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Nutrition – some science for you

September 9, 2011

As most of our readers are training for distance events, I’m going to talk about energy requirements for one hour plus events rather than sprinting.

At the cellular level the muscles use adenosine triphosphate (ATP) in their contraction, but after 3 seconds the store in the muscle itself is used up, the next 15 seconds comes from creatine phosphate also stored in muscles, so enough for a short burst of energy i.e. a throwing or jumping event. After this ATP must be gained from carbohydrates, proteins and fats all of which can be delivered to the muscle cell and broken down for energy. Proteins do not generally give much to energy expenditure, unless it is a very prolonged endurance event such as an Ironman when they may supply 10% of total energy. The majority of our energy whilst exercising comes from fat and carbohydrates, but the balance depends on the intensity we are working at.

As almost all of the bodies energy at rest is provided by fat oxidation, this is the most efficient source of energy and ATP production. The main drawback though is that it is a slow system and uses more oxygen that carbohydrate metabolism does. So the body has to use carbohydrates even at low rates of exertion, meaning the stores of carbohydrates in the form of glycogen
are very important.

When intensity increases the body demands more ATP for energy, which must be produced faster and is done so by increasing the amount of carbohydrates and glucose. As the requirement for energy outstrips the supply of oxygen lactic acid begins to build up as it is an incomplete metabolic process, eventually this will prevent muscle contraction so exercise has to stop. Luckily gradual increases in training can increase the bodies tolerance to lactic acid, meaning you can work harder for longer, but at some stage the ‘Krebs cycle’ begins and the oxygen debt needs to be repaid.

Glycogen is also important as this can step in to help control blood glucose levels when carbohydrates have been depleted. The glycogen levels are controlled by the liver and helps to product new glucose from glycerol and amino acid residues. Low blood glucose levels can lead to physiological stress which can result in the release of hormones such as cortisol which in turn can have a negative effect on immunity.

So keeping high levels of glycogen stores are important when extending the duration of the exercise. In the 1930’s studies showed that exercise time to exhaustion can be lengthened if you increase a diet containing a high proportion of carbohydrates, which in turn were higher than those on a high fat / protein diet.

When training for distance events the diet should be based around carbohydrates, ideally between 55-60% of the energy required. The carbs shold be a mix of complex and simple sources, mainly due to the problems consuming soley complex carbs. A daily requirement of between 8-10g / kg of bodyweight during hard periods of training (sessions lasting in excess of 60 minutes), but
also taking on carbohydrates at intervals after the first 30 minutes either in the form of drinks or gels. This helps to maintain the levels of glucose circulating in the blood and prevents glycogen depletion, as once these have been used they cannot be replaced until after you have stopped training.

After training muscles need to be refilled as soon as possible with glycogen. The first thing you should have is a carbohydrate rich food. The recommended level is between 50g – 100g in the first hour or 1-2g / kg. Protein after exercise is also required to repair the muscles broken during exercise, consumption in the region of 1.5g / kg of bodyweight will aid in the initial repair, with the balance gained from regular diet. If too much protein is consumed there is no adverse effects as it is just ‘lost’ through the normal body process.

The main message in the run up to events it is worth testing different drinks and gels and recording how you ran on your long runs in training, plus trying different dietary combinations in the run up to a race can also be hugely beneficial.

Hydration strategies are also very important, both planning for pre and post event can lead to a very successful day.

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