The type of food eaten can also influence your metabolism. The food you eat can either be burned to liberate energy, converted into body weight, or it can be excreted. All foods release heat when they are burned. This release of heat is measured in kilocalories. A calorie is a unit of heat. Practically speaking this unit is too small to be useful therefore the kilocalorie is the preferred unit in metabolite studies. Not all foods are burned completely to produce energy. Some of them are only partially degraded to provide building blocks to support repair and growth. Generally speaking, the calories coming from protein are used for maintenance, repair, and growth of new tissues and organs. Calories from carbohydrates are used for energy. Calories from conventional sources of fat are prone to be stored as fat since they already have the same molecular structure as body fat. The heat liberated from a particular food is thus a measure not only of its energy content but also of its tendency to be burned as heat. This is known as the thermogenic effect.

Increased thermogenesis means increased heat production, which correlates with increased oxygen consumption, and an increased metabolic rate. The more heat your body produces, the more oxygen it needs, because heat cannot be liberated in the absence of oxygen. Food efficiency is simply a measure of how efficiently a particular food is converted to body weight. Foods with high food efficiency are prone to be converted to body weight while foods with low food efficiency are prone to be burned as energy.

Understanding how your body will use the calories you consume can help you in setting up your nutritional program. Simply counting calories will not lead to body fat loss. The heat liberated from a particular food, whether it is fat, protein, or carbohydrate, is determined by its particular molecular structure, and this structure determines its thermogenic effect. The higher the thermogenic effect of any particular food, the higher your metabolic rate. Know what you are consuming, but more importantly, know how your body will use your consumed calories. A method of determining the “fuel mix” being used has been developed, called the respiratory quotient (RQ), which gives us a way to measure the relative amounts of fats, carbohydrates, and proteins being burned for energy.

The respiratory quotient is a measure of the ratio of the volume of carbon dioxide expired, to the volume of oxygen consumed. Because the amount of oxygen used up for the combustion of fat, carbohydrate and protein will be different, differences in the RQ will indicate which nutrient source is being predominantly used for energy purposes. The formula for calculating RQ is:

RQ = volume of CO2 expired ÷ volume of O2 utilized

(Eq. 1-1)

The RQ for carbohydrates is 1.0, whereas the RQ for fat is 0.7. Fat has a lower RQ value since fatty acids require more oxygen for oxidation than the amount of carbon dioxide produced. The RQ for energy production from protein is about 0.8. The average person at rest will have an RQ of about 0.8; however, this is from using a mixture of fatty acids and carbohydrates for energy production, not from protein as the number may indicate. Remember, protein/amino acids are not usually used for energy. In a normal diet containing carbohydrate, fat, and protein, about 40% to 45% of the energy is derived from fatty acids, 40% to 45% from carbohydrates, and 10% to 15% from protein. However, this rate of energy production will vary depending on diet, physical activity, and level of physical training.

Research indicates that when the diet is high in carbohydrates, the RQ is higher, and therefore more energy is being produced from carbohydrates. When the diet is low in carbohydrates and higher in fat, more energy is produced from fat. Additionally, training intensity will affect the energy source during exercise. Exercise rate below 60% of maximum oxygen uptake (VO2 max) results in RQ of about 0.8, which indicates an equal portion of energy derived from fatty acids and carbohydrates. As training intensity increases above 60% VO2 max, more carbohydrate is used for energy. Exercise intensity at 100% VO2 max (which can only be sustained for minutes) yields a RQ of 1. You must also keep in mind that amino acids, in particular the BCAAs (branch chain amino acids that aid in recovery), are also being used for energy during exercise and at rest–perhaps as much as 10% or more during exercise.

In general, physical conditioning lowers the RQ, which means more energy is being obtained from fatty acids in the trained individual. However, more energy is also being obtained from protein in the trained individual. Carbohydrate is always being used for energy. For example, when comparing the RQ of untrained individuals vs. trained individuals during exercise, the RQ of the untrained individuals was 0.95, and the RQ of the trained individuals was 0.9. This means that while both groups were using mostly carbohydrate for fuel during exercise, the trained individuals were using a higher amount of fatty acids for energy. At rest, fatty acids are the predominant energy source in most people; as exercise begins, carbohydrate utilization increases. High intensity exercise uses more carbohydrate, while low to moderate intensity exercise uses fatty acid and carbohydrate for energy. While this discussion of RQ is very brief, you can see that the energy substrate utilization of the body is quite varied, and both composition of the diet and intensity of physical activity will determine which energy substrates are used. Therefore, it is easy to see why different sports require different dietary considerations.

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