Masterclass With Masterjohn: Energy Metabolism
Masterclass With Masterjohn: Energy Metabolism is a course with 39 lessons on the biochemistry of how we burn carbohydrate and fat for fuel.
This is a 39-lesson series on the nuts and bolts of how we burn carbohydrate and fat for fuel, complete with videos, slides, and transcripts so that you can pick whatever format works best for you and learn how the system works at your own pace.
The audio and video of the first three lessons of this course are available to everyone for preview, and the premium features are available to preview for the first lesson. Premium features after lesson 1, and all parts of lessons 4-39, are for Masterpass members only. To learn more about the Masterpass, click here.
Here are the 39 lessons (migration in progress):
The first MWM Energy Metabolism lesson answers the question, why do we have to eat such an enormous amount of food?
The answer is to comply with the second law of thermodynamics. If you have a chemistry background, you should recognize this as a light review of the thermodynamics unit from a general chemistry class, with its most essential concepts teased out and packed into a half-hour lesson. If you don't, you can use this as a basic foundation for understanding the biochemistry to follow.
The lesson relates the 2nd law to food coloring dispersing in water, how a hydropower plant operates, ATP production, and why we need to eat our body weight in food more than once a month. In the process, we have a little fun.
The second MWM energy metabolism looks at how we use enzymes to exert exquisite control over what happens inside our bodies.
If the second law of thermodynamics holds that entropy is always increasing, why don't we reach maximum entropy right away? Why do we observe any order at all? The activation energy represents the resistance to change that can be found in any substance. We exploit the concept biologically by maintaining a body temperature that provides insufficient energy for most relevant reactions to go forward without catalysis, and imposing upon this backdrop an expansive repertoire of enzymes that can, in a regulated fashion, lower the energy barriers sufficiently for reactions to go forward.
This lesson looks at how they do that, and how we regulate their activity.
The third MWM Energy Metabolism lesson provides an overview of the basic objectives of using the citric acid cycle and the electron transport chain to make ATP. We start here because, no matter whether we burn protein, carbs, or fat, these two interrelated systems are what is shared in common.
The fourth MWM Energy Metabolism explores the first two steps of the citric acid cycle and explains how the rate of ATP production is regulated according to the abilities of the electron transport chain. Together with the upcoming lesson five, it explains how cells regulate their ATP production according to their needs and abilities. In the course of exploring this theme, we examine the role of reactive oxygen species in diabetes.
The fifth MWM Energy Metabolism lesson explores the third and fourth steps of the citric acid cycle and explains how the rate of ATP production is regulated according to the cell’s need for ATP. Together with lesson four, it explains how cells regulate their ATP production according to their needs and abilities. In the course of exploring this theme, we look at the role of AMP kinase (AMPK) in promoting energy uptake during exercise. We consider how AMPK and reactive oxygen species, so often at odds with one another, work together to produce fitness in response to exercise.
You have to squeeze your brain muscles extra hard for this one, but it pays off.
This lesson looks at the third step of the citric acid cycle in much more detail, digging into the organic chemistry concepts involved in the conversion of isocitrate to α-ketoglutarate. We dive deep into this because it’s the only way to explain why this step parts ways with most other decarboxylation reactions in that it does not require thiamin (vitamin B1).
This, in turn, provides a basis for understanding why burning carbohydrate for fuel requires twice as much thiamin than burning fat, and why high-fat, low-carbohydrate, ketogenic diets can be used to overcome problems with thiamin deficiency or defects in thiamin-dependent enzymes. We conclude by looking at how this step allows the interconversion of amino acids and citric acid cycle intermediates, the role of vitamin B6 in this process, and the use of enzymes known as transaminases to diagnose B6 deficiency and liver dysfunction.
The alpha-ketoglutarate dehydrogenase complex is marvelously complex and incredibly rich in details that are relevant to the big picture of metabolism and to many issues of health and disease. Today, we break down what actually happens so that we can spend all of the next lesson discussing the rich array of relevant principles it brings to light.
This complex is so rich in biochemical concepts and relevance to health and disease. Having done the dirty work of looking at its organic chemistry mechanisms in the last lesson, here we explore broadly applicable biochemistry principles like energetic coupling and substrate channeling. We look at how thiamin deficiency, oxidative stress, arsenic, and heavy metal poisoning can affect metabolism, and how to recognize markers of these processes in blood or urine. We make the subtle yet critical distinction between oxidative stress and oxidative damage. We look at the role of this complex in Alzheimer’s disease.
We then turn to the product of this complex, succinyl CoA, to examine how it provides an entry into the cycle for odd-chain fatty acids and certain amino acids and an exit out of the cycle for the synthesis of heme. In doing so, we look at the roles of vitamins B12 and B6 in these processes, the use of methylmalonic acid to diagnose B12 deficiency, and the ability of B6 deficiency to cause sideroblastic anemia.
This lesson addresses the curious case of why CoA makes a brief cameo in the citric acid cycle during the formation of succinyl CoA only to leave again in the next step. We dig into the chemistry underlying the high-energy thioester bond that CoA forms with acyl groups, which explains more broadly one of the key roles of sulfur in energy metabolism. We conclude by looking at how the appearance of CoA allows us to harness energy released during the decarboxylation of alpha-ketoglutarate to form ATP directly during “substrate-level phosphorylation,” or, alternatively, to use energy from ATP to invest in the synthesis of heme.
This lesson looks at the fundamental principle that atomic oxygen is the limiting factor for the release of carbon dioxide in metabolism, and when we don't have enough we take it from water. This will become very relevant when we cover fats versus carbohydrates, because they consume different amounts of water and release different amounts of carbon dioxide for this very reason. That, in turn, relates to a number of health endpoints such as the functions of vitamin K and biotin, delivery of oxygen to tissues, and the stress placed on the lungs during breathing.
Here, we look at the principle in the citric acid cycle. In doing so, we see that, while textbooks only point to two water molecules consumed, a third water molecule is irreversibly consumed to donate oxygen to the cycle via phosphate.
Now we take it clinical: how do we use what we've learned so far to interpret the section of a urinary organic acids test that reports the citric acid cycle metabolites?
We begin by looking at the underlying chemistry to explain the curious absence of oxaloacetate on these tests. We conclude by mastering the ability to spot three unique patterns: energy overload, oxidative stress, and thiamin deficiency.
Since carbs are richer in oxygen than fat, they consume less water in their metabolism and release more carbon dioxide. Carbon dioxide puts stress on the lungs and its generation should be restricted in the case of lung injury to allow healing. This calls for a low-carbohydrate, high-fat diet. On the other hand, carbon dioxide is needed to support the action of vitamin K and biotin, and to promote delivery of oxygen to tissues during exercise.
In our first glimpse into glycolysis and beta-oxidation, we find that understanding the basic chemical makeup of these molecules is deeply relevant to how we would manipulate the diet in many contexts of health and disease.
The pyruvate dehydrogenase complex catalyzes the one decarboxylation step that carbohydrate undergoes to generate acetyl CoA, which accounts for the one carbon dioxide molecule produced in carbohydrate metabolism that is not produced during the metabolism of fat. It also accounts for why burning carbs requires twice as much thiamin as fat. In fact, the pyruvate dehydrogenase complex is remarkably analogous to the alpha-ketoglutarate dehydrogenase complex, sharing all the same cofactors and catalyzing virtually the same reactions. In this lesson, we look at why this has to be true and how it works. This provides the foundation for our deeply practical look at thiamin in the next lesson.
Did you realize that thiamin deficiency can be caused by your environment? In the old days, beriberi was associated with the consumption of white rice. Nowadays, refined foods are an unlikely cause of thiamin deficiency because they are fortified. We associate deficiency syndromes such as Wernicke’s encephalopathy and Korsakoff’s psychosis primarily with chronic alcoholism. Yet there are regional outbreaks of thiamin deficiency among wildlife attributed to poorly characterized thiamin antagonists in the environment. Thiamin-destroying amoebas can pollute water, thiamin-destroying bacteria have been isolated from human feces, and thiamin-destroying fungi have also been identified. Could toxic indoor molds and systemic infections play a role as well?
Thiamin deficiency is overwhelmingly neurological in nature and hurts the metabolism of carbohydrate much more than fat. Indeed, preliminary evidence suggests thiamin supplementation can help mitigate glucose intolerance. Ketogenic diets are the diets that maximally spare thiamin and are best characterized as treatments for neurological disorders. Anecdotally, ketogenic diet-responsive neurological problems sometimes arise as a result of infection. Could ketogenic diets be treating problems with thiamin or thiamin-dependent enzymes? One must exercise caution here: fat contains little thiamin, and ketogenic diets can actually cause thiamin deficiency if they don’t contain added B vitamins. The relationships between thiamin, glucose metabolism, and neurological health are remarkable and desperately need our attention.
One of the advantages of carbohydrate over fat is the ability to support the production of lactate. This is so important that carbohydrate is physiologically essential to red blood cells and certain brain cells known as astrocytes. For the same reason, it plays an important role in supporting the energy requirements of the lens and cornea, kidney medulla, and testes, and supports the quick boosts of peak energy needed during stressful situations that include high-intensity exercise. The biochemical role of lactate is to rescue NAD+ during times when NAD+ becomes limiting for glycolysis and glycolysis becomes a meaningful source of ATP. Through the Cori cycle, lactate can extract energy from the liver’s supply of ATP and deliver it to other tissues such as skeletal muscle in the form of glucose. This lesson fleshes out the physiological and biochemical roles of lactate and serves as a foundation for the next lesson, which explores the role of carbohydrate in supporting sports performance.
“Anaplerosis” means “to fill up” and refers to substrates and reactions that fill up a metabolic pathway as its own substrates leak out for other purposes. The citric example is a central example of this because its intermediates are often used to synthesize other components the cell needs. On a mixed diet where carbohydrate provides much of the energy, pyruvate serves as the main anaplerotic substrate. During carbohydrate restriction, protein takes over. Fat is the least anaplerotic of the macronutrients because the main product of fatty acid metabolism, acetyl CoA, is not directly anaplerotic. There are several very minor pathways that allow some anaplerosis from fat, but they are unlikely to eclipse the need for protein to support this purpose during carbohydrate restriction. Thus, carbs and protein are the two primary sources of anaplerosis. This means carbs can spare the need for protein, and that protein requirements rise on a carb-restricted diet.
Can athletes fat-adapt their workouts? This lesson lays down the principles of exercise biochemistry and physiology needed to understand the importance of the three energy systems supporting energy metabolism in skeletal muscle: the phosphagen system (ATP and creatine), anaerobic glycolysis (dependent on carbs), and oxidative phosphorylation (dependent on carbs, fat, or protein). We discuss why maximal intensity always depends on carbs if the intensity and duration are sufficient to deplete phosphocreatine concentrations, and clarify the window of time and intensity that can be fat-adapted. This sets the foundation for the next lesson, which looks at the evidence of how carbohydrate restriction and ketogenic diets impact sports performance.
Can fat fuel intensity in a competitive athlete? This lesson takes a critical look at the commonly cited evidence in favor of a neutral or beneficial effect of low-carbohydrate or ketogenic diets on sports performance, as well as key pieces of conflicting evidence. Bottom line? Fat can fuel duration, but probably can never fuel your peak intensity, just as the physiology would predict.
In this lesson, we examine the entire glycolytic pathway. We use as our theme the transfer of oxygen from phosphate to newly generated water. This explains why the standard stoichiometry of glycolysis found in textbooks show it generating two water molecules, and ties the information together with the analogous principles from substrate-level phosphorylation in the citric acid cycle and the relative differences in water consumption and carbon dioxide generation between fat and carbohydrate. As with our discussion of the citric acid cycle, we also reveal why the standard stoichiometry of glycolysis is misleading and why, when we account for atoms rather than molecules, we find glycolysis to be net water-neutral.
In this lesson, we examine beta-oxidation in its simplest form: the breakdown of a long-chain, saturated fatty acid. We see once again the principle that the oxygen content of a molecule determines how much water its metabolism consumes and how much carbon dioxide its metabolism releases. In beta-oxidation, we consume one water per round and release no carbon dioxide. This reflects the fact that fatty acids are not hydrates of carbons like sugars are, which is where the name carbohydrate comes from.
This lesson covers the regulation of glycolysis. The principle regulation occurs at phosphofructokinase, which guards the gate to the first irreversible, committed step to burn glucose for energy. What governs it? Energy. If you need more ATP, you burn more glucose; if you don’t, you don’t.
If the cell has glucose beyond its needs for energy, it uses it for the pentose phosphate pathway, which allows the production of 5-carbon sugars and antioxidant defense if needed, or stores it as glycogen if there is room. If not, glucose-6-phosphate accumulates and shuts down hexokinase. This, together with low AMPK levels, causes glucose to get left in the blood. The other key regulated step of glycolysis is pyruvate kinase, where the primary purpose of regulation is to prevent futile cycling between steps of glycolysis and gluconeogenesis. On the whole, glycolysis and glucose uptake are regulated primarily by energy status and secondarily by glucose-specific decisions about the need for glycogen or for the pentose phosphate pathway. Since we mostly use glucose for energy under most circumstances, the key regulation of the pathway is the regulation of phosphofructokinase by energy status. This means glucose uptake is largely driven by energy status, and our decisions about preventing hyperglycemia should center on total energy balance.
This lesson covers the regulation of beta-oxidation. The primary regulation of beta-oxidation occurs at the mitochondrial membrane, where fatty acids are transported into the mitochondrion. Acetyl CoA carboxylase governs both the formation of fatty acids from non-carbohydrate precursors and the transport of fatty acids into the mitochondrion. Its product, malonyl CoA, is a substrate for fatty acid synthesis in the cytosol but a regulator of fatty acid transport in the mitochondrion. Thus, there are two isoforms of acetyl CoA carboxylase that are regulated similarly. The cytosolic isoform plays a direct role in fatty acid synthesis and the mitochondrial isoform regulates beta-oxidation. This ensures that the two processes are regulated reciprocally, so that one is shut down to the extent the other is activated, thereby preventing wasteful futile cycling. The primary regulator of acetyl CoA carboxylase activity is, as you might expect by this point, energy status. When a cell needs more energy, it lets fatty acids into the mitochondrion. When it has too much, it shuts down fat-burning.
Remarkably, we know from dietary studies that we get the most insulin from eating carbohydrate, yet we know from molecular and cellular studies that insulin secretion is primarily triggered by the ratio of ATP to ADP inside the pancreatic beta-cell. The former implies that insulin is a response to glucose, while the latter implies that insulin is a response to total energy availability.
What can explain this discrepancy?
In this lesson, we explore the possibility that it is the anatomy and physiology that drive the dietary effect of carbohydrate rather than the biochemistry. Carbs are wired to get soaked up by the pancreas when blood sugar rises above the normal fasting level once the liver has taken its share to replete hepatic glycogen, whereas fats are wired to go primarily to the heart and muscle when those organs need energy and to go primarily to adipose tissue otherwise. The combination of circulatory routes and the relative expression of glucose transporters and lipoprotein lipase by different tissues likely directs fat to the pancreatic beta-cell as a source of ATP only during extreme hyperglycemia or when it exceeds adipose storage capacity due to obesity, insulin resistance, or very high-fat meals.
The pancreatic beta-cell does have a diversity of complicated and often controversial secondary biochemical mechanisms that “amplify” the insulin-triggering effect of ATP, and carbs are more versatile at supporting these mechanism than fat. These likely make a contribution to the dietary effect, but they strike me as unlikely to be the primary driver of the dietary effect.
Thus, insulin is a response mainly to carbohydrate availability but also to total energy availability, and this driven mainly by the anatomy and physiology but also by the biochemistry.
Seeing insulin as a response to cellular energy status will eventually help us broaden our view of insulin as a key governor of what to do with that energy that goes far, far beyond regulating blood glucose levels.
Most people interested in health and nutrition know that insulin clears glucose from the blood into cells, but it is much less widely appreciated that insulin also makes you burn that glucose for energy.
Insulin stimulates the translocation of GLUT4 to the membrane of skeletal muscle, heart, and adipose cells, and activates hexokinase 2. GLUT4 increases the rate of glucose transport across the cell membrane and hexokinase 2 locks the glucose into the cell, making sure that glucose travels inward rather than outward. Insulin stimulates glycogen synthase, causing you to store glucose as glycogen, but it also stimulates pyruvate dehydrogenase, causing you to burn pyruvate for energy. The key determinant of which one of these you do is the energy status of the cell. Glucose 6-phosphate is needed to activate glycogen synthase, and it only accumulates if high energy status is inhibiting phosphofructokinase. If low energy status is stimulating phosphofructokinase, the net effect of insulin is to irreversibly commit glucose to glycolysis, and then to stimulate the conversion of pyruvate to acetyl CoA, which then enters the citric acid cycle to allow the full combustion of the carbons and maximal synthesis of ATP. Thus, if you need the energy, the net effect of insulin is to make you burn glucose to get that energy.
Insulin prevents fat-burning in part by locking fat in adipose tissue and in part by shutting down transport of fatty acids into the mitochondrion inside cells.
By downregulating lipoprotein lipase (LPL) at heart and skeletal muscle and upregulating it at adipose tissue, insulin shifts dietary fat away from heart and muscle and toward adipose tissue. By downregulating hormone-sensitive lipase in adipose tissue, it prevents the release of free fatty acids from adipose tissue into the blood.
At the cellular level, insulin leads to the phosphorylation and deactivation of AMPK. Since AMPK inhibits acetyl CoA carboxylase, insulin-mediated deactivation of AMPK leads to activation of acetyl CoA carboxylase and the conversion of acetyl CoA to malonyl CoA. Malonyl CoA inhibits carnitine palmitoyl transferase-1 (CPT-1) and thus blocks the transport of fatty acids into the mitochondrion.
Nevertheless, all of these steps are also regulated at the most fundamental level by energy status, as covered in lesson 22. Further, insulin stimulates the burning of carbohydrate for energy, as covered in lesson 24. So, is insulin’s blockade of fat-burning sufficient to cause net fat storage, or does this critically depend on energy balance? This question will be answered in the next lesson.
In a caloric deficit, the low energy status of muscle and heart will lead them to take up fat rather than adipose tissue, even in the presence of insulin. Insulin combined with low energy status will promote the uptake of glucose in skeletal muscle over adipose tissue and will promote the oxidation of glucose rather than its incorporation into fat.
Some advocates of the carbohydrate hypothesis of obesity have argued that glucose is needed to form the glycerol backbone of triglycerides within adipose tissue. Although glucose can serve this role, it isn’t necessary because adipose glyceroneogenesis and hepatic gluconeogenesis can both provide the needed glycerol phosphate. Further, low energy status promotes the use of glycerol as fuel and high energy status is needed to promote the formation of glycerol from glucose. Finally, fatty acids are needed to store fat in adipose tissue and they overwhelmingly come from dietary fat in almost any circumstance.
Insulin can only promote de novo lipogenesis, the synthesis of fatty acids from other precursors such as carbohydrate, in the context of excess energy, and this pathway is minor in conditions of caloric deficit, caloric balance, or moderate caloric excess. Thus, although insulin does promote storage of fat in adipose tissue, it doesn’t directly affect energy balance, and energy balance is the determinant of how much fat you store overall.
The pentose phosphate pathway provides a deep look into a stunning array of essential roles for glucose.
In it, glucose becomes the source of NADPH, used for antioxidant defense, detoxification, recycling of nutrients like vitamin K and folate, and the anabolic synthesis of fatty acids, cholesterol, neurotransmitters, and nucleotides. At the same time, glucose also becomes the source of 5-carbon sugars, used structurally in DNA, RNA, and energy carriers like ATP, coenzyme A, NADH, NADPH, and FADH2. DNA is needed for growth, reproduction, and cellular repair; RNA is needed to translate genetic information from DNA into all of the structures in our bodies; the energy carriers constitute the very infrastructure of the entire system of energy metabolism.
This lesson covers the details of the pentose phosphate pathway, how it operates in multiple modes according to the relative needs of the cell for ATP, NADPH, and 5-carbon sugars, the role of glucose 6-phosphate dehydrogenase deficiency and thiamin deficiency in its dysfunction, and what it means for the importance of glucose to human health.
Insulin is commonly seen as a response to blood glucose whose primary role is to keep blood glucose within a narrow range. This view of insulin fails to account for its many roles outside of energy metabolism that govern long-term investments in health. The biochemistry and physiology of insulin secretion suggest, rather, that insulin is a gauge of short-term energy status and energetic versatility. Since glucose can only be stored in small amounts and since it is the most versatile of the macronutrients in its ability to support specialized pathways of energy metabolism, it makes sense that it would be wired to the pancreas as the primary signal of short-term energy status and energetic versatility. In this lesson, we review the unique uses of glucose and the mechanisms of insulin signaling to synthesize them into a more nuanced view of the role of insulin than is typically presented.
Gluconeogenesis is extremely expensive. Three steps of glycolysis are so energetically favorable that they are irreversible. Getting around them requires four gluconeogenesis-specific enzymes and the investment of a much larger amount of energy. Overall, six ATP worth of energy are invested to yield glucose, a molecule that only yields 2 ATP when broken down in glycolysis. This lesson covers the details of the reactions as well as the rationale for investing so much energy. One of the most pervasive themes in biology is the drive to conserve energy. That we will spend this much energy synthesizing glucose is a testament to how essential it is to our life and well being.
Lesson 30: Gluconeogenesis Occurs When the Liver is Rich in Energy and the Body is Deprived of Glucose
Since gluconeogenesis is extremely expensive, it has to be tightly regulated so that it only occurs when both of two conditions are met: 1) the liver has enough energy to invest a portion into synthesizing glucose, and 2) the rest of the body is in need of that glucose.
Since the liver is the metabolic hub of the body that also plays a major role in anabolic synthesis and nitrogen disposal, it also regulates glycolysis and gluconeogenesis according to whether amino acids are available to supply energy in place of glucose and whether there is sufficient citrate and associated energy for biosynthesis. This lesson covers how insulin, glucagon, alanine, citrate, fructose 2-6-bisphosphate, ATP, ADP, and AMP regulate the flux between glycolysis and gluconeogenesis.
The last lesson covered how insulin, glucagon, and allosteric regulators from within the liver ensure that the liver only engages in gluconeogenesis when it can and when it needs to. This lesson focuses on an additional layer of regulation: cortisol.
Cortisol is the principal glucocorticoid in humans. Glucocorticoids are steroid hormones produced by the adrenal cortex that increase blood glucose. Cortisol has multiple actions on the liver, muscle, adipose, and pancreas that all converge on making glucose more available to the brain.
Among them, it increases movement of fatty acids from adipose to the liver, which provide the energy for gluconeogenesis, and the movement of amino acids from skeletal muscle to the liver, which provide the building blocks for gluconeogenesis. Cortisol serves both to antagonize insulin, thereby acutely increasing gluconeogenesis, and to increase the synthesis of gluconeogenic enzymes, which amplifies all other pro-gluconeogenic signaling and increases the total capacity for gluconeogenesis. In fact, even the day-to-day regulation of gluconeogenesis by glucagon is strongly dependent on normal healthy levels of cortisol in the background.
Since gluconeogenesis is an extremely expensive investment with a negative return, it makes sense that the body would regulate it as a stress response, and thus place it under control by cortisol. This raises the question of whether carbohydrate restriction increases cortisol. Several studies are reviewed in this lesson that indicate that 1) there may be an extreme level of carbohydrate restriction that always increases cortisol, and 2) carbohydrate restriction definitely increases cortisol in some people. It may be the case that other stressors in a person’s “stress bucket” determine whether and how strongly the person reacts to carbohydrate restriction with elevated cortisol.
In conditions of glucose deprivation, such as fasting or carbohydrate restriction, ketogenesis serves to reduce our needs for glucose. This reduces the need to engage in the energetically wasteful process of gluconeogenesis, which would otherwise be extremely taxing on our skeletal muscle if dietary protein were inadequate.
Ketogenesis mainly occurs in the liver. The biochemical event that leads to ketogenesis is an accumulation of acetyl CoA that cannot enter the citric acid cycle because it exceeds the supply of oxaloacetate. The set of physiological conditions that provoke this biochemical event are as follows: free fatty acids from adipose tissue reach the liver, providing the energy needed for gluconeogenesis as well as a large excess of acetyl CoA. Oxaloacetate, with the help of the energy provided by free fatty acids, leaves the citric acid cycle for gluconeogenesis. These events increase the ratio of acetyl CoA to oxaloacetate, which leads to the accumulation of acetyl CoA that cannot enter the citric acid cycle and therefore enter the ketogenic pathway.
This pathway results in the production of acetoacetate, a ketoacid. Acetoacetate can then be reduced to beta-hydroxybutyrate, a hydroxyacid, in a manner analogous to the reduction of pyruvate, a ketoacid, to lactate, a hydroxyacid. Acetoacetate is an unstable beta-ketoacid just like oxalosuccinate (covered in lesson 6) and can also spontaneously decarboxylate to form acetone, a simple ketone that is extremely volatile and can evaporate through the lungs, causing ketone breath.
This lesson covers the basic mechanisms of ketogenesis and sets the ground for the forthcoming lesson on the benefits and drawbacks of ketogenesis in various contexts.
The production of ketones in the liver frees up coenzyme A (CoA) that would otherwise be captured in the accumulating acetyl CoA, and the conversion of acetoacetate to beta-hydroxybutyrate frees up NAD+ that would otherwise be trapped as NADH as the production of NADH in beta-oxidation exceeds that oxidized in the electron transport chain. This allows free CoA and NAD+ to keep beta-oxidation running rapidly. The ketone bodies then traverse through the inner mitochondrial and plasma membranes through the monocarboxylate transporter 1 (MCT1), and possibly through the voltage-dependent anion channel (VDAC) in the outer mitochondrial membrane.
They travel through the blood to ketone-utilizing tissues, where they get into the mitochondria through the same transporters that allowed them to exit the liver. Beta-hydroxybutyrate must be converted to acetoacetate to undergo further metabolism in the ketone-utilizing tissue. Acetoacetate is converted to acetoacetyl CoA using the CoA from succinyl CoA. It is then split to two acetyl CoA by beta-ketothiolase, the same exact enzyme that catalyzes the opposite conversion during ketogenesis.
The acetyl CoA then enters the citric acid cycle. The use of succinyl CoA causes the loss of one ATP that would otherwise have been synthesized in the substrate-level phosphorylation step of the citric acid cycle. Nevertheless, most ATP energy is conserved and ketones represent an efficient means of transporting energy between tissues.
The physiological purpose of ketogenesis is to spare the loss of lean mass that would otherwise occur during prolonged fasting. Meeting the glucose requirement of the brain entirely by gluconeogenesis from amino acids would cause us to lose 2.2 pounds of lean mass every day. This would cause us to die much more quickly from starvation than we actually do. Ketones can fuel 75% of the brain’s energy requirement. Glycerol from triglyceride hydrolysis and acetone derived from ketogenesis can together spare 55% of the amino acids that would otherwise be used to sustain the brain’s remaining glucose requirement. The acidity of ketones requires the kidney to hydrolyze glutamine to neutralize it with ammonia, however, and this attenuates the sparing of lean mass. In fact, it doubles the amount of protein breakdown over that used to synthesize glucose alone. When considering all this, ketogenesis cuts down the amount of lean mass lost during fasting 5-fold, allowing us to survive for a much longer time during fasting than we would be able to without ketones.
Ketone homeostasis has two central objectives: 1) keep ketone levels high enough to feed the brain, and 2) keep ketone levels low enough to avoid the serious and life-threatening condition of ketoacidosis. While proportion of circulating ketones taken up by skeletal muscle, heart, and other tissues during fasting is initially high, these tissues limit their absolute uptake of ketones and may even lower their uptake in response to increased availability of free fatty acids. As a result, ketone concentrations rapidly outpace production rates, and this is exactly what allows them to reach high enough concentrations to feed the brain. To out compete glucose for transport across the blood brain barrier, they also act on the liver to suppress glucose output, causing blood glucose to lower. While high ketones and low glucose favor maximal penetration of ketones into the brain, the threat of ketoacidosis requires a negative feedback loop. Thus, ketones suppress adipose tissue lipolysis, restraining their own production so that blood concentrations stay in the sweet spot to safely nourish the brain.
Ketones have a dark side: ketoacidosis. And it does NOT only happen in diabetes. Ketoacidosis is a serious and life-threatening medical condition wherein ketones accumulate to such high levels that they overwhelm the body’s natural buffering capacity and sink the pH of the blood to dangerous and possibly fatal levels. Ketoacidosis is most often associated with poorly controlled diabetes. Contrary to many popular claims, however, it is not limited to diabetes. Alcohol abuse with malnourishment, fasting during pregnancy or lactation, and in rare cases low-carbohydrate diets can be causes of ketoacidosis. This lesson covers the science behind ketoacidosis and reviews several case reports to illustrate what it looks like in practice.
Why were the Inuit never in ketosis, despite their traditional high-fat diet? That question is answered in this lesson. The answer provides a stunning example of human evolution and makes it clear that evolution does not “want” us in a constant state of ketosis.
CPT-1a deficiency is a genetic disorder in the ability to make ketones and to derive energy from fatty acids needed to make glucose during fasting. In its severe form, it is extremely rare, dangerous, and fatal if not treated with frequent feeding and often a high-carbohydrate, low-fat diet. A much more mild form of CPT-1a deficiency known as “the Arctic variant” is only found in the Arctic and it is nearly universal in the Arctic. It causes a serious impairment in the ability to make ketones, dramatically raises the risk of developing hypoglycemia while fasting, and causes a three-fold increase in infant mortality. Yet virtually everyone native to the Arctic has it and it is usually asymptomatic.
What is utterly stunning about this is that this variant took hold of the Arctic in one of the strongest selective sweeps ever documented in humans. This means that evolution judged this variant as better suited to the Arctic environment than almost any human gene has ever been suited to any environment. How on earth can an impairment in fat metabolism be well suited to an environment that forces a high-fat diet on its inhabitants? Complete the lesson to find out.
The ketogenic diet was first proposed by Russell M. Wilder at the Mayo Clinic in 1921. The goal was to mimic the the physiology of fasting in a way that was more sustainable over time than completely abstaining from food. The application was to treat epilepsy. The use of fasting to treat epilepsy dates back to the era of Hippocrates, but at the turn of the 20th century physicians were documenting its effects more clearly and new methods of chemical analysis were revealing that ketone bodies were peed out in the urine during fasting. Wilder proposed that acetoacetate acted as an anesthesia on the brain and thereby blunted the occurrence of seizures.
Wilder was an expert in diabetes, and the predominant view at that time was that diabetes should be treated with dietary measures to suppress ketogenesis, thereby preventing diabetic ketoacidosis, while using only the minimum carbohydrate necessary for this purpose to reduce stress on the pancreas. Wilder took the work on the anti-ketogenic diet used in diabetes and turned it on its head to develop the ketogenic diet to treat epilepsy. This high-fat, moderate protein, low-carbohydrate diet proved remarkably effective at the time, with initial reports of the Mayo Clinic suggesting that 95% of patients benefited and that almost all of them could be treated without the need for drugs. Within a few years, the Mayo reports were revised to suggest that approximately half of patients with good compliance benefited, and that almost a quarter of the total patient pool had to be excluded from their analysis due to poor compliance. Still, it was remarkably effective in the responders and it is the only dietary treatment that has ever been shown to treat epilepsy without medicine.
Ketogenic Diets fell by the wayside in the 40s through the 90s, but the late 90s and early 00s saw a resurgence in their use to treat epilepsy, and research on their effects mushroomed. Today, there are seven randomized controlled trials showing their benefits. These studies suggest that a large subsection of children who do not respond well to antiepileptic drugs do respond well to the ketogenic diet. Studies tend to estimate these children are 20-50% of the patient pool, though some studies suggest even higher rates. Preliminary evidence suggests similar efficacy in adults, and similar efficacy in children for several less strict diets, such as the MCT oil diet, the modified Atkins diet, and the low-glycemic index, low-carbohydrate diet.
Why are the ketogenic diet and these related diets effective? We’ll tackle the mechanisms in our next lesson, and that will serve as the bridge to evaluating the efficacy of the ketogenic diet for many other conditions.
This lesson will eventually be used as a bridge toward explanations of why ketogenic diets work for epilepsy once this course continues on. For now, here is how neurons get excited.