• Pyruvate dehydro- — ^
  • Q,0Amino acids kinase complex ^ <

Pyruvate NAD*


Ketone bodies

  • Pyruvate dehydro- — ^
  • Q,0Amino acids kinase complex ^ <

Pyruvate NAD*


Alcohol Fatty Acids

Fatty acids O-O- Alcohol


Citrate cycle NADH NAD+

  • CoA Succinyl-CoA
  • Triglycerides

Fatty acids O-O- Alcohol



Citrate cycle NADH NAD+

Ketone bodies


—CoA Succinyl-CoA


e a-Ketoglutarate

Oxidative phosphorylation


Respiratory chain

1/2 O2 H2O

How Food Energy Is Used

The adult body makes and uses ~85 kg (187 lb) of adenosine triphosphate (ATP) per day. The energy in ATP (A) is stored in the high-energy bonds between the phosphates; the terminal bond has the highest energy. Hydrolysis of these bonds (B) yields ~8 kcal (33.47 kJ) per 1 mol of ATP under physiological conditions. Additional energy can be obtained by further breakdown of ADP (adenosine diphos-phate) to AMP (adenosine monophos-phate)—this reaction is of lesser significance, though. In a reversal of the above hydrolysis, the energy released during the metabolic breakdown of energy nutrients is used to synthesize ATP by attaching a phosphate group to ADP. Even though in a healthy person about 95 % of the energy nutrients consumed are absorbed, only part of that energy is converted into ATP energy (C). Fifty percent of the metabolizable energy (also called physiological calorific content) of the metabolized energy nutrients is immediately converted to heat energy. A few percent are used for digestion, modification, distribution, and storage of energy nutrients. This postprandial thermic effect of food (TEF) used to be called "specific dynamic action." The TEF of proteins is 14-20%, of carbohydrates 4-10%, and of fats 2-4%. The actual energy transduced to ATP energy comes from the remaining ~40 %. One, therefore, always consumes considerably more energy than will ultimately end up as ATP. Individual differences in the efficiency of transduction determine whether a person "burns calories easily" or not and thereby affect weight. For more than 100 years, the energy content of nutrients has been determined using bomb calorimetry. The temperature changes in the medium surrounding the bomb calorimeter are measured. The measured heat difference is called the calorific value and reflects the energy content. For nutrients like lipids and carbohydrates, which are completely oxidized to CO2 and H2O in metabolism, the actual (physiological) calorific value (D) is identical to the bomb calorimetric calorific value, assuming complete absorption. For carbohydrates, it is 400 kcal (1680 kJ)/100 g, for lipids 930 kcal (3890 kJ)/100 g. Proteins do not have a uniform caloric value since proteins differ in their composition of amino acids. On average, though, it is similar to that of carbohydrates: for instance, the physiological calorific value of casein is 4.25 kcal (17.8 kJ)/g. Nitrogen (N) contained in proteins cannot be fully oxidized by the body. It is used to make urea, which is excreted by the kidneys. Since the N in urea is not oxidized, some of the energy consumed with proteins is always lost through the urine. Therefore, for proteins, the actual, physiological calorific value is always lower than the calorific value determined by bomb calorimetry.

A. Adenosine triphosphate

B. ATP hydrolysis

ATP H2O ADP P 8 kcal

  • 33.47 kJ)
  • C. Food Energy -
Energy Food Make Atp

I- D. Use of Nutrients for Making ATP


ATP formed/ 100 g nutrient

Physiological calorific value/100 g nutrient

Required nutrient energy/1 mol ATP

Carbohydrates (starch)


410 kcal (1720 kJ)

17.4 kcal (73.2 kJ)

Fat (tristearate)


930 kcal (3890 kJ)

18.1 kcal (75.7 kJ)

Protein (casein)


425 kcal (1780 kJ)

20.8 kcal (87.3 kJ)

Energy Requirements

Human energy requirements can be divided into three essential components (A): basal metabolism, energy required for physical activity, and energy used for food-induced thermo-genesis. There are a number of additional minor factors, often neglected because of their lesser share in the basal metabolism, like increase in metabolic rate due to body, organ, or muscle growth. Mental activity, though perceived as work, does not impact energy metabolism.

The basal metabolic rate (BMR) is by no means constant, but varies between individuals and over time. Sleeping induces a 10% decrease compared to being awake. Intense cold causes a 25% increase. Temperatures above 30° C (86° F) cause an increase of 0.5% per additional °C. Women, having more body fat, generally have a lower BMR compared to men. Until ages 4-5, basal metabolism increases significantly in relation to body weight, then decreases slowly until age 20-25. With increasing age, a decrease in metabolically active tissue (mainly muscle tissue) further reduces BMR. Individual variations in the significance of thermogenesis versus ATP synthesis account for people's different levels of metabolic efficiency.

Energy consumption does correlate with body weight (B), but depends predominantly on lean body mass. Since increasing body weight usually goes along with an increase in metabolically inactive fatty tissue, body mass increases usually do not result in large increases in energy consumption.

Most people greatly overestimate energy use through physical activity (C). Nowadays, high-energy-use activi ties are, for the most part, restricted to professional or semi-professional athletes.

Energy consumption is determined by the number of muscle fibers used and the intensity of their use. This type of activity always requires increased oxygen consumption resulting in an increase in respiratory rate and heart rate (D). The one-dimensional, sustained training of individual muscle groups as commonly practiced by body builders does not represent extreme activity and therefore increases energy use only minimally.

The classic method for measuring energy consumption is direct calorime-try. A closed, insulated chamber with controlled air supply represents a closed system in which the law of energy conservation applies: all energy produced must ultimately be transformed into heat. Since measuring this heat energy is expensive and complicated, today, indirect calorimetry is used nearly exclusively. This method is based on the fact that a defined amount of oxygen is required to produce a specific amount of energy. One can, therefore, determine energy use by measuring oxygen consumption. Recently, a new doubly labeled water technique (DLW) was developed that allows for very precise BMR measurement through detection of excreted 2H218O molecules. A drawback is the high price of 18O.

A. Energy Metabolism —

2500 n


Food-induced 6-10%

Physical activity

Basal metabolism


  • B. Energy Use and Body Weight -
  • kcal/d) 3500

3000 2500 2000 1500 1000

3000 2500 2000 1500 1000

50 60 70 80 90 100 110 120 130 Body weight (kg)

- C. Energy Use and Activity Levels -




D. Metabolic Rate and Physical Activity

Physical activity

Respiratory Oxygen rate used


Sleeping, lying down, sitting, driving, standing up, iron

¿Lightly active

Moderately active

Walking (6 kmh = 3.7 mph), bicycling, tennis, cleaning

Very active

Hiking uphill with backpack, digging, swimming (3 kmh = 1.9 mph), playing basketball

Heart rate


Calories consumed (kcal/min)

<2.5 1.0-1.1

  1. 5-5.0 2.6-2.9 2.5 -3.0
  2. 0-7.5 4.1-4.3


Extremely active

Running, climbing Unusually and extremely active Active to exhaustion

J 1500-






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