Marks Daily Apple
Serving up health and fitness insights (daily, of course) with a side of irreverence.
19 May

A Metabolic Paradigm Shift, or Why Fat is the Preferred Fuel for Human Metabolism

There’s a good reason so many people (mostly the sugar-burners, whose disparate group includes fruitarians, veg*ans, HEDers, body-builders, most MDs, the USDA and virtually every RD program in the country) can’t seem to grasp why a lower carb, Primal approach to eating is a better choice for health and fitness: their fundamental paradigm – the core theory that underpins everything else in that belief system – is flawed. They remain slaves to the antiquated notion that glucose is the king of fuels, so they live their lives in a fear of running low. The truth is, fat is the preferred fuel of human metabolism and has been for most of human evolution. Under normal human circumstances, we actually require only minimal amounts of glucose, most or all of which can be supplied by the liver as needed on a daily basis. The simple SAD fact that carbs/glucose are so readily available and cheap today doesn’t mean that we should depend on them as a primary source of fuel or revere them so highly. In fact, it is this blind allegiance to the “Carb Paradigm” that has driven so many of us to experience the vast array of metabolic problems that threaten to overwhelm our health care system.

It boggles my mind that such a large segment of the so-called health and fitness community would continue to defend high carbohydrate diets with such tenacity. It should all be very obvious by now. The studies keep piling up indicating that carbohydrate intake is the major variable in determining body composition and that excess glucose from carbohydrate intake (especially from processed grains and sugars) is the primary culprit in obesity and in many disease processes. It follows logically that if you can limit carb intake to a range of which is absolutely necessary (and even up to 50 grams a day over) and make the difference up with tasty fats and protein, you can literally reprogram your genes back to the evolutionary-based factory setting you had at birth – the setting that offered you the opportunity to start life as a truly efficient fat-burning organism and to continue to do so for the rest of your life as long as you send the right signals to your genes. Becoming an efficient fat-burner is the major premise of the Primal Blueprint eating and exercise strategies.

But logic doesn’t rule when you are stuck in the Carb Paradigm, so I still see some misguided bloggers decrying the Primal Blueprint eating strategy as potentially harmful for its relatively low carb intake or stating that my advice to “generally keep carbs under 150 grams a day unless you’re an athlete” is ridiculous. How many more times do I have to overhear a trainer advising a still-portly client to “eat 5 or 6 small meals throughout the day, always with some carbs, so you keep your blood sugar up and don’t go into starvation mode.”? It’s time to stop this nonsense and reframe the current views of human metabolism to accurately reflect the two and a half million years of evolution that shaped the current human genome – a perfect DNA recipe that fully expects us from birth to function largely on fats.

It’s time for a Metabolic Paradigm Shift within the health and fitness world.

The Faulty Carb Paradigm “Logic” Goes Something Like This

The basic underlying assumption is that glucose is the preferred fuel of most cells; BUT, because we can’t store very much glucose (as glycogen in liver and muscles), we need to provide a continuous source of glucose in the form of exogenous carbohydrate (high carb meals) to keep the brain, blood, and certain organs humming along and the muscles primed for activity. AND, if we don’t feed ourselves enough carbohydrate every few hours, our blood sugar will drop and we’ll go into “starvation mode” and cannibalize our precious muscle tissue. AND any lack of regular glucose refilling (i.e. skipping a meal or fasting) will cause cortisol to rise, which will have additional deleterious effects. FURTHERMORE, an excess of glucose in the bloodstream is known to raise insulin and will predispose excess calories (from all sources) to be stored as fat. THEREFORE, we should also be doing a lot of moderate-to-heavy cardio or lifting activity most days to burn off this excess stored body fat. HOWEVER, if we want to be ready and able to exercise frequently and strenuously to burn off our stored fat, we need to eat lots of complex carbohydrates between workouts to refill our glycogen stores. And ULTIMATELY, the only way to lose weight is to restrict calories (calories in<calories out), BUT if you’re working out regularly, it’s almost impossible to maintain a calorie-restricted regimen and still be able to work out hard enough to burn appreciable calories. Sheesh.

Sure, there are exceptions, like the driven and genetically gifted types, who can train long hours, refuel on carbs and not add much body fat (hey, I was one). But unless you love to work out incessantly and have really lucky familial genes, the Carb Paradigm is an unsustainable and ridiculous literal and figurative treadmill, a self-fulfilling prophecy for most people who tend to gain weight steadily and insidiously over the years and wonder why. If you are one of the 60+% of the American population who is overweight, the above scenario plays itself out because you have spent your life programming your genes in the direction of being an effective sugar burner and, as a result, have become dependent on a fresh supply of sugar (carbs) every few hours. Naturally, in the presence of all that glucose, and provided you actually do some exercise, your genes will eventually get the signals to up-regulate the enzyme systems, pathways and receptors involved in sugar-burning and fat storage and they’ll down-regulate all those involved in accessing and burning fat for energy. Of course, that doesn’t make it right, but it sure makes it appear as if glucose is king. What makes it worse, if you don’t exercise, you head down the path to insulin resistance and/or obesity.

The Problem: The Basic Assumption of the Carb Paradigm is Wrong

Glucose is not the preferred fuel of muscle cells under normal human resting metabolic conditions or even under most normal human movement patterns (exercise). Fat is. Sure, given an unlimited supply of glucose and regular refilling of glycogen stores, skeletal muscle will burn through it during exercise the same way a fire burns through kindling when that’s all you have to offer. The body can shift carbohydrate oxidation to keep up with intake. But skeletal muscle can burn fat with great efficiency (and far less oxidative fallout) at relatively high outputs for very long bouts. Cardiac muscle actually prefers ketones, and the brain can run just fine (maybe even optimally) on a blend of ketones and minimal glucose.  Our survival as a species has depended on these evolutionary adaptations away from glucose dependency. Entire civilizations have existed for ages on what is practically a zero-carb diet. Think about this: there is actually no requirement for any “essential dietary carbohydrates” in human nutrition. It’s possible to live a very long and healthy life never consuming much – if any – in the way of carbs, provided you get adequate dietary protein and fat. The same can’t be said for going too long without protein or fat. Cut too far back on either of those macronutrients and you will eventually get sick and die.

The Evolutionary Model

Fat and protein were the dominant macronutrients (when food was even available) over the majority of our two-and-a-half million years as evolving humans. The lack of regular access to food and a scarcity of carbohydrates for much of this time necessitated that we adapt efficient pathways to readily store and access body fat for energy if we were to survive day-to-day and generation-to-generation. Our movement patterns were such that we never required large amounts of glucose or that we needed to store very much glycogen. It was predominantly fats, ketones and the minimal infusion of glucose via gluconeogenesis that got us here. Dietary carbs were insignificant. In fact, when you consider how ridiculously small the body’s glycogen reservoirs are, you understand that it would have been impossible for us to survive as a species if glucose were truly the “preferred” fuel. The liver, the main back-up glycogen/glucose storage facility for the brain and other glucose-burning organs, can only store about 100 grams of glycogen. Less than a day’s worth. Your muscles can only hold another 350-500 grams, barely enough to run for 90 minutes at a reasonable clip, and that glycogen isn’t even available to provide fuel for the brain. Meanwhile, we have a virtually unlimited storage capacity for fat (like 100,000 grams or close to a million calories on some people). The reason glycogen storage wasn’t necessary is because, between our copious fat storage capability, easy access to fats as fuel, gluconeogenesis and ketones, we just didn’t need much. Evolution tends not to reward structures or functions that take up unnecessary space or waste energy.

So How Much Glucose Do You Really Need?

Much less than most people assume. At any one time, the total amount of glucose dissolved in the bloodstream of a healthy non-diabetic is equivalent to only a teaspoon (maybe 5 grams). Much more than that is toxic; much less than that and you pass out. That’s not much range for a so-called “preferred” fuel, is it? Several studies have shown that under normal low MET conditions (at rest or low-to mid- levels of activity such as walking and easy work) the body only needs about 5 grams of glucose an hour. And that’s for people who aren’t yet fat-adapted or keto-adapted. The brain is the major consumer of glucose, needing maybe 120 grams a day in people who aren’t yet on a low carb eating program. Low carb eating reduces the brain’s glucose requirements considerably, and those who are very low carb (VLC) and keto-adapted may only require about 30 grams of glucose per day to fuel the brain (and little-to-none to fuel the muscles at <75% max efforts). Twenty of those grams can come from glycerol (a byproduct of fat metabolism) and the balance from gluconeogenesis in the liver (which can actually make up to a whopping 150 grams a day if you haven’t metabolically damaged it with NAFLD through fructose overdosing). Bottom line, unless you are a physical laborer or are training (exercising) hard on a daily basis, once you become fat-adapted, you probably don’t ever need to consume more than 150 grams of dietary carbs – and you can probably thrive on far less. Many PBers do very well (including working out) on 30-70 grams a day.

The Fat Paradigm

The Fat Paradigm, under which the human species has thrived quite effectively for two and a half million years, recognizes that human metabolism is pre-programmed by evolution to be primarily fat-based (the real preferred fuel). In other words, our genes expect us to function optimally when we consume fats and can easily access our stored fat. The Fat Paradigm acknowledges that the body is able to manufacture adequate glucose as needed. It acknowledges that most typical human movement patterns can be fueled almost entirely by fats and/or ketones (PDF) if need be, but can draw on glycogen when energy bursts are required (and which can then be replaced over time). It acknowledges that fat (and cholesterol) are not the proximate cause of heart disease. It acknowledges that fat cells are designed to release stored fatty acids as required, especially during times of scarcity or fasting. It allows for intermittent fasting as a means of accelerating fat loss without sacrificing muscle tissue. It increases insulin sensitivity, modulates energy and mood swings, and allows for a normal and healthy drop in hunger and cravings. There is a downside, however: you can’t train long and hard day-in and day-out in the fat paradigm.

Now then, having explained all this, please understand that I am not carb phobic. I actually permit more carbs in the Primal Blueprint than many other low carb eating strategies. I prefer to view carbs as the “elective” macronutrient, as a tool to use to manipulate your glycogen levels as needed. Low carb isn’t even the main objective of eating in the PB: eliminating grains, sugars and seed oils are the primary objective. Of course, when you get rid of that crap and naturally limit your carb intake to veggies, root tubers and a few fruits, you almost invariably decrease carbs to under 150 grams a day. And that emulates our ancestral dietary intake.

I came up with a simple Carbohydrate Curve a few years ago that offers a pretty concise picture of where most people ought to fall if they are seeking optimum health and energy, depending on their size, weight, sex, age, goals, etc. Now, many hundreds of thousands of user experiences later, I am finding that the Curve is pretty much spot on for a large segment of the population.

When I say generally that a chronic intake of over 150 grams of carbs can lead to insidious weight gain over a lifetime, I am factoring in the concept that many people are at the effect of a familial genetic predisposition to storing fat easily under the carb paradigm (the 60+% overweight). I am also factoring in the drop in metabolism that happens naturally with age, as well as the fact that PBers don’t NEED to purge and refill glycogen stores every day via exercise. Yes, there are some people (a small percentage of outliers) who might maintain pretty decent body composition at up to 300 grams a day on little exercise. I would bet that they also are selective about the carb sources and do a better job of controlling overall calories, so there’s little excess to store. For most of the population, that 150 mark remains a good average level for maintaining ideal body composition.

Well, that was a lot to digest today. You see where I’m going with this. I need your help in showing the health community that their basic assumptions are wrong and that they need to make a Metabolic Paradigm Shift. I’m sure there will be lots of specific questions, so bring  ‘em on and I’ll do a follow up post in a week or two.

You want comments? We got comments:

Imagine you’re George Clooney. Take a moment to admire your grooming and wit. Okay, now imagine someone walks up to you and asks, “What’s your name?” You say, “I’m George Clooney.” Or maybe you say, “I’m the Clooninator!” You don’t say “I’m George of George Clooney Sells Movies Blog” and you certainly don’t say, “I’m Clooney Weight Loss Plan”. So while spam is technically meat, it ain’t anywhere near Primal. Please nickname yourself something your friends would call you.

  1. hi, just wondering how all this can be incorporated for people/myself with hypothyroidism? I used to be an easy 96lbs, even after two kids…Now 8 months post birth of my second child and on .75mcg synthroid, I can no longer eat like I used to. Even pulling back on my carb intake (one cheat day per week) and exercising x6 week, my weight keeps going up. I never had belly rolls before and that’s what I have begun to sport.

    amy wrote on September 6th, 2013
  2. So… If my half blood ancestors “yugoslav”, if they had a diet which was largely vegetables and meat was lesser availability. Relying on pulenta and other processed foods along with white bread as a side to every meal. Relying on these as staple foods that are fround at in this day and time “white bread for e.g. would the man

    mitch wrote on October 3rd, 2013
  3. So… If my half blood ancestors “yugoslav”, if they had a diet which was largely vegetables and meat was lesser availability. Relying on pulenta and other processed foods along with white bread as a side to every meal. Relying on these as staple foods that are fround at in this day and time “white bread for e.g. would the many generations of this style of high carb eating have an impact on the way my boddy handles these processed forms of carbs that have been the way of my ancestors?

    mitch wrote on October 3rd, 2013
  4. I workout on the weights for about an hour. I workout so hard, I’m drenching in sweat.

    I then go for a two-hour walk.

    I need the carbs to give me that energy during and after that exercise (both brain and muscles).

    (I also make sure to include plenty of lost minerals, vitamins and salt.)

    You need carbs for that energy.


    Then marathon runners would notice a difference.

    You don’t think they have tried fats?

    I have.

    It ain;t enough.

    Human evolution didn’t include what we are putting our bodies through, like working out on the weights, and other strenous exercise.

    Carbs are also cheap to eat, and they break down in the body faster.

    Fats also take longer to break down to give us that energy.

    If I was going all day, and not eating as much, then, yeah, the fats would come into play–which they are there for.

    But even then, I would need the carbs for added energy because fats can’t do it very well.

    Carbs also make us feel better (moods), among other benefits.

    Heck, right now I’m feeling a little drained, so I’m gong to go have a bowl of oatmeal and a glass of milk.

    Sorry, Mark, but you are wrong on this one.

    Perry wrote on November 22nd, 2013
  5. Orthodox thinking is a hard cycle to break, people will stick because it is wired into us as a safety mechanism. Great article! Thanks

    Ben wrote on December 16th, 2013
  6. Hi – I am just starting this diet and really want to be successful with it. Just wondering what you eat for a quick lunch and breakfast – I work, have to 2 young children and so do not have the time to prepare anything lavish for these meals?

    Many thanks

    Lisa Clapp wrote on January 22nd, 2014
  7. On time post here Mark. I have been shifting my daily intake of carbs and my clients as well to a very low level. I have seen the positive results in doing so, and will continue to use this in my regime. Love all the information you put out on your blog. Very informative. I am a happy reader.

    Thanks again,


    Ontarian Hawkins wrote on February 16th, 2014
  8. Mark,

    I’m enjoying all the benefits of the paleo journey so far. But I also enjoy reading formal research articles in peer-reviewed medical journals. Geeky I know.

    You mentioned in “A Metabolic Paradigm Shift . . .” that “studies keep piling up indicating that carbohydrate intake is the major variable in determining body composition . . ”

    I’m not trying to be obnoxious, but anyway you can refer to actual studies on that?


    Russ Yeager wrote on February 16th, 2014
  9. I’m very confused with all of this. I want to change over to primal eating habits to better my health and lose weight. I’m so confused as to what I should and shouldn’t eat, I still like some carbs therefore what carbs can I eat and be safe. There’s almost too much info out there for us to read. I’ve been reading for days now and still cannot sit down and put together a meal plan for myself. Not to mention my teenage daughter will not eat alot of vegetables so making two meal plans each week is tough. Thoughts? Help!

    Blondie wrote on June 12th, 2014
    • Safe carbs to eat would rate low on the Glycemic Index.

      An easy way to jumpstart low carb eating would be the Atkins diet. I lost over 30lbs in a few months and there are plenty books and websites that will lay out full meal plans. It’s not heavy on veggies and after you lose the weight you want, you can switch over to a full paleo diet to not gain the weight back.

      Aaron wrote on June 12th, 2014
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    jennifer wrote on August 10th, 2014
  11. Glucose is ESSENTIAL FOR LIFE.Carbs are the go to food.And with keto bodies your brain SUCK and subfunctions.Carbs are friends especially for those of us who are working hard.NEVER EVER EEEEEEEEEEEEEEEEEEEEEEVER a performance of muscular development is OPTIMAL with fat as a prime energy source.Sorry man you are making a tremendous mistake.

    Jotun Heim wrote on December 22nd, 2014
  12. I’m New to the PB, actually coming across this by accident as I’m a holistic/sports nutrition student. I have to agree with the Gov guidelines being a little to much in regards to carb consumptions advocating 200+

    I personally have found even as a weight lifter and future fitness competitor lifting 6x a week I cannot handle 130grams a carbs every single day I tend to get hypoglycemic episodes despite being active. A very good coach suggested cycling my carb intake and it has def helped my situation as far as weight management and keeping my blood sugars stable.

    Great article, I look forward to going over some more of your work.

    Nicole wrote on January 2nd, 2015
  13. I eat white rice and potatoes EVERYDAY, however, I don’t eat any sugar.
    No matter how rice and potatoes I eat, I don’t gain weight. It is a fallacy to say white rice and other starches cause weight gain. In fact, it is fats and sugars that cause weight gain.
    Try it yourself.

    Donna Miller wrote on January 9th, 2015
  14. Exercise is needed but if you are using exercise as means to lose or maintain weight, then you are on the wrong diet.
    When eating the right foods, you’ll never gain weight.
    Stay away from dairy, sugar, and fat. Limit meat consumption (a little goes a long way) but eat some protein with every meal.
    Rice and potatoes won’t make you fat, however, the sugar and cream in your coffee will. the dessert you eat will. The butter and oils you cook with will. Dairy has a lot of fat as well, so avoid it. Stay away from anything sweet, including fruit and fruit juice. And don’t eat anything that will irritate your stomach.
    Being healthy is eating foods that (for you) won’t cause inflammation (low grade inflammation or otherwise).
    (this information is from Christopher MacDonald: author.

    Donna Miller wrote on January 9th, 2015
  15. Your article is very good for normal people. I tried some of those low carb diets when I was young and within a couple of days I would turn stark white and be barely able to function. That isn’t to say that I did well with too many sweets either. i once just gave up sugar and sweets, like desert type things and that worked great. I felt wonderful and actually lost a little weight without even dieting.

    Okay, skip to the present. I now well into my senior years and have become obese and have other health problems. Since doctors don’t really seem to be much help I have gotten into supplements and found some which really help my particular health problems, one of which is that I was cold all of the time. They give me a very low dose of thyroid which is just enough to barely keep me in the normal range and tell me that is enough. It probably isn’t and i need to find another doctor who will correct that problem, but part of the reason that I was cold all of the time is that my metabolism didn’t do very well at burning fat, especially now that I am old. When I was young it didn’t seem to be a problem, except when I would try something like the Atkins Diet. I was healthy, active and my weight ranged from normal weight to 10 or 20 excess pounds. I had to work pretty hard to keep slim, but I managed and all and all my body worked great.

    Well, anyway,some of the supplements that I am taking which help to keep me warm change my metabolism into a fat burner. I finally noticed that I didn’t crave sweets much but I sometimes craved fats. Fortunately, I have never liked junk kind of fats, like deep fried foods or anything like that, but I do like my butter and olive oil spread and I have noticed that I probably eat more fats than I used to. I guess the point of all of this is that some people may not be able to be fat burners, rather than carbohydrate burners, even though it works for most people.

    For me high doses of Alpha Lipoic Acid, Acetyl L-Carnitine and CoQ10 seem to have enabled me to be a fat burner, at least I believe these are the ones that are doing it. I also take other supplements, so I can’t be sure exactly which ones are responsible for this change in my metabolism, but you need to be aware that eating the way that you reccomend may not work for everyone if they have health or metabolic problems. You might do some research and at least suggest the proper supplements to adjust their metabolism if eating that way is a problem for some of your readers. I really remember how sick those kind of diets made me when I was a healthy young person and not only did I feel terrible, but people could just look at me and tell that I was really sick. Be careful what you recommend.

    Kathy457 wrote on February 28th, 2015
  16. So how much athlete should eat daily carbohydrates in your opinion?

    i mean an endurance athlete like playing soccer .

    Jacek wrote on March 26th, 2015
  17. I recently saw a video presentation by Nora Gedgaudas which totally agrees with this assessment- take a look:

    Doug Brunelle wrote on April 4th, 2015
  18. Over the last 20-25 years we have been repeatedly told to cut back on fat and increase fibre (usually found in bread, cereals, etc). Over the past 20-25 years we have seen an increase in the number of fat people. Is the connection really that simple?

    (I used to think the fat explosion had been caused by everyone quitting cigarettes and needing something else to do with their mouths – like eat biscuits!)

    Ciaran Guilfoyle wrote on April 15th, 2015
  19. Great post Mark!!!

    I would also add that it’s important to consider the other functions fats serve in the body beyond providing energy, like the absorption and storage of fat-soluble nutrients like Vitamins A, D, E, and K.

    Keep spreading the good word! We must hope and trust that good science and positive real-world results will win out in the end.

    Mark Pompeo wrote on July 14th, 2015
  20. Sources, please.

    Lauren wrote on September 2nd, 2015
  21. LOL! So if I get two visitors, and one is really irritating so I get rid of it as soon as possible, and the other is kinda nice and I keep it around longer, you think I prefer the first? That’s backwards.

    BTW, one way to irrefutably know that someone is irrationally biased is if they want those who disagree to STFU.

    Bill DeWitt wrote on June 18th, 2011
  22. Thank you, Lyle. It’s good to see someone else out there who has some sense (or education).

    And by the way, everyone, your anecdotal stories are not evidence for anything.

    Seth wrote on June 18th, 2011
  23. My husband and I do competition hiking on difficult trails.
    I’m half the person he is, he is twice as tall with long legs.
    He is on a high carb diet of grains, I’m primal, running on fat as fuel.

    He runs away from me on trails, most of the time even wins the race because of his size but with a high price of exhaustion, cravings, and several days, up to a week to recover from this race, with muscle/joint pain.
    I, on the other hand, feel NOTHING after the race is done and could just keep going, or repeat the entire race the very next day. I don’t even have to eat after the race, while he chows down on sweets right after it’s over. I have 0 down time.

    Tell me which fuel is the one that would make Grok survive???

    Primal Palate wrote on July 10th, 2011
  24. Mark, you just need to shut the fuck up so hard.

    If you just cover your ears and your eyes, then he won’t be there any more, and you don’t have to be afwaid of the big bad caveman any more!

    Mike Ellwood wrote on July 20th, 2011
  25. Primal, you might like the book “Born to Run” by Christopher McDougall, though the Tarahumara do not seem to eat a diet based on high animal products but do eat in moderation and are not the usual sugar runners or fat in size.

    mhikl wrote on February 4th, 2013
  26. the idea of living was to search for food and eat it.

    even if you run after an animal you would at some point stop to eat it.

    at some point after your running just like your husband you will have to stop and eat.

    What is the difference, apart from the fact he is a winner and you are not!

    wayne wrote on February 9th, 2013

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