Pyramid Science

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Saturday, December 27, 2008

Energy Fuels

Amino Acid - Protein Synthesis
Blood Flow Physics

Energy for exercise is provided by three main fuels:
  • Carbohydrate
    • 4kcal/g
  • Fat
    • 9kcal/g
  • Protein
    • 4kcal/g
The amount and proportion of each fuel used depends on the type, duration and intensity of exercise, your fitness level and your diet. For aerobic activities, all three fuels may be broken down (although protein makes a significantly smaller contribution than fat and carbohydrate). For anaerobic activities, only phosphocreatine (PC) and glycogen are broken down. The proportion of carbohydrate (glycogen) used increases with exercise intensity and decreases with exercise duration. The main cause of fatigue during aerobic exercise is usually glycogen depletion and/or dehydration. The main cause of fatigue during anaerobic exercise is initially PC depletion and/or lactic acid build up, but after several exercise sets, fatigue is eventually also due to glycogen depletion. For almost all types of exercise, performance is limited by the amount of glycogen in the muscles. Low pre-exercise stores lead to early fatigue, reduced training intensity and reduced training gains. Ensuring full glycogen stores before each training session can help delay fatigue and improve performance. This can be achieved by a high carbohydrate diet.

Summary - carbo power

For optimal training gains, ensure that muscle glycogen stores are fully restored during the period between training sessions. The length of time taken to refuel glycogen stores between training sessions depends on the intensity and duration of exercise (ie, the degree of depletion) and the amount and timing of carbohydrate intake and your fitness level. It takes longer to refuel following high intensity and/or prolonged exercise and will be increased by a low carbohydrate diet and low fitness levels. The recommended carb intake for athletes and active people is 60-70% of total energy intake. Practically, this means about 6-10g carb/kg body weight/day.

Glycogen refuelling is faster in the 2hr period following the exercise and a carb intake of at least 1g/kg body weight is recommended during this time. For efficient refuelling, continue to consume a minimum of 50g carb every 2hrs. Leave approx 2-3hrs between last meal and training. Consume a further 50g to help maintain blood sugar levels and delay fatigue and improve performance. For strenuous exercise lasting more than 60-90 mins consume 30-60g carb (liquid or solid form) to help maintain longer performance.

Choice of type depends on the nutritional package (and other contained nutrients). The glycemic index (GI) is a measure of the speed of absorption and the consequential rate of rising blood sugar. The nearer 100 the faster this absorption. The main part of any diet should comprise carb foods with an overall good nutritional package: bread, cereals, pulses, starchy vegetables, fruit and low fat dairy products. Carbs with high GI are good during and after exercise as they are absorbed quickly. Athletes with high energy and carb requirements should include mix of high and low bulk carbs in the diet.


Proteins make up part of the structure of every cell in the body including about three quarters of the dry weight of muscle. They also form enzymes and hormones and are continually broken down into their constituent amino acids and recycled as new proteins or energy substrates. Some protein is lost every day and must be replaced in the diet and most protein is broken down to provide energy when glycogen is in short supply (dieting and prolonged intense exercise). Exercise increases protein breakdown (catabolism) and therefore dietary requirements. The exact type depends on the type, frequency, duration and intensity of exercise, fitness level and energy and carb intake.

The protein needs of athletes is greater than those of sedentary people and intake is between 1.2 to 1.7g/kg body weight/day (12-15% energy intake is recommended). Strength training increases protein needs more than aerobic training. Intake of 1.4 to 1.7g/kg for strength athletes, 1.2 to 1.4g/kg for endurance athletes. Any increased protein needs should be able to be met from a balanced diet that is meeting energy needs. High protein intake surplus to requirements do not enhance muscle strength, size or mass and do not offer any advantage. Protein supplements are not necessary for most athletes and do not automatically enhance performace or strength, but may be useful if energy and protein requirements cannot be met by food alone.


There is no evidence to support claims for amino acid supplementation. Vitamin and mineral requirements depend on age, body size, activity level and individual metabolism. Dietary Reference Values (DRV) should be used only as a guide for the general population and not targets since they do not take into account the needs of the athlete. Regular and intense exercise increases the requirement for number of vitamins and minerals, though there is no official recommendation for athletes. A low intake can adversely affect health and performance and a high intake exceeding requirements will not necessarily improve performance. A well planned and balanced diet that meets energy needs is likely to provide sufficient vitamins and minerals.

Supplements should not take the place of a balanced diet

Supplements containing 100-200% Dietary Reference Intake (DRI) may be useful for athletes consuming less than 2000kcal/day and those with erratic eating habits, food intolerances or restrictive diets (vegan). Vitamins A, D and B6 and a number of minerals may be toxic in high doses (more than 10 x DRI). Indiscriminate supplementation may lead to nutritional imbalances and deficiences.

Vitamins - Overview

Increased free radicals are produced during exercise and these may be responsible for post exercise muscle soreness (DOMS: Delayed Onset Muscular Soreness). Excessive amounts may also increase the risk of heart disease, certain cancers and premature ageing. Antioxidant nutrients can help prevent free radical damage. High dietary intake of antioxidant rich foods is recommended: aim is at least 5 portions of fruit and vegetables a day, with moderate amounts of vegetables oils, oily fish, nuts and red wine. Optimal doses for antioxidant nutrients are unknown and the value of supplements not yet clear.

Vitamin A is essential for normal colour vision and for cells in the eye that enable sight in dim light. It promotes healthy skin and mucous membranes lining the mouth, nose and digestive system and is found in meat, eggs, whole milk, cheese, oily fish, butter and margarine.


Dehydration impairs performance and health and fluid losses depend on duration and intensity of exercise, temp and humidity, body size, fitness level. This can be as high as 1-2l/hr and always start well hydrated and continue drinking at regular intervals early on and drink plenty afterwards to replace fluid losses. Water suitable fluid for moderate exercise of up to 1hr. More intense exercise of more than 1hr should include dilute salt (sodium chloride). Carbohydrate can speed up water absorption and provide additional fuel. Optimal concentration for fluid replacement is 4-8g carb/100ml and 40-110mg sodium/100ml. A compromise should be made between function and taste as is often made in commercial drinks.

Tonicity is a measure of the ability of a solution to exert an osmotic pressure upon a membrane. Hypotonic and isotonic sports drinks are the most suitable when rapid fluid replacement is the priority. Carb drinks based on glucose polymers also replace fluids, but provide greater amounts of carb (10-20%) at lower osmolality (a measure of the number of dissolved particles in fluid). Most suitable for prolonged intense exercise (>90min) when fuel replacement is the priority and fluid losses are small.

Alcohol before any exercise has a negative effect on strength, endurance, co-ordination, power and speed. It raises the risk of injury. Excess body fat is a major disadvantage in most sports and fitness programmes and it reduces power, speed and performance. Very low body fat does not guarantee improved performance. There appears to be an optimal fat range for every individual and cannot be predicted by any standard linear relationship. There are three components to body fat:
  • Essential (tissue structure)
  • Gender specific (hormonal function)
  • Storage (energy)
The minimum %age fat for men is 5% and for women is 10%. In normal health these range 13-18% and 18-25%, respectively. Many athletes fall below these recommended ranges. Very low fat levels are associated with hormonal imbalance in both sexes (amenorrhea, infertility, reduced bone density, increased risk of osteoporosis). Very low fat diets can lead to deficient intakes of essential fatty acids and fat soluble vitamins. In the long term, fat and calorie restriction can result in other nutritional imbalances, depleted glycogen stores, chronic fatigue, loss of lean tissue and reduced performance. A fat intake of 15-30% of energy needs is recommended for athletes and active people. Unsaturated fatty acids should make up majority of the fat intake, with saturated fatty acids and trans fatty acids kept to a minimum.


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