LeClaire Institute

Module 3 · Antinutrients

Antinutrients are natural compounds in plant foods that reduce how well you absorb or use a nutrient: phytate, oxalate, tannins, lectins, trypsin inhibitors and goitrogens. Each is shown in ball-and-stick form or as a labeled cartoon, with what it does, who should pay attention, where it is found, and how soaking, sprouting, fermenting and cooking reduce it. The figure is on the left and the lesson script on the right; the quiz is at the end.

Previous lesson: Module 3 · Additives and E-Numbers

Figures12345678910
1
Figure 1

The Antinutrient Family Tree

Six natural compounds, grouped by what they block.

Antinutrientsnatural plant compounds that block nutrientsBind mineralsiron, zinc, calcium · Figs. 3–5Block protein digestionFig. 6Block iodine useFig. 7PhytateFig. 3OxalateFig. 4TanninsFig. 5LectinsFig. 6Trypsin inhibitorsFig. 6GoitrogensFig. 7Grains, beanssoak, sproutSpinach, rhubarbboilTea, red winebetween mealsRaw kidney beanssoak, then boilRaw soybeanscookBrassicas, cassavacook; iodine
What the tree shows
Three things they blockAntinutrients are compounds that plants make for their own reasons, such as storing phosphorus or deterring insects, which also reduce how well you absorb or use a nutrient. They bind minerals, block protein digestion, or get in the way of iodine.
The bottom row is the fixEach has a kitchen answer: soaking, sprouting, fermenting, boiling or simply cooking. Traditional ways of preparing grains and beans grew up partly because they work.
Not on the labelAntinutrients occur naturally in whole foods, so they never appear in the ingredient list or the Nutrition Facts panel.
2
Figure 2

What Makes an Antinutrient

The minerals they grip, and three ways to block a nutrient.

Starting point

The minerals they hold on to

ZincZn²⁺

IronFe³⁺

CalciumCa²⁺

Minerals travel through your gut as small positive ions. Many antinutrients carry negative oxygen groups that grip these ions, making a clump too big or too insoluble to cross the gut wall, so the mineral leaves in the stool instead (see Trace Minerals and Major Minerals).

Three ways to block a nutrient
How it worksAntinutrientsNutrient affected
Grips mineral ions in the gutphytate, oxalate, tanninsiron, zinc, calcium, magnesium
Blocks a digestive enzyme or sticks to the gut walltrypsin inhibitors, lectinsprotein; lectins can also cause stomach upset
Gets in the way of the thyroidgoitrogensiodine
Dose and context
A problem mainly when diets are narrowFor most people eating a varied diet, antinutrients matter little. They matter more when one staple supplies most of the calories, when mineral intake is already low, or in vegan diets that rely on whole grains and beans for iron and zinc.
Often useful tooMany of the same compounds are studied for benefits: phytate and tannins are antioxidants, and the foods richest in them (whole grains, beans, tea) are linked to better health (see Bioactives).

zinc iron calcium · ions drawn to the same scale as the molecules on this page · tap one to enlarge it

3
Figure 3

Phytate

Inositol with six phosphates: the seed’s phosphorus store.

The two building blocks

1

InositolC₆H₁₂O₆

2

PhosphateH₃PO₄

  1. 1Inositol: a six-carbon ring with an OH on every carbon, a close cousin of glucose…
  2. 2…and phosphate, the same group that is in DNA and ATP.
Phytate

Phytic acid, C₆H₁₈O₂₄P₆

344
  1. 3The same inositol ring…
  2. 4…with a phosphate on all six carbons. In the gut each phosphate loses hydrogens and carries negative charges, which grip zinc, iron and calcium ions.

What it is: Phytic acid, or phytate in its salt form, is how seeds store phosphorus: it holds most of the phosphorus in grains, beans, nuts and seeds, ready for the sprouting plant.

In your body: Humans make very little phytase, the enzyme that cuts phosphates off phytate, so most phytate passes through intact, taking bound zinc and iron with it. It is the main dietary factor that lowers zinc absorption. It also has an upside: it is an antioxidant, and diets rich in whole grains and beans are linked to lower risk of several chronic diseases.

Too much: High-phytate diets with little meat can lead to zinc and iron deficiency, especially where one unrefined grain or bean is the main food.

Found in: Bran of whole grains (wheat, rice, oats), beans, lentils, soy, nuts and seeds (sesame, almonds). Refined white flour has much less, because the bran is removed.

carbon hydrogen oxygen phosphorus · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it

4
Figure 4

Oxalate

A tiny acid that locks up calcium.

Oxalate

Oxalic acid and oxalate

1

Oxalic acidC₂H₂O₄

2

OxalateC₂O₄²⁻

3

Calcium oxalateCaC₂O₄

  1. 1The smallest acid with two acid groups: just two carbons and four oxygens.
  2. 2In food and in the body it loses both hydrogens, leaving oxalate with two negative charges…
  3. 3…which clamp onto a calcium ion. Calcium oxalate barely dissolves, so it forms crystals.

What it is: A small acid that many plants store, sometimes as needle-like calcium oxalate crystals that deter animals from eating them.

In your body: Oxalate binds calcium in the gut, so the calcium in spinach is poorly absorbed, only about a fifth as well as the calcium in milk. Oxalate that is absorbed leaves in urine, where it can join calcium to form kidney stones; most kidney stones are calcium oxalate. Your liver also makes oxalate, and very high doses of vitamin C are partly turned into it.

Too much: A risk mainly for people who form kidney stones, who are often advised to limit high-oxalate foods, drink plenty of fluid, and eat calcium-rich foods with them, which binds oxalate in the gut. Rhubarb leaves are poisonous, partly from their oxalate; eat only the stalks.

Found in: Spinach, rhubarb, Swiss chard, beet greens, beets, almonds, cashews, cocoa, sweet potatoes, black tea.

carbon hydrogen oxygen calcium · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it

5
Figure 5

Tannins

Polyphenols that grip iron and proteins.

Tannins

The two kinds of tannin building block

12

Gallic acidC₇H₆O₅

34

CatechinC₁₅H₁₄O₆

  1. 1Three OH groups side by side on a ring: a strong grip for iron.
  2. 2Hydrolyzable tannins are many gallic acids joined through their acid group to a sugar.
  3. 3Catechin has two neighboring OH groups on this ring, the same iron grip…
  4. 4…and condensed tannins are chains of catechin-type units linked end to end.

What it is: Large polyphenols that bind proteins. That binding is the dry, puckering feel of strong tea, red wine and unripe fruit, called astringency.

In your body: Tannins bind nonheme iron in the gut. Tea with a meal can cut the iron absorbed from that meal by more than half; coffee has a similar effect through related polyphenols. Heme iron from meat is much less affected.

Too much: Mainly a concern for people with low iron, who can drink tea and coffee between meals rather than with them. Tannins in sorghum and beans also make some protein harder to digest.

Found in: Black and green tea, red wine, coffee (related compounds), cocoa, pomegranate, persimmons, grape skins and seeds, sorghum, colored beans, walnuts.

carbon hydrogen oxygen · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it

6
Figure 6

Lectins and Trypsin Inhibitors

Proteins that stick to the gut or plug an enzyme.

Lectins: proteins that grab sugars

Sugars on the gut wall

Lectins are proteins that bind specific sugars. The cells lining your gut are coated with sugar chains, and some lectins clamp onto them; they resist digestion and can damage the lining.

Cartoon, not to scale: each hexagon is a whole sugar; each blue shape is a whole lectin protein.

Lectins

What it is: Sugar-binding proteins found in many plants, highest in raw legumes. The best known is phytohemagglutinin in red kidney beans, so named because it makes red blood cells clump.

Too much: Raw or undercooked kidney beans can cause severe nausea, vomiting and diarrhea within a few hours; the US FDA notes as few as four or five raw beans can do it. Beans warmed but not boiled, as in some slow cookers, can be worse than raw. Fully cooked beans are safe: boiling destroys almost all lectin activity, and canned beans are already cooked.

Found in: Red and white kidney beans (most), other dried beans, lentils, peanuts, soybeans, raw wheat germ. Tomatoes and potatoes have small amounts that cause no known problems.

Trypsin inhibitors: a plug in the enzyme

1 · Normal digestion

Trypsin, an enzyme from your pancreas, cuts proteins into pieces in the small intestine (see Protein).

2 · Blocked

A trypsin inhibitor is a small protein that fits tightly into trypsin’s cutting site, so less protein is digested.

3 · Cooked

Heat unfolds the inhibitor, so it no longer fits, and digestion works again.

Cartoon, not to scale: each bead is a whole amino acid; the brown shape is the enzyme.

Trypsin inhibitors

What it is: Proteins in seeds that block protein-digesting enzymes, probably to protect the seed from insects.

In your body: They lower how much protein you can digest. Animals fed raw soybeans grow poorly and their pancreas enlarges as it tries to make more enzyme.

Too much: Rarely a problem in people, because soy and other legumes are almost always cooked, toasted or fermented, which inactivates most of the inhibitor.

Found in: Raw soybeans and other raw legumes, raw egg white (ovomucoid), raw potatoes; tofu, soy milk and tempeh have much less after heating.

7
Figure 7

Goitrogens

Compounds that get in the way of iodine.

Goitrogens

Iodide and two goitrogens

IodideI⁻

1

ThiocyanateSCN⁻

23

GoitrinC₅H₇NOS

  1. 1Thiocyanate is a small negative ion about the size of iodide, so it competes with iodide for the pump that pulls iodide into the thyroid.
  2. 2Goitrin’s C=S group interferes with the enzyme that attaches iodine to make thyroid hormone…
  3. 3…it forms from a glucosinolate in brassica vegetables when they are cut or chewed.

What it is: Goitrogens are compounds that can enlarge the thyroid (cause goiter) by reducing how much iodine it takes up or uses. Most come from glucosinolates, the sulfur compounds that give cabbage family vegetables their bite (see Bioactives).

In your body: The thyroid needs iodide to make thyroxine (T4) and T3 (see Trace Minerals). Goitrogens slow that, so with too little iodine the gland grows to compensate.

Too much: A real problem mainly where iodine intake is low and a goitrogenic food is a staple, notably cassava, whose cyanide-releasing compounds are turned into thiocyanate. Normal servings of cooked broccoli, kale or cabbage are not a concern for people who get enough iodine.

Found in: Raw cabbage, kale, Brussels sprouts, turnips, rutabaga, collards, bok choy; cassava; millet; soy isoflavones have a weak effect in lab studies.

iodine carbon hydrogen oxygen nitrogen sulfur · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it

8
Figure 8

Soaking, Sprouting, Fermenting, Cooking

The kitchen fixes, and which foods to pair.

Traditional ways to cut antinutrients
MethodWhat it doesReduces
Soakingwater draws out water-soluble compounds and wakes up the seed’s own phytase; pour the water awayphytate, oxalate, tannins, lectins (partly)
Sproutingthe seed breaks down its phytate store to feed the growing shootphytate, trypsin inhibitors, lectins
Fermentingmicrobes and their phytase break compounds down; sourdough bread has much less phytate than yeast breadphytate, trypsin inhibitors, tannins, goitrogens
Boilingheat unfolds proteins; soluble compounds leach into the water, which is thrown awaylectins, trypsin inhibitors, oxalate, goitrogens
Milling and peelingremoves the bran or skin, where many antinutrients sit, but also removes fiber, vitamins and mineralsphytate, tannins
Pairing foods
Vitamin C with plant ironVitamin C in the same meal counteracts phytate and tannins and raises nonheme iron absorption (see Trace Minerals).
Calcium with oxalateEating dairy or another calcium source with high-oxalate foods binds the oxalate in the gut, so less is absorbed.
Tea and coffee between mealsWaiting an hour or so after an iron-rich meal keeps their polyphenols away from the iron.
Heat-stable or not

Lectins and trypsin inhibitors are proteins, so thorough cooking destroys most of them. Phytate, oxalate and tannins are small, sturdy molecules that survive heat; they are reduced mainly by soaking, sprouting, fermenting, or boiling and discarding the water.

9
Figure 9

Where antinutrients act in the body

Tap a number on the cell to see what that part does; the list below it gives every other place in the body.

Most antinutrients act inside the gut, on the food you eat with them; a few reach the kidneys or thyroid.

Inside a cell

A typical animal cell, not to scale. Real cells hold thousands of ribosomes and hundreds of mitochondria.

Some plant compounds block nutrients or target parts of the cell.

  1. Lectins bind sugars on the cell surface; raw kidney-bean lectin damages gut cells, but cooking destroys it.
  2. Ricin, from castor beans, shuts down ribosomes. It is not a food, but it shows how lectin-type proteins can attack a cell.
  3. Phytates and oxalates hold onto iron, zinc and calcium in the gut, so fewer get carried into cells.
  4. Cyanide from raw cassava or bitter almonds blocks the last step of ATP making; proper processing removes it.
  5. Goitrogens such as thiocyanate from raw cruciferous vegetables and cassava can reduce iodine uptake in thyroid cells, mostly a concern when iodine is low.
  6. The liver's enzymes detoxify many plant compounds; oxalate cannot be broken down and is passed out.

Everywhere else in the body

Each item, the places outside the cell where it is found or used, and what it does there.

Phytate

  • Small intestine. Binds iron, zinc, calcium and magnesium so less is absorbed.
  • Large intestine. Bacteria break some down.
  • Kidneys. The little that is absorbed may help prevent calcium kidney stones.

Oxalate

  • Small intestine. Binds calcium in the meal.
  • Large intestine. Some bacteria break oxalate down.
  • Kidneys. Absorbed oxalate leaves in urine, where it can join calcium in kidney stones, the most common kind.

Tannins

  • Mouth. Bind saliva proteins: the dry, puckering feel of strong tea or red wine.
  • Small intestine. Bind iron from plant foods, proteins and digestive enzymes.
  • Large intestine. Bacteria break them into smaller compounds.

Lectins

  • Small intestine. Raw kidney-bean lectin binds the gut lining and causes vomiting and diarrhea; cooking destroys it.
  • Blood. Some lectins clump red blood cells in the lab, which gave them their name (agglutinins).

Trypsin inhibitors

  • Small intestine. Block trypsin and chymotrypsin, so protein is digested less.
  • Pancreas. In animals fed raw soy, the pancreas works harder and grows.

Goitrogens

  • Thyroid. Thiocyanate blocks iodine uptake and goitrin blocks hormone making, mainly a problem when iodine is low.

Cyanogenic glycosides

  • Gut. Release cyanide from raw cassava, bitter almonds and apricot kernels.
  • Liver. Turns cyanide into thiocyanate.
  • Thyroid. That thiocyanate competes with iodine.
  • Nerves. Long exposure from badly processed cassava causes konzo, a paralysis of the legs.
10
Figure 10

Putting It All Together

The whole lesson in one table, plus a few extras.

The six antinutrients
AntinutrientWhat it isWhat it blocksMain sourcesBest fix
Phytateinositol with six phosphates; the seed’s phosphorus storezinc, iron, calciumwhole grains, beans, nuts, seedssoak, sprout, ferment (sourdough)
Oxalatea small two-carbon acidcalcium; kidney stonesspinach, rhubarb, chard, almonds, cocoaboil and drain; eat with calcium
Tanninspolyphenols built from gallic acid or catechinnonheme iron; some proteintea, red wine, sorghum, colored beansdrink tea between meals
Lectinssugar-binding proteinsgut lining; can cause vomitingraw kidney beans and other raw legumessoak, then boil hard
Trypsin inhibitorsproteins that plug trypsinprotein digestionraw soybeans and legumes, raw egg whitecook
Goitrogensglucosinolate products such as goitrin and thiocyanateiodine use by the thyroidraw brassicas, cassava, milletcook; get enough iodine
Worth knowing

Same compound, two sides

Phytate, tannins and glucosinolates are antinutrients in one lesson and bioactives in the next: whether they help or hinder depends on the amount, the rest of the diet and how the food is prepared.

Cassava and bitter almonds

Some plants make cyanogenic glycosides, which release cyanide when crushed. Cassava is made safe by soaking, grating, fermenting and drying; raw bitter almonds and apricot kernels should not be eaten.

Green potatoes

Potatoes that turn green in the light also make solanine, a bitter glycoalkaloid that is not destroyed by cooking. Cut away green parts and sprouts, and discard potatoes that taste bitter.

General education, not medical advice. Facts follow mainstream sources (US FDA, NIH Office of Dietary Supplements, WHO, standard nutrition and food-science texts).

Quiz

Tap a question to see the answer

1. What is an antinutrient?

A natural compound in food that reduces how well you absorb or use a nutrient (Figure 1).

2. Why do phytate, oxalate and tannins hold on to minerals?

They carry negative oxygen groups that grip positive mineral ions such as Zn²⁺, Fe³⁺ and Ca²⁺ (Figure 2).

3. What is phytic acid built from?

An inositol ring with a phosphate on each of its six carbons (Figure 3).

4. Which mineral does phytate lower the most?

Zinc, along with iron and calcium (Figure 3).

5. Which antinutrient is high in spinach, and what does it bind?

Oxalate; it binds calcium (Figure 4).

6. What are most kidney stones made of?

Calcium oxalate (Figure 4).

7. Why drink tea between meals rather than with them if your iron is low?

Tea tannins bind nonheme iron in the meal (Figure 5).

8. How do you make dried kidney beans safe?

Soak them, then boil them hard; warming without boiling is not enough (Figure 6).

9. What does a trypsin inhibitor do, and what inactivates it?

It plugs trypsin so less protein is digested; cooking inactivates it (Figure 6).

10. What do goitrogens affect?

How the thyroid takes up or uses iodine (Figure 7).

11. Why does sourdough bread have less phytate than ordinary yeast bread?

The long fermentation activates phytase, which breaks phytate down (Figure 8).

12. Which antinutrients are destroyed by cooking, and which survive it?

Lectins and trypsin inhibitors (proteins) are destroyed; phytate, oxalate and tannins mostly survive (Figure 8).

Next lesson: Module 3 · Processing →