The only part of Physics which is even remotely relevant to the study of Nutrition [remember, it
is “the Science of Nutrition”] is the Laws of Thermodynamics which describe behavior of
energy in the Universe. Most students of introductory Nutrition tend to be confident that they
should be able to understand the concepts of nutrition without having to learn physics, and they
are probably correct. Not withstanding that objection, I feel that you will appreciate some concepts
of nutrition better if you have been exposed to the Laws of Thermodynamics, even if you don't
remember them. So, you “get” to hear about these laws.
The 1st Law of Thermodynamics states that energy is neither created nor destroyed, but is merely converted from one form to another. Although we repeatedly state that Humans consume energy, they obviously don't; that would require that we destroy energy by consuming it. We actually take energy in one form [the energy stored in chemical bonds in food substances] and convert it to other forms [mostly energy stored in a chemical bond in ATP molecules], then convert it again into yet another form [such as the energy used to think about, and to try to remember the key concepts necessary and sufficient to get a good grade in this course].
The 2nd Law of Thermodynamics states that any time energy is converted from one form to another, a portion is lost as heat. Basically this says that you simply cannot capture all of the energy stored in a double cheeseburger, large fries & diet cola as energy stored in ATP. This also means that if you consume 2,000 Calories from food per day, you cannot expect to use all 2,000 of them to sustain life (resting energy use, activities of daily living or ADLs, plus exercise or intentional physical activity). Some of the energy at each conversion is lost as heat, some of which you can use to help maintain your body temperature at 98.6 °F, and the rest is completely lost. The published guidelines for recommended daily energy intake are already adjusted to account for this lost energy.
This leads us to the concept of efficiency, which is defined as the “(amount of useful energy output) divided by (the total energy input).” Ecological studies which have addressed this concept have reported a 10% efficiency in conversion of food biomass to biomass of the organism doing the eating. This is not energy efficiency, but makes the point that about 10% of the energy available from your diet is used for growth and repair of tissues, leaving less than 90% for use as energy. I have seen a few references to 60% energy efficiency for Humans, but have not reviewed the sources of the data. Modern (1970's) coal-fired power plants are only about 40% efficient in converting energy stored as chemical bonds in coal to electricity fed to the power distribution grid, so if Humans are 60% efficient, that is an impressive number!
The most important part of Chemistry for understanding bioenergetics in Humans is the concept
of Oxidation - Reduction reactions (Redox). Even if you have taken
Chemistry, you probably don't feel that you really understand oxidation - reduction, but don't worry
about that, I don't either! The basic concept is that oxidation reactions which release net
energy are coupled to reduction reactions which consume net energy,
and the energy released by the oxidation reactions provides the energy to drive the reduction
reactions. By extension, it is coupled oxidation - reduction reactions which drive metabolism in
living creatures. Confused yet? Hopelessly confused, somewhat confused, somewhat unconfused,
hopelessly unconfused, none of the above?
In my never-ending efforts to keep you only slightly confused, I offer my infamous
“Fire in the Fireplace” story.
“We all know that it is a pleasant experience to sit back and enjoy a roaring fire in the fireplace (weather permitting; August is not a good time for building a fire in the fireplace, in most locations), so I set out to build a roaring fire to impress my girl friend at the time. I piled a bunch of logs in the fireplace, then sat down to enjoy the fire. But there was no fire. Ah ha! I remembered from Boy Scouts that you need kindling to start a good fire, so I removed the logs, put in the kindling and put the logs back. Again I sat down to enjoy the fire, but there was still no fire. Humm, perhaps my friends who were never Boy Scouts were right, and you need wadded up newspaper to start a decent fire. So I emptied the fireplace, wadded up a newspaper, put it in the fireplace, threw the kindling on top of the paper, added logs. Now maybe I can enjoy the fire, but there is still no fire. Back to my Boy Scout days, I was supposed to be able to start a fire [to earn some badge] with no more than 3 matches. Ah ha! I need 3 matches. The match, of course, has no fire until you strike it. Moving the ‘strike anywhere’ match across the hearth uses muscle energy to cause enough friction to heat the yellow tip of the match head hot enough to cause the Phosphorous to burst into flame [the first redox]. The heat of the burning Phosphorous heats the blue part of the match head enough to cause the Sulfur to begin burning [second redox]. The heat of the burning Sulfur heats the matchstick to cause it to burn [third redox]; the heat of the burning matchstick causes the paper to burn [fourth redox]; the heat of the burning paper causes the kindling to burn [fifth redox]. Finally, the heat of the burning kindling ignites the logs [sixth redox], and at long last, I can enjoy the roaring fire. Unfortunately, my girl friend at the time became annoyed at my lengthy process to achieve a such a simple task, and went home. And now you should have an intuitive grasp of the concept of redox.” [And in Native American tradition, the signal that the story is over: “That's all I have to say about that.”]
Even oxidation reactions generally require some energy to be added to get the otherwise energy releasing reaction started. This is “energy of activation,” like the friction on the match head to light the fire in the fireplace. The total energy released minus the energy of activation is the net energy released. For example, the first two steps in burning of sugar in living creatures are the phosphorylation of glucose [to glucose 6-phosphate, then to fructose 1,6-biphosphate] which uses the energy from the oxidation of 2 ATP molecules, and activates the energy-capturing pathway [each subsequent step gets its activation energy from the previous step].
Metabolism consists of catabolic and anabolic reactions. The catabolic reactions are decomposition where a large molecule becomes smaller molecules. This releases net energy so the reaction is exothermic, and is an oxidation reaction. Anabolic reactions are synthesis reaction where small molecules become a larger molecule. Such reactions absorb net energy so are endothermic, and are examples of reduction reactions. As you are supposed to remember oxidation and reduction reactions tend to be coupled reactions. The catabolic reactions in the pathways of glucose metabolism (oxidation ultimately to Carbon dioxide and water) are coupled to reduction (ultimately) of ADP to ATP. Conversely, anabolic reactions in cells are coupled to the oxidation of ATP to ADP to provide the energy for the anabolic reactions. The digestion of food in the digestive system is technically not metabolism because the contents of the digestive system are outside the body.
There are numerous units of measurement for energy, most of which are not even “interesting.” That, however, will not keep me from listing them for you. I expect you to forget these as soon as possible after reading them; at least, I would not recommend remembering them unless you are planning to become a world class Trivial Pursuit master.
• We start with the calorie, with a small “c”, or “scientific” calorie. This used to be considered to be the metric unit for energy, and has been in common use in laboratories for over a century. By definition, one calorie is the amount of heat energy necessary to raise the temperature of 1 gram (1g) of water one Celsius degree (1 C°) starting at standard conditions (20 °C, 1 atmosphere pressure). Its symbol is “cal.” The apparatus used to measure calories in food is the bomb calorimeter [pronounced ‘Cal-or-im-eter’], which is immersed in 10 liters of water. We measure the increase in the temperature of the water to guess how many calories are in the food sunstance.
• The Calorie, with a capital “C”, or dietary calorie is a kilocalorie (1,000 cal). To distinguish it from the scientific calorie, we use the symbol “Cal.” One Cal of heat would raise the temperature of 1.0 liters of water one Celsius degree [1g water is 1 ml (milliliter)]. When we burn one Peanut Butter Sandwich Girl Scout Cookie® in the bomb calorimeter, we will get 170 Calories. An average daily intake of 10 serving of these cookies will provide more than enough Calories to allow you to join the over-weight population in the future, while supporting your local, friendly Girl Scout Troop generously today.
• The Official [Système Internationale, or SI] metric unit of energy is now the joule [pronounced “jewel”], symbolized as j, or sometimes J. One Kilocalorie (1 kcal, or 1 Cal) is equal to 4.2 j. After this sentence and with a little luck, you may never use (or even hear) the term “joule” again.
• In the English system of weights and measures, the unit of energy is the BTU, or British Thermal Unit. It is defined as the energy as heat needed to raise one pound (1 lb) of water one Farenheit degree (1 F°). The next time you see the BTU it will describe the heat output of a furnace or air conditioner. Incidently, the only country in the entire World where the English system of weights and measures is still used is the United States of America. However, the Congress passed a law declaring the metric system to be the official system in 1986. They also had passed a similar law in 1976, and before that in 1856. [dates approximate]
• If you have natural gas piped into your home, you have met the therm. A therm is the energy as heat which can be released by burning 100 ft3, or CCF of gas [the first C is Latin for 100, the second C is cubic and the F is feet]. One therm is 100,000 BTU. Your supplier of natural gas bills you for the therms delivered.
• Actually, the watt is a unit of power, or energy per unit time, but is included here anyway because it involves energy. A watt (1 w) is one joule per second. For those of you who don't do algebra on Monday mornings, this means that a 100 w light bulb uses 100 j/sec. After 3,600 seconds (1 hr), the 100 w bulb will have used 360,000 j [about 85.7 Cal]. Your electric company [not the one on the Monopoly© board, nor on PBS television; the one who keeps sending you bills for kilowatt-hours consumed] bills you in kilowatt-hours, where a kilowatt-hour is the energy involved at 1,000 w for 1 hr. Leaving a 100 w light bulb on for 1 hour uses 0.1 kw-hr. A 60 w bulb left on for one hour uses 0.06 kw-hr, although the “energy saver” flourscent bulb of equal brightness would have used only 0.013 kw-hr. [This is supposed to be useful, although irrelevant, trivia.] Most Nutrition textbooks suggest that a typical American will use the equivalent of about 200 kw-hrs each day for metabolism, ADLs, and intentional activity.
Life requires large amounts of energy just for homeostasis, which is your largest energy need
[you use over 4,000 - 4,500 kw-hrs each month] other than the large energy demands of all that
strenuous physical exercise you probably don't do [you use as much as another 1,000 - 2,000 kw-hrs
per month for this]. Roizen & Oz state that “Only 15 percent to 30 percent of your calories
are burned through intentional physical activity” (You, on a Diet, pg 129), leaving 70
to 85 percent for sustaining Life. Homeostasis refers to maintaining a
reasonably consistent internal environment.
BMR (RMR) is Basal Metabolic Rate (Resting Metabolic Rate), and includes all energy demands of the body doing “nothing.” While doing nothing, you will be carrying out numerous metabolic (chemical) processes. You will also circulate blood and breathe, control your internal environment using your nervous system plus hormones, as well as carry on digestion and elimination of wastes.
ADLs are Activities of Daily Living, which are “the basic activities of caring for oneself: eating, dressing, bathing, using the bathroom ("toileting"), moving back and forth from a bed to a chair ("transferring"), and remaining continent.” (downloaded from link to The Federal Long Term Care Insurance Program at “www.ltcfeds.com/assets/glossary.html”). Dietary thermogenesis is “the energy expended in acquiring, eating and digesting food.” It seems to me that this should not be listed as separate energy demand [it is included in ADLs as ‘eating’]; and every accounting I have seen where dietary thermogenesis is accounted for as a separate “line item” yields total daily Calorie needs at levels guaranteed to increase the “obesity crisis.”
Exercise is any activity beyond that needed to sustain life, or as Roizen & Oz put it “intentional exercise.” If you are enrolled in traditional college courses in a lecture hall or laboratory, and park at the closest available parking space, walking to class counts as an ADL. If you park further from the door than the closest space, the distance you must walk to reach the closest parking space is intentional exercise, and therefore counts as exercise.
Direct measurement of RMR is relatively straight-forward: the test subject sits in a sealed room [resembling a telephone booth from old Superman movies], while air is circulated into and out of the room, with measurement of CO2, or O2, concentration in the intake and exhaust air. The increase in CO2, or decrease in O2, in the exhaust compared to the intake can be used to calculate the total energy burned at rest.
A recent study (Haugen et al, 2003. Variability of measured resting metabolic rate, Am J Clin Nutr 2003; 78:1141-4. downloaded from www.ajcn.org on February 3, 2009), published in the American Journal of Clinical Nutrition, was designed to determine the day-to-day variability in RMR. The study included 10 men (mean age = 40.0, mean weight = 78.2 kg [172.4 lbs], mean % body fat = 20.7 and 24 women (mean age = 36.9, mean weight = 68.0 kg [149.9 lbs], mean % body fat = 32.2). The grand mean (both sexes over two morning visits) was 1,509.7 Cal/day. Morning is the “standard” time to measure RMR, after a 12-hr fast, 12-hr abstention from caffeine & 12 h postexercise. Actual measured BMR turns out to be 1,500 Cal/day. Using the estimating method in Brown (2008. Nutrition Now, 5th ed. ©Thompson Learning, Inc. pg. 8-4), we would get 1,611 or 7% more than the measured value.
The description of 'Inactive' in Nutrition Now, 5th ed (Table 8.2 pg. 8-5) comes close to the definition of ADLs, so we could accept the 30% of BMR value [corrected for the about 10% overestimation in other values compared to measured values, or 27%]. This gives us 1,500 Cal/day (measured) for BMR plus 400 Cal/day (calculated) for ADLs, or 1,900 Cal/day [compared to the 1,800 - 3,000 Cal/day (depending on activity level) in the 2005 USDA Food Guide Pyramid, 1,600 - 2,800 Cal/day (depending on activity level) in 2000, & 1,200 - 1,400 Cal/day (depending on activity level) in 1950]. Not too surprisingly, the American waist size has been steadily increasing since the mid to late 1960's. I strongly encourage returning to the 1950 energy guidelines.
Scott & Djurisic, in the Journal of Exercise Physiology, (2008. The metabolic oxidation of glucose: thermodynamic considerations for anaerobic and aerobic exergy expanditure [sic]. JEPonline 2008; 11:34-43 downloaded from www.asep.org/journals/JEPonline/ on February 2009) quote 3.0 to 5.0 MET, where a MET is defined as “a multiple of resting metabolic rate,” (pg. 36) or described “as 1 kcal per minute so that it can be viewed as also having a duration component” (pg. 36). Thus an accurate accounting of the energy demands of exercise require a minute by minute summation of energy burned. The other option is to use the Roizen & Oz estimates of 15% to 30% of total Calories for intentional exercise. Adding these Calories in, our total daily energy needs range from 2,235 Cal (minimal intentional exercise) to 2,710 (very active). Again, these numbers impress me as over-estimated, because since the USDA Food Guide Pyramid was increased above the 1950s numbers (1,200 - 1,400 Cal/day) the average weight of Americans has steadily increased.
Thinking as a Biologist rather than as a Nutritionist, I find the discussions of “energy in foods” in Nutrition textbooks, and other reliable sources of nutrition information, to be a bit misleading. Some non-reliable sources [including a book that was a New York Times best-seller written by a Medical Doctor (Dr. Atkins)] are dangerously misleading, in that hundreds of women died within the first year after publication of the book by following the Dr. Atkins' diet advice [plus an unknown number whose deaths were not attributed (on the death certificate) to the diet, but whose death certificates list other causes (which were side-effects of the diet).
The problem is that Nutritionists apparently consider the 'energy nutrients' to include carbohydrates, protein, and fats. Biologically, the energy nutrients are only carbohydrates and fats, because the biological chemistry of energy capture in ATP molecules is based on glucose (a carbohydrate), as the substrate. Almost all carbohydrates can be digested to disaccharides and soluable fiber; disaccharides are then converted to monosaccharides in the cells. Carbohydrates, in the food supply, which cannot be digested to sugars are considered to be fiber. Fiber can not be used as an energy source by living creatures [other than termites]. Fats are easily converted to glycogen (animal starch, a complex carbohydrate) or to sugar analogs, which can serve as substrates for the biochemistry of energy capture as ATP. An important chemical characteristic of carbohydrates and fats is that they contain only Carbon (C), Hydrogen (H), Oxygen (O), and sometimes phosphate groups (PO4), or Pi -the same 'Pi' in the following reactions:
ADP + Pi → ATP
ATP → ADP + Pi
Proteins, on the other hand also contain Nitrogen (N) and sometimes Sulfur (S). You can utilize amino acids (digestion products of protein) as an energy source, but you must first remove any element(s) not found in carbohydrates and fats. Removal of Nitrogen is deamination, and produces the toxic byproduct, ammonia (NH3). The ammonia must be converted quickly to a less toxic substance, urea [as in uremic poisoning, a potentially fatal condition]. Sulfur can be removed as sulfate or as sulfite [free radicals, or oxidants, capable of causing serious damage, leading to accelerated aging or potentially to cell death]. Once the amino acid has been stripped of all elements not found in carbohydrates and fats, it can be converted [at relatively high energy cost] to sugar analogs to be used in energy capture metabolism. You are designed to use protein as an energy source only in emergencies - that is, starvation relative to the biological energy nutrients: both carbohydrate starvation and lipid starvation! Not only should protein not be on the list of energy-providing nutrients, it certainly should not be listed second, implying that is more important as a energy source than fats. [I suspect that fats became listed as third because of the urban legend that dietary fats move essentially unchanged from the stomach to the waist-line, and to the plague deposits in your arteries; neither of which is true]
The biological chemistry of energy metabolism can be summarized as follows:
| ||glycolysis1||citric acid cycle2
As to the question of what foods contain energy, ALL foods contain energy, but not always in a form suitable for capture as ATP. There are no foods which do not contain energy; in fact, most non-foods also contain energy. Nutrition Now, 5th notes that, if you have grilled food on a barbeque, “you have probably observed firsthand the high level of stored enery in fats… [because] drips from high-fat foods cause bursts of flames to shoot up from the grill.” (pg. 8-6) If you have toasted marshmallows at a bonfire, you have also had the opportunity to observe the high level of stored energy in carbohydrates, because marshmallows often erupt into flame. But when the hamburger slips off the grill onto the coals below, have you ever seen protein produce flames? Yet protein stores as much energy per gram as do carbohydrates. This word picture, while accurate, illustrates something different than amount of “energy stored.” As a question for thought [this means think about it], what does it illustrate? [hint: there are two different correct answers hidden above in this lecture.]
While physiologic processes provide the source of energy for cells, as we have already discussed,
the body also has mechanisms for dealing with energy intake in excess of energy needs. These
processes are mediated by the liver, and fall into short-term and long-term storage of the surplus.
The liver can remove excesses both of sugars and of fatty acids from the hepatic portal vessel.
Some sugar can be stored in the liver, to be released as blood glucose drops. Excess fatty acids
in the blood stream can also be withdrawn by the liver, and converted back to fats for storage in
the liver. When the liver stores of glucose exceed its capacity, the excess is converted to glycogen
(animal starch) and stored in the liver, but this requires insulin so diabetics cannot make glycogen.
All storage by the liver is short-term.
Excess glycogen in the liver is converted to fatty acids, then either converted to fat for short-term storage in the liver, or returned to the blood stream. Excess fat stored in the liver is also converted back to fatty acids and returned to the blood stream. When the liver adds surplus fatty acids to the blood stream, it also signals the fat tissues (adipose) to convert the fatty acids to fat for long term storage. The liver also can release glycogen to be taken up for short-term storage by skeletal muscle.
The liver can store limited amounts of amino acids, but does no processing of them. The only known site of amino acid deamination and processing as an energy source is in those cells which cannot find any other energy source.
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revised: 10 Aug 2010