Nutrition For Endurance Sports – Metabolism

In the previous article I set the foundation for the series about diet and endurance sports. Today I’ll be more precise and write about energy metabolism.

I’ll mention the main diet-related causes of fatigue and in the next part describe them in more detail, alongside the recommendations how to avoid them.

Human body needs energy to live. Other than an everyday food intake, energy can be stored in the form of all macronutrients: carbohydrate, fat and protein. There is 50-100 times more energy stored in fat than in the form of glycogen, bodily carb storage. (1) In an average quantity of 15 kg of fat in the body of an adult there is stored 130 000 kcal of energy. An average energetic value present in glycogen is around 2000 kcal, and we can find it in 500 g of this compound.

Body primarily uses carbohydrate and fat as energy sources, while protein have building, enzyme, and transport roles. Protein can also be used as an energy source in a small amount, but we’ll leave them out of the equation for simplicity sake.

ATP and creatine phosphate

Energy is stored in the form of ATP (adenosine 3-phosphate). ATP is the basic energy unit in the body down to which all nutrients are metabolised to. Energy is stored in the form of creatine phosphate as well. It serves as a “reserve” for ATP, providing it the phosphate group. The quantities of the last two compounds are very limited and enough for only several seconds of maximum intensity activity. All these different energy sources show complexity of human energy metabolism and its exceptional adaptability to different circumstances. The goal of these adaptations is to give the body enough of easily accessible energy in the time of need and restore it during recovery time, relying on other energy sources.

Fuel partitioning

Energy source, let’s call it fuel, used in the given moment, is almost never a single nutrient, but a combination of the before mentioned ones. The quantity of certain nutrients depends on several things. First of all, the accessibility of nutrients defined by diet, then intensity and duration of the activity, and also body stores of glycogen, fat and protein. (2)

In periods of rest, body uses almost exclusively fats as an energy source. (3) In low intensity activities, carbohydrates get involved as well. As intensity rises, so does the quantity of carbs in the total energy spent. There are a few reasons for this. The most important ones being slower energy production and higher oxygen level needed to produce the same number of ATP molecules from fat than from carbs. It’s logical that in high intensity conditions, when the speed of energy production is essential, and oxygen intake is a limiting factor, the body prefers this type of ratio of nutrient expenditure.

Carohydrate availability

Carbohydrate availability is an essential factor in all endurance sports. It depends on glycogen reserves, intake via food and drinks, and the speed of consumption.

Even though some say fat is the most important energy source during long activities, in the very long ones like long triathlon (Ironman) and ultramarathons a large part of energy comes from carbs. Also, endurance sports sometimes demand periods of high intensity, when carbs accessibility becomes crucial. To ensure optimal performance, you should fill your carbs reserves before competition, but also ingest them during the competition.

Glycogen sparing

A phrase often found in literature is „glycogen sparing“ – the ability to use as much fat in the fuel as possible. That is because fats are almost an unlimited energy source and this spares the limited glycogen reserves. You should also know that carbohydrate absorption from external sources is very limited and its maximum is around 90 g/h of activity (it seems that the number does not vary with body weight). Energetically, this means that we can use only 350 kcal per hour from food and drinks. For comparison, running a marathon spends 1 kcal per kilogram of body weight with every kilometer. (4) A 60 kilo athlete running a marathon in over 2 hours spends around 1200 kcal/hProtein and fat, be it long chain or medium chain fatty acids, are not an adequate energy source during the activity because of their very slow absorption and/or side effects such as digestion problems.

Fuel optimisation

One of the main body adjustments to endurance training is enhancing the fat content in total energy spent. (5) A well trained athlete will burn more fat in the same amount of energy (calories) spent when compared to their less trained colleague. This is due to higher mitochondria density, concentration of key enzymes in electron transport chain, proliferation of capillaries in the muscles, and a greater concentration of carnitine transferase and transport proteins for fatty acids. (6)

Some will say: „So marathoners are so thin because they burn more fat in training!“.

Of course not.

Marathoners and other endurance athletes work on maintaining low body weight or they lower it for greater efficiency of movement and speed. And they do it with a lot of carbs in their diet. Even if they wanted to gain weight (and who would want that?) they have a hard time because of high energy consumption.

Performance limiting factors

Limiting factors connected to diet which cause fatigue and drop in performance intensity in endurance sports are as listed (from the most common): (7)

  • spending glycogen reserves
  • dehydration
  • low blood glucose
  • gastrointestinal problems (nausea, diarrhea, intestinal cramps, vomiting)
  • hyperthermia
  • hyponatremia

In the next part I’ll explain the causes of these factors and recommend strategies on how to avoid them.


  1. Horowitz, Jeffrey F., and Samuel Klein. “Lipid metabolism during endurance exercise.” The American journal of clinical nutrition 72.2 (2000): 558s-563s.
  2. Flatt, J. P. “Body composition, respiratory quotient, and weight maintenance.” The American journal of clinical nutrition 62.5 (1995): 1107S-1117S.
  3. Klein, S., et al. “Palmitate and glycerol kinetics during brief starvation in normal weight young adult and elderly subjects.” Journal of Clinical Investigation 78.4 (1986): 928.
  4. Margaria, R., et al. “Energy cost of running.” Journal of Applied Physiology 18.2 (1963): 367-370.
  5. Jansson, E., and L. E. N. N. A. R. T. Kaijser. “Substrate utilization and enzymes in skeletal muscle of extremely endurance-trained men.” Journal of Applied Physiology 62.3 (1987): 999-1005.
  6. Hawley, John A. “Adaptations of skeletal muscle to prolonged, intense endurance training.” Clinical and experimental pharmacology and physiology 29.3 (2002): 218-222.
  7. Jeukendrup, Asker E., Roy LPG Jentjens, and Luke Moseley. “Nutritional considerations in triathlon.” Sports Medicine 35.2 (2005): 163-181.