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Fuelling Your Exercise

Diet is important in all stages of training, before, during and after, to provide adequate energy and ensure fluid levels are maintained. Nutrition plays a crucial role in achieving long-term fitness goals, but to maximise exercise performance we must understand how we metabolise during exercise.

Carbohydrate and fat are the most important fuel sources for active muscles. Both are stored in the body and can be mobilized when energy demand increases. They are used by muscle tissues concurrently, but the predominant fuel source depends on exercise intensity and duration, and general aerobic fitness and diet. More intense exercise like sprinting favours carbohydrate usage, while longer less intense activities such as long-distance running will utilize fat stores as carbohydrate reserves are depleted within 60-90 minutes (“hitting a wall”). The more fit the athlete, the more likely he/she is to burn fat, but the proportion of each substrate used preferentially is also reflective of their quantities in the diet.

Many studies have shown that exercise and performance in many physical activities are enhanced by strategies that help maintain carbohydrate stores. If you undertake mostly intense or explosive activities, such as high-intensity interval training or circuit training, a high-carbohydrate, low-fat diet will maximise carbohydrate stores. Carbohydrate is stored as glycogen in the liver and muscles but is limited to a few hundred grams. During exercise, this is converted to simple sugars (glucose) for use by active cells. If glycogen stores are inadequate, the athlete may experience heavy/tired muscles, fatigue and overall poor performance. To optimise these stores the diet should be based around high-carbohydrate foods, ideally comprising up to 60% of total energy intake, mostly from complex (starchy) sources rather than simple (sugary) varieties. Starchy foods include:

• Breakfast cereals, porridge
• Bread, crackers
• Potatoes, pasta, rice
• Beans, peas and lentils
• Root vegetables (parsnip, sweet corn)

Sugar and sugary foods can be included in the diet as snacks, and are the preferred food/beverages immediately before and after a sporting event:

• Jam/marmalade/honey
• Boiled sweets, jellies
• Mineral/sport/sweetened drinks

If the competition is anticipated to last greater than 90mins these foods and drinks should also be taken during exercise to maintain body stores.

“Carbohydrate loading” is the practice of super-loading bodily stores of glycogen to help offset fatigue and exhaustion during competition. Typically it can postpone fatigue and extend the duration of exercise by 20% and improve performance in events longer than 90mins. In the 3-4 days before a specific event training should be tapered, halving the time spent each day so that none is undertaken on the day immediately before competition. In addition to this, a particularly high carbohydrate is advised during this period.

The importance of adequate carbohydrate intake applies not only to the preparation stage before an event but also during activities and in recovery. Glycogen storage may occur more efficiently in the hours after exercise so consuming a source of high carbohydrates should be a priority for athletes in this window. Fewer complex carbohydrates may be favourable at this time over starchier varieties and may take the form of a sports drink, fruit, white bread and jam etc.

Body stores of fat, in contrast to carbohydrate, are virtually unlimited and even in lean athletes are generally more than 5% body weight (often between 15-30% in the non-athlete population). Fat is an important source of energy particularly for sports of long duration (i.e. over 90mins). A hypothesis that long-term high-fat diets may improve competition performance has been suggested, however, the evidence is currently unconvincing, especially considering other ramifications of a high-fat diet (weight gain, cardiovascular disease). Since body stores of fat are generally adequate, a moderate-fat diet where energy from fat is between 30-35% of total energy is advisable. Complete avoidance of fat in the diet will eventually lead to a deficiency of some fat-soluble vitamins and essential fatty acids, which are needed for normal body function. Plant-derived fats, e.g. olive/rapeseed/sunflower oil, nuts and seeds are the preferred source from a cardiovascular health perspective.

A related side effect of the increased energy turnover in exercise is an increase in body temperature, which must be dissipated. This is achieved by perspiration (sweating), where sweat excreted onto the skin then evaporates, thereby utilizing some of the excess heat, and thus lowering temperature. It is not surprising that considerable amounts of fluid (up to 1.8 litres per hour) and electrolytes (e.g. sodium) are lost. Dehydration is an important factor for anyone undertaking medium to longer duration activities to be consciously aware of, as thirst will not be stimulated until 2% of body weight is lost. Even below 2% of fluid losses are sufficient to impair exercise capacity by up to 10%. Sports drinks contain glucose and sodium and can be useful to help maintain carbohydrate, fluid and electrolyte levels before, during and after a competition.

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