If you’re on a low-carbohydrate, ketogenic or carnivore diet (and even if you’re not), you’ve may have heard of the term gluconeogenesis. Since the word sounds similar to “glucose,” it may instill a sense of fear in carbo-phobes. We associate glucose with sugar. In fact, keto dieters sometimes fear that eating certain foods may even kick them out of ketosis due to the process of gluconeogenesis raising insulin.
Truth is, gluconeogenesis is harmless, and actually is a necessary process that allows us to create our own fuel to keep certain bodily processes going. Let’s take a deep dive into gluconeogenesis, looking at some of the science behind the term and the physiology behind the process.
What is Gluconeogenesis?
Glucose is a simple sugar. In the body, a lot of glucose is present as blood glucose and serves as a major fuel source for organs, tissues, and metabolic pathways and processes. While we can break down food substances such as carbohydrates into glucose for fuel, our body can also create its own glucose from non-carbohydrate carbon sources; this process is known as gluconeogenesis—or GNG for short.
To better understand gluconeogenesis, let’s start with a short refresher on glycolysis—the process by which we break down carbohydrates (glucose) to produce energy. During this process, which takes place in the cytoplasm of mitochondria, a glucose molecule is energized by adding ATP.
Then, a cascade of steps occurs involving the production of the following molecules, in order: conversion of glucose-6 phosphate to fructose-6 phosphate to fructose 1,6-bisphosphate to glyceraldehyde-3 phosphate to 1,3-bisphosphoglycerate to 3-phosphoglycerate, then 2-phosphoglycerate, to phosphoenolpyruvate (PEP). Finally, PEP is acted on by an enzyme known as pyruvate kinase, and we are left with is something called pyruvate (important to remember) and two molecules of ATP, the energy currency for the cell.
The gluconeogenesis pathway is essentially a reversal of glycolysis with the exception of a few well-regulated steps. Most gluconeogenesis (about 90%) happens in the liver, and the remaining 10% occurs in the kidney.
In particular, three crucial irreversible steps occur in gluconeogenesis. Irreversible means that once these reactions occur, they can't "go back," since doing so would require too much energy. These steps are:
- The conversion of pyruvate to phosphoenol pyruvate (PEP) by pep carboxykinase
- Conversion of fructose 1,6-bisphosphate to fructose-6 phosphate by fructose 1,6-bisphosphatase
- Conversion of glucose 6-phosphate to glucose by glucose 6-phosphatase
These reactions are important; they prevent a “futile cycle” where we produce glucose, only to break it down again.
Under aerobic conditions (meaning, with lots of oxygen present), pyruvate can enter a process called the tricarboxylic acid cycle (a.k.a TCA cycle or citric acid cycle). This involves the conversion of pyruvate to acetyl-coA with an enzyme known as pyruvate dehydrogenase.
Aerobic metabolism predominates when you go out for a lower-intensity run, for example. However, anaerobic (meaning, with low to no oxygen) conditions create a scenario where pyruvate is converted to lactate by lactate dehydrogenase and eventually lactic acid and acidic hydrogen ions. Anaerobic glycolysis occurs, for example, when you're mid-way through a 30 second all-out sprint. Remember pyruvate and lactate—these will become important later on.
Blood Glucose Maintenance
There is a strong misconception that certain low- and no-carb diets eliminate the body’s need for glucose, and that people on these diets have no need for it. It is important to realize that this statement is true only for dietary carbohydrates, for which there is no real “requirement.” However, even carnivores and keto-dieters need some glucose.
A typical “normal” blood glucose range is between 3.3 and 5.5mM (60 - 90 mg/dl). If blood glucose falls to a level below about 2mM (about 36 mg/dl), unconsciousness will occur. While you might hear some people claim that dietary sugar is “the enemy,” blood glucose is keeping you alive; don’t let the term “sugar” instill a sense of fear.
In a typical diet, blood glucose is normally maintained from day-to-day by ingesting carbohydrates. When we feel a bit groggy, low on energy, or need a “pick me up,” most people reach for a quick snack high in the form of carbohydrates, maybe even something with a little sugar—a “quick hit” of energy.
Carbohydrates are a quick and easily-metabolized form of energy, and will lead to a blood sugar rise or maintenance. We aren’t recommending stuffing your face with candy-bars though. Consuming a diet adequate in quality, unprocessed carbohydrates is one way that humans and animals maintain the blood glucose needed to function. Whole food sources of carbohydrates also can have several other benefits.
In addition, while the brain, heart, and muscles can use fuel sources like fatty acids and ketones for energy, some tissues in the body can only use glucose.
Red blood cells, the kidney medulla, the lens and cornea of the eye, and the testicles are some examples of tissues that need glucose to function, since they cannot use ketone bodies as a fuel. They don't have the necessary transporters to get ketones in, or the enzymes to break ketones down.
While neurons and other central nervous system cells can use ketones, and are actually quite good at it, their high energetic requirement requires some glucose to be present and ready to be used as a fuel in addition to ketone bodies. For example, the brain uses about 120g per day of glucose—about 60% of the total glucose consumed by the body under resting conditions!
Many argue that this is why humans “need carbohydrates” in order to function, but remember, this simply means glucose—not necessarily coming from dietary carbohydrates!
A certain amount of blood sugar is also required just to keep us alive and conscious. Severe hypoglycemia (a condition characterized by extremely low blood sugar levels) can have devastating consequences and symptoms such as dizziness, fainting, and confusion.
But how are blood glucose levels maintained when we go without food for long periods? Most of us aren’t able to consume food throughout the night since we have better things to do...like sleep. In addition, our body needs some way to keep blood glucose up in between meals. How do we meet this need for glucose when it isn’t being supplied from external sources?
Ketogenesis and Glycogenolysis
Usually, our small blood glucose requirement is met by two processes—the first involves the release (and metabolism) of free fatty acids from adipose tissue and ketone bodies from the liver.
While free fatty acids (FFAs) and ketone bodies can’t directly contribute to the production of glucose (gluconeogenesis) or be used as a direct energy source by some tissues, their role is super important.
During fasting and other low-energy situations, producing ketones and metabolizing FFAs allows us to spare what little glucose is available for cells and tissues that depend on it like red blood cells and neurons.
The “glucose sparing” effect of ketones is also one of the qualities that make exogenous ketones a particularly effective exogenous fuel source. Consumed alone or alongside carbohydrates, exogenous ketones may spare muscle glycogen during prolonged intense exercise.
The second process involves producing glucose, but is different than gluconeogenesis. This process is termed glycogenolysis, the breakdown (lysis) of stored glucose (glycogen).
Glycogen is stored in the muscle and liver in tiny, branched structures called “granules.” In total, we store about 350g - 500g of glycogen in our liver (20%) and skeletal muscle (80%). Between meals, during exercise and “fight or flight” situations, and in the early stages of fasting—our body calls upon stored glycogen to provide the fuel we need. The process involves a sequential breakdown of the glycogen “branches” by enzymes until single free molecules of glucose can be released into the bloodstream.
Think of glycogen like money in a wallet. As you continue to stay awake and active (purchase things), you keep taking money out of the wallet. A five here, a one there, until you’re left with no cash. Unless you make a deposit (eat something) to promote glycogen synthesis, once the cash is gone, you’re out of luck. Glycogen depletion is like running out of cash.
Gluconeogenesis is our body’s clever way of getting around this dilemma—providing a way to produce energy de novo when other pathways have been exhausted.
Glycogen stores are limited. And, while we have nearly “unlimited” fat stores from which we can metabolize FFAs and ketones, remember that some cells do have a small glucose requirement. Therefore, it only makes sense that our bodies would have evolved a way to make some available for “emergency” situations.
During times of low energy availability, like fasting or starvation, gluconeogenesis would have allowed us to create some glucose from substrates circulating throughout our body. These substrates are things like proteins (amino acids) from the breakdown of structures like muscle tissue and other compounds that are released when adipose tissue is broken down (a process known as lipolysis).
Gluconeogenesis also plays a role in low-carbohydrate and ketogenic diets, where it allows us to maintain blood glucose levels despite very little dietary carbohydrates being consumed.
A low-carbohydrate diet, in some sense, “mimics” low energy availability without malnutrition.
Carbohydrates aren’t coming in, forcing the body to find alternative routes of energy production. Interestingly, some drugs for type 2 diabetes target gluconeogenesis—inhibiting the process of glucose formation and stimulating glucose uptake.
GNG can provide the small amount of glucose necessary for the tissues that require it, and the rest of the body’s energy can be supplied from the breakdown of fat and the production of ketone bodies to fuel tissues like the heart and skeletal muscle.
What are the Substrates for GNG?
During gluconeogenesis, we make glucose from non-carbohydrate carbon substrates, but what exactly are these gluconeogenic substrates used in the “glucose factory?”
Our body has several, but the main product from which glucose is made is pyruvate. Pyruvate is a biological molecule involved in several energy-producing pathways. It’s also a breakdown product of aerobic glycolysis. You may be more familiar with pyruvate as it is involved in the production of lactate (lactic acid). During exercise or physical activity, we breakdown glucose (glycolysis) to produce energy (ATP). One of the byproducts of this process is pyruvate. Pyruvate can also come from TCA cycle intermediates like oxaloacetate.
We can also generate pyruvate (and then glucose) by breaking down amino acids (AAs) like alanine and glutamate—these are known as glucogenic amino acids.
When degraded, the carbon skeletons of these amino acids can be converted into pyruvate and enter metabolic pathways. Pyruvate can also be generated through the breakdown of glycerol, a molecule that is released when adipose (fat) tissue is broken down into fatty acids (known as lipolysis). Glycerol accounts for about 20% of total glucose production during prolonged fasting.
The final way we generate pyruvate is through lactate. Often (incorrectly) maligned for its role in muscle soreness and fatigue, lactate (sometimes incorrectly called lactic acid) can actually serve as a fuel source during exercise and also be used in the process of gluconeogenesis. This process is known as the Cori cycle. However, we don’t just make lactate during exercise—it’s always being produced at a constant, low level even under resting conditions, but our body can clear it out or metabolize it.
How is glucose made from the breakdown of the gluconeogenic substrates and eventually, from pyruvate? The steps are basically a reversal of glycolysis. Rather than breakdown free glucose top-down, we create it from the bottom-up.
First, pyruvate is generated from gluconeogenic precursors like lactate, alanine, glutamate, and glycerol in the liver or kidney, and also through citric acid cycle (TCA cycle) intermediates through the formation of oxaloacetate (OAA). The first step of gluconeogenesis involves the formation of oxaloacetate by the carboxylation of pyruvate (with the enzyme pyruvate carboxylase). OAA is then reduced to a molecule known as malate, which is then shuttled out of the mitochondrial membrane. Then, an enzyme known as malate dehydrogenase turns malate into oxaloacetate.
Once produced, OAA is then acted on by another gluconeogenic enzyme known as phosphoenolpyruvate carboxykinase (PEP carboxykinase or PEPCK) to produce phosphoenolpyruvate (PEP). Once PEP is formed, the enzymes involved in the metabolism of glucose (glycolysis) start to work in the reverse direction.
Gluconeogenesis “ends” at the production of a molecule known as glucose 6-phosphate. Glucose 6-phosphate can then either be stored as glycogen, or broken down in one final step into free glucose, a reaction that is catalyzed by an enzyme known as glucose 6-phosphatase present in the endoplasmic reticulum of the mitochondria.
This process requires quite a bit of energy and time—we must breakdown six molecules of ATP to turn pyruvate back into one molecule of glucose.
GNG During Fasting and Ketosis
Typically, most individuals maintain their blood glucose needs by consuming dietary carbohydrates.
However, if you’re consuming a low-carb or ketogenic diet or participating in regular intermittent fasting / time restricted eating, gluconeogenesis is likely happening at the cellular level, producing glucose to “compensate” for what you’re not eating. Let’s take a look at how GNG operates under certain dietary conditions.
Intermittent and Prolonged Fasting
There are several biological, physiological, and cellular benefits to intermittent fasting, many of which occur due to the fact that during fasting, we burn our own body fat for fuel. This occurs because after a prolonged period without eating, blood glucose (and insulin) levels begin to decline. However, blood glucose never drops to zero, and remember—we always need some glucose. Enter, gluconeogenesis.
Nearly all glucose after an overnight fast will come from our body’s own production—whether breaking down stored glycogen or creating glucose through GNG.
At this point, each metabolic pathway will contribute about half—50% for GNG and the other 50% from glycogenolysis.
How much glucose is provided by GNG vs. glycolysis will also depend on your pre-fast glycogen stores and the amount of physical activity or exercise you’re engaging in during the fast, among other factors. You can deplete glycogen stores quicker through exercise, and GNG will “kick in” sooner during a fast. In the same way, if you exercise before a fast or start with already low muscle and liver glycogen stores, it’ll take a lot less time to run low. Fat burning and GNG will start sooner.
Eating a ketogenic diet by definition means your body is producing ketone bodies, which then can be used by various body tissues as a source of energy.
Ketosis greatly diminishes the reliance on carbohydrates, and certain tissues will begin to require less glucose to function—preferring ketones instead. However, this doesn’t mean that the keto diet removes the glucose requirement of some other tissues; gluconeogenesis still occurs to some degree even if you’re the most well-oiled fat-burning machine. For instance, red blood cells lack mitochondria, and therefore can’t use ketone bodies as an energy substrate.
The metabolic processes of ketogenesis and gluconeogenesis are compatible, both running simultaneously.
The only difference is that on a keto diet, ketones are “promoted” as the preferred fuel source, saving the small amount of glucose for where it’s needed.
Certain body tissues can run on the “credit” of ketones, while saving the glucose “cash” for those that can’t use ketone bodies.
In the initial stages of a low-carb or ketogenic diet (the fat-adaptation period), internal glucose from GNG and glycolysis will provide a good portion of glucose as an energy substrate. At this point, your ability to produce and utilize ketones isn’t quite optimized. However, after a few weeks, ketogenesis hits full force, and your body becomes a ketone-burning (and producing) machine.
It might seem paradoxical, but rates of GNG might actually increase on a ketogenic or low-carb diet, despite blood glucose levels dropping. Why does this happen? Essentially, all of the glucose you’re producing is being used for a purpose—whether it be maintaining an adequate blood glucose, providing energy for glucose-reliant tissues, or replenishing muscle glycogen.
A ketogenic diet prevents the need for excess gluconeogenesis, since this would require a lot of extra energy. Remember, producing a single glucose molecule from pyruvate requires six ATP molecules. In addition, ketones generate more energy (ATP) per gram than glucose. While 100g of glucose provides about 8.7kg of ATP, the same amount of BHB (the ketone body, beta-hydroxybutyrate) can yield 10.5kg, and acetoacetate (another ketone body) about 9.4kg.
This is why some body tissues (or the body as a whole) might prefer ketogenesis vs. other energy-producing metabolic pathways.
Myth Busting: Will Excess Protein Kick You Out of Ketosis?
Many have heard the popular misconception that eating excess protein on a ketogenic diet will “kick you out” of ketosis. This fear arises from the fact that certain amino acids can be used in gluconeogenesis. The theory goes that eating more protein than is “required” means the extra amino acids will be used to produce glucose and lead to an insulin spike, inhibiting ketone production.
The short answer to the protein and GNG dilemma? No.
Even if you’re in ketosis, gluconeogenesis is still occurring. It’s not as if some switch is flipped where GNG is either “on” or “off.” Furthermore, our body has ways of balancing and mediating the GNG response so that blood glucose doesn’t rise too quickly.
If you’re fairly insulin sensitive, any glucose response to a high-protein meal is dealt with accordingly. GNG also occurs slowly, and it’s unlikely you’d experience high blood sugar in response to a single high protein meal.
And, GNG doesn’t happen because it can—it’s not a supply driven process. Rather, GNG happens only when it needs to—it’s demand driven.
For instance, with high levels of glucose and ADP are present, GNG is inhibited. Unless you need glucose, you won’t produce any more than is necessary. If you’re keto-adapted, your demand drops sharply.
So, there may be one time to worry more about protein and GNG, when you’re initially adapting to a ketogenic diet and haven’t built the metabolic machinery necessary to fully utilize fat and ketones as your primary fuel source.
Nevertheless, studies have shown that, even under the “optimal” gluconeogenic conditions (after an overnight fast), ingesting a high protein-no carbohydrate meal failed to significantly increase GNG.
Another nail in the coffin for this myth is the fact that there are more preferred gluconeogenic substrates than amino acids.
For instance, lactate is more readily preferred compared to amino acids. Studies have shown that overall, the carbon sources from protein breakdown contribute very little to endogenous glucose production after a meal.
Even if (or when) gluconeogenesis increases slightly on a low-carb, high-fat diet, blood glucose is either decreased or unaffected.
A diet containing 30% protein and 70% fat has been compared to one containing “moderate” amount of protein (12%), and it was shown that while GNG was higher, blood glucose concentrations were actually significantly lower.
So, a ketogenic diet containing moderate protein intake is nothing to fear, and probably has more benefits than risks—especially if you’re looking to build lean muscle mass. However, the recommendation for protein on keto is to keep intake at about 10% - 15% of total calories (or 20% - 25% on a more liberal keto diet), with the rest coming from dietary fat.
A well-formulated, low-carbohydrate or ketogenic diet can have several health benefits, not to mention that many people report reaching new highs in mental and athletic performance after adopting these lifestyles.
Along with intermittent fasting, they’re all strategies for human optimization. But diving deep into these topics means certain concepts will come up that make physiology sound a bit confusing.
Gluconeogenesis is not a simple process to understand, but knowing how and why it works along with some of the common misconceptions about the diet, you can optimize your ketogenic or low-carbohydrate eating patterns for maximum benefit.
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