Bioenergetics And Healthy Nutrition: You Make Fat From Sugar But Not Sugar From Fat

Sweden rejects low fat diet

Sweden Becomes First Western Nation to Reject Low-fat Diet Dogma in Favor of Low-carb High-fat Nutrition. Click image for link to related article.

Healthy nutrition is one key to a long happy life, and don’t think that it’s fat that makes you fat; low fat foolishness made America fat. For many years I have been interested in Biochemistry, and my triathlon training heightened this interest. As you exercise you burn fuel to generate energy to do work (moving, thinking, making cells, excreting wastes). In fact, you burn these fuels all the time, but more as you exercise. The trick to learning Biology is building a mental picture of the thing that you are trying to understand, be it the insides of a worm (very interesting), the contents of a cell (even more interesting), or the metabolic pathways that determine where everything you eat and everything you are will end up between meals (fascinating).

contents of cells

Contents of animal and plant cell. From:

The details of these things are mind-bogglingly complex, but the underlying principles are fairly straight forward. For instance, we eat to provide ourselves with fuel, which we burn immediately or store to burn later. There are really only three major fuels, sugars, fats, and proteins. Each is different, and if you want to understand your body, learn to optimize its function, and avoid bonking in an Ironman race, you (or your coach) need to understand the basic principles of these fuel systems. Here is a simple word picture pulled out of my head, after many years of studying this subject:

  1. You eat sugars (as glucose or starch, for instance), burn (make energy to do things) what is immediately available, or you squirrel it away as sugar in the short-term storage form of glycogen, mainly in your liver, or you convert it to fat (which is all over the place, and in somewhat different places in men versus women) for long-term storage.
  2. You eat fats, and burn or store them, in this case as fat.
  3. You eat proteins, break them down into their component parts to make your proteins from their bits and pieces, and then burn, excrete, or store bits and pieces of the components of the remainder as glycogen or fat.
  4. When you run out of food you first burn the liver glycogen, then make essential sugar (glucose) from protein components, using the energy in fat to do this. Some tissues can use fats for energy and some have to burn glucose for energy. So you first burn your liver glycogen, and then start to burn proteins and fats, to allow you to keep doing things until the next meal.
  5. When you use proteins to make sugar, with the aid of fat energy for the manufacturing process, you have to eliminate some nitrogen waste from protein components. These proteins will have to be rebuilt later, when dinner arrives.
    metabolism chart available for free on the Internet with many underlying links to further detais

    KEGG Metabolism Chart – Take a look. It is magic!!! From:

    This whole process is called energy metabolism, and this area of study is called Bioenergetics.

  6. And so the dance of metabolism goes on from meal to meal, being simple in principal, but each small part of this machinery can be the subject of study for a human life-time.

Did you notice one odd thing, you make fat from sugar, but you can’t (essentially) make sugar from fat! This makes things very complicated, and obliges the organism to breakdown and then rebuild proteins all the time, along with which comes the nitrogen waste problem. I have pondered this issue for years, always assuming it is just a left-over from our evolution (stuck on a fitness peak). This goes back to the evolution of our mitochondrial bacterial ancestors, when we were ‘cobbled together’ by a series of environmentally mandated decisions to which our ancestors were subjected, such that their progeny would survive. Our inability to make sugar from fat is a consequence of an accounting problem built into the Kreb’s cycle (a piece of metabolic magic), where almost all of the carbon atoms from fat are turned into carbon dioxide and excreted in your expired air, and are not, therefore, available to make sugar. That’s the reason, carbon accounting, based upon a critical, and ancient biochemical decision that is built into our genes! This whole show is run by layers and layers of regulatory circuits, with hormones such as insulin controlling the overall behavior of the system.

Isn’t that interesting? But interesting or not, it’s important. It is the stuff of bonking during a race, and the source of the explanation of mechanisms of diabetes, obesity, and a host of other metabolic diseases. If you want to understand your body as an athlete, a little time spent learning Bioenergetics might provide you with some useful insights. And it’s fun!

-k Your Medical Mind


  1. Translation problem! In English English “bonking” has come to have another meaning. It is the process whereby we make babies. Two people do it together.
    It reads odd in England.

  2. I would like to share one of my favorite motivational fitness quotes with you and your readers: “The man who can drive himself further once the effort gets painful is the man who will win.” – Roger Bannister

  3. The term bonk for cycling fatigue is presumably derived from the original meaning “to hit”, and dates back at least half a century. A 2005 video issued by the British Transport Films Collection contains several old films, one of which entitled “Cyclists Special”, a color film produced in 1955, tells the story of a party of cyclists touring the English countryside. At one point they stop for refreshments and the film’s commentator states that if they didn’t rest and eat they would get “the bonk”.
    The term is used colloquially both as a noun (“hitting the bonk”) and a verb (“to bonk halfway through the race”). The condition is also known to long-distance (marathon) runners, who usually refer to it as “hitting the wall”. The British may refer to it as “hunger knock,” while “hunger bonk” or “bunger honk” was used by South African cyclists in the 1960s.

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Disclaimer: As a veterinarian, I do not provide medical advice for human animals. If you undertake or modify an exercise program, consult your medical advisors before doing so. Undertaking activities pursued by the author does not mean that he endorses your undertaking such activities, which is clearly your decision and responsibility. Be careful and sensible, please.