Cooking, Fibre and Digestive Tolerance

Table of Contents

1. The Boiling Problem, Measured
2. Myrosinase and the Heat Window
3. Shredding, Storage and the Six-Hour Clock
4. Vitamin C and the Rest of the Panel
5. Roasting: Why It Works
6. Frozen Sprouts and the Blanching Question
7. Fibre, Bile Acids and the Gut
8. Why Sprouts Cause Wind, and What To Do
9. FODMAPs and Irritable Bowel Syndrome
10. Goitrogens and the Thyroid, in Proportion
11. Building Tolerance
12. A Practical Cooking Guide
13. Key Research Papers
14. Connections
15. Featured Videos

The Boiling Problem, Measured

The definitive study is Song and Thornalley's 2007 work at the University of Essex. They quantified the seven major glucosinolates in broccoli, Brussels sprouts, cauliflower and green cabbage by liquid chromatography with tandem mass spectrometry, then put the vegetables through the storage and cooking treatments people actually use.

The results were unusually clean:

• Steaming, microwaving and stir-frying produced no significant loss of glucosinolates. Not a small loss — no significant loss.
• Boiling produced significant losses, and the mechanism was leaching rather than destruction. Roughly 90 percent of the glucosinolates that disappeared from the vegetable were recovered in the cooking water.
• Storage at room temperature or in a domestic fridge for seven days cost relatively little — a 9 to 26 percent decline.

That 90-percent figure is the single most useful number on this page. The compounds were not destroyed by heat; they were washed out. They were sitting in the pan when it was tipped into the sink. Hanschen and colleagues later mapped the kinetics of that leaching and degradation during boiling of Brassica oleracea vegetables in detail, confirming the same picture with more resolution.

The practical consequence writes itself. Do not boil Brussels sprouts. If you do boil them, use very little water and keep it — a soup, a sauce, a gravy — and you have recovered most of what you would otherwise have poured away.

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Myrosinase and the Heat Window

Glucosinolates do nothing by themselves. The enzyme myrosinase, stored in separate cells in the intact plant, converts them into isothiocyanates when the tissue is broken. Myrosinase is a protein, and heat denatures proteins, so any cooking hot enough and long enough destroys it.

When myrosinase is gone, the glucosinolates are not wasted — they pass into the colon, where bacteria carry out the same conversion. But bacterial conversion is less efficient and varies enormously between people. Studies consistently find that the isothiocyanate yield from cooked crucifers is substantially lower than from raw or lightly cooked ones. So preserving some myrosinase activity is worth doing.

There is a genuinely surprising piece of biochemistry here, from Matusheski, Juvik and Jeffery in 2004. Crushing raw Brassica tissue at room temperature does not produce only isothiocyanates. A second protein, epithiospecifier protein (ESP), diverts the reaction toward biologically inert nitriles instead. ESP is more heat-sensitive than myrosinase. Heating broccoli to 60 °C before crushing knocked out ESP activity, which simultaneously increased sulforaphane formation and decreased the nitrile. Heating to 70 °C and above destroyed both, because by then myrosinase itself was gone.

That work was done on broccoli, and no one has published the equivalent temperature curve for Brussels sprouts, so it should not be quoted as a sprout result. What it establishes as a principle is that there is a window — gentle heat can be better than none, and hard heat is worse than gentle. In a domestic kitchen, that translates into: brief steaming, or roasting where the outside browns while the centre stays short of prolonged high temperature, beats both a long boil and, arguably, eating them entirely raw.

There is also a cheap trick that works regardless. Mustard, horseradish and wasabi contain their own active myrosinase. A teaspoon of mustard stirred into cooked sprouts, or a mustard vinaigrette poured over them, supplies the enzyme the cooking destroyed. This has been demonstrated for cooked Brassica vegetables generally, and it costs nothing.

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Shredding, Storage and the Six-Hour Clock

One more finding from Song and Thornalley deserves separate billing, because it contradicts a piece of standard advice.

Finely shredded Brassica vegetables lost glucosinolates rapidly as they sat: up to 75 percent over six hours of post-shredding dwell time. Part of that loss appeared as isothiocyanates — the myrosinase reaction proceeding in the bowl rather than in your mouth — and isothiocyanates are volatile and unstable, so a good deal simply evaporates.

This matters because the “chop and wait forty minutes” advice, which is sound for broccoli and garlic where the goal is to let the enzyme finish its work before heat arrives, becomes counterproductive if it turns into “shred in the morning and cook at dinner”. Meal-prep habits that involve shredding a bag of sprouts on Sunday for the week are quietly throwing away a large fraction of the interesting chemistry.

The workable version: whole sprouts keep well in the fridge for a week with modest losses. Halve or shred them shortly before cooking. If you want the chop-and-rest benefit, minutes are plenty; hours are a loss.

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Vitamin C and the Rest of the Panel

Brussels sprouts are a genuinely excellent source of vitamin C. Using United States Department of Agriculture reference values, raw sprouts carry roughly 85 milligrams per 100 grams, which is more than an orange weight for weight, and cooked sprouts still carry a large fraction of that. A cup of cooked sprouts comfortably covers an adult's daily requirement.

Vitamin C is the most fragile thing in the vegetable: water-soluble, heat-labile and easily oxidised. It is lost by exactly the same mechanism as the glucosinolates — leaching — plus genuine thermal destruction on top. Petersen's comparison of sous vide processing, steaming and boiling in broccoli florets found the same hierarchy that holds throughout this literature: methods without a large volume of cooking water retain more.

The rest of the panel, again from USDA reference values per 100 grams of raw sprouts:

• Folate — around 60 micrograms, a solid contribution to the daily requirement and one of the better non-leafy sources. See folate.
• Vitamin K — roughly 175 micrograms, one of the highest of any commonly eaten vegetable. This is the one with a genuine drug interaction, covered in full on the vitamin K and warfarin page.
• Potassium — around 390 milligrams, useful for anyone managing blood pressure. See potassium.
• Fibre — about 3.8 grams raw, close to 4 grams in a cup of cooked sprouts.
• Energy — about 43 kilocalories per 100 grams raw. Sprouts are filling out of all proportion to that.

Folate, like vitamin C, is water-soluble and leaches. The same rule covers both: keep the water out, or keep the water.

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Roasting: Why It Works

Roasting is the method that rehabilitated this vegetable, and there are four separate reasons it beats boiling rather than one.

1. No cooking water, so no leaching. The glucosinolates, vitamin C, folate and potassium stay in the sprout. This is the largest single effect and it is entirely mechanical.
2. Browning creates flavour that was not there before. High dry heat caramelises the sprout's own sugars and drives Maillard reactions between sugars and amino acids, generating hundreds of nutty, savoury, roasted aroma compounds. Boiling generates none of these; it only removes.
3. Sweetness suppresses bitterness. This is a measured perceptual effect, not a metaphor, and caramelisation supplies the sweetness from inside the vegetable.
4. The sulfur volatiles never build up. The smell of overcooked sprouts comes from prolonged wet heat breaking sulfur compounds down into hydrogen sulfide and related molecules. A hot, fast roast does not give that chemistry time.

The technique that matters: halve the sprouts, toss them in enough oil to coat, lay them cut side down on a preheated tray so the flat face is in direct contact with hot metal, and use a genuinely hot oven. Crowding the tray steams them in their own moisture and produces a soft, pale, disappointing sprout. Give them room.

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Frozen Sprouts and the Blanching Question

Frozen Brussels sprouts are blanched before freezing — a short immersion in boiling water or steam to inactivate the enzymes that would otherwise cause deterioration in the freezer. That step is not optional; without it, frozen vegetables develop off-flavours within months.

The consequence is that frozen sprouts arrive with myrosinase already destroyed and with some glucosinolate loss from the blanch itself. Goodrich, Anderson and Stoewsand measured glucosinolate changes in blanched broccoli and Brussels sprouts and documented exactly this.

That is not an argument against frozen sprouts. Freezing preserves vitamin C well, frozen vegetables are often processed within hours of harvest, and a frozen sprout that gets eaten beats a fresh one that rots in the drawer. It is an argument for two things: cook frozen sprouts by roasting from frozen rather than boiling, which avoids adding a second leaching step to the one already applied; and add mustard or another raw crucifer to the finished dish, which restores the enzyme the blanch removed.

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Fibre, Bile Acids and the Gut

A cup of cooked sprouts supplies close to four grams of fibre, a mixture of insoluble cellulose and hemicellulose with a soluble fraction that includes pectins and fructans. The insoluble portion adds bulk and speeds transit; the soluble portion is fermented by colonic bacteria into short-chain fatty acids, principally butyrate, which is the preferred fuel of the cells lining the colon. That is the ordinary, well-supported case for eating vegetables with fibre in them, and it is covered more fully under gut health.

Brassica vegetables also bind bile acids, which is one proposed route to lower cholesterol: bile acids bound in the gut are excreted rather than reabsorbed, and the liver must draw on cholesterol to make replacements. Kahlon and colleagues at the USDA's Western Regional Research Center measured this directly in a laboratory digestion model. Relative to cholestyramine, the bile-acid-binding drug used as the positive control, binding on a dry-matter basis was around 13 percent for collard greens, kale and mustard greens, 10 percent for broccoli, and about 8 percent for Brussels sprouts and spinach, with cabbage lowest at around 5 percent. A companion finding is more actionable: steam cooking significantly improved bile acid binding compared with the raw vegetables.

Two caveats keep that honest. It is an in-vitro measurement, not a cholesterol trial in people. And the numbers are relative to a drug specifically designed for the job, so “8 percent of cholestyramine” is a real but modest effect.

On fibre and colorectal cancer, the evidence deserves to be stated precisely rather than in the usual vague form. Aune and colleagues' dose-response meta-analysis of 25 prospective studies found a relative risk of 0.90 (95% CI 0.86 to 0.94) per 10 grams of total dietary fibre a day, and 0.83 (0.78 to 0.89) for each three daily servings of whole grains. But when fibre was broken down by source, cereal fibre carried the association (0.90) and vegetable fibre did not — 0.98, with a confidence interval spanning 1. So the honest reading is that the colorectal-cancer case for fibre rests mainly on cereal fibre and whole grains such as brown rice, oats and whole wheat, and that Brussels sprouts contribute to total fibre without being the part of the evidence that carries the finding. Sprouts have their own, separate case as a cruciferous vegetable, made on the detox enzymes page.

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Why Sprouts Cause Wind, and What To Do

This is real, it is normal, and it has two distinct causes that get confused with each other.

Cause one: fermentation. Human digestive enzymes cannot break the bonds in fructans and raffinose-family oligosaccharides, so these carbohydrates arrive intact in the colon, where bacteria ferment them and produce hydrogen, carbon dioxide and, in some people, methane. This is the same process that produces the beneficial short-chain fatty acids. The gas is a by-product of something you want happening.

Cause two: sulfur. The smell is a different matter, and it comes from the sulfur compounds that make a sprout a sprout. Overcooking generates them in the pan; gut bacteria generate hydrogen sulfide from sulfur-containing compounds downstream. Roasting rather than boiling reduces the first source considerably.

What actually helps:

• Increase intake gradually. The gut microbial community adapts over weeks. Going from no sprouts to a large plate produces a much worse result than building up.
• Smaller portions, more often. Six sprouts three times a week is better tolerated than eighteen sprouts once.
• Cook them properly and do not overcook them. Roasting produces markedly less sulfurous smell than a long boil.
• Chew thoroughly. More surface area, more complete digestion of the parts that can be digested.
• Give it time. Most people who eat cruciferous vegetables regularly stop noticing within a few weeks.

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FODMAPs and Irritable Bowel Syndrome

For people with irritable bowel syndrome, the fermentable carbohydrates in Brussels sprouts are not a background nuisance but a genuine trigger. Sprouts carry fructans, one of the FODMAP groups — fermentable oligosaccharides, disaccharides, monosaccharides and polyols.

The compositional groundwork is Muir and colleagues' 2009 analysis, which measured short-chain carbohydrates across common vegetables and fruits by high-performance liquid chromatography and provided much of the data the low-FODMAP diet is built on. The clinical evidence is Halmos and colleagues' 2014 controlled feeding trial in Gastroenterology, which showed that a diet low in FODMAPs reduced symptoms in people with IBS.

The important practical point is that FODMAP tolerance is dose-dependent, not all-or-nothing. Brussels sprouts are not banned on a low-FODMAP diet; a small portion typically falls within the tolerated range while a large portion does not. The Monash University group that developed the diet publishes tested serving thresholds and revises them as foods are re-analysed, so their app rather than any web page — including this one — is the place to check a current number.

Two things worth saying to anyone in this position. First, the low-FODMAP diet is designed as a short elimination phase followed by systematic reintroduction to find your thresholds, not as a permanent restriction; staying in the elimination phase indefinitely narrows the diet and starves the microbiota of the substrates it needs. Second, if sprouts are a problem for you, the cruciferous chemistry is available from lower-FODMAP relatives — kale, bok choy, arugula, radish and the green tops of turnips are generally better tolerated.

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Goitrogens and the Thyroid, in Proportion

Brussels sprouts do contain compounds that can interfere with the thyroid, and this is one of the places where the internet is both right about the mechanism and badly wrong about the magnitude.

The mechanism is straightforward. Progoitrin, one of the two glucosinolates the Dutch breeders targeted for taste reasons, breaks down to goitrin, which interferes with the incorporation of iodine into thyroid hormone. Separately, the breakdown of indole glucosinolates yields thiocyanate, which competes with iodide for uptake by the thyroid's sodium-iodide symporter.

The most useful quantitative assessment is Felker, Bunch and Leung's 2016 review in Nutrition Reviews, and it is worth reading closely because it does not simply reassure. Its findings:

• Radioiodine uptake by the thyroid was inhibited by 194 micromoles of goitrin, but not by 77 micromoles — so there is a threshold, and it is well above trivial exposures.
• Collards, Brussels sprouts and some Russian kale contain enough goitrin to potentially decrease thyroid iodine uptake at sufficient intake. Sprouts are explicitly on the higher-goitrin side of the family.
• Turnip tops, commercial broccoli, broccoli rabe and Brassica oleracea kale contain less than 10 micromoles of goitrin per 100-gram serving and were judged of minimal risk.
• The thiocyanate contribution from indole glucosinolates was estimated as small relative to background plasma thiocyanate, which in most people comes overwhelmingly from other sources — smoking above all.

It is worth noting, in the interest of reading a source properly, that two of that review's three authors were affiliated with a California vegetable grower. That is a disclosure worth making, and it cuts in a useful direction: a grower-affiliated review that nonetheless names Brussels sprouts among the higher-goitrin vegetables is not being softened.

How much would you have to eat for this to matter? The published cases of diet-induced hypothyroidism involve intakes far outside normal eating. The most-cited is a 2010 report in the New England Journal of Medicine of myxedema coma — the extreme end of hypothyroidism — in a patient who had been consuming very large daily quantities of raw bok choy. Raw, in enormous quantity, sustained over months. That is the shape of the risk, and it is nothing like a portion of roasted sprouts at dinner.

Three things reduce the risk further, and one increases it:

• Cooking helps. Heat destroys myrosinase, so less goitrin is generated in the first place, and boiling leaches away the precursors. Panduang and colleagues examined exactly this trade-off in cabbage and Chinese kale — cooking reduces goitrin but also destroys isothiocyanates — and used response-surface modelling to find household conditions that balance the two, landing on brief steaming at 80–100 °C for cabbage.
• Adequate iodine is protective. Goitrin and thiocyanate act by interfering with iodine handling. Where iodine intake is sufficient, the effect is far less consequential. Iodised salt, dairy, eggs, fish and seaweed all contribute.
• Ordinary portions are far below any documented threshold. Nobody has shown thyroid harm from eating normal amounts of cooked crucifers.
• The one group that should pay attention is people with existing hypothyroidism or a goiter combined with low iodine intake, particularly if they eat large quantities of raw crucifers — the green-smoothie pattern, where a blender makes it easy to consume several hundred grams of raw kale or cabbage daily without noticing. Even then the answer is to cook them and to secure iodine intake, not to give them up.

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Building Tolerance

If sprouts currently disagree with you, the sequence that works is unglamorous and reliable.

1. Start with two or three, roasted, once or twice a week. Well below whatever your current threshold is.
2. Hold that for two weeks. The microbial adaptation is real but not instant.
3. Increase slowly, one or two sprouts at a time. If symptoms appear, go back a step and hold longer.
4. Keep them well cooked while you are adapting. Roasted or steamed until properly tender is easier on the gut than raw shredded sprouts, which are the hardest version.
5. Eat them alongside other food, not on an empty stomach.
6. Keep the rest of the diet varied. A microbiota with a broad substrate supply handles new fermentable material better than one accustomed to a narrow diet.

If you have IBS, do this within the reintroduction phase of a properly structured low-FODMAP plan rather than by trial and error, ideally with a dietitian who works with the protocol.

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A Practical Cooking Guide

1. Roast as the default. Halved, oiled, cut side down on a preheated tray in a hot oven, not crowded, until the faces are deeply browned and the loose outer leaves are crisp.
2. Steam when you want them plain. Five to seven minutes for halves. No significant glucosinolate loss, and the texture stays intact.
3. Stir-fry shredded sprouts. Fast, hot, no water, and the shredding happens minutes before cooking rather than hours.
4. Microwave if you are in a hurry. It performed as well as steaming in the Essex measurements — a small amount of water, covered, a few minutes.
5. Do not boil. If you must, use minimal water and use the water afterwards.
6. Add mustard at the end. It supplies the myrosinase cooking destroyed, and it tastes right with sprouts anyway.
7. Buy small, tight, firm sprouts and store them whole in the fridge. Cut or shred shortly before cooking.
8. Try them raw and shaved for a completely different food — maximum enzyme activity, maximum vitamin C, and no cooking losses at all. Dress with lemon and olive oil, add toasted nuts and something salty.
9. Roast frozen sprouts from frozen. Do not thaw and do not boil; they have already had one heat treatment.

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Key Research Papers

1. Song L, Thornalley PJ. Effect of storage, processing and cooking on glucosinolate content of Brassica vegetables. Food and Chemical Toxicology. 2007;45(2):216-224. — doi:10.1016/j.fct.2006.07.021
2. Hanschen FS, Kühn C, Nickel M, Rohn S, Dekker M. Leaching and degradation kinetics of glucosinolates during boiling of Brassica oleracea vegetables and the formation of their breakdown products. Food Chemistry. 2018;263:240-250. — doi:10.1016/j.foodchem.2018.04.069
3. Matusheski NV, Juvik JA, Jeffery EH. Heating decreases epithiospecifier protein activity and increases sulforaphane formation in broccoli. Phytochemistry. 2004;65(9):1273-1281. — doi:10.1016/j.phytochem.2004.04.013
4. Oliviero T, Verkerk R, Dekker M. Isothiocyanates from Brassica vegetables — effects of processing, cooking, mastication, and digestion. Molecular Nutrition & Food Research. 2018;62(18). — doi:10.1002/mnfr.201701069
5. Oliviero T, Verkerk R, Vermeulen M, Dekker M. In vivo formation and bioavailability of isothiocyanates from glucosinolates in broccoli as affected by processing conditions. Molecular Nutrition & Food Research. 2014;58(7):1447-1456. — doi:10.1002/mnfr.201300894
6. Yuan GF, Sun B, Yuan J, Wang QM. Effects of different cooking methods on health-promoting compounds of broccoli. Journal of Zhejiang University SCIENCE B. 2009;10(8):580-588. — doi:10.1631/jzus.b0920051
7. Petersen MA. Influence of sous vide processing, steaming and boiling on vitamin retention and sensory quality in broccoli florets. Zeitschrift für Lebensmittel-Untersuchung und -Forschung. 1993;197(4):375-380. — doi:10.1007/bf01242064
8. Goodrich RM, Anderson JL, Stoewsand GS. Glucosinolate changes in blanched broccoli and Brussels sprouts. Journal of Food Processing and Preservation. 1989;13(4):275-280. — doi:10.1111/j.1745-4549.1989.tb00106.x
9. Kim JS. Effect of steaming and ultrasound treatment on sulforaphane and allyl isothiocyanate content of Brussels sprouts (Brassica oleracea var. gemmifera). Journal of the East Asian Society of Dietary Life. 2023;33(5):365-374. — doi:10.17495/easdl.2023.10.33.5.365
10. Kahlon TS, Chapman MH, Smith GE. In vitro binding of bile acids by spinach, kale, brussels sprouts, broccoli, mustard greens, green bell pepper, cabbage and collard greens. Food Chemistry. 2007;100(4):1531-1536. — doi:10.1016/j.foodchem.2005.12.020
11. Kahlon TS, Chiu MCM, Chapman MH. Steam cooking significantly improves in vitro bile acid binding of collard greens, kale, mustard greens, broccoli, green bell pepper, and cabbage. Nutrition Research. 2008;28(6):351-357. — doi:10.1016/j.nutres.2008.03.007
12. Aune D, Chan DSM, Lau R, Vieira R, Greenwood DC, Kampman E, Norat T. Dietary fibre, whole grains, and risk of colorectal cancer: systematic review and dose-response meta-analysis of prospective studies. BMJ. 2011;343:d6617. — doi:10.1136/bmj.d6617
13. Muir JG, Rose R, Rosella O, Liels K, Barrett JS, Shepherd SJ, Gibson PR. Measurement of short-chain carbohydrates in common Australian vegetables and fruits by high-performance liquid chromatography (HPLC). Journal of Agricultural and Food Chemistry. 2009;57(2):554-565. — doi:10.1021/jf802700e
14. Halmos EP, Power VA, Shepherd SJ, Gibson PR, Muir JG. A diet low in FODMAPs reduces symptoms of irritable bowel syndrome. Gastroenterology. 2014;146(1):67-75.e5. — doi:10.1053/j.gastro.2013.09.046
15. Felker P, Bunch R, Leung AM. Concentrations of thiocyanate and goitrin in human plasma, their precursor concentrations in brassica vegetables, and associated potential risk for hypothyroidism. Nutrition Reviews. 2016;74(4):248-258. — doi:10.1093/nutrit/nuv110
16. Panduang T, Phucharoenrak P, Karnpanit W, Trachootham D. Cooking methods for preserving isothiocyanates and reducing goitrin in Brassica vegetables. Foods. 2023;12(19):3647. — doi:10.3390/foods12193647
17. Chu M, Seltzer TF. Myxedema coma induced by ingestion of raw bok choy. New England Journal of Medicine. 2010;362(20):1945-1946. — doi:10.1056/NEJMc0911005
18. Cartea ME, Velasco P. Glucosinolates in Brassica foods: bioavailability in food and significance for human health. Phytochemistry Reviews. 2007;7(2):213-229. — doi:10.1007/s11101-007-9072-2

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Connections

• Brussels Sprouts — the main topic page.
• Brussels Sprouts — Benefits Deep Dive — the hub for these four articles.
• Detox Enzymes and DNA Damage — what the compounds you are trying to preserve actually do.
• Vitamin K, Warfarin and Drug Interactions — the other “should I worry” question.
• Why Brussels Sprouts Stopped Tasting Bitter — progoitrin as a flavour problem rather than a thyroid one.
• Brussels Sprouts: History and Origins — from hand-picked stalks to frozen bags.
• Cauliflower: FODMAPs, Goitrogens and Tolerance — the same questions for the mannitol-rich relative.
• Cabbage: Goitrogens and Safe Intake — the thyroid question across the family.
• Cabbage: Glucosinolates and Sulforaphane — myrosinase and the chop-and-rest question in detail.
• Irritable Bowel Syndrome — where FODMAP tolerance is handled properly.
• Gut Health — fibre, fermentation and short-chain fatty acids.
• Hypothyroidism — the condition the goitrogen worry is about.
• Goiter — iodine, the thyroid and what goitrin is named after.
• Vitamin C — the most fragile thing in the pan.
• Folate — the other water-soluble vitamin that leaches.
• Brown Rice — the cereal fibre that actually carries the colorectal association.
• Kale — a lower-FODMAP crucifer.
• Bok Choy — the vegetable in the 2010 case report, eaten raw and in kilograms.

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