Another way of looking at endurance–training fueling strategies.

The human body can metabolize a number of different substrates as fuels for endurance training and performance. If we look at the relative combustibility of fuels, or the ratio of carbon dioxide expired, to the amount of oxygen inspired, we get an idea of how efficient the combustion of the fuel source has been in terms of carbon dioxide production. Clearly, laboratory-based treadmill testing of runners in a laboratory setting where the amount of oxygen (O2) consumed or inspired can be compared to the amount of carbon dioxide (CO2) expired shows that when the Respiratory Quotient or RQ of CO2 expired/O2 inspired approaches 1.00, the main fuel being consumed is usually a carbohydrate.

For the purposes of this article, I have examined the energy yields from oxidation of fats or carbohydrates, normally referred to as respiration in biological organisms, as equivalent to the energy yields from the oxidation or combustion of exactly the same chemical fuels in inorganic situations.

The aerobic consumption or oxidation of glucose, and its resulting energy yield, can be represented by:

C6H12O6 (Glucose) + 6O2 → 6CO2 + 6H2O +36 ATP. In this case, RQ=6(CO2)/6(O2) =1.0, or unity.

What has emerged recently is that the formerly demonized substance, lactate, C3H5O3– can also be combusted as a carbohydrate. It is basically half of one glucose molecule, with an extra proton (H+) chopped off as well, rendering the resulting ‘molecule’ as a negatively charged lactate molecule with an accompanying hydrogen ion. This newly formed entity, lactate, C3H5O3-, combusts in oxygen according to the following balanced equation:

3 C3H5O3– +3O2→3 CO2 +3H2O +OH-, with RQ= 3/3= 1.0.

Because it is only half the size of its precursor 6-carbon glucose molecule, lactate doesn’t require a transport protein to cross the cell membrane, and the net energy yield on a pro-rata carbon-hydrogen basis, ends up being much the same as for glucose.

Other common fuels in the human body can include fatty acids of all types, and proteins to a lesser extent, under conditions where glucose-based fuels are relatively low. The aerobic consumption or oxidation of fatty acids, and the relative energy yields per molecule, can be approximated by looking at two separate equations: C16H32O2 (palmitic acid) + 23O2 → 16CO2 + 16H2O +129 ATP: Here, RQ=16/23=0.696. The triglyceride form of palmitic acid, with three palmitic acid chains joined to one glycerol molecule, glycerol tripalmitate, provides the most common form of palmitic acid storage in the body and has a slightly different RQ.

Although it provides more than three times the net energy yield that one molecule of palmitic acid does, its RQ differs very little, at 0.703. The equation can be shown as C51H98O6 + 72.5 O2 → 51 CO2 + 49 H2O + 402 ATP, and RQ will be 51/72.5, or 0.703.

Usually, the most available carbohydrate in the human body is glucose, C6H12O6, which follows the exact ratio of carbon atoms to hydrogen atoms to oxygen atoms implied in the term ‘carbo-hydrate’: 1carbon: I hydrate or 1Carbon: 1H20. So all carbohydrates have the same relative ratios of Carbon to Hydrogens to Oxygen: C:2H:O.

The third substrate type commonly used as a fuel source by the body is protein, and although it is only accessible as a fuel when relative deprivation of carbohydrate substrate exists, once the process of glucose-formation from protein (gluconeogenesis) starts, oxidation of substrate is better than for fats. The equation for proteins as a substrate can be represented by the following equation, for the RQ of albumin:

C72H112N18O22S + 77 O2 → 63 CO2 + 38 H2O + SO3 + 9 CO(NH2)2, with RQ=63/77, or 0.82.

So, in one way, we can rank these three substrates in terms of their combustibility, or burnability, no matter which way they end up being oxidized; by organic respiration or inorganic combustion.

 Effectively, 100% of carbohydrates, including lactate, can be burnt up with oxygen to expire as many molecules of CO2 in expiration as molecules of O2 drawn in with inspiration. All carbon atoms in the original substrate get to combine with oxygen, so the combustibility of carbohydrates is as high as it can get, at 100%.

With proteins, we have just shown that 82% of the substrate’s carbon content gets oxidized.

With fats, the percentage of carbon atoms that gets oxidized to form CO2 is only about 70%.

According to conventional thought, fats can’t be a good endurance fuel because they don’t completely combust with oxygen to form carbon dioxide and water as products. They also require far more oxygen per molecule to combust as well, which is not a problem at sea level where oxygen is plentiful, but is a problem at high altitudes, especially for climbers who often stay immobilized for long periods while they work their muscles isometrically, rather than dynamically, as in regular aerobic activities like running or cycling, where the rhythmic contractions of working skeletal muscles get plenty of oxygen-rich blood deep into the metabolic tissues.

Why Endurance Capacity is most defined by one’s ability to utilize fats

If we haven’t a very well-developed ability to utilize fats as fuel, we are limited to our extremely limited carbohydrate storage capacity. The body has a very limited endogenous supply of carbohydrate, stored in the muscles and liver, and when fully topped up, it equates to around 2000kcal or only 500 grams; enough for only about an hour of intense exercise at or just above the anaerobic threshold.

From routine oxygen uptake tests that often rely on expired gas analysis with face-masks, it is usually assumed that aerobic exercise can usually be divided into two types of substrate, just by the RQ that is derived from the ratio of expired CO2 volume to the volume of inspired O2.

At the very lowest, slowest effort-levels, fatty acids are the primary fuel, and in trained athletes with a good fat-burning capacity, this is borne out by an almost homogenous RQ of about 0.7, through all aerobic intensities till sub-maximal heart rates at about 80%-85% intensity, indicating that the primary fuel is fat. As higher aerobic work output rises, the muscles switch to burning more combustible fuels like glucose and its derivative, lactate; the RQ readings at the point where glucose starts to get burnt rapidly change from about 0.7 to 1.0 almost straight away, and RQ remains at 1.0 until local skeletal muscle acidosis makes aerobic activity cease.

From routine oxygen uptake tests that often rely on expired gas analysis with face-masks, it is usually assumed that aerobic exercise can usually be divided into two types of substrate, just by the RQ that is derived from the ratio of expired CO2 volume to the volume of inspired O2.

At the very lowest, slowest effort-levels, fatty acids are the primary fuel, and in trained athletes with a good fat-burning capacity, this is borne out by an almost homogenous RQ of about 0.7, through all aerobic intensities till sub-maximal heart rates at about 80%-85% intensity, indicating that the primary fuel is fat. As higher aerobic work output rises, the muscles switch to burning more combustible fuels like glucose and its derivative, lactate; the RQ readings at the point where glucose starts to get burnt rapidly change from about 0.7 to 1.0 almost straight away, and RQ remains at 1.0 until local skeletal muscle acidosis makes aerobic activity cease.

This graphic both show the increasing ability of the body to use fats as primary fuel, at steadily increasing % of maximal capacity, year by year as the body adapts its metabolism to fat substrates rather than carbohydrates. In a way, if an athlete is carbohydrate-dependent, he or she will not find endurance training easy at even very low intensities. Training the body and mind to patiently cover long distances slowly for long periods, without ingesting carbohydrates, will result in a training response of increased fat-burning capacity, and increased absolute endurance capacity; very useful for all events that last long enough to deplete carbohydrate stores.

Enter KETONES.

In the absence of enough blood glucose, usually derived from carbohydrates, the alpha-cells of the pancreas produce the hormone glucagon, which stimulates the rapid formation of 3 and 4-carbon fatty acids called ketones in the liver.There are three known ketones in the human body that can provide a ready source of rapid energy to the brain and body in the absence of glucose. The molecules are so small they do not require the MCT (monocarboxylase transporter) molecules to cross cell membranes that most longer carbon-chain fats require.

The three ketones and their relative RQ assays are:

  1. 3-hydroxybutyric acid: 2 C4H8O3+ 9 O2= 8 CO2 + 8 H2O RQ= 8/9 = 0.8888
  2. Acetoacetic Acid: C4H6O3+ 4 O2= 4 CO2 + 3 H2O             RQ=4/4 = 1.0
  3. Acetone: C3H6O + 4 O2= 3 CO2+ 3 H2O                             RQ=3/4 =  0.75

The two main ketone bodies are acetoacetate (AcAc) and 3-beta-hydroxybutyrate (3HB), while acetone is the third, and least abundant, ketone body.

If we look at the two main ketones, we see an RQ or combustibility Index, averaging between 0.88888 and 1.00000, or 0.9444. So at least 94.4% of the carbon composition in ketones gets fully oxidized, presuming the two most abundant human ketones are present in equal quantities. The portion that doesn’t get oxidized is in the carboxylic acid tail (COOH-) attached to the remaining line of two or three carbon atoms.

In other words, there are very good reasons why these very short-chain fatty acids can be considered as an excellent high-energy fuel for endurance athletes.

Other great reasons for utilizing ketones as a primary fuel when exercising aerobically are:

  1. the RQ is more than high enough to sustain all aerobic exercise intensities below the anaerobic threshold, even at the most advantageous aerobic intensity for mitochondrial development in the Fast Twitch Aerobic Type IIA muscle fibres, the sub-threshold high-aerobic intensity between 75-80% of maximal heart rate intensity.
  2. the production of damaging ‘free oxygen radicals’ or ‘negative ions’ (unattached electrons) is far less with ketone –based metabolism when compared to glucose metabolism

Because in theory all training during a typical Lydiard-based aerobic buildup is performed below the anaerobic threshold, and because ketones adequately bridge the power output gap between the RQ of regular fatty acids and carbohydrates, all training can be done in this base phase without the need to resort to carbohydrate as a primary fuel.

 

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