Animals get glucose from three main sources: digesting carbohydrates in food, breaking down stored energy reserves, and manufacturing it internally from non-sugar molecules like fats and proteins. The balance between these sources varies dramatically across species. A cow grazing on grass, a hummingbird drinking nectar, and a bear deep in hibernation are all keeping their cells fueled with glucose, but through very different strategies.
Glucose From Food
The most direct route is eating carbohydrates. When an animal consumes sugars or starches, digestive enzymes break those molecules into simple sugars, primarily glucose, which then pass through the intestinal wall into the bloodstream. This process is relatively straightforward for omnivores and frugivores eating sugar-rich or starchy foods. Pigs, for example, absorb glucose from starch-heavy meals much the way humans do, though the speed of absorption differs depending on whether the carbohydrate source is pure sugar, table sugar, or corn starch.
Herbivores face a bigger challenge. The bulk of their diet is plant fiber, especially cellulose, which is essentially glucose molecules locked together in a bond that animal enzymes cannot break. Cellulose contains enormous potential energy, but almost no multicellular animal can digest it on its own. Instead, herbivores rely on symbiotic microorganisms living in their digestive tracts. These bacteria and other microbes produce specialized enzymes that ferment plant cell walls, releasing energy the animal can use. Horses, for instance, have a highly developed cecum and large intestine where microbial fermentation happens after the stomach. Ruminants like cows take a different approach, fermenting food in a multi-chambered stomach before it even reaches the small intestine.
What makes ruminant digestion especially interesting is that these animals absorb very little glucose directly from their gut. Instead, microbial fermentation produces short-chain fatty acids, particularly propionate, which the liver then converts into glucose. Propionate alone can account for 32% to 73% of all glucose the liver produces in cattle. So a cow chewing grass is ultimately running on glucose, but that glucose was assembled in the liver from fermentation byproducts rather than absorbed from the intestines the way it would be in a dog or a human.
Making Glucose From Scratch
Every animal can manufacture glucose internally through a process called gluconeogenesis, which literally means “new glucose creation.” This happens primarily in the liver and, to a lesser extent, the kidneys. The raw materials are non-sugar molecules circulating in the blood: leftover fragments from protein digestion, the glycerol backbone released when fat is broken down, and lactate produced by muscles during exercise.
In animals with simple stomachs (dogs, cats, humans, rodents), the liver preferentially uses lactate, glycerol, and certain amino acids, especially alanine, as building blocks. Lactate gets recycled through what’s known as the Cori cycle: muscles produce lactate during intense activity, the liver scoops it up and converts it back to glucose, and that glucose returns to the muscles for fuel. Glycerol, released when stored fat is broken down, undergoes a two-step conversion in the liver before entering the glucose-production pathway. Amino acids stripped from dietary or body protein get funneled into the same pathway after being chemically rearranged.
For ruminants, gluconeogenesis isn’t just a backup system. It’s the primary source of blood glucose at all times, since so little glucose is absorbed directly from their food. This makes the ruminant liver a glucose factory running around the clock, with propionate from gut fermentation serving as the dominant raw material.
Glycogen: The Short-Term Reserve
When glucose is abundant after a meal, animals store the excess as glycogen, a densely branched molecule packed into liver and muscle cells. In a typical mammal, glycogen makes up about 5% of the liver’s wet weight and roughly 1% of skeletal muscle. For a 70-kilogram human, that works out to around 350 grams in the liver and 90 grams in muscle tissue. Other mammals store glycogen in similar proportions relative to body size.
Liver glycogen and muscle glycogen serve different purposes. The liver breaks down its glycogen and releases glucose into the bloodstream to keep the rest of the body supplied between meals. Muscle glycogen stays local, fueling only the muscle fibers where it’s stored. This is why an animal sprinting from a predator burns through muscle glycogen quickly but can’t tap its liver stores fast enough to power that burst of speed. Once glycogen runs low, the body shifts toward gluconeogenesis and fat metabolism to keep glucose levels stable.
How Insects Handle Glucose Differently
Insects don’t circulate glucose in their blood the way mammals do. Instead, the main sugar in insect hemolymph (the insect equivalent of blood) is trehalose, a molecule made of two glucose units bonded together. The fat body, an organ that functions like a combined liver and fat storage depot, synthesizes trehalose and releases it into circulation. When tissues need energy, specialized enzymes split trehalose back into glucose on demand.
This system has a practical advantage. Trehalose is more chemically stable than glucose and resists the kind of unwanted reactions that high glucose levels can cause in mammalian blood. It acts as both a transport sugar and a rapid energy reserve. When a flying insect needs a sudden burst of fuel, trehalose in the hemolymph is quickly hydrolyzed to glucose and burned by flight muscles.
Blood Glucose Levels Across Species
Not all animals maintain glucose at the same concentration. Reptiles typically run the lowest, with blood glucose between 60 and 100 mg/dL, though this fluctuates significantly with temperature and feeding status. Mammals sit in a middle range, generally between 100 and 200 mg/dL. Birds run surprisingly high, with normal levels between 200 and 500 mg/dL, reflecting their intense metabolic demands for flight and maintaining a high body temperature.
These differences aren’t accidental. They reflect each group’s metabolic rate and energy strategy. A bird’s cells consume glucose at a furious pace, so maintaining a higher blood concentration ensures fuel is always available. A cold-blooded reptile basking in the sun has far lower energy demands and can function perfectly well with less circulating sugar.
How Glucose Enters Cells
Getting glucose into the bloodstream is only half the job. It still has to cross cell membranes to be used as fuel, and animal cells rely on a family of transporter proteins embedded in their surfaces to pull glucose inside. Different tissues use different transporters tuned to their specific needs.
Brain cells use a high-affinity transporter that grabs glucose even when blood levels dip, ensuring the brain never runs short of its primary fuel. This transporter has the highest turnover rate of any in the family, meaning it shuttles glucose molecules across the membrane faster than other versions. Pancreatic cells and liver cells use a different transporter with a much lower affinity for glucose, which lets them sense how much glucose is circulating and respond accordingly. Fat cells and muscle cells use yet another version that normally sits inside the cell and only moves to the surface when insulin signals that blood glucose is elevated. This is the mechanism that fails in insulin resistance.
Hibernation: Switching Away From Glucose
Hibernating animals offer a striking example of how flexible glucose sourcing can be. As chipmunks transition from active life into hibernation, their primary energy source shifts from carbohydrates to stored fat. The liver pivots from glucose metabolism to producing ketone bodies from fat, which most tissues can burn as an alternative fuel.
Paradoxically, hibernating chipmunks actually have higher blood glucose and higher liver glycogen stores than active ones. This isn’t because they’re using more glucose. It’s because they’re using less. Their cells become less sensitive to insulin, reducing glucose uptake and consumption. Glycogen accumulates in the liver, heart, kidneys, lungs, and muscles precisely because it isn’t being burned. The animal essentially shelves its glucose system and runs almost entirely on fat-derived fuel for months, preserving glucose reserves as a backup rather than a primary energy source.
This metabolic flexibility, the ability to switch between glucose, fat, and protein as fuel depending on circumstances, is one of the defining features of animal metabolism. Whether an animal is eating, fasting, sprinting, or sleeping through winter, glucose remains available through some combination of diet, internal production, and stored reserves.