The only part of Physics which is 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 the bond between the second and third phosphates in ATP], 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, given that
you probably would not even take the course were it not required for your Nursing degree].
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 hamburger, 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 for BMR, ADLs, and exercise. Some of the energy at each conversion is lost as heat, which
you can use to help maintain your body temperature at 98.3 °F [the 98.3° value is
based on data from wellness clinics in and around Boston [study published in the mid to late 1980s
in either the Journal of the American Medical Association (JAMA) or in the New
England Journal of Medicine], while the more familiar 98.6° is based on patients in hospital
(these patients were probably sick at the time, since hospitals are rather expensive hotels if
you're not sick)].
This leads us to the concept of efficiency, which
is defined as the (amount of useful energy ouput) 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 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.
In my never-ending efforts to keep you 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 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 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 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 glycolysis 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 glycolysis 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 guess from the above discussion, these tend to be coupled reactions. The
catabolic reactions in the pathways of glucose metabolism (glycolysis, citric acid cycle, and
electron transport system) are coupled to reduction either of ADP to ATP or of “electron
carriers” which carry the electrons to the electron transport system where they are oxidized in
reactions coupled to the reduction of ADP. 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
lumen of the digestive system is 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 addicted to trivia.
We start with the calorie, with a small “c”, or
“scientific” calorie. This used to be considerd 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.” As you recall [or not], the apparatus used to measure calories in food is the
bomb calorimeter, which is immersed in about 10 liters of water.
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 10 liters of water one Celsius degree.
[1g water is 1 cm3]. Obviously [at least to me], we would want to immerse our bomb
calorimeter in exactly 10 liters of water, so each Celsius degree of increase in water temperature
represents 1 Calorie [when we burn one Peanut Butter Sandwich Girl Scout Cookie® in the
bomb calorimeter, we will get an increase of 56.7 Celsius degrees, so a single serving (3 cookies)
will provide 3 times 56.7 = 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] 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 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 [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.
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 about 200 kw-hrs per day.
Life requires large amounts of energy just for homeostasis, which is your largest energy need other
than the large energy demands of all that strenuous physical exercise you probably don't do. Using a
modified version [ignoring Brown's estimate of dietary thermogenesis] of the estimates from your text
(pg 8-5), an inactive individual uses about 77% of their daily energy expenditure for basal
metabolism (BMR), while an active person uses about 57% of their daily energy expenditure for BMR.
[I ignored Brown's estimates of dietary thermogenesis because I suspect she, and other authors of
Nutrition texts, over-estimate both BMR and dietary thermogenesis. From a Biology point of view
rather than a Nutrition point of view, I think dietary thermogenesis ought to be included in BMR or
ADLs anyway.] Meanwhile, back at the ranch… [for all of you who don't remember the serial
Western at the movie theater's Saturday Matinee], 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 BMR (including dietary thermogenesis) plus
Activities of Daily Living (ADLs).
Most references (both Biology and Nutrition) describe the Basal Matabolism Rate (BMR)
as the energy burned by the body “at rest;” this is also called Resting Metabolic Rate
(RMR). Direct measurement 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. (Actually, there are other methods of measuring, such as a ventilated hood, and even a
handheld device.) 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. For every 6 moles of
CO2 (324 g CO2) released, or 6 moles of O2 (192 g O2)
consumed, by the subject, 219 kcal will have been captured in ATP molecules. Assuming that ATP is
produced in response to ATP being oxidized to ADP, we conclude the subject has burned 219 Cal.
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 by Dr. LaFrance 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 ± 10.9 [the ±
number is the standard deviation (about 67% of all values for similar populations can be expected to
lie within 1 standard deviation of the mean)]; mean weight = 78.2 kg [172.4 lbs] ± 9.5; mean %
body fat = 20.7 ± 10.1) and 24 women (mean age = 36.9 ± 10.9; mean weight = 68.0 kg [149.9
lbs] ± 12.2; mean % body fat = 32.2 ± 10.6). The grand mean (both sexes over two morning
visits) was 1,509.7 ± 33.7 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 (pg. 8-4) [10 men at a factor of 11
plus 24 women at a factor of 10 (more algebra on an early Monday morning)] we would get 1,611
or 7% over the measured value.
The description of 'Inactive' in your text (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 for
BMR plus 400 Cal/day 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.
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/ by Dr. LaFrance 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).
It occurs to me that I have been assuming that you understand what I am
refering to as BMR and ADLs. In case you don't remember, or haven't yet learned these terms, I will
explain:
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 anabolic metabolic processes, and supplying energy of activation for catabolic
metabolic processes. You will also circulate blood and breathe, exercise CNS & hormonal control
of homeostatic functions, 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)
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 (at a commuter campus), 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.
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 who died from proximate causes, secondary to 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 biochemistry of energy capture in ATP molecules is based on glucose, a
monosaccharide, 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. 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: 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.
The biochemistry of energy metabolism can be summarized as follows:
| glycolysis | citric acid cycle Krebs cycle |
ETS | |
| substrate | glucose | pyruvate | electrons |
| product | pyruvate electrons |
electrons | ATP |
| by-products | - | H2O, CO2 | - |
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. Your text 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?
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 serum glucose drops. Excess serum fatty acids can also be
withdrawn by the liver, and converted 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
storage in the liver, or returned to the serum. 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 blood
stream, it also signals the adipose to convert the fatty acids to fat for long term storage. The
liver also can release glycogen to be taken up for 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 cells which cannot find
any other energy source.
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