You will learn
#1 Why your body makes it own carbs
#2 The different ways your body makes its own carbs
#3 How your body uses those carbs to accomplish many awesome tasks
Carbohydrates are an energy source that humans have eaten more or less of throughout evolutionary history. How much we ate depended on the season, geography, weather, climate, and our successes and failures hunting big game, small critters or catching insects.
Like every other animal, we had to evolve an optimal foraging strategy (OFS) that could ensure our food gathering efforts helped us live in good enough health to make babies, babies with a fighting chance to do the same. Food sources of carbohydrates like starchy tubers, fruit and honey are reasonably assumed to have been part of the human OFS when available.
This raises questions like: Were carby foods a fallback option when times were hard? Or were they our main source of calories? Can their role during our evolution tell us how much of them we should eat now? Or is that too much of a stretch?
These questions are best asked with the understanding that humans are the most successful apex predators of all time and have an obligate need for animal sourced foods [1, 2]. We have a more extensive post on carbohydrate intake during the paleolithic if you want to take a deeper dive on the matter.
We won’t get to the bottom of those questions today but a good place to start is to understand how we make, store and ‘burn’ our carbs. This will provide food for thought.
1.How you make your carbs
Humans don’t technically have to eat any dietary carbs because our bodies have a process to make their own called gluconeogenesis. You can check out our more extensive post on gluconeogenesis and how it’s involved with blood sugar control and general metabolic health.
A simple gluconeogenesis definition states that it’s the process of building glucose anew. But how? And using what? Other types of sugars? No!
A more comprehensive gluconeogenesis definition adds that this simple sugar molecule (glucose) is actually built from other molecules that aren’t sugars, like amino acids (from protein) and glycerol (from fat). Gluconeogenesis is the net production of sugars.
Where does gluconeogenesis occur?
Gluconeogenesis occurs in the liver mainly, as well as in the kidneys and the gastrointestinal tract (GI) . However, those are only the final locations for the assembly of glucose – not representative of the whole process. A more complete answer to the question Where does gluconeogenesis occur? starts with ‘in the liver, kidneys and GI…’ but goes on to highlight the ‘supply chain’ upstream of those organs.
Amino acids can be pulled directly from the bloodstream or obtained by breaking down protein from muscle and other protein-rich tissues. Glycerol can also be pulled directly from the bloodstream or it can find its way from adipocytes (fat cells) that break down stored triglycerides to liberate it.
Since the process of gluconeogenesis requires raw materials (substrates) be shipped to those assembly centers (gluconeogenic organs), gluconeogenesis regulation isn’t restricted to those organs. It really starts upstream at the level of fat cells (adipocytes). They are tasked with integrating hormonal and metabolic signals for proper gluconeogenesis regulation, then liberating the goods for subsequent assembly in those particular organs.
Type 2 diabetics with high blood sugars may be told that their gluconeogenesis regulation is ‘off’ and that this is a problem with their liver. However, this neglects the elephant in the room: adipocytes that are unable or unwilling to retain the substrates that eventually end up being pumped out of the liver as an excess of glucose [4, 5]. It also neglects the fact that kidneys and the GI tract have minor but significant gluconeogenic roles .
A lesser known process called glyconeogenesis (not gluconeogenesis) synthesizes glycogen, the storage form of glucose energy in animals, not from glucose but from non-glucose molecules.
Glyconeogenesis is the process whereby glycogen is synthesized anew, not from dietary or gluconeogenesis-derived glucose, but from non-glucose molecules like lactate and amino acids.
It seems to be a sort of redundant mechanism our bodies evolved to make glycogen – never hurts to have a plan B so to speak!
Glyconeogenesis is a minor pathway contributing to glycogen storage compared to gluconeogenesis. Glyconeogenesis appears to be a ‘recycling pathway’ using lactate to make glycogen from the glucose used up by fast twitch muscle fibers [7, 8].
Say you use up lots of glucose while sprinting, the leftover lactate can be turned back into glucose or glycogen; glyconeogenesis turns it back into stored carbs instead of ready-to-use carbs (glucose). Very little is known about this pathway compared to gluconeogenesis. Information may quickly evolve.
2.How you store your carbs
As mentioned, glucose can be made from scratch and pumped out into the bloodstream to feed cells that are clamoring for it or heavily rely on it compared to other fuels (obligate glucose users).
Glucose can also be stored for later use, but not in its simple form as a sugar molecule. It must be stored as a large, complex branching chain of glucose molecules strung together called glycogen.
The process that takes simple glucose and turns it into glycogen is called glycogenesis.
Glycogen is how glucose energy is stored in animals including humans. So humans store glucose as glycogen and plants store it as starches; starches are also a similar type of long branching chain of glucose which plants typically use as their preferred form of stored energy. Humans on the other hand store the vast majority of energy as fat (triglycerides).
Unlike glyconeogenesis, a glycogenesis definition must highlight that it is the dominant mechanism for converting glucose to glycogen and that this glycogen isn’t made anew but from pre-existing glucose. In the liver, kidneys or GI tract, glycogenesis primarily occurs so that it can be broken down quickly in a responsive manner to moment-to-moment energy demands. This is rather like how a shop keeps a stock of goods in the back so that it doesn’t have to place an order every time a customer asks for an item.
Glycogen in muscles is basically there for a quick source of energy during intense athletic efforts that cannot be met by fat energy alone.
Storing glycogen isn’t cheap, in a sense. 1g of glycogen requires 3g of water stored alongside it . This is costly in terms of weight and space and carries 2 to 3 times fewer calories per gram compared to fat.
This is not to say that glycogen is a better or worse form of stored energy than fat. It’s simply fulfilling different needs: fat is biased towards long-term energy needs and glycogen towards shorter-term ones.
Ever heard people complain about ‘only losing water-weight’ and not fat? This is because glycogen is stored with water in a 3:1 ratio and most weight-loss initially come from ‘water weight’ because glycogen is used up before fat.
This is due to carbs having higher ‘oxidative priority’ than fat (which we’ll get to in a bit). This just means that when both are available, glycogen will be used up first and faster. This is especially true when going from a standard American diet (SAD) to a well-formulated low-carb or ketogenic diet which is glycogen depleting .
A human can maximally store about 15 g/kg of body weight of glycogen. An average non-obese male in his 20s, weighing 65 kg (~145 lbs) and loading up on carbs like crazy, can store close to 1 kg (2.2 lbs) of glycogen, mainly in his muscles and liver .
That’s about 3,000 kcals, so approximately ⅔ or more of his daily energy needs. Clearly, only having glycogen as a stored form of energy to rely on for humans is not a good strategy and thus we didn’t only evolve that one.
We also evolved to have a much larger energy ‘tank’, namely adipose tissue (fat tissue). So in comparison, that same male at 13% body fat has 8.45 kg (~19 lbs) of fat on him, equivalent to more than 76,000 kcals! That’s 25 times more stored energy and can keep him alive for weeks or months! The image below is not scientifically validated but it is good enough to ballpark body fat percentages.
And remember, in this example we’re comparing maximum glycogen storage to a low/normal body fat percentage. So the difference could be far greater when comparing the average person with more than 25% body fat.
3.How you burn your carbs
Once carbs are made and stored it’s eventually time to burn them. ‘Burn’ is not a scientific term so it’s worth clarifying the different meanings it may hold.
Glucose is first fermented. That may sound slow if you’re thinking of cheese or wine, but it’s actually fast. Fermentation happens in the absence of oxygen or when oxygen is insufficient. This fermentation process is called glycolysis. It happens in the cytoplasm of our cells – not in the engines (mitochondria) that float around the cytoplasm.
Glycolysis doesn’t produce a lot of the energy currency per unit ATP, but it does make a lot of it fast. Think of glycolysis as the first gear: not an efficient way to drive on a long road trip but great for accelerating fast.
The end-product of this ‘acceleration’ is lactate which you could think of as ‘exhaust fumes’. However, lactate is not a waste product, because it can be recycled and used by a variety of cells and has important signaling properties .
Glycolysis, however, doesn’t have to end up producing ATP and lactate; it can produce ATP and pyruvate. Picture pyruvate as half a glucose molecule. There’s still energy available from pyruvate. This gets shuttled to mitochondria floating around the cytoplasm so that the remaining energy can be extracted from it – this time using oxygen in a process called oxidative phosphorylation (OxPhos).
Glycogenolysis is the the breakdown of glycogen into the simple sugar glucose for immediate energy use. This can happen in the liver and muscle. In the liver, glycogenolysis keeps glucose metabolism functioning properly by releasing glucose into the blood as needed. In the muscle, glycogenolysis releases glucose for the cell to burn, crucial for the ‘fight-or-flight’ response that occurs when we’re fearful or performing an intense athletic feat.
How does glycogenolysis happen? Let’s take muscle cells first.
Glucocorticoids like cortisol are released by the adrenal glands sitting atop of the kidneys and go on to induce the release of catecholamines (a class of neurotransmitters). These then cause a chain reaction, swiftly breaking down the glycogen chains into glucose.
In the liver, cortisol ends up doing the opposite! It leads to the increase of glycogen storage rather than spurring glycogenolysis which decreases glycogen storage.
Oxidative priority was mentioned above because this is an important part of how we burn our carbs. Oxidative priority refers to which fuels get used first when all are equally available. Carbs come in 3rd, in front of fat but after protein . Alcohol is 1st technically but it can be ignored here as it’s obviously not a viable fuel for humans.
The oxidative priority also explains why when someone is trying to lose fat, glycogen is used up before fat loss is ramped up. This may be due to an intentional or unintentional restriction of calories and the use of carbohydrate restricted diets such as low-carb high-fat (LCHF), ketogenic (keto) and protein-sparing modified fast (PSMF). Check out doctor Ted Naiman’s excellent food pyramid for carbohydrate restricted diets.
#1 Humans make their own carbs through gluconeogenesis and the lesser known glyconeogenesis pathways
#2 Glucose is actually built from other molecules that aren’t sugars, like amino acids (from protein) and glycerol (from fat)
#3 Carbs can be stored in the liver and muscle but only in small amounts compared to our fat
#4 To burn carbs we use glycolysis which is inefficient but fast, and helps maintain glucose homeostasis, the fight-or-flight response and intense athletic feats
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