Nutrient Metabolism Overview

This article actually represents the entirety of Chapter 4 of the Revised Rapid Fat Loss Handbook.

In this chapter, I want to give readers a very brief and simplified overview of human metabolism and nutrient use. Which, for those who know a lot about the topic will realize, is an understatement of vast proportion. The complexities of human metabolism can and do fill up hundreds of pages in physiology books and this chapter should be taken with that in mind.

The Basics: Energy and Building Blocks

Very simplistically speaking, we can divide the uses of the nutrients (discussed last chapter) into three categories, of which I only really want to talk about two. One category, which I won’t discuss much has to do with the vitamins and minerals which both act, essentially, as nuts and bolts in the body. They fulfill any number of different roles; depending on which one you’re talking about. While critical to human health, they simply aren’t that important to the topic of this book. If you’re interested, go get yourself a book on vitamins and minerals and go to town. All I’m going to say is that you should make an effort to ensure your vitamin and mineral intake. A basic one-per-day multivitamin should probably be good “nutritional insurance” for everyone, the obsessed can look at versions containing mega-doses or what have you of the different nutrients.

The second category is for use as building blocks. Most parts of the human body are in a constant state of breakdown and buildup and nutrients must come in to the body to provide building blocks for those processes. One I imagine all readers are familiar with is that of calcium (a mineral) being the building block for bones. Additionally, skeletal muscle, organs and many hormones have amino acids (coming from protein) as their building blocks. As well, both fats and cholesterol play a role as a building block for cell membranes and a few other substances in the body.

The third category, and the one I’ll spend the most time on in this chapter, is as an energy (fuel) source. Even as you sit reading this and growing bored, your body is using energy at some rate. So your brain, your heart and other organs, skeletal muscle, liver and even your fat cells are using energy, although the rates at which each uses energy varies from high (e.g. brain, liver) to extremely low (e.g. fat cells). Surprisingly and quite contrary to common belief, at rest skeletal muscle doesn’t burn that many calories. The idea that adding muscle mass will turn you into a calorie burning inferno is simply incorrect.

Where Does the Energy Come From?

So where does that energy come from? At the lowest level of cellular function, the only form of energy that your cells can use directly is something called adenosine triphosphate (ATP). I doubt that factoid is very helpful to readers except perhaps as the answer to a Trivial Pursuit or game show question. If you happen to sit around having polite conversation about ATP, please send me an email: I want to hang out with you.

Of more use to us, the body generates ATP from the burning (oxidation or combustion to use a more sciency term) of either glucose from carbohydrate or fatty acids from fats. Under specific circumstances protein can be used to produce ATP, either directly or via the conversion to either glucose or fat (usually protein is converted to glucose to be used for fuel). I’ll come back to this below.

With a few exceptions that I’ll talk about in a second, every tissue in your body can use either carbohydrate or fat for fuel. What determines which they use? For the most part, it’s the availability of carbohydrates: when carbs are available (because you’re eating plenty of them), those tissues will use carbohydrates, in the form of glucose, for fuel. When carbs are not available (because you’re restricting them), the body will switch to using fat for fuel. That fat can either come from your diet or from the fat stored on your butt or stomach. This has another implication that is often forgotten in weight/fat reduction programs: when you eat more carbohydrates, your body uses less fat for energy; when you eat less carbohydrates, your body uses more fat for energy.

So what about those exceptions? A few tissues in your body such as the brain/central nervous system and one or two others can’t use fatty acids for fuel; they can only use glucose. The brain is the main one I want to talk about here. It’s usually (and incorrectly) stated that the brain can only use glucose for fuel, and this is true if you only consider glucose, amino acids, and fat as potential fuel sources. But this leaves out a fourth, extremely important, fuel source: ketones (also known as ketone bodies). Ketones are made from the breakdown of fat in the liver and function as a fat-derived fuel for the brain during periods of starvation/carbohydrate restriction.

I’ll talk about starvation in more detail in a second but I want to mention that, after a few weeks in ketosis (a state where ketones build up in the bloodstream such that fuels such as the brain start using them for energy), the brain can derive 75% of its total energy from ketone metabolism. The other 25% comes from glucose.

So Aren’t Carbohydrates Essential?

At this point you may be slightly confused about the role of carbohydrates in the diet. In the last chapter, I stated that carbohydrates weren’t an essential nutrient and above I mentioned that a few tissues can only use glucose and that even the brain gets about 25% of its total fuel requirements from glucose after adaptation to ketosis. So if those tissues still require glucose for energy, you may be wondering how carbohydrates aren’t essential in the diet. Remember from the last chapter what the two requirements of an essential nutrient are

  • That nutrient is required for the proper function of the body.
  • The body can’t make that nutrient in sufficient quantities.

The second criterion is the reason that dietary carbohydrate is not an essential nutrient: the body is able to make as much glucose as the brain and the few other tissues need on a day-to-day basis. I should mention that the body is not able to provide sufficient carbohydrate to fuel high intensity exercise such as sprinting or weight training and carbs might be considered essential for individuals who want to do that type of exercise.

So how is the glucose made? The answer is a biochemical process with the unwieldy name of gluconeogenesis, which simply means the making of new glucose. This process primarily occurs in the liver. When necessary, the body can make glucose out of a number of other substances including glycerol (which comes from fat metabolism), lactate and pyruvate (which comes from carbohydrate metabolism), and certain amino acids (from protein).

Which brings me back around to the topic of protein as a fuel source for the body. Readers may have read that “carbohydrates spare protein” and this is part of the basis for that claim: when carbohydrates are being eaten in sufficient quantities, the body has no need to break down protein for fuel. By extension, when carbohydrates are being restricted for whatever reason, some proportion of protein will be used to make glucose, leaving less to be used for building blocks. This has an important implication for dieting, namely that protein requirements go up when you’re restricting either calories or carbohydrates.

What About Starvation?

Now seems like as good of a time to talk about starvation, the consumption of zero food. I should mention that therapeutic starvation (as it was called) was tried during the middle of the 20th century for weight loss, frequently causing rather rapid losses of weight. But it had an unfortunate problem, which I’m going to address below. For now, let’s look at starvation and what happens.

So let’s say you stop eating anything and look at what happens (a much more detailed examination of this and many other topics can be found in my first book The Ketogenic Diet). Over the first few hours of starvation, blood glucose and insulin levels both drop. This signals the body to break down glycogen (stored carbohydrate) in the liver to release it into the bloodstream. As well, the body starts mobilizing fat from fat cells to use for fuel. After 12-18 hours or so (faster if you exercise), liver glycogen is emptied. At this point blood glucose will drop to low-normal levels and stay there. Blood fatty acids will have increased significantly by this point.

After a day or so, most cells in the body, with a few exceptions, are using fatty acids for fuel. Obese individuals may derive over 90% of their total fuel requirements from fat while leaner individuals may only derive about 75% of the total from fat. So far so good, right, the body is mobilizing and utilizing an absolute ton of fatty acids for fuel: 90% of your total energy expenditure if you’re fat and 75% if you’re lean (I’ll talk about what fat and lean is in another chapter).

There must be a drawback and here it is: the few tissues that require glucose are getting it via gluconeogenesis in the liver. As above, gluconeogenesis occurs from glycerol, lactate, pyruvate and amino acids. Now, if the person isn’t eating any protein, where are those amino acids going to have to come from?

That’s right, from the protein that is already in the body. But recall from last chapter that there really isn’t a store of protein in the body, unless you count muscles and organs. Which means that, during total starvation, the body has to break down protein tissues to provide amino acids to make glucose. The body starts eating its own lean body mass to make glucose to fuel certain tissues. This is bad.

Now, as fatty acids start to accumulate and be burned in the liver, ketones will start to be produced. Initially, for reasons totally unimportant to this book, muscles will use the majority of ketones that are produced. As I mentioned above, after a few weeks, the brain will adapt so that it is using ketones and deriving most of its fuel from them; the small remainder comes from the glucose being produced via gluconeogenesis.

Now, the adaptation to ketosis occurs for a profoundly important reason. Once again, much of the glucose produced in the body is from amino acids which are coming from the protein in muscle (and to a lesser degree, organs). If such a breakdown continued in the long term, so much muscle would be lost that the individual who was starving would be unable to move. Quite in fact, the loss of too much lean body mass (muscle and organs) causes death. The shift to using ketones decreases the need to break down body protein to make glucose.

As I mentioned above, therapeutic starvation was often used in the cases where rapid weight loss was needed. And while it did generate rather high levels of weight and fat loss, it had as a problem the loss of excessive body protein. So researchers decided to find a way to try and generate similar levels of weight/fat loss while sparing LBM. And that’s the topic of the next chapter.



7 thoughts on “Nutrient Metabolism Overview

  1. Very interesting thanks!

    I must admit though that I am trying to figure where Cortisol comes into it. This is an understanding and research thing because I have Addisons..


  2. “When necessary, the body can make glucose out of a number of other substances including glycerol (which comes from fat metabolism), lactate and pyruvate (which comes from carbohydrate metabolism), and certain amino acids (from protein).”

    I was wondering why the body can’t simply use glycerol to maintain its requirement of glucose in lieu of carbohydrates. Why is protein necessary? Is it simply the fact that the body can’t produce the remaining 25% needed fast enough with glycerol? Do the amino acids and glycerol combine in some fundamental way? Both questions refer to that particular context.


  3. Eric: The process is simply very rate limited. On average, the body will make ~10% of total fat broken down in a day into glucose from glycerol. So if you broke down a whopping 180 grams of fat in a day, you could get at most 18 grams of glucose. And you’ll usually get less than that. So other substrates are required.

  4. I have another quick question from an interview.

    “In one of the all-time classic studies (the Minnesota semi-starvation study), men were put on 50% of their maintenance calories for 6 months. It measured the largest reduction in metabolic rate I’ve ever seen, something like 40% below baseline. Yet at no point did the men stop losing fat until they hit 5% body fat at the end of the study.”

    Is it possible to permanently alter your metabolism in a negative way? Also, by 40% below baseline did you mean 40% below regular BMR? For instance, I weigh 350 lbs so my BMR would be roughly 3500 calories. 40% reduction would leave me at 2100. I know you can’t be exact, but how long would you guess I’d have to be dieting using your Rapid Fat Loss Diet to reach that metabolic deficit?

    Thank you very much for answering my questions.

  5. When you reach 5% bodyfat, you might get there. And no you can’t do permanent damage. Things always recover when weight/fat is regained. Even moving calories back to maintenance will undo some of the adaptive change (for example, the post-obese at weight maintenance often still show a 5% reduction in metabolic rate for their weight).

    Unfortunately, most of the reduction (in the Minnesota study case, 25%) is due simply to weighing less. That never goes away unless you regain the weight. A smaller body simply burns fewer calories.

  6. You said “After a day or so, most cells in the body, with a few exceptions, are using fatty acids for fuel.”

    But I wonder how these fatty acids could directly used as energy OR be converted to ATP, without being converted to Glucose first..!!

    and if we believe above theory, why there would be a need for ketosis then ?

    Simply put, Please explain me the difference in these two processes ;-
    a. Fat burning in liver through Ketosis ?
    b. Fatty acids used directly in cells as energy ?

  7. All nutrients have to be converted to ATP for fuel use as ATP is the only fuel that can be burned directly for energy by any cell. Fatty acids are burned through mitochondrial oxidation in various cells; they don’t have to be converted to glucose first (in fact, fatty acids can’t be converted to glucose in the first place).

    Ketones are produced when an excess of acetyl-coa is produced in the liver and the liver forms them into ketones (which can be used to generate ATP as well in various cells).

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