Reading #1 Introduction to Science

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Digestive Efficiency

The “availability” to the body of the ingested macronutrients determines their ultimate caloric yield. Availability refers to completeness of digestion and absorption. Normally about 97% of carbohydrates, 95% of lipids, and 92% of proteins become digested, absorbed, and available to the body for energy. Large variation exists for protein ranging from a high of 97% for animal protein to a low 78% for dried peas and beans. Furthermore, less energy becomes available from a meal with high fiber content. Considering average digestive efficiencies, the net kCal value per gram for carbohydrate equals 4.0, 9.0 for lipid, and 4.0 for protein. These corrected heats of combustion comprise the “Atwater Factors,” named after the scientist who first studied the energy release from food in the calorimeter, and in the body.

Energy Value of a Meal

Table 1. Method of calculating the caloric value of a food from its composition of nutrients.


Atwater Factor



Total grams

In one gram

KCal• g-1




0.04 g


(0.04 x 4.0=0.16)




0.13 g


(0.13 x 9.0=1.17)




0.21 g


(0.21 x 4.0=0.84)

Total kCal per gram: 0.16 + 1.17 + 0.84 = 2.17 kCal

Total kCal per 100 grams: 2.17 x 100 = 217 kCal

If the composition and weight of a food are known, the caloric content of any portion of food or an entire meal can be termed using the Atwater factors. Table 1 illustrates the method for calculating the kCal value of 100 g (3.l5 oz) of vanilla ice cream. Based on laboratory analysis, vanilla ice cream contains about 4% protein, 13% lipid, and 21% carbohydrate, with the remaining 62% water. Thus, each gram of ice cream contains 0.04 g protein, 0.13 g lipid, and 0.21 g carbohydrate. Using these compositional values and the Atwater factors the kCal value per gram of ice cream is determined as follows: The net kCal value indicate that 0.04 g of protein contains 0.16 kCal (0.04 x 4.0 kCal•g-1), 0.13 g of lipid contains 1.17 kCal (0.13 x 9 kCal•g-1, and 0.21 g of carbohydrate contains 0.84 kCal (0.21 g x 4.0 kCal•g-1. Combining the separate values for the nutrients yields a total energy value for each gram of vanilla ice cream equal to 2.17 kCal (0.16 + 1.17 + 0.84). A 100-g serving yields a caloric value 100 times as large, or 217 kCal. Increasing or decreasing portion sizes or adding rich sauces or candies, or, conversely, adding fruits or calorie-free substitutes will affect the kCal content accordingly. Fortunately, the need seldom exists to compute the kCal value of foods because the United States Department of Agriculture (USDA) has already made these determinations for most foods.

Calories Equal Calories

When examining the energy value of various foods, one makes a rather striking observation with regard to a food’s energy value. Consider, for example, five common foods: raw celery, cooked cabbage, cooked asparagus spears, mayonnaise, and salad oil. To consume 100 kCal of each of these foods, one must eat 20 stalks of celery, 4 cups of cabbage, 30 asparagus spears, but only 1 tablespoon of mayonnaise or 4/5 tablespoon of salad oil. The point is that a small serving of some foods contains the equivalent energy value as a large quantity of other foods. Viewed from a different perspective, to meet daily energy needs a sedentary young adult would have to consume more than 4000 stalks of celery, 800 cups of cabbage, or 30 eggs, yet only 1.5 cups of mayonnaise or about 8 ounces of salad oil! The major difference among these foods is that high-fat foods contain more energy with little water. In contrast, foods low in fat or high in water tend to contain relatively little energy. An important concept, however, is that 100 kCal from mayonnaise and 100 kCal from celery are exactly the same in terms of energy.

Also note that a calorie reflects food energy regardless of the food source. Thus, from an energy standpoint, 100 calories from mayonnaise equals the same 100 calories in 20 celery stalks. The more one eats of any food, the more calories one consume. However, a small quantity of fatty foods represents a considerable quantity of calories; thus, the term “fattening” often misdescribes these foods. An individual's caloric intake equals the sum of all energy consumed from either small or large quantities of foods. Celery would become a “fattening” food if consumed in excess!

For Your Information

Equivalents for 100 Calories

• 20 stalks of celery • 2 bites (1/16) of a Big Mac

• 4 cups cooked cabbage • 9 oz skim mile

• 1-tablespoon mayonnaise • 5 oz whole milk

Heat Produced by the Body


The principles of human heat production is summarized below:

Calorimetry involves the measurement of heat dissipation, which is a direct measure of Calorie expenditure. One can measure heat directly (direct calorimetry) or the amount of oxygen consumed (indirect calorimetry) to indicate caloric expenditure by the body.

Direct Calorimetry

All of the body's metabolic processes ultimately result in heat production. Consequently, we can measure human heat production similarly to the method used to determine the caloric value of foods in the bomb calorimeter (refer to Figure 1, above).

The human calorimeter illustrated in Figure 2 consists of an airtight chamber where a person lives and works for extended periods. A known volume of water at a specified temperature circulates through a series of coils at the top of the chamber. Circulating water absorbs the heat produced and radiated by the individual. Insulation protects the entire chamber so any change in water temperature relates directly to the individual’s energy metabolism. For adequate ventilation, chemicals continually remove moisture and absorb carbon dioxide from the person’s exhaled air. Oxygen added to the air recirculates through the chamber.

Professors Atwater (a chemist) and Rosa (a physicist) in the 1890s built and perfected the first human calorimeter of major scientific importance at Wesleyan University (Connecticut). Their elegant human calorimetric experiments relating energy input to energy expenditure successfully verified the law of the conservation of energy and validated the relationship between direct and indirect calorimetry. The Atwater-Rosa Calorimeter consisted of a small chamber where a subject lived, ate, slept, and exercised on a bicycle ergometer or treadmill. Experiments lasted from several hours to 13 days; during some experiments, subjects performed cycling exercise continuously for up to 16 hours expending more than 10,000 kCal! The calorimeter's operation required 16 people working in teams of eight for 12-hour shifts.

Direct measurement of heat production in humans has considerable theoretical implications, but limited practical application. Accurate measurements of heat production in the calorimeter require considerable time, expense, and formidable engineering expertise. Thus, the calorimeters use remains generally inapplicable for human energy determinations for most sport, occupational, and recreational activities. Also, direct calorimetry cannot be applied for large-scale studies in underdeveloped and poor countries. Great need exists for total nutritional and energy balance assessments under a variety of deprivation conditions, particularly undernutrition and starvation. In the 90 years since Atwater and Rosa published their papers on human calorimetry, other methodology evolved to infer energy expenditure indirectly from metabolic gas exchanges (see next section). For example, the modern space suit worn by astronauts, in reality a “suit-calorimeter,” maintains respiratory gas exchange and thermal balance while the astronaut works outside an orbiting space vehicle.

Indirect Calorimetry

All energy-releasing reactions in the body ultimately depend on oxygen utilization. By measuring a person’s oxygen uptake during steady-rate exercise, researchers obtain an indirect yet accurate estimate of energy expenditure. Indirect calorimetry remains relatively simple and less expensive to maintain and staff compared to direct calorimetry. Closed-circuit and open-circuit spirometry represent the two common methods of indirect calorimetry.

Closed-Circuit Spirometry

Figure 3 illustrates the technique of closed-circuit spirometry developed in the late 1800's and now used in hospitals and research laboratories to estimate resting energy expenditure. The subject breathes 100% oxygen from a prefilled container (spirometer). The equipment consists of a "closed system" because the person rebreathes only the gas in the spirometer. A canister of soda lime (potassium hydroxide) placed in the breathing circuit absorbs the carbon dioxide in the exhaled air. A drum attached to the spirometer revolves at a known speed and records oxygen uptake from changes in the system's volume.

During exercise, oxygen uptake measurement using closed-circuit spirometry becomes problematic. The subject must remain close to the equipment, the breathing circuit offers great resistance to the large gas volumes exchanged during exercise, and the relatively slow speed of carbon dioxide removal becomes inadequate during heavy exercise.

Open-Circuit Spirometry

The open-circuit method remains the most widely used technique to measure oxygen uptake during exercise. A subject inhales ambient air with a constant composition of 20.93% oxygen, 0.03% carbon dioxide, and 79.04% nitrogen. The nitrogen fraction also includes a small quantity of inert gases. Changes in oxygen and carbon dioxide percentages in expired air compared to inspired ambient air indirectly reflect the ongoing process of energy metabolism. Thus, analysis of two factors volume of air breathed during a specified time period, and composition of exhaled air provide a useful way to measure oxygen uptake and infer energy expenditure.

Three common open-circuit, indirect calorimetric procedures measure oxygen uptake during physical activity:

  • Portable spirometry

  • Bag technique

  • Computerized instrumentation

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