Health

Fuel for Exercise: Bioenergetics and Muscle Metabolism

All energy originates from the sun as light energy. Chemical reactions in plants (photosynthesis) convert light into stored chemical energy. In turn, humans obtain energy either by eating plants or by eating animals that feed on plants. Nutrients from ingested foods are provided in the form of carbohydrates, fats, and proteins. These three basic fuels, or energy substrates, can ultimately be broken down to release the stored energy. Each cell contains chemical pathways that convert these substrates to energy that can then be used by that cell and other cells of the body

Energy is released when chemical bonds—the bonds that hold elements together to form molecules—are broken. Substrates are composed primarily of carbon, hydrogen, oxygen, and (in the case of protein) nitrogen. The molecular bonds that hold these elements together are relatively weak and therefore provide little energy when broken. Consequently, food is not used directly for cellular operations.

The amount of carbohydrate utilized during exercise is related to both the carbohydrate availability and the muscles’ well-developed system for carbohydrate metabolism. All carbohydrates are ultimately converted to the simple six-carbon sugar, glucose (figure 2.1), a monosaccharide (one-unit sugar) that is transported through the blood to all body tissues. Under resting conditions, ingested carbohydrate is stored in muscles and liver in the form of a more complex polysaccharide (multiple linked sugar molecules), glycogen

To be useful, free energy must be released from chemical compounds at a controlled rate. This rate is primarily determined by two things, the availability of the primary substrate and enzyme activity. The availability of large amounts of a substrate increases the activity of that particular pathway. An abundance of one particular fuel (e.g., carbohydrate) can cause cells to rely more on that source than on alternatives.

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Conclusion

The human body stores little oxygen. Therefore, the amount of oxygen entering the blood as it passes through the lungs is directly proportional to the amount used by the tissues for oxidative metabolism. Consequently, a reasonably accurate estimate of aerobic energy production can be made by measuring the amount of oxygen consumed at the lungs.

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