Minggu, 17 Juli 2011
Sports nutrition part I
It's often an iterative process because you initially have no idea where to find the source but in reading "Nutrition for Sport and Exercise" by Dunford and Doyle, I think I have got quite close to finding it. One thing I very much like about the book is that, not only do they cite their references, but for the most important studies they mention, they also give a brief overview of the paper in question.
Anyway, for my own benefit as well as hopefully yours, I thought I would summarize the points that I found most interesting in the context of an Ironman / Marathon.
ATP is the currency of energy in the body and is produced by the three metabolic pathways:
1) Creatine Phosphate. A very fast acting system but of short duration (10 seconds) and slow to replenish (by oxidation). This explains why you see sprinters gasping for air after running.
2) Aerobic glycosis. As its name suggests, this system does not require oxygen although the lactic acid that is produced as a by-product is subsequently available for oxydation. Perhaps this explains the effectiveness of series training, with the rest in between allowing oxidation (and therefore removal) of the lactic acid. After a few minutes acidosis reduces the ability of the enzymes to catalyze the metabolic reaction and fatigue sets in. The source of fuel is carbohydrate - either directly in the form of glucose or as glycogen (the form in which glucose is stored in the muscles and the liver). Glycogen yields 3 ATPs while glucose only 2. This system is the preferred energy source for fast twitch muscle fibers. The brain relies on glucose as its energy source (except in the case of starvation or severe carbohydrate restriction).
3) Carbohydrates can also be metabolized by oxidative phosphorylation, the preferred energy system for slow twitch muscle fibers. Fats (stored as tryglycerides in adipose tissues) and proteins (which are catabolized directly) can also by oxidized. Alcohol - the 4th macronutrient - cannot be stored and therefore must be oxidized directly. Carbohydrates and proteins yield 4.2 kcal per gram, alcohol 7.0 kcal per gram and fats at 9.4 kcal per gram are the most energetically dense, although they require more oxygen to metabolize. Proteins are the least favoured source of energy (because they have many other important uses) but typically make up 3-5% of the energy generated. Oxidation produces many more ATPs than the other metabolic pathways (for example, glucose produces 36 and glycogen 37) but it is a much slower process than glycosis, requiring 124 chemical reactions.
The proportion of fat / carbohydrates being oxidized may be calculated by measuring the Respiratory Exchange Ratio (RER) or the ratio between oxygen consumed and carbon dioxide exhaled. The point at which the highest proportion of calories derived from fat is obtained is quite different from the point at which the maximum (absolute) number of calories from fat oxidation is achieved.
It is possible to accurately estimate the number of calories being burnt using the RER and the VO2 (the amount of oxygen consumed). The more slow twitch muscle fibers an athlete has, the higher his potential VO2max or maximum oxygen uptake. Excess calories from food (over and above the Resting Energy Expenditure (REE) and energy used in exercise) are stored as fat.