Your blood has both triglycerides and glucose that it can use for energy.
At rest, the body uses both for energy, though certain tissues (such as the brain) tend to rely more on glucose for energy.
As you ramp up activity, the body releases hormones like adrenaline that cause the release of more triglycerides and glucose from stores in the liver, fatty tissue, to keep the levels stable in the blood stream.
Thus, burning of both sources gets ramped up as the body uses it.
At a certain point, around 60-65% Vo2max intensity (e.g. lactate threshold) for normal people, the body switches from burning « both » sources to mostly carbohydrates and that increases towards 100% Vo2max. However, elite endurance runners can run at around 90% Vo2max without hitting lactate threshold.
This tells us a couple things:
- Since triglycerides provide more energy per unit, your body can ramp up consumption of triglycerides for energy as you become more proficient at distance running.
- Since you can pump up glycogen stores, your body can ramp up increases of these too as you become more proficient at running.
Remember that distance running increases mitochondria in muscles. This serves multiple functions.
- Mitochondria can effectively « buffer » metabolic acidosis generated from ATP->ADP, glucose oxidation, etc. (remember that lactate does not cause metabolic acidosis). This is one of the effects on why elite distance runners can run at a higher percentage of their Vo2max before reaching lactate threshold.
- More mitochondria per muscle means that more energy can be input in the form of triglycerides.
- Upregulation of enzymes. Enzymes are likely the limiting rate within muscles controlled much of the action as you may have learned in biology and chemistry.
- The main « part » where this becomes important is the fact that if you are training distance running you are mainly emphasizing low intensity work which means you are emphasizing aerobic type enzymes and beta oxidation. This means that the glycolytic enzymes that are more emphasized by anaerobic work at higher Vo2max are NOT emphasized, and as such there is going to be a limited « rate » on the processing of glucose within muscle cells. This makes sense, because elite endurance runners should not have the anaerobic capacity of sprinters.
Remember that most energy generation is provided not in glycolysis, but in the citric acid cycle and electron chain transport. Glucose, when split into 2 parts will provide 2 Acetyl-CoA. Glucose yields approximately 34/36/38 ATP depending on your sources, while the primary fatty acid store in the body, palmitate, yeilds closer to 106-109 ATP per cycle turnover.
So going back, what do we know. If say, we look at « energy usage » at rest we would see:
1 unit of glucose and 1 unit of triglycerides is used (again, tissue dependent so I’m generalizing).
As you ramp up exercise towards aerobic threshold, depending on your level of training you utilize a bit more triglycerides than glucose for various reasons.
This may lead to a ratio such as say 3 units of glucose vs 4-5 units of triglycerides (again, generalizing — elite endurance runners may burn more triglycerides)
As you pass lactate threshold, your body starts to turn down triglycerides production and increase lactate production. (Note that It’s only after lactate removal is exceeded by lactate production that there is an issue — muscle have lactate transporters in the muscle membrane to transport it out into the blood stream and then the liver can reconvert lactate into glucise via the Cori cycle. Also, the brain and heart can use lactate as a « fuel »).
This would be say burning 5-6 units of glucose vs. 2-3 of triglycerides.
The race evidence for increased triglyceride consumption in elite endurance runners is that they don’t hit « the wall » around mile 19-20 in marathoning where glucose stores typically run out for your recreational runners. In fact, they don’t hit the wall at all during the marathon, which means triglyceride consumption likely increases significantly more than a 3:4-5 unit ratio like I described above.
- The body simulatenously increases consumption of carbohydrates and fat during exercise until lactate threshold. In the untrained, training with a pre-workout carbs decreases fat oxidation but not in trained.
- Over time, the body increases the ratio of fat:carbohydrates consumed for energy under lactate threshold.
- You CANNOT emphasize the fat:carb ratio by training fasted; fasted just means you can train at a lower speed compared, but fatty acid oxidation stays consistent (see Shea4it’s link below).
- If you’re running for fat loss, stay under lactate threshold & do a shit ton of running. (Though for most it’s more applicable to fix their diet…).