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Introduction

Upon waking the body is in the postabsorptive or fasting state¹. This means that an individual has completely absorbed all nutrients from their last meal, and that energy requirements must be satisfied by nutrients already in present in the body. This paper looks at how energy is provided for those people that do not eat breakfast and are in the postabsorptive state.

The Absorptive State

After eating the body absorbs nutrients from food for about four hours, this is called the absorptive state. Glucose is freely available for energy production, and the body stores excess nutrients. Many processes occur during this state:

Glucose is oxidised by cells to produce ATP.

The liver converts glucose to glycogen and triglycerides.

Fatty acids, circulated as lipoproteins by the liver, are stored in adipose tissue.

Adipose tissue stores unused glucose as triglycerides, and some glucose is stored in muscles as glycogen.

Many amino acids are converted by the liver to keto acids, which can be used in the Kreb’s cycle for ATP production, or are used to synthesise glucose or fatty acids. Some amino acids are used to make proteins – either in the liver or other parts of the body.

During the absorptive state the blood glucose level is easily maintained.

The Postabsorptive State

The normal blood glucose level is between 70 – 120 mg/dl², about four hours after eating this level begins to drop. The body’s ultimate aim is to maintain blood sugar levels thereby maintaining normal ATP production, which uses glucose as its main source of fuel. When no glucose is being absorbed from food it is either made in the body and after a short time conservation of glucose begins and ATP is generated using other sources.

Glucose Production

There are a number of ways that the body can produce glucose when none is being absorbed from food, these include:

The breakdown of glycogen from the liver. This is a major source of glucose for the body, and can be supplied for up to four hours.

The breakdown of glycerol stored in adipose tissue in a process called lipolysis.

The breakdown of lactic acid, produced by muscles during anaerobic exercise, through gluconeogenesis in the liver.

The breakdown of amino acids, derived from skeletal muscle and other cells, through gluconeogenesis in the liver.

Blood glucose levels cannot be maintained for a great deal of time using these methods without further metabolic changes¹. Thus major adjustments must be made to conserve glucose whilst other methods are used to produce ATP¹.

ATP production without glucose

The body can generate ATP in the absence of glucose using the following methods:

Lipolysis which releases fatty acids and glycerol. The fatty acids, unlike the glycerol, cannot be used to produce glucose but most cells can oxidise them, feed them into the Kreb’s cycle as acetyl CoA, and produce ATP.

Cardiac muscles can produce ATP in the presence of oxygen from lactic acid.

Liver cells are able to oxidise amino acids to produce ATP.

Fatty acids are converted to ketone bodies by the liver; these can be used by the heart, kidneys and other tissues to produce ATP. The nervous system is also able to oxidise ketone bodies to produce ATP if fasting lasts for more than a day.

Glycogen can be broken down by skeletal muscle cells to glucose 6-phosphate, which can, through the process of glycolysis, be used to produce ATP.

If the person does not eat for a prolonged period, the body begins to mobilise fatty acids. The nervous system continues to use glucose, or ketone bodies, whilst the rest of the system can oxidise fatty acids releasing carbon dioxide and water to release energy. As a result of the glucose sparing and use of fatty acids a person can fast for several weeks, provided they drink water, and the blood glucose level does not fall by more than 25% of its normal range¹.

Hormones that regulate the fasting state

During the fasting state blood glucose levels require sustenance. Insulin production, which decreases the blood sugar level by causing cells to absorb glucose, falls away, whilst production of hormones that raise blood sugar levels and counter the effects of insulin starts to rise. The main hormones released in the postabsorptive state include:

Glucagon, which stimulates glycogen breakdown to glucose (glycogenolysis) in hepatocytes and muscle fibres, as well as stimulating gluconeogenesis (the synthesis of glucose from non-carbohydrate precursors) in hepatocytes and kidney cortex, cells.

Epinephrine which stimulates glycogenolysis in hepatocytes and muscle fibres, and stimulates the breakdown of fatty acids (lipolysis) in adipose cells.

Cortisol which stimulates the breakdown of proteins to amino acids in most body cells, stimulating gluconeogenesis in hepatocytes and the kidney cortex cells, as well as promoting lipolysis.

Other hormones involved in lipolysis include norepinephrine, human growth hormone, and thyroid hormones.

Conclusion

It is seen that the person who does not eat breakfast can survive throughout the morning until lunchtime and beyond. During fasting the body spares glucose, maintaining blood sugar levels, and employs other sources to feed the Kreb’s cycle for the production of ATP. The nervous system is particularly reliant on the maintenance of blood glucose levels, but it can use ketone bodies. Fasting can be sustained for weeks given the blood glucose level does not fall 25% below its normal range, and the body remains hydrated.

References

Tortora, G.J., Grabowski, S.R., Principles of Anatomy and Physiology - 8^th Edition, Harper Collins, NY, 1996.

Willis Hurst, J (Ed.), Medicine for the Practicing Physician – 3^rd Edition, Butterworth-Heinemann, USA, 1992.