Most cells use glucose for ATP synthesis, but there are other fuel molecules equally important for maintaining the body's equilibrium or homeostasis. Indeed, although the oxidation pathways of fatty acids, amino acids, and glucose begin differently, these mechanisms ultimately converge onto a common pathway, the TCA cycle, occurring within the mitochondria (Figure 1). As mentioned earlier, the ATP yield obtained from lipid oxidation is over twice the amount obtained from carbohydrates and amino acids. So why don't all cells simply use lipids as fuel?
In fact, many different cells do oxidize fatty acids for ATP production (Figure 2). Between meals, cardiac muscle cells meet 90% of their ATP demands by oxidizing fatty acids. Although these proportions may fall to about 60% depending on the nutritional status and the intensity of contractions, fatty acids may be considered the major fuel consumed by cardiac muscle. Skeletal muscle cells also oxidize lipids. Indeed, fatty acids are the main source of energy in skeletal muscle during rest and mild-intensity exercise. As exercise intensity increases, glucose oxidation surpasses fatty acid oxidation. Other secondary factors that influence the substrate of choice for muscle include exercise duration, gender, and training status.
Another tissue that utilizes fatty acids in high amount is adipose tissue. Since adipose tissue is the storehouse of body fat, one might conclude that, during fasting, the source of fatty acids for adipose tissue cells is their own stock. Skeletal muscle and adipose tissue cells also utilize glucose in significant proportions, but only at the absorptive stage - that is, right after a regular meal. Other organs that use primarily fatty acid oxidation are the kidney and the liver. The cortex cells of the kidneys need a constant supply of energy for continual blood filtration, and so does the liver to accomplish its important biosynthetic functions.
Despite their massive use as fuels, fatty acids are oxidized only in the mitochondria. But not all human cells possess mitochondria! Although that may sound strange, human red blood cells are the most common cells lacking mitochondria. Other examples include tissues of the eyes, such as the lens, which is almost totally devoid of mitochondria; and the outer segment of the retina, which contains the photosensitive pigment. You may have already guessed that these cells and tissues then must produce ATP by metabolizing glucose only. In these situations, glucose is degraded to pyruvate, which is then promptly converted to lactate (Figure 2). This process is called lactic acid fermentation. Although not highly metabolically active, red blood cells are abundant, resulting in the continual uptake of glucose molecules from the bloodstream. Additionally, there are cells that, despite having mitochondria, rely almost exclusively on lactic acid fermentation for ATP production. This is the case for renal medulla cells, whose oxygenated blood supply is not adequate to accomplish oxidative phosphorylation.
Finally, what if the availability of fatty acids to cells changes? The blood-brain barrier provides a good example. In most physiological situations, the blood-brain barrier prevents the access of lipids to the cells of the central nervous system (CNS). Therefore, CNS cells also rely solely on glucose as fuel molecules (Figure 2). In prolonged fasting, however, ketone bodies released in the blood by liver cells as part of the continual metabolization of fatty acids are used as fuels for ATP production by CNS cells. In both situations and unlike red blood cells, however, CNS cells are extremely metabolically active and do have mitochondria. Thus, they are able to fully oxidize glucose, generating greater amounts of ATP. Indeed, the daily consumption of nerve cells is about 120 g of glucose equivalent, which corresponds to an input of about 420 kilocalories (1,760 kilojoules). This figure accounts for 60% of glucose utilization (or 20% of the energy needs of the human body in the resting state). However, most remaining cell types in the human body have mitochondria, adequate oxygen supply, and access to all three fuel molecules. Which fuel, then, is preferentially used by each of these cells?
Blaxter, K. Energy Metabolism in Animals and Man. Cambridge: Cambridge University Press, 1989.
Holmes, F. L. Lavoisier and the Chemistry of Life. Madison: University of Wisconsin Press, 1985.
Krebs, H. Nobel Prize Lecture (1953). Nobelprize.org, 2010.
Kresge, N., Simoni, R. D., & Hill, R. L. ATP synthesis and the binding change mechanism: The work of Paul D. Boyer. Journal of Biological Chemistry 281, e18 (2006).
Lusk, G. The Elements of the Science of Nutrition, 4th ed. Philadelphia: W. B. Saunders, 1931.
Luz, M. R. M. P. Glucose as the sole metabolic fuel: A study on the possible influence of teachers' knowledge on the establishment of a misconception among Brazilian high school stucents. Advances in Physiological Education 32, 225–230 (2008) doi:10.1152/advan.00050.2007.
Luz, M. R. M. P. et al. Glucose as the sole metabolic fuel: The possible influence of formal teaching on the establishment of a misconception about the energy-yielding metabolism among Brazilian students. Biochemistry and Molecular Biology Education 36, 407–416 (2008) doi:10.1002/bmb.20235.
Oliveira, G. A. et al. Students' misconception about energy yielding metabolism: Glucose as the sole metabolic fuel. Advances in Physiological Education 27, 97–101 (2003 doi:10.1152/advan.00009.2003.
By Lisa Maloney Updated December 06, 2018
Technically, a calorie is defined as the amount of heat necessary to raise the temperature of one gram of water by one degree centigrade. But in layperson's terms, the calorie is a unit of how much energy your food gives you – and if you eat more energy than your body uses, your body stores the excess as fat. So it's just a bit ironic that of the three macronutrients (fat, protein and carbohydrate), it's fat that gives the most energy – or calories – per gram.
Fat provides the most energy of all the macronutrients, at a whopping 9 calories per gram. That's why even a small amount of food or condiments that are rich in fat can pack so many calories. For example, a single tablespoon of creamy peanut butter has a whopping 191 calories, more than 75 percent of which come from its 16.44 grams of fat. Carbohydrates and protein each pack just 4 calories per gram, less than half the energy in a gram of fat.
Each macronutrient has its place in a healthy diet, and the fact that fat contains more calories than carbohydrates and protein doesn't make it inherently bad. In fact, Dietary Guidelines for Americans 2015-2020 recommends that 25 to 35 percent of the calories in your diet should be made up of fats. The trick is to make sure you consume healthy unsaturated fats, like those you get in nuts, avocados and fish. Less than 10 percent of your daily calories should come from saturated fats, which you'll find in many meats and fast food. As much as possible, limit or exclude trans fat, which makes foods shelf-stable but has a serious impact on your health.
Carbohydrates are the first and quickest form of energy that your body turns to when it needs fuel. Because of this, simple or fast-acting carbohydrates from foods like candy, baked goods and juices can cause a spike and then a crash in blood sugar and energy levels. Complex carbohydrates, which include whole grains and legumes, raise your blood sugar levels more slowly, give you more sustained energy and even help you feel full longer. According to the Dietary Guidelines for Americans, 45 to 65 percent of your daily calories should come from carbohydrates.
Your body can burn protein as energy, but it's also used for growth and maintenance. Think of it as a set of building blocks that your body can use for renovations. Protein takes a long time to digest, and it elevates your blood sugar levels very little, so while it's not your body's preferred quick-acting fuel, high-protein snacks like jerky and soy nuts won't cause you to peak and crash, either. According to Dietary Guidelines for Americans, 10 to 35 percent of your daily calories should come from protein.
References Writer BioLisa is a retired personal trainer with more than 4,000 hours of hands-on experience working with a variety of clients, from sports teams to weight loss and post-rehab populations. She's also a professional writer. Published credits in the health field include Livestrong.com, Feel Rich, SheKnows, Precor.com, and the East Coast magazine Breathe.
1
1) The molecule that serves as the major source of readily available fuel for neurons and blood cells is ________.
2
2) Which of the choices below is not a fate of carbohydrate taken into the body?
D) conversion to a nucleic acid
3
3) Dietary fats are important because they ________.
B) help the body absorb fat-soluble vitamins
4
4) Cholesterol, though it is not an energy molecule, has importance in the body because it ________.
A) is a stabilizing component of the plasma membranes and is the parent molecule of steroid hormones
5
5) Which of the following statements best describes complete protein?
D) must meet all the body's amino acid requirements for maintenance and growth
6
6) The term metabolism is best defined as ________.
D) biochemical reactions involved in building cell molecules or breaking down molecules for energy
7
7) The term metabolic rate reflects the ________.
A) energy the body needs to perform only its most essential activities
8
8) When proteins undergo deamination, the waste substance found in the urine is mostly________.
9
9) It is important to ensure that your diet is adequately rich in vitamins because ________.
C) most vitamins are coenzymes needed to help the body utilize essential nutrients
10
10) Oxidation-reduction reactions are catalyzed by which of the following enzymes?
A) dehydrogenases and oxidases
11
11) Which of the choices below describes the pathway of cellular respiration (the complete oxidation of glucose)?
A) glycolysis, Krebs cycle, electron transport chain, oxidative phosphorylation
12
12) Anabolism includes reactions in which ________.
B) larger molecules or structures are built from smaller ones
13
13) Catabolism would be best described as a process that ________.
C) breaks down complex structures to simpler ones
14
14) The primary function of cellular respiration is to ________.
D) break down food molecules and generate ATP
15
15) The process of breaking triglycerides down into glycerol and fatty acids is known as ________.
16
16) Which of the following mechanisms produces the most ATP during cellular respiration?
A) oxidative phosphorylation
17
17) Lipogenesis occurs when ________.
D) cellular ATP and glucose levels are high
18
18) Oxidative deamination takes place in the ________.
19
19) Transamination is the process whereby the amine group of an amino acid is ________
C) transferred to a keto acid
20
20) Glycogen is formed in the liver during the ________.
21
21) Which of the following is a normal consequence of the activation of the heat-promoting center?
A) release of epinephrine
22
22) Gluconeogenesis is the process in which ________.
B) glucose is formed from noncarbohydrate precursors
23
23) Glycolysis is best defined as a catabolic reaction based upon the ________.
B) conversion of glucose into two molecules of pyruvic acid
24
24) What is the outcome of ketosis?
25
25) Which of the choices below happens during the absorptive state?
A) Anabolic processes exceed catabolic ones.
26
26) In the case of a person who consumes a normal, balanced diet, proteins are essential to the body for all of the following except ________.
27
27) The most abundant dietary lipids are ________.
28
28) Loss of heat in the form of infrared waves is termed ________.
29
29) Which hormone directs essentially all the events of the absorptive state?
30
30) Prostaglandins play a role in ________.
D) control of blood pressure
31
31) Which of the following is the most important function of the liver?
32
32) As the body progresses from the absorptive to the postabsorptive state, only the ________ continues to burn glucose while every other organ in the body mostly switches to fatty acids.
33
33) In gluconeogenesis, during the postabsorptive state, amino acids and ________ are converted to glucose.
34
34) In the liver, the amine group of glutamic acid is removed as ________ in the oxidative state.
35
35) Which of the choices below is not a mechanism of heat production?
36
36) Heat-loss mechanisms do not include ________.
D) vasoconstriction of peripheral blood vessels
37
37) The amount of ________ produced is probably the most important hormonal factor in determining BMR.
38
38) When ketone bodies are present in the blood and urine in large amounts, it usually indicates increased metabolism of ________.
39
39) Many factors influence BMR. What is the most critical factor?
C) the ratio of surface area to volume (weight) of the body
40
40) The primary reason elderly people should decrease their caloric intake is that ________.
A) muscle mass and metabolism decline with age
41
41) Which of the choices below is not a major route of heat exchange?
42
42) Which of the following statements is a false or incorrect statement?
A) The amino acid pool is the body's total supply of amino acids in the body's proteins.
43
43) Which of the following molecules are considered key molecules at metabolic crossroads?
A) glucose-6-phosphate, pyruvic acid, acetyl CoA
44
44) Which of the following nutrients yield the highest amount of energy per gram when metabolized?
45
45) Which of the following does not occur in the mitochondria?
46
46) Which of the following is not true of beta oxidation?
C) It involves the anabolism of fats.
47
47) Select the correct statement about proteins.
C) Proteins will be used by most cells for ATP synthesis if insufficient carbohydrates are ingested.
48
48) Oxidation reduction reactions ________.
B) may involve the loss of hydrogen and electrons
49
49) What process primes a molecule to change in a way that increases its activity, produces motion, or does work?
50
50) Which of the choices below is not a source of glucose during the postabsorptive state?
C) absorption of glucose from the GI tract
51
51) Which of the following is correct?
B) Oxidation of FADH2 eventually yields four ATP via oxidative phosphorylation.
52
52) Which of the following food groups are considered good sources of complete proteins?
D) eggs, milk, yogurt, meat, and fish
53
53) Conditions that promote the oxidative deamination and energy use of amino acids include ________.
C) excessive amounts of protein in the diet
54
54) When a person's hypothalamic thermostat is set to a higher level and the actual body temperature is below that level, the person may ________.
55
55) Glucose can be obtained from ________.
56
56) Which of the following is not a function of LDLs?
A) transport cholesterol from the peripheral tissues to the liver
57
57) Which of the following best defines negative nitrogen balance?
A) Protein breakdown exceeds protein synthesis.