Antinutrients: Lectins, Phytates, and Oxalates Explained

Plants can’t run away from predators. So over millions of years, many evolved chemical defenses: compounds that make them less attractive to insects, fungi, and animals that might eat them.

Antinutrients are some of those compounds. The name sounds alarming, but it’s just a functional category. These are molecules that interfere, to some degree, with nutrient absorption or digestion. In very large amounts, some cause direct harm.

The key phrase is “in very large amounts.” Because the foods that contain antinutrients (beans, lentils, whole grains, leafy greens, nuts, seeds) are also among the most consistently health-associated foods in human nutrition research.

The question isn’t whether antinutrients exist. They do. The question is whether they matter at the amounts found in a normal, varied diet. For most people, current evidence suggests the answer is: not much.

Phytates: The Mineral Binders

Phytic acid (also called phytate when in its salt form) is found in whole grains, legumes, nuts, and seeds. It’s the plant’s way of storing phosphorus. Chemically, it has a strong affinity for binding minerals (particularly zinc, iron, and calcium) forming insoluble complexes that reduce absorption.

This is a real effect. Studies show that phytate can reduce iron absorption by 50-65% and zinc absorption by 15-35% in controlled settings (Hambidge et al., 2010, Food and Nutrition Bulletin). People eating plant-based diets with high phytate intake have lower iron bioavailability per gram of iron consumed than people eating diets with heme iron from meat.

But here’s an important point: reduced absorption doesn’t automatically mean deficiency. The body adapts by upregulating absorption when dietary levels are lower. And you can actively reduce phytate content through food preparation.

Methods that reduce phytates:

• Soaking: reduces phytate in legumes by 30-70%
• Sprouting (germination): activates the plant’s own phytase enzyme, which breaks down phytic acid
• Fermentation: highly effective, often reducing phytates by 50-100% (this is one reason traditional sourdough bread has significantly lower phytate content than commercial yeast bread)
• Cooking: reduces phytate content, though less dramatically than soaking or fermenting

The fermentation-basics article explains the mechanisms behind why fermented foods like sourdough and tempeh are lower in antinutrients than their unfermented equivalents.

Pairing plant-based iron sources with vitamin C also helps. Vitamin C converts iron to a more absorbable form and partially counters phytate inhibition of iron absorption (Cook and Reddy, 2001, American Journal of Clinical Nutrition).

Oxalates: The Calcium Complication

Oxalic acid (oxalate) is found in high concentrations in spinach, rhubarb, beet greens, Swiss chard, and to a lesser extent in almonds, cashews, and sweet potatoes. Like phytate, it binds minerals (primarily calcium) in the gut, reducing their absorption.

The absorption of calcium from spinach is very low, around 5%, compared to 30-35% from dairy and 40-60% from low-oxalate vegetables like kale and broccoli (Weaver et al., 1999, Journal of Nutrition). This doesn’t mean spinach is unhealthy. It offers other nutrients, including folate, vitamin K, and magnesium. It just means it’s not a reliable calcium source.

For most people, oxalates are absorbed in small amounts, processed by the liver, and excreted in urine without incident. However, people prone to calcium oxalate kidney stones (the most common type) may benefit from limiting very high-oxalate foods. About 80% of kidney stones are calcium oxalate, and elevated urinary oxalate (hyperoxaluria) is a risk factor. If you have a history of these stones, a registered dietitian or physician can guide you on managing oxalate intake. This is a specific clinical consideration, not a general population warning.

Boiling high-oxalate vegetables reduces oxalate content by 30-87%, with the oxalate leaching into the cooking water. Steaming is less effective. So cooking method matters if you’re specifically managing oxalate intake.

Lectins: The Most Misunderstood Antinutrient

Lectins are proteins that bind to carbohydrates. They’re found in virtually all plant foods, and in many animal foods too. The concern about lectins in popular media has grown substantially, largely driven by Dr. Steven Gundry’s “Plant Paradox” hypothesis, which claims lectins in cooked plant foods cause widespread harm.

The scientific consensus doesn’t support this conclusion.

There is one clear situation where lectins matter: raw or undercooked kidney beans. They contain very high concentrations of phytohaemagglutinin (PHA), a lectin that causes severe gastrointestinal illness (nausea, vomiting, and diarrhea) even in small amounts. As few as 4-5 raw kidney beans can cause symptoms (Rodhouse et al., 1990, Epidemiology and Infection). Canned beans are safe because they’re fully cooked during the canning process. Slow cookers run at temperatures that may not fully deactivate PHA. Bring kidney beans to a rolling boil first.

Beyond raw kidney beans, the evidence for lectins causing harm in properly cooked foods is weak. Most dietary lectins are deactivated by cooking. The lectins that survive cooking in small amounts are mostly not absorbed intact from the gut and don’t reach systemic circulation in meaningful quantities in healthy people.

Studies in populations eating high-legume diets consistently show health benefits, not harm. The Blue Zones (populations with exceptional longevity and low rates of chronic disease) all consume substantial amounts of legumes as a dietary staple (Buettner, 2015). This doesn’t prove causation, but it’s hard to reconcile with a theory that lectins in properly cooked beans cause serious systemic damage.

Tannins: The Fourth Type

Tannins are polyphenols found in tea, coffee, red wine, legumes, and some grains. They bind iron and some proteins, reducing absorption. They’re why black tea consumed with a meal can reduce non-heme iron absorption by 50-90% in controlled studies (Morck et al., 1983, American Journal of Clinical Nutrition).

For most people, this is a minor concern. But for people at risk of iron deficiency (particularly those eating plant-based diets) spacing tea and coffee consumption away from iron-rich meals by an hour or two makes a practical difference.

Tannins also have antioxidant properties, which is one reason tea and coffee consumption is associated with some health benefits in observational studies. This is a good example of the broader point about antinutrients: the same compound that inhibits something can also have beneficial properties.

The Big Picture

Here’s what the antinutrient conversation misses most often: the foods that contain antinutrients are among the most protective foods in the human diet.

Legumes are associated with reduced cardiovascular disease risk, improved glycemic control, and lower all-cause mortality in large prospective studies. Whole grains consistently outperform refined grains in metabolic health outcomes. Leafy vegetables are correlated with better cognitive aging outcomes.

The concept of bioavailability is relevant here. Nutrients in food exist in a matrix. Absorption isn’t a fixed number. It depends on the whole food, the meal context, the individual, and preparation method. Antinutrients are one factor in that matrix. But they’re competing against a long list of positive factors in the same foods: fiber, polyphenols, vitamins, minerals, and prebiotic compounds that feed the gut microbiome.

The concern about antinutrients is most legitimate in narrow clinical contexts: people with diagnosed deficiencies of zinc or iron in populations where plant foods dominate a limited diet, or people with kidney stone history and very high oxalate intake. In a varied diet with adequate protein and whole-food diversity, antinutrients are a minor footnote.

Cook your beans. Soak your legumes before cooking. Vary your plant foods. Eat some vitamin C alongside plant-based iron sources. You’ve done essentially everything needed to address antinutrients in a practical way.