The Real Numbers: Protein, Fibre and Vitamin D in Mushrooms

Ask ten people what mushrooms are nutritionally good for and you'll get ten different half-right answers — "protein," "vitamin D," "fibre," "low calorie," usually said with the confidence of a fact rather than the tentativeness of a guess. The numbers behind these claims are real, but they get flattened in casual conversation to the point of being almost useless. So here they are properly, per 100 grams, the way we were taught them.

Calories and macros: genuinely low, for a specific reason

A raw button mushroom runs about 22 calories per 100g. Oyster mushroom is a bit higher at 33, shiitake around 34, cordyceps closer to 39. None of these numbers are dramatic on their own — a banana is roughly 90 calories per 100g — but the reason mushrooms sit low isn't a trick of measurement. It's water content. Fresh mushrooms are close to 90% water by weight, with no starch and very little sugar. What calories exist come mostly from a small amount of protein and fibre, with fat almost negligible — most species sit under 2% fat, and what fat is present skews toward unsaturated fatty acids.

This matters for a fairly unglamorous but practical reason: mushrooms have close to zero cholesterol and a genuinely low calorific load per unit of volume, which is why they show up so often in weight-conscious meal plans. It's not a magic property. It's arithmetic.

Protein: real, but the fresh-vs-dry distinction actually matters

This is where most casual claims go wrong. On a dry-weight basis, mushrooms average 30-35% protein — a genuinely high number, comparable to legumes. But nobody eats dry mushroom powder as their base food; fresh mushrooms are eaten wet, and because they're roughly 90% water, the protein per 100g fresh weight drops to somewhere between 2 and 4 grams depending on species — oyster mushroom at 3.3g, button at 3.1g, shiitake lower at 2.2g.

That's a legitimate but modest contribution — closer to what you'd get from a glass of milk than a chicken breast. The place where the dry-weight protein density actually shows up in a meaningful serving is in dried mushrooms and mushroom powder, which is exactly why we use milled mushroom powder as an ingredient in our cookies, nutribars and soup mixes rather than relying on the fresh product or, worse, a flavouring extract that carries none of the nutrition. A 40g mushroom protein nutribar built on concentrated powder is doing real nutritional work that a bowl of fresh sautéed mushrooms, eaten alone, would need several servings to match.

One more detail worth knowing if you're building a diet around it: mushroom protein contains all nine essential amino acids and is particularly rich in threonine and valine, but runs low in the sulphur-containing amino acids methionine and cysteine. That's not a flaw exactly — it's simply why nutritionists describe mushrooms as a protein contributor rather than a complete standalone protein source, the same caveat you'd apply to most single plant-based foods.

Fibre: this is where mushrooms genuinely outperform expectations

Fibre content varies more by species than most people expect. Wood ear mushroom, unusually, carries close to 5.8g of fibre per 100g — high enough that it's used specifically for texture and bulk in Chinese and Northeast Indian cuisine. Oyster mushroom sits around 2.3g, shiitake and morel both close to 2.5-2.8g, and even the humbler button mushroom contributes about 1g. Mushroom fibre includes beta-glucans, the same class of soluble fibre studied for its effects on cholesterol and immune signalling in oats and barley — though the clinical evidence for mushroom beta-glucans specifically is earlier-stage than the oat research, so we're careful not to overstate it.

Combined with the absence of starch and low sugar content, this fibre profile is the actual mechanism behind the "diabetic-friendly" reputation mushrooms carry in Indian dietary advice — not a mystical property, just a low-glycemic, high-fibre food that doesn't spike blood sugar the way a starchy vegetable might.

Vitamin D: real, but conditional

This is the claim that gets most garbled in translation. Mushrooms contain ergosterol, a compound that converts to vitamin D2 when exposed to ultraviolet light — sunlight or UV lamps. A mushroom grown and harvested entirely in the dark (which describes most commercially farmed mushrooms, including the enoki you'd find in a supermarket) has negligible vitamin D. A mushroom that's been sun-exposed after harvest, even briefly, can develop meaningfully more. Shiitake in particular is noted for developing higher vitamin D2 levels with deliberate sun exposure post-harvest — some producers do this on purpose, laying trays of shiitake out in daylight for a few hours before packing.

So "mushrooms have vitamin D" is true but incomplete. It's true of sun-exposed mushrooms and largely untrue of mushrooms grown and sold without that step. If a product's vitamin D content matters to you specifically, look for suppliers who mention UV or sun exposure explicitly rather than assuming it by default.

Minerals: the quiet strength of the category

The mineral profile is genuinely one of the stronger nutritional arguments for mushrooms, and it's underdiscussed relative to protein and vitamin D. Potassium content runs high across most species — oyster mushroom at 420mg per 100g, morel at 411mg, both ahead of a medium banana. Selenium is a standout: mushrooms are one of the few reliable vegetarian sources of selenium, a mineral otherwise concentrated mostly in animal protein and Brazil nuts, and important for immune function and antioxidant activity. Copper is present in high enough amounts that a 100g serving can supply a meaningful fraction of daily requirement.

None of this makes mushrooms a superfood in the marketing sense of the word — we try to avoid that language deliberately. What it makes them is a genuinely useful, low-calorie, fibre-rich, mineral-dense food category that earns its place in a diet on real numbers rather than vibes. That's the version of the nutrition story we'd rather tell, even if it's less exciting than the one-line claims that usually circulate.