Glucose is made in three fundamentally different ways depending on context: plants build it from carbon dioxide and sunlight, your body synthesizes it from non-sugar raw materials like protein and fat, and factories produce it by breaking down corn starch with enzymes. Each process arrives at the same simple sugar molecule, but the machinery and starting ingredients are completely different.
How Plants Make Glucose
Plants are the original glucose factories. They pull carbon dioxide from the air through tiny pores in their leaves and combine it with water using energy captured from sunlight. This happens in two stages: light-dependent reactions that harvest solar energy, and a second set of reactions called the Calvin cycle that actually assembles glucose.
In the Calvin cycle, an enzyme called rubisco (the most abundant protein on Earth) attaches a carbon dioxide molecule to a five-carbon sugar. The resulting six-carbon compound immediately splits into two three-carbon molecules. These three-carbon pieces then get an energy boost from the light-harvesting stage, converting them into a small sugar building block called G3P. Some G3P molecules are used to regenerate the starting five-carbon sugar so the cycle can continue. The rest are combined to build glucose. The whole cycle must turn six times, fixing six carbon dioxide molecules, to produce one molecule of glucose.
How Your Body Makes Glucose
Your body doesn’t just rely on food for glucose. Your liver, and to a lesser extent your kidneys, can build brand-new glucose molecules from scratch using a process called gluconeogenesis. This kicks in whenever blood sugar drops, particularly between meals, during sleep, and during exercise.
The three main raw materials are lactate (a byproduct of muscle activity), glycerol (released when fat cells break down stored fat), and amino acids from protein. Alanine is the most important amino acid for this purpose in the liver, while glutamine plays a bigger role in the kidneys. These starting materials all get funneled through a series of chemical steps that essentially run the glucose-burning pathway in reverse, though with a few key workarounds that require extra energy. Building one glucose molecule from scratch costs the equivalent of six ATP molecules, your cells’ energy currency.
After an overnight fast, your body produces roughly 10 to 11 micromoles of glucose per kilogram of body weight every minute. About 80% of that comes from the liver, with the kidneys contributing around 20%. During prolonged fasting (beyond 24 hours or so), the kidneys ramp up significantly and can account for about 40% of total glucose production.
When Gluconeogenesis Ramps Up
For the first several hours after your last meal, your liver mostly draws on its stored glycogen, a starch-like reserve of pre-made glucose. But glycogen stores are limited. Somewhere between 12 and 36 hours after eating, depending on your activity level and how full those stores were to begin with, the liver’s glycogen runs out and gluconeogenesis becomes the primary source of blood glucose. This is the same metabolic shift that makes prolonged fasting feel different from simply skipping a meal.
Hormones That Drive Production
Several hormones tell your liver to ramp up glucose production. Glucagon, released by the pancreas when blood sugar falls, is the primary signal. Adrenaline and noradrenaline trigger a rapid burst of glucose release during stress or physical danger. Cortisol and growth hormone contribute over longer time frames by promoting fat breakdown (which provides glycerol) and reducing how efficiently muscles absorb glucose, ensuring more stays available for the brain.
This system is what goes wrong in type 2 diabetes. The liver overproduces glucose even when blood sugar is already high. Metformin, the most widely prescribed diabetes medication, works by dialing down gluconeogenesis in the liver. It specifically interferes with the liver’s ability to convert lactate and glycerol into glucose, which is a major reason it lowers blood sugar so effectively.
How Factories Make Glucose
Commercial glucose production starts with starch, most commonly from corn. The process involves breaking the long starch chains into individual glucose molecules using a combination of heat and enzymes, in what amounts to an industrial version of digestion.
The process has two main stages. First, corn starch is mixed with water and heated above 110°C (230°F) in the presence of a heat-stable enzyme that chops the long starch chains into shorter fragments. This liquefaction step takes about 60 to 90 minutes at around 95°C. In the second stage, called saccharification, a different enzyme clips those shorter fragments into individual glucose molecules. The result is glucose syrup, which can be further processed, crystallized, or converted into other sweeteners like high-fructose corn syrup.
This two-enzyme approach replaced older acid-based methods because it produces a cleaner, higher-glucose product with fewer unwanted byproducts. The same basic technique works with wheat, potato, or rice starch, though corn dominates in the United States due to cost and availability.
Making Glucose in the Lab
Researchers have begun experimenting with producing glucose entirely outside of living organisms, essentially replicating what plants do but using lab equipment instead of leaves. One approach uses the same rubisco enzyme from plants, immobilized inside tiny microfluidic channels, to continuously fix carbon dioxide into glucose precursors. These systems can produce the three-carbon building blocks that plants use to assemble glucose, though scaling this up to meaningful food production remains far off. The work is motivated by potential applications in space colonization and food security, where growing crops may not be practical.