Info

retardation

*RDA = recommended dietary allowance.

*RDA = recommended dietary allowance.

needs to maintain a positive nitrogen balance, the recommendation adds 30 grams of protein per day. Similarly, a nursing mother requires an additional 20 grams of protein per day to maintain a high level of milk production.

In addition to tissue wasting, protein deficiency may also decrease the level of plasma proteins, which decreases the colloid osmotic pressure of the plasma. As a result, fluids collect in the tissues, producing a condition called nutritional edema. Table 18.3 summarizes the sources, uses, and requirements for carbohydrate, lipid, and protein nutrients.

O^1 Reconnect to chapter 15, Exchanges in the Capillaries, pages 605-606.

99 What are the physiological functions of proteins?

^9 What is a negative nitrogen balance? A positive nitrogen balance?

^9 How can inadequate nutrition cause edema?

Energy Expenditures

Carbohydrates, fats, and proteins supply energy. Since energy is required for all metabolic processes, energy availability is of prime importance to cell survival. If the diet is deficient in energy-supplying nutrients, structural molecules may gradually be consumed, leading to death. On the other hand, excess intake of energy supplying nutrients may lead to obesity.

Energy Values of Foods

The amount of potential energy a food contains can be expressed as calories (kal'o-rez), which are units of heat. Although a calorie is defined as the amount of heat required to raise the temperature of a gram of water by 1 degree Celsius (°C), the calorie used to measure food energy is 1,000 times greater. This large calorie (Cal.) equals the amount of heat required to raise the temperature of a

Oxygen -

Stirrer-

Water -

Oxygen -

Stirrer-

Water -

Electric wires for ignition of sample

-Food sample

-Insulation

Platinum dish

Figure 18.7

A bomb calorimeter measures the caloric content of a food sample.

Electric wires for ignition of sample

. Thermometer

-Food sample

Calories Used during Various Activities

Calories Used during Various Activities

-Insulation

Platinum dish

Figure 18.7

A bomb calorimeter measures the caloric content of a food sample.

kilogram (1,000 grams) of water by 1°C (actually from 14.5°C to 15.5°C) and is also equal to 4.184 joules, with a joule being the international unit of heat and energy. This unit is sometimes called a kilocalorie, but it is customary in nutritional studies to refer to it simply as a calorie.

Figure 18.7 shows a bomb calorimeter, which is used to measure the caloric contents of foods. It consists of a metal chamber submerged in a known volume of water. A food sample is dried, weighed, and placed in a nonreac-tive dish inside the chamber. The chamber is filled with oxygen gas and submerged in the water. Then, the food is ignited and allowed to oxidize completely. Heat released from the food raises the temperature of the surrounding water, and the change in temperature is measured. Since the volume of the water is known, the amount of heat released from the food can be calculated in calories.

Caloric values determined in a bomb calorimeter are somewhat higher than the amount of energy that metabolic oxidation actually releases because nutrients generally are not completely absorbed from the digestive tract. Also, the body does not completely oxidize amino acids but excretes parts of them in urea or uses them to synthesize other nitrogenous substances. When such losses are considered, cellular oxidation yields on the average about 4.1 calories from 1 gram of carbohydrate, 4.1 calories from 1 gram of protein, and 9.5 calories from 1 gram of fat. The fact that more than twice as much energy is derived from equal amounts by weight of fats as from either proteins or carbohydrates is one reason why avoiding fatty foods helps weight loss. Fats also encourage weight gain because they add flavor to food, which can cause overeating.

99 What term designates the potential energy in a food?

^9 How can the energy value of a food be determined?

^9 What is the energy value of a gram of carbohydrate? A gram of protein? A gram of fat?

. Thermometer

Activity

Calories (per Hour)

Walking up stairs

1,100

Running (jogging)

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