Mushrooms are weird. Honestly, they aren't plants, they aren't animals, and they definitely don't fit the profile of most "starchy" things we grow in the dirt. If you’ve ever found yourself staring at a pack of portobellos while trying to stick to a keto diet, you’ve probably wondered about the carbs in a mushroom. Most people assume all vegetables—or fungus, in this case—have at least a little sugar or starch. But mushrooms are a different beast entirely. They are basically water and fiber held together by a savory, meaty texture.
They’re a cheat code.
If you are tracking macros, you probably know that a medium-sized white button mushroom has about 0.3 grams of total carbohydrates. That is practically nothing. You could eat a literal mountain of them and still stay in ketosis. But it isn't just about the low number; it’s about the kind of carbohydrates they contain. Unlike a potato, which is loaded with rapidly digesting starch, the carbohydrates in fungi are mostly structural. We are talking about chitin and beta-glucans. These don't behave like bread. They don't spike your insulin. Instead, they feed your gut bacteria.
What Most People Get Wrong About Carbs in a Mushroom
People often lump all mushrooms together into one nutritional bucket. That's a mistake. While most varieties are incredibly low-carb, the density varies. Take the common white button mushroom (Agaricus bisporus). It’s the baseline. According to the USDA FoodData Central database, 100 grams of raw white mushrooms contains roughly 3.3 grams of carbs. But wait. About 1 gram of that is dietary fiber. This leaves you with a net carb count of just 2.3 grams.
Compare that to a Chanterelle.
Chanterelles are spectacular, but they are slightly more "expensive" in the carb department, coming in closer to 7 grams per 100g. It’s still low, but if you’re a stickler for details, it matters. Most of the time, when we talk about carbs in a mushroom, we are really talking about the net impact on blood sugar. Dr. Joel Fuhrman, who often discusses the "G-BOMBS" (Greens, Beans, Onions, Mushrooms, Berries, Seeds) diet, highlights mushrooms specifically because they contain compounds that actually inhibit aromatase, an enzyme that produces estrogen. This has nothing to do with carbs, but it shows why these things are health powerhouses beyond just being "low calorie."
The Fiber Factor: Chitin and Beta-Glucans
The structure of a mushroom is built from chitin. It’s the same stuff found in the shells of lobsters and crabs. Humans can't digest chitin easily, which is why mushrooms have that distinct "snap" when you bite them. Because your body doesn't break it down into glucose, it doesn't count toward the metabolic load that high-carb foods do.
Then there are the beta-glucans.
These are soluble fibers. They’ve been studied extensively for their ability to improve insulin sensitivity. It’s a bit ironic. You’re eating a carbohydrate that actually helps your body handle other carbohydrates better. Research published in the Journal of Affective Disorders even suggested a link between mushroom consumption and lower odds of depression, potentially due to these bioactive compounds. When you look at the carbs in a mushroom, you have to see past the raw number on the label. You're looking at a complex matrix of fiber that supports your microbiome.
Cooking Changes Everything (Sorta)
You can't just look at raw numbers. Nobody eats a raw Portobello like an apple. Well, maybe someone does, but it’s not common. When you cook a mushroom, it loses a massive amount of water. This concentrates everything.
If you sauté a cup of mushrooms, they shrink down to about a third of their size. Suddenly, that "cup" of mushrooms has triple the nutrients and triple the carbs by volume. However, the total carbohydrate count remains the same as long as you aren't adding flour or sugary glazes. A big mistake people make in restaurants is ordering "low-carb" mushrooms that have been tossed in balsamic reductions or thickened gravies. That’s where the hidden sugars live.
- White Button: 2.3g net carbs per 100g.
- Shiitake: These are the "heavyweights." They have about 7g of net carbs per 100g raw. They are much denser and have a more intense flavor.
- Oyster Mushrooms: Very light. About 3g net carbs.
- Portobello: Basically a grown-up white button. About 2.5g net carbs.
Why Shiitakes Are the Outlier
If you’re on a strict medical ketogenic diet, you actually need to be careful with Shiitakes. They are delicious. They are umami bombs. But they contain significantly more starch than their cousins. 100 grams of dried shiitakes—which is a lot, admittedly—can have upwards of 60 grams of carbs. Obviously, most of that is because the water is gone, but the ratio is still higher. Stick to the "wet" mushrooms if you are trying to keep your numbers as low as possible.
The Glycemic Impact: Why the Carb Count is Misleading
The Glycemic Index (GI) of mushrooms is incredibly low. Most varieties sit at a GI of around 10 to 15. For context, pure glucose is 100, and a slice of white bread is about 75. When you eat the carbs in a mushroom, your blood sugar barely moves. This makes them a "free food" for many diabetics.
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I remember talking to a nutritionist who specialized in Type 1 diabetes. She mentioned that many of her patients don't even bolus (inject insulin) for mushrooms because the fiber-to-carb ratio is so favorable. The body just processes them so slowly. It’s a slow burn.
- They provide satiety because of the volume.
- The glutamate (natural MSG) tells your brain you are eating meat.
- They are loaded with B vitamins (B2, B3, B5).
- They are the only produce source of Vitamin D (if they were exposed to UV light).
Real World Application: Swapping Carbs for Fungi
The "Mushroom Blend" is a real trend for a reason. You take 50% ground beef and 50% chopped mushrooms. You mix them. You make a burger. You’ve just cut the calories and the fat, but more importantly, you’ve added volume without adding significant carbs. This is the most practical way to think about the carbs in a mushroom. They aren't just something to "limit"; they are a tool to replace higher-carb fillers like breadcrumbs or rice.
Some people use large Portobello caps as bun replacements. It works, though it's messy. Others use sliced King Oyster mushrooms as "scallops" or "noodles." Because the carb count is so low, you can use these as a vehicle for fats like butter, olive oil, or cream sauces without the metabolic disaster that comes from mixing high fats with high starches (the "pizza effect").
A Word on "Mushroom Powders"
You see them everywhere now. Lion's Mane, Reishi, Cordyceps. These are supplements, not dinner. But if you're looking at the carbs in a mushroom powder, be careful. Many cheap brands use "mycelium on grain." This means they grow the mushroom roots on rice or oats and then grind the whole thing up. You aren't just getting mushroom; you're getting the starchy grain they grew on. Always look for "100% Fruiting Body" to ensure you're getting the actual fungus and not a pile of hidden grain carbs.
Limitations and Nuance
It is worth noting that some people have trouble digesting the specific types of carbs in a mushroom. If you have IBS or follow a low-FODMAP diet, mushrooms can be tricky. They contain mannitol, which is a sugar alcohol. For most people, this is fine. For others, it leads to bloating. It’s a reminder that "low carb" doesn't always mean "easy to digest" for every single person.
Also, wild mushrooms are a different story. If you’re foraging (which you shouldn't do unless you’re an expert), the nutritional profiles can vary wildly based on the soil and the tree the mushroom is growing on. Stick to the cultivated stuff for predictable macros.
Moving Forward With Mushrooms
If you want to maximize the benefit of the low carbs in a mushroom, stop boiling them. Boiled mushrooms are slimy and sad. Roast them at 400 degrees Fahrenheit with plenty of salt and a high-smoke-point oil. This carmelizes the small amount of natural sugars they do have, creating that deep brown "crust" that makes them taste like a steak.
Start by replacing one starchy side dish a week with a big pan of roasted Cremini mushrooms. Don't worry about the portion size. You could eat twenty of them and still be well under the carb count of a single slice of toast. Focus on the "wet" varieties like buttons, oysters, and portobellos for the lowest carb impact. If you are using them in a stir-fry, add them first to let the water cook out before adding your sauce; this prevents the dish from getting soggy and keeps the flavor concentrated. Check your labels on any pre-packaged mushroom snacks, as many "mushroom jerky" products are loaded with added honey or cane sugar, which completely negates the natural low-carb advantage of the fungus itself.