Glucan is a type of polysaccharide, which is a long chain of glucose molecules linked together. It’s one of the most common carbohydrate structures in nature, found in everything from the starch in bread to the cell walls of mushrooms. The difference between one glucan and another comes down to how those glucose molecules are connected, and that small structural detail determines whether your body uses it for quick energy or whether it passes through your digestive system delivering other health benefits along the way.
Alpha-Glucans vs. Beta-Glucans
Glucans split into two major families based on the type of chemical bond holding their glucose chains together: alpha-glucans and beta-glucans. Alpha-glucans include familiar substances like starch and glycogen. Your body breaks these down easily into glucose for energy. They’re the reason a bowl of rice or a potato fuels your muscles and brain.
Beta-glucans are structurally different. The glucose units are joined at an angle that human digestive enzymes can’t break apart, which means beta-glucans pass through your stomach and small intestine mostly intact. This makes them a form of dietary fiber, and it’s the reason they have such distinct effects on cholesterol, blood sugar, and immune function. When people talk about “glucan” in the context of health and nutrition, they almost always mean beta-glucan.
Where Beta-Glucans Come From
Beta-glucans show up in two broad categories of food: cereal grains and fungi. Among grains, barley and oats are the richest sources. Barley contains 2 to 20 grams of beta-glucan per 100 grams of dry weight, with about 65% of that being water-soluble. Oats contain 3 to 8 grams per 100 grams dry weight, and a notably high 82% is water-soluble. Other grains lag behind considerably: rye has 1.3 to 2.7 grams, sorghum 1.1 to 6.2 grams, and wheat just 0.5 to 1.0 gram.
On the fungal side, mushrooms like shiitake, maitake, and reishi are well-known sources. Baker’s yeast is another significant one. The beta-glucans in mushrooms and yeast have a different branching pattern than those in oats and barley, and this structural difference is important because it changes what the beta-glucan does in your body. Cereal beta-glucans are best known for their effects on cholesterol and blood sugar. Fungal and yeast beta-glucans are the ones most studied for immune system activity.
How Beta-Glucan Lowers Cholesterol
The cholesterol-lowering effect of oat and barley beta-glucan is one of the best-documented benefits. When you eat these soluble fibers, they dissolve in your gut and form a thick, viscous gel. This gel traps bile acids, which are substances your liver makes from cholesterol to help digest fat. Normally, bile acids get reabsorbed in the lower part of your small intestine and recycled. Beta-glucan prevents that reabsorption, so the bile acids leave your body in stool instead.
Your liver then needs to pull cholesterol out of your bloodstream to manufacture replacement bile acids. The net result is lower LDL cholesterol. This mechanism is well enough established that the U.S. FDA allows foods containing beta-glucan from oats or barley to carry a heart health claim, provided they deliver at least 3 grams of beta-glucan per day.
Effects on Blood Sugar
That same viscous gel also slows down how quickly your body absorbs glucose after a meal. Beta-glucan delays gastric emptying, meaning food moves out of your stomach more slowly. It also lengthens the time it takes nutrients to travel through your intestines. The thick gel layer reduces how quickly digestive enzymes can reach starch molecules and limits how fast glucose can cross the intestinal wall into your bloodstream.
The practical result is a flatter blood sugar curve after eating. Instead of a sharp spike followed by a crash, glucose enters your blood more gradually. This also reduces the amount of insulin your body needs to release in response. For people managing blood sugar levels, incorporating oat or barley beta-glucan into meals can meaningfully blunt postmeal glucose and insulin responses.
Beta-Glucan and the Immune System
Fungal and yeast beta-glucans interact with your immune system in a fundamentally different way than cereal beta-glucans do. Your immune cells, particularly macrophages (the cells that patrol for threats and engulf foreign invaders), have receptors on their surface that recognize the branching structure of fungal beta-glucans. When beta-glucan binds to these receptors, the macrophage internalizes it and activates a cascade of immune signaling. This can increase the activity of other immune cells and ramp up the body’s antifungal defenses.
Research has identified that different branching patterns activate different receptors. The beta-glucans found in yeast cell walls, for instance, bind to a specific receptor and trigger internal signaling pathways that modulate genes related to immune defense and the recruitment of infection-fighting white blood cells. This “biological response modifier” activity is why beta-glucan supplements derived from yeast or mushrooms are marketed for immune support, and why researchers have been investigating them as potential adjuncts in cancer treatment. Early clinical trial data from 2023 to 2025 has shown that beta-glucans used alongside immunotherapy drugs improved overall survival in melanoma and lung cancer patients, though this research is still in relatively early stages.
Solubility Matters
Not all beta-glucan behaves the same way once you eat it, and solubility is the key variable. Water-soluble beta-glucan, the dominant form in oats, dissolves in your gut and creates that viscous gel responsible for cholesterol and blood sugar benefits. Insoluble fiber, by contrast, speeds up transit time through the intestines and adds bulk to stool. Oat beta-glucan is about 82% water-soluble, which is why oats have become the poster child for soluble fiber’s cardiovascular benefits. Barley’s beta-glucan is roughly 65% water-soluble, still effective but slightly less so on a gram-for-gram basis.
Cellulose, incidentally, is also technically a beta-glucan. It’s the structural fiber in plant cell walls. But cellulose is completely insoluble and has a simple, straight-chain structure that gives it none of the cholesterol or immune effects associated with the branched beta-glucans in oats or mushrooms. This is a good illustration of why “glucan” is such a broad term: the same basic building block, glucose, can produce substances with wildly different properties depending on how the pieces are connected.
Safety and Typical Intake
Beta-glucans are considered safe whether consumed as part of whole foods or as supplements. No adverse effects have been reported from eating oat or barley beta-glucan at normal dietary levels. Because beta-glucan is a soluble fiber, consuming large amounts can cause mild digestive symptoms like gas or bloating, particularly if you increase your intake quickly. Starting with smaller portions and building up gradually tends to minimize this. There is no established upper intake limit for beta-glucan, and the 3-gram daily target for heart health benefits is easily achievable through a bowl of oatmeal or a barley-based side dish.