Energy storage in the body

Ok, I think I’ll talk about the bodies energy systems. From where does our energy come , what energy system does the body use for various activities, how is it stored?

This is going to be another thumbnail sketch of my understanding of it.

Swimming movement comes from muscle contraction. All energy for muscle comes from inputted energy that is derived from food.

Energy is the body is stored in five different ways:

ATP (Adenosine triphosphate)
CP (Creatine phosphate)

ATP synthase
ATP synthase

ATP is actually used for muscle contraction. It’s stored in the muscles (and liver) and once it’s used it has to be reconstituted or replaced, but there is a large store and it is readily replaced.

CP’s main function is the repair of the used ATP. There’s only enough in the body to last for a few seconds of all-out effort such as a sprint.

Once the CP is used ATP has to be repaired or replaced from other sources. The main sources are carbohydrates.

Glycogen is also called blood sugar but is stored in both the blood and liver and used in the muscles. It is derived from glucose which in turn is derived from carbohydrates (sugars/starch) and to be used it turns back to glucose .

If you have a pretty good diet, you should have sufficient muscle glycogen to provide energy for two to three hours. However regular heavy training depletes this store, which is why serious training, like Channel training requires a serious increase in food intake. (for example jumping my normal  approximately 2 and a half thousand Calories per day to anywhere from 6 to 8 thousand daily last year, as for most Channel swimmers. Blood and liver glycogen takes time to get into the muscles, so endurance athletes have to keep the store up by a constant supply of glucose. For cyclists, running out of glycogen is called the bonk (or the knock in my racing days).

Fat also supplies ATP. (White) fat is a very dense energy store. It’s the bodies emergency energy store. However it requires oxygen to be converted and it’s slower than carbohydrates (which can be converted both with and without oxygen, aerobic and anaerobic). It’s not useful for sudden energy demands so can only be used for lower energy rate requirements. (This is one of the reasons why swimming often isn’t a good exercise for losing weight, since even poor swimmers will get a high cardiac rate due to poor breathing or technique). Fat is used by endurance athletes by staying well below the anaerobic threshold.

A primary reason of the developed world weight issue is because humans have excess food for the first time ever. Extra food metabolised is stored as fat against an emergency future requirement.

So Channel and marathon swimmers will start to metabolise fat once the glycogen is used, and since the body can only metabolise a certain amount of carbohydrates per hour. This the reason we lose a lot of weight over a short period (4 kgs for me in 2 days). Carbohydrate metabolism is improved by a 4:1 carb:protein ratio.

Protein however isn’t stored as a free resource in the body but is all used (in muscles etc). Protein is used mainly therefore as a muscle repair energy source. But if someone trains with low glycogen stores too often, muscle is used leading to muscle loss.

Any corrections delightedly accepted.

Heart rate training zones


8 thoughts on “Energy storage in the body

  1. A couple of queries:

    1. At what point does the body story energy from food as fat? Does it always try to replenish glycogen stores until they’re full before storing as fat? Can energy be converted from fat, protein and carb intake in to glycogen stores?

    2. Do glycogen stores need to be empty for the body to use fat stores for energy?

    3. “Protein is used mainly therefore as a muscle repair energy source. But if someone trains with low glycogen stores too often, muscle is used leading to muscle loss.”
    Presumably your glycogen stores need to be empty before fat is used for energy? How do we ensure fat stores are used for energy over protein stored?


    • Hi Rich,

      1. Fat is stored whenever the calorie intake is greater than expenditure. Regardless of the macros (carbs, fat, protein) of the food, glycogen is replaced first. So fat and protein are used for energy also. Protein metabolisation for example works more efficiently when carbs are added.
      2. Yes… with a but. Superstarches are a long molecule chain carbohydrate that allow the body to access fat stores before depletion of glycogen.
      3. Energy is produced by glycogen first, then fat, then protein. Protein isn’t stored like fat or glycogen. Hope this helps.


  2. (I understand this is an old entry, but it’s one of the first search results for energy storage.)

    Just a few notes:

    Creatine phosphate is abbreviated as PCr. That’s all I have to say about that.

    Glycogen is a polysaccharide made of long branched chains of glucose. Glycogen is stored in muscle tissue, then broken down into glucose for transport and use. Glucose is “blood sugar,” not glycogen. Both glycogen and glucose are carbohydrates. Starch is the form of stored glucose in plants, where glycogen is the form of stored glucose in animals. (Carbohydrates is a family of organic compounds composed of carbon, hydrogen, and oxygen, which chemical energy in living organisms. Carbs == sugars == saccharides.)

    All macronutrients, protein included (with the exception of some glucose for the purposes of replenishing intramuscular glycogen), are converted to fat and stored if they’re not utilized. Not *all* protein is used. Some forms of protein may be excreted if it’s in excess, but for the most part it gets converted to fat and stored as adipose tissue. You are correct in that muscle tissue may be utilized as energy, but it’s really only done in the absence of available glycogen and fat stores. This type of wasting typically doesn’t occur unless an individual’s body composition is at dangerously low levels.


    • Muscles do not export glucose but instead will use the energy and either export alanine or lactate. Glycogen is not considered a carbohydrate but instead a Protein-Carbohydrate Complex Glycogenin is the protein component. Liver is the main storage of Glycogen for the purpose of Blood Glucose regulation and therefore is the only organ that exports glucose (some kidney and small intestines but negligible). Protein do not get excreted but instead the amine group in each amino acid is toxic if its not conjugated and ammonia is converted to urea in the urea cycle in the liver and excreted by the kidneys through blood filtration. Carbon skeletons get recycled through out the body so depending on individual metabolism and dietary deficiencies the body has methods of compensating for these deficiencies. As far as essentials of the body there are 9 amino acids that the human body cannot produce due to the loss of specific enzymes that are necessary for their pathway these you must intake through diet (10 if you consider Arganine as human body does not produce enough for development). The reason we intake more Glutamic Acid (Glutamate) and Aspartic Acid (Aspartate) is for the proper absorption and digestion of other proteins as Glutamate is the final amino acceptor in Amino Acid degradation. There are also 2 fat molecules that are essential in the diet as the body cannot produce these but are essential to survival (Lenoleic acid and Lenolenic acid). The reason that you should eat your vegetables is because they contain vitamins, can also supplement in a multivitamin daily. Vitamins serve as cofactors to the important body enzymes required for metabolism.

      I honestly thought the original article was extremely on point but if you are going to correct someone and attempt to drop knowledge on science and metabolism should make sure that you are correct first.

      If anyone has contradictory or additional info please feel free to add.


      • Thanks for your contribution ans sorry it took me a week to getting around to approving it.
        As I said in the original article, I was trying to give a basic understanding, which is all I have. Not trying to understand this often leads to swimmers, (I’m concerned with distance or marathon swimmers) making decisions about feeding and or nutrition based entirely on what the information passed around their peer group. Not that I’m any more expert, but I wanted (back then in 2011) to at least put information out, and because I wasn’t in any way expert, I tried to err on the side of caution. I appreciate your feedback. I have a much longer 4-part series on this subject for marathon swimmers that I started but have yet to complete due to time constraints and the amount of time to try to understand the background.


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