Fat Metabolism & Endurance
31 Jan 2004
In 1971 Froberg and Mossfeldt (1) had already documented the use of intramuscular triglycerides reserves during a prolonged effort (85 km of cross country skiing).
In fact, triglycerides can be stored inside muscle fibers in the form of small drops surrounding mitochondria, inside of which they become oxidized.
Next to glycogen “granules” we find these small drops of fat (triglycerides), all of them necessary to produce the energy needed for muscular activity.
A man of 75 kg has muscular stores of (on average) 400 g of glycogen (about 1600 Kcal) and 300 g of triglycerides (corresponding to 2700 Kcal).
With proper nutritional strategies it is possible to increase both glycogen and triglyceride stores within muscles by 50%, remarkably improving endurance performances.
Free fatty acids (FFA) are released from subcutaneous adipose stores in the blood together with intramuscular triglycerides, and with the guidance of albumine they reach muscle fibers to be used as fuel for their activity.
The amount of FFA utilized by muscles under effort is proportional up to a certain limit (about 1.5 mmol/l) to their concentration in the blood, although the limiting factor to fatty acids’ use in energy production is their carrier system through fibrocell membranes.
With the help of specific training, it is possible to improve the efficiency of FFA transfer from subcutaneous adipose stores to mitochondria in muscle cells. A correct training into Soglia (high) and Medio (medium) intensities is then mostly applied to this purpose.
Vukovic and Costill in 1993 (2) proved that the intake of 90 g of fats, 3 hours prior to a 60 minute effort on cycloergometer at 70% of VO2max, allowed a decrease by 40% in utilized glycogen in comparison with a control trial (intake of 45 g of carbohydrates in a light breakfast).
Muoio and Pendergast in 1994 (3) verified an increase in endurance in a group of athletes after a 7-day diet rich in fats (38% of total Kcal was fats, 50% was CHO and 12% was protein). The running time to exhaustion was shown to increase by 31% in comparison with the one obtained with the usual diet and by 20% in comparison with a diet rich in CHO (73% of total Kcal).
Such data do not lessen the importance of carbohydrates in a cyclist’s diet: if glycogen stores are insufficient, the athlete is not able to perform high intensity efforts, as demonstrated by dozens of scientific publications and on-the-road practice.
Likewise, if the rider assumes a diet that is too poor in fats, his endurance performances will be negatively affected.
(1) Acta Physiol.Scand. 1971; 82: 167-171
(2) J.Appl.Physiol. 1993; 75: 1513-1518
(3) Med.Sci.Sports.exerc. 1994; 26: 81-88