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. all this is no big revelation is it? its really just common sense. don’t eat processed foods, sugar is bad for you etc.

    jan wrote on May 13th, 2012
  2. @jan

    to some extent it is common sense, as you say, just lay off the processed foods and sugar
    but wheat? that doesn’t seem like common sense to most people (that i’ve spoken to anyway)
    the majority of people, imo, consider wholewheat to be an important staple of the weatern diet, and to limit it or stop eating it altogether ‘does not compute’ in their minds

    baz wrote on May 14th, 2012
    • baz, I recently read that ancient wheats (as mentioned in religious texts and still can be found today) had twenty-something chromosomes but the stuff today has in the forties. Three of those were not selected for but came about anyway – they induce addiction, pleasure and weight gain.

      The majority does not make it right and the majority does not listen to the needs and reactions of the body as they did in even the recent past. That is where we are trying to travel on a primal diet.

      I found the sugar addiction the most difficult to overcome. It took me three years living in Borneo where high sugary foods were not easily available but within a year I was addicted again upon returning home.

      On a primal raw extremely low carb and 80% fat ketogenic diet, it took but a few months. As a sugar fueler for forty years, I was amazed how easy is was to shed my addictions on this path.

      mhikl wrote on February 4th, 2013
  3. I think you should make mention of the effect of carbs on people with diabetes and those with pre-diabetes or severe carbohydrate intolerance. I have severe carbohydrate intolerance and I keep my carbs at around 20g a day and never go over 30g. Going higher for me means weight gain, carb cravings and over eating and for Type 2 diabetics it would lead to unstable blood sugars.

    I am trying to eat primal but I have to do it very low carb. I restrict fruit to small amounts of berries and I do not eat root vegetables – e.g potatoes, carrots, swedes.

    Diane Smith wrote on May 31st, 2012
  4. Thank you for writing this. I get so tired of people spreading the misinformation about carbs. I’ve been overweight for most of my life. Limiting my carbs has been the only thing that has helped me lose weight and pretty much keep that weight off. On top of that, my headaches, heart palpitations, and depression have gone away. Even nutritionist keep this lie going which I don’t understand!

    Elizabeth wrote on June 5th, 2012
  5. I’m so glad I found you.

    Beki Amador wrote on June 14th, 2012
  6. This is a bit of a late read for me and a late reply to make this comment but there are a few things i wanna say:

    Firstly i don’t see any referencing or any sort of indication that this is anything more than what could possibly be someone playing a practical joke on the chubbier people in the community.

    Secondly, i do like this concept and being very interested in nutrition myself find that it does add up in some unseen ways and open my eyes a little bit as to why we rely on glycogen despite such minute stores in the body when we can retain far more fat for fuel.

    Thirdly, i feel that this is dangerous for a “diet” without exercise as people may begin to look that little bit thinner as in their waist size drops a little and yeah sure, their blood sugar levels will inevitably decrease but their body fat percentage will significantly rise if caution isn’t taken leading to CHD’s, high blood pressure and never achieving that six pack or dropping that “cellulite look”.

    Finally, if you are still working on such a thing, please contact me through e-mail as i am very interested in the article and am currently working on a very extensive database for both nutrition and exercise in relation to BMR and making this work as effectively and easily for any individual wanting to use it.

    I have used the USDA database, am currently importing thousands of exercises to try and incorporate everything i can into it from calories burned turning a page in a book to calories burned when eating a sandwich. My database already includes converters, the food database, individual BMR and a weight loss/gain/maintain system. I am putting in an individual RDA for every possible ingredient in food from Ash to Sodium to make it the most extensive and useful system to date.

    If you would like to contact me about this, i would be more than happy to work with you over the internet to include this “Primal lifestyle” as an option within this database and therefore provide more testimonials for creating the awareness that you are so sure of.

    As priorly mentioned, i have not considered this before and have therefore not researched it which is why i have yet to say if i agree or disagree with it however, testimonials are proving to be a strong case.

    Please contact me on

    Just as an FYI, i have a keen interest in nutrition and have been working towards my degree in sports science since i could with an aim to go into Personal Training, with my interests being in Nutrition i do like to be clued up about it hence the comments and my database.

    Thank you

    Ben Wadeley wrote on June 22nd, 2012
    • Ben W, sounds like a lot of work and it has been done to death the past sixty years. Conventional means haven’t worked so far, so what ever makes you think it will with your efforts. I don’t mean to be nasty; the world has only gotten fatter and sicker following modern paradigms.

      The PB by Mark is more than just this one article and if you spend time reading much of his efforts before judging, then you will have a better education than trying the research path you are presently on.

      Conventional ways to health have not worked over the past sixty or more years. Thinking outside the box is hard for some to do but in these times of cancer, obesity and tired health, it is become the only thing to do when convention means have been disproven by their own failure.

      mhikl wrote on February 4th, 2013
  7. Great informative post if your information was only 50% correct.

    There is more than enough scientific research out there to suggest what you are professing is probably wrong and probably dangerous.

    I agree the sugars, manufactured fructose, over consumed fats/oils and processed food (dangerous mix of everything that is poor in nutrition) could be very dangerous to our health but ‘carbs’ are not the issue when they are from natural plant forms i.e. the potato. The potato is a complete natural package of carbs, protein, fat, minerals and vitamins and any other nutrient deficiencies can be added by eating vegetables, fruits, nuts and seeds.

    What we are missing is that the better the nutrient rich food we eat then the better we are going to be in terms of heath, vitality and maintaining a true weight to support our own individual requirements.

    Forcing our body to run in a glucose deficient state only replicates very sparse times in our evolution and the glucose deficient state of turning fat to energy is a safety net to get you through lean times rather than a way of maintain true function.

    wayne wrote on July 5th, 2012
    • Wayne, conventional research says the only answer to cancer is cutting, burning and poisoning and the first two have been used for over 110 years, the latter uses the same poisons used in exterminations in WWII concentration camps. And year by year the number of people getting cancer in their life times has risen from one in twenty (circa 1900), one in seven (1970) to greater than one in three today and the rate is growing. Fewer than one in three following the three conventions are alive five years after diagnosis. Pitiful.

      PB offers an alternative to the diet and health measures that have made the world fat and sick. The trend is only getting worse following conventional propaganda. And since you believe in conventional research as it now stands, studies are coming out that cancer feeds on sugar, carbohydrates, starch or what ever name you want to call the stuff. High carb foods demand minerals and vitamins for their assimilation and a conventional or SAD diet based upon the USDA pyramid even when fast food is limited (it is acceptable as a part of USDA recommendations), makes it impossible to get enough of these essential elements with out supplementation. On Primal, essential nutrients can be got from natural unprocessed foods high in animal fats, moderate animal protein and low plant based carbs.

      Remember that it is big pharma doing the research; neither the government nor the universities do meaningful research anymore and both are reliant upon big pharma for their monetary support. The fox is in charge of the hen house so thinking people are beginning to put less faith in the medical powers that have failed in their relentless quest to monetize illness.

      Scientific peer review has made it difficult to impossible for true new research and new ideas, (that do not meet the dogma of the day), getting published. We live in sorry times where science has been usurped by the interests of conventional thinking because of the money being made by old, failing norms.

      Less then one in ten can think outside the box. The modern Galileo, Copernicus or Semmelweis doesn’t stand a chance. Incredible that any thinking person would propose gluttony with sugars over measured nutritionally packed foods from animals and low carb plants.

      mhikl wrote on February 4th, 2013
      • >>And since you believe in conventional research as it now stands, studies are coming out that cancer feeds on sugar, carbohydrates, starch or what ever name you want to call the stuff.

        There are more ingredients needed for cancer to become a killer than just a food source. As a human I for one need water and oxygen as a priority!!

        >>Less then one in ten can think outside the box. The modern Galileo, Copernicus or Semmelweis doesn’t stand a chance. Incredible that any thinking person would propose gluttony with sugars over measured nutritionally packed foods from animals and low carb plants.

        There are modern day thinkers and experimentalist proving you are WRONG at this very moment.

        One is here

        Get out of the dogma and learn EVERY aspect to food!

        wayne wrote on February 9th, 2013
  8. This was great. I love that you mention Kuhn. It isn’t often I run across someone else that has read, let alone references his Structure of Scientific Revolutions.

    runnergerl wrote on July 10th, 2012

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