Fiber

Dietary fiber is plant material that resists digestion. Soluble and insoluble types differ. Excess raw fiber can irritate a fragile gut; raw carrot is a specific tool, not a license for bran megadoses.

How far resistant starch gets fermented depends on which colonic bacteria a person carries. In an in vitro study of four dominant starch-degrading bacteria from the human colon, only one Firmicutes species (named in the reference title) stimulated breakdown of type 2 and type 3 resistant starch by the other three species in co-culture. Adding that species to fecal bacteria from a volunteer who lacked its close relatives greatly raised type 3 resistant starch fermentation.

Practice

Cooked food first when the gut is weak. Carrot salad as a mechanical binder for some people. See Carrot, Digestion, Gut, Endotoxin.

In an uncontrolled study of people with constipation of no known cause, those who stopped or cut back on fiber had more frequent bowel movements and less bloating and straining. Those who kept eating a high-fiber diet saw no change. See Constipation.

Inulin

Inulin is a fructan-type soluble fiber found in foods such as chicory root and Jerusalem artichoke, commonly marketed as a prebiotic. Separately, inulin has long served as a marker molecule for measuring extracellular space in muscle tissue in physiology research, since it distributes into extracellular water but is excluded from cells; Gilbert Ling and coworkers used a low-concentration-inulin probe method, among several other independent methods, to establish that the extracellular space of frog sartorius muscle is close to 9-10%.

Antinutrients

Lectins are plant proteins that bind carbohydrates on cell surfaces; beans and other legumes are a major dietary source. Anti-lectin diets exaggerate the risk for most people eating properly cooked food; raw or undercooked kidney beans specifically are the real, well-documented lectin hazard, and standard boiling neutralizes it. A 2018 French military-base outbreak traced to undercooked kidney beans in chili con carne sickened 200 of 1,700 diners (a 12% attack rate) within a median 3.3 hours of the meal; the raw beans tested at 66,667 HAU/gram of the lectin phytohaemagglutinin, over 150 times the roughly 400 HAU/gram threshold considered potentially toxic, confirming that undercooking, not lectins in general, was the actual hazard.

Peat's actual concern with beans: phosphate, not lectins

"Lectins" specifically is more a paleo/anti-grain framing than a term central to Ray Peat's work. His own stated caution about beans was different: phosphate load, not lectin content:

Beans and meat are phosphate dangerous.

— Ray Peat

That fits his broader framework better than a lectin-avoidance frame: high phosphate relative to calcium is the recurring mineral-balance concern in his writing on legumes and muscle meat alike, not lectin protein content specifically.

Practice (Lectins)

Cook legumes thoroughly; this destroys the genuine lectin hazard in raw beans regardless of any broader dietary philosophy. Balance bean-heavy meals with calcium-rich food given the phosphate load. See Digestion, Phytate, Calcium.

Oxalic acid molecular structure

Oxalates are plant-derived salts (oxalic acid bound to minerals) that bind calcium in the gut and, in susceptible people, can form kidney stones or tissue crystals. Spinach, chard, beet greens, nuts, and chocolate are common high-oxalate foods.

Context

No specific Peat commentary naming oxalate or oxalic acid was found. His general framework for plant antinutrients (see Phytate) treats cooking and mineral balance, not blanket avoidance, as the practical response: boiling high-oxalate greens and discarding the water reduces oxalate content substantially, and pairing them with calcium-rich food limits how much free oxalate reaches the gut lining and kidneys. A lab analysis of nine raw and cooked vegetables found boiling cut soluble oxalate content by 30-87%, clearly outperforming steaming (5-53%) and baking (no measurable loss, tested on potatoes); discarding the cooking water matters, since nearly all of the lost oxalate ends up dissolved in it rather than destroyed. Roughly 75% of kidney stones are primarily calcium oxalate, the reason this reduction matters clinically for stone-prone individuals.

Practice (Oxalate)

Cook high-oxalate greens rather than eating them raw in large amounts; balance oxalate-heavy foods against adequate calcium and magnesium intake. People with a history of oxalate kidney stones need individualized medical guidance beyond general dietary rules.

Phytate (phytic acid) in seeds, grains, and legumes binds calcium and other minerals in the gut, lowering their absorption. Soaking, fermenting, and cooking reduce it to varying degrees depending on the grain.

Which grains Peat flagged

Asked about the safest grain choices, Peat singled out oats specifically for high phytate interfering with calcium absorption, and ranked alternatives by how they're processed:

Masa harina (best), white rice or oats. The phytic acid in the oats block absorption of much of the calcium; cooking the oats much longer than usual might improve its nutritional value.

— Ray Peat

Masa harina (nixtamalized corn: corn soaked and cooked in an alkaline solution, traditionally calcium hydroxide) ranked best because that processing measurably increases calcium content and digestibility, not just because corn is the base grain: a controlled lime-cooking study found phytic acid dropped about 35% while ionizable calcium rose 400-478% and ionizable iron rose 52-77%. Asked whether soaking oats the same way would help similarly: boiling to start the process, rather than just soaking, matters for retaining the added calcium.

Practice (Phytate)

Do not build a diet around raw bran and unprocessed seed/grain meals. Where grains are eaten, prefer nixtamalized corn or well-cooked white rice over oats; long-cook oats if using them, and pair grain-heavy meals with dairy or other easy calcium sources. See Calcium, Iron, Zinc, Magnesium.

Oats

Oatmeal is oat porridge: starch with more fiber and unsaturated fat than white rice. Health culture sells it as heart-protective breakfast. For many people it is a heavy, flat-energy starch with little mineral upside next to milk, fruit, and eggs.

Ranking

Fruit, juice, milk, and eggs beat a large oat bowl when the goal is warm metabolism and easy digestion. Oats with sugar and milk are less bad than dry oats with seed oil "granola." Phytate and fiber load matter for mineral absorption.

Practice (Oatmeal)

If oats are used, cook them soft with milk and sugar; do not build every morning on dry oat bulk.

Prefer orange juice and eggs when pulse and temperature are low. See Orange juice, Eggs, Sugar.

Pectin

Pectin is a soluble fiber found in fruit, especially in the skin and core of apples and in pears.

Effect on serotonin

Pectin tends to increase serotonin by irritating the intestine; raw apples, high in pectin, can cause the gut to release serotonin into the blood, while well-cooked apples have much less of this effect. Fruit-related sore throat and migraine reactions some people experience have been linked to this serotonin-releasing effect.

Psyllium

Psyllium husk is a soluble fiber used as a bulking agent for constipation and to bind irritants such as excess carotene in the gut.

Uses

Washed and cooked bran or psyllium husk was suggested as a way to reduce carotene from carrots without discarding the vegetable. In animals, psyllium husk added to the diet has been described as reducing diarrhea in cases of inflammatory bowel disease, alongside pregnenolone and progesterone. Fiber such as psyllium can also stimulate peristalsis, which tends to be sluggish when body temperature is low.