Energy for exercise is derived from two food sources, carbohydrates and fats - protein is available but rarely used. For each fuel there are two sources, intramuscular and peripheral (from blood). Glucose comes either from the blood sugar or from glycogen breakdown in the liver to prevent low glucose levels (hypoglycaemia).

Peripheral fat stores may be subcutaneous (under the skin) or intra-abdominal. Releasing fat from peripheral stores is termed lipolysis- intramuscular fats are in the form of triglycerides.

During exercise all four sources are utilised. Which is used depends on

• The intensity and duration of exercise
• The training status of the muscles
• The general diet- percentage of fates and carbohydrates
• The time interval between the last meal and exercise.

In addition to during the exercise, after exercise energy expenditure remains higher than following a period of rest.

Exercise itself induces hormonal changes that impact on the fuel utilised and food intake.

Fats require more oxygen to burn than carbohydrates- so in high intensity exercise more carbohydrates are burnt – in low intensity exercise when oxygen supply is maintained fats are the fuel of choice.

Our resting oxygen requirement is 3.5ml Oxygen/Kg Body weight / minute -referred to as 1 MET.

The act of eating, desk work, driving your car doubles that requirement to 7mls Oxygen/Kg Body weight (2 METS) - average daily activity 3-4 METS, hard squash game 13 METS and so on.

Increasing exercise increases heart rate and oxygen consumption – until there comes a time when you are unable to increase heart rate and oxygen consumption is maximal and anaerobic metabolism occurs (VO2Max).

The anaerobic threshold (AT) commences at about 75-80% of your attained maximal heart rate.

At rest you are at about 5-8% of VO2max. A comfortable walking pace is 25%VO2max, 40%VO2max is known as “fat burning zone”, 45-70% as the aerobic training zone and 70-75%VO2max is high intensity at which stage reliance on carbohydrate fuel is high and lactic acid is produced.

As intensity of work increases so does lactic acid levels and muscle failure in the untrained occurs.

Assuming that we undertake exercise without a pre-exercise meal then the following fuels are used as exercise increases.

Work Intensity Fuel Used Source Calories

Low Fat Peripheral 85%
Glucose Blood 15%

Moderate Glucose Muscle 30-50%
Fat Stores 50-70%

High Glycogen Muscle 75%

During walking a high percentage of fat is used although caloric consumption is low (3-6 calories per minute). 1gram of fat yields 9 calories.

One hour of walking requires 360 calories (85% fat) ie 306 calories = 34 Grams fat.

Traditionally recommended diets contains 50% carbohydrate, 30% fat, 20% protein. The writer believes that this carbohydrate level should be reduced to 40% or less in order to prevent the epidemic of obesity, glucose intolerance and type 2 diabetes.

As working muscles use 85% fat for energy after 20-30 minutes of this exercise level circulating fat levels are reduced.

To restock circulating fat levels adrenaline and noradrenaline are released which causes fat cells to breakdown and release fats into blood stream (Lipolysis).

Carbohydrates in the diet however release insulin which opposes the action of adrenaline and noradrenaline.

As work intensity increases both fat and carbohydrate burning with carbohydrate percentage increasing, overall fat consumption continues to increase.

Working muscles require more oxygen – therefore more blood – increases heart rate.

Muscles are only about 25% efficient in terms of work. The remaining 75% of calories are lost in sweating.

Shunting of blood to skin to dissipate heat reduces the amount of blood supplied to peripheral fat stores so muscle begins to breakdown its own fat stores.

Although contributing only 25% of calories maximum total fat burning occurs at higher intensity of exercise (75% maximum heart rate) as more total calories are required. The longer the exercise takes the more fat is burnt as glycogen stores become depleted and less glucose is produced

Carbohydrates in the diet result in insulin release (to metabolise the glucose).

Insulin opposes the action of adrenaline and noradrenaline and is antililypolytic shutting down fat release and promoting fat storage in peripheral cells.

As carbohydrate in food stimulates insulin production it is preferable to exercise in the fasting state if weight loss is your main aim and indeed to continue to fast after exercise.

The Post Exercise Period

After an aerobic session of exercise more oxygen is consumed than occurs after a period of rest. Fat burning provides the bulk of the excess oxygen consumption.

The greater the exercise intensity the greater the degree of increased fat burning during the post exercise period which may last up to three hours.

Exercise levels that deplete glycogen stores results in the body repleting these stores after exercise and not using carbohydrates for fuel with the body continuing to burn more fat.

When undertaking high intensity exercise the oxygen requirement of the muscles outstrips the supply, the muscle changes its metabolism and energy is produced without oxygen (anaerobic metabolism).

Under anaerobic conditions muscle metabolism increases lactic and pyruvic acid. Accumulation of these acids in the body can be dangerous to the heart.

Lactic acid is removed from the blood by

• Working muscles buffering themselves
• Liver removing lactic acid and converting it back to glucose
• Non-working muscles may take up lactic acid using it for fuel and conserving glucose
• Heart can use some lactic acid for fuel

Under lactic acidosis the heart does not respond to endogenous or exogenous catecholamines.

A novel form of exercise (Neocardial exercise) maximises the burning of fat whilst protecting the body against the dangerous build up of acids.

By utilising short periods of high intensity exercise (3 to 5 minutes) interspersed with one minute of low level of work lactic acid accumulation is minimised.

The high intensity work level increase fat burning and the heart conditions better at high intensity anaerobic work.

This form of exercise allows you to lose weight without dieting - however caloric restriction will increase weight loss. It is not suitable for diabetics or the non obese.

Dr M A Neaverson
Copywrite 2006 Neocardia

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