. It's all good. Stay away from the extremes and try to include everything in there in some quantity.
                          Last edited by ChocoTaco369; 06-12-2012, 01:16 PM.
                          Don't put your trust in anyone on this forum, including me. You are the key to your own success.

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                          • #14
                            Originally posted by mpurcell4u View Post
                            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.
                            This is a very dangerous stance to take.

                            The body does not require carbohydrate to survive. That is true - it can convert protein into glucose, and your brain can run on a mix of ketones and glucose (it can't run on pure ketones - it will always devour protein for some glucose in the absence of carbs).

                            Similarly, your body also doesn't need saturated fat to survive. It's plenty happy converting other sources into palmitic acid and will continue to do so - if you don't mind the inflammation.

                            You also don't need to take in dietary cholesterol. The body can make its own.

                            You won't find many people on this site saying saturated fat and dietary cholesterol are evil though, right? We praise the almighty egg and juicy steak around here. So why do we demonize the poor misunderstood carbohydrate?

                            Don't fall into the above trap. You don't need carbs, and you don't need saturated fat or cholesterol. However, to function in optimal health and maximize performance, you should consume all these things.


                            If you're happy forcing your body to devour your lean muscle mass to fuel a glucose-starved brain, that's up to you. I can tell you from experience my strength, mood and physical performance is much higher NOT needlessly starving my body of a macronutrient. Feel free to experiment on yourself.
                            Don't put your trust in anyone on this forum, including me. You are the key to your own success.

                            Comment


                            • #15
                              I've a few questions.

                              1.
                              Originally posted by Jaysond
                              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 all VO2 Max tests give the energy source breakdown, or might some just tell you the overall oxygen consumption?

                              2. If one stayed low carb, does the body maintain a certain amount of muscle glycogen storage through glyconeogenesis, or are the muscles left almost completely "empty"?

                              3. Can glycogen from the liver be moved quickly (or at all) into the blood stream for muscle work in the absense of sufficient muscle glycogen?

                              4. Is there a good generalization about when gluconeogensis would use dietary protein, rather than muscle mass to do its thing?
                              Would most (gluconeogensis) occur after a protein meal and top up liver stores?

                              On a side note, my understanding was that "preferred fuel" means what the body will prioritize, not what it "likes" the best.
                              For example ethanol is burnt before carbohydrate. Technically it's "preferred" over carbs, it doesn't mean you should go on a high alcohol, low carb diet

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