Why Plant Nutrients Aren’t What They Seem: The Truth About Antinutrients and Absorption
When you look at a nutrition label, the numbers may not tell the whole story. The iron, zinc, or vitamin A listed does not always translate to what your body can absorb and use. Plants contain compounds known as antinutrients that can block or reduce the absorption of important nutrients. In addition, certain plant vitamins come in forms that are not readily usable by humans, which means you may be getting less benefit than you think.
Understanding how antinutrients work and how to work around them is essential if you are choosing to include plants in your diet. Knowing that you may not be getting what you think, and that other factors are at play, will help you make better choices about how to meet your nutritional needs.
What are antinutrients?
Antinutrients are naturally occurring compounds found in plants. They are part of a plant’s defense system, designed to protect against insects, animals, and humans. These compounds can irritate the gut, interfere with enzymes, or block mineral absorption.
Phytates: Found in grains, legumes, nuts, and seeds. They bind iron, zinc, and calcium, making them unavailable to the body.
Oxalates: High in spinach, beets, almonds, and peanuts. They bind calcium and magnesium to form insoluble salts. When intake exceeds about 100 mg, the body cannot fully detoxify them. Excess oxalates are stored in tissues as sharp crystals called raphides, which can cause pain, kidney stones, thyroid issues, and joint problems.
Lectins and tannins: Present in beans, legumes, and tea. They disrupt digestion, reduce nutrient bioavailability, and in higher amounts may irritate the gut lining, contribute to leaky gut, and trigger immune reactions. Tannins can also interfere with protein and iron absorption, sometimes causing nausea, headaches, or irregular digestion.
These effects can lead to gut issues like bloating, gas, abdominal pain, diarrhea, constipation, or long-term nutrient deficiencies. I prefer not to consume plants for this reason. But if you do, it is important to know that unexplained symptoms may be tied to these compounds.
Food combinations that reduce or enhance absorption
How you combine foods makes a difference in nutrient absorption.
Tea and coffee: Polyphenols and tannins strongly inhibit non-heme iron absorption when consumed with meals. A cup of black tea with spinach or coffee with oatmeal lowers usable iron. Waiting one hour before or after eating reduces this effect.
Calcium-rich foods: Large amounts of calcium compete with iron. Pairing milk with fortified cereal or yogurt with whole-grain toast can lower iron uptake.
Phytate-rich meals: Eating beans with whole grains, like hummus and pita or rice and beans, reduces zinc and iron absorption unless steps like soaking or sprouting are used.
Some combinations improve absorption:
Vitamin C with plant iron: Adding lemon juice to lentils, tomatoes to bean stew, or bell peppers to quinoa improves non-heme iron uptake.
Heme iron from animal foods is not affected by these inhibitors, which is why including meat or seafood with plant foods increases total iron absorption.
Plant nutrients are not always usable forms
Plants often provide nutrients in precursor or analog forms that require conversion before the body can use them. A precursor is an inactive version of a vitamin or fat. The conversion is often inefficient, and in some people nearly nonexistent.
Vitamin A: Plants provide beta-carotene, which must convert to retinol. Up to 45 percent of people are poor converters, meaning that eating carrots or sweet potatoes does little to raise vitamin A status for them.
Omega-3 fats: Flaxseeds and chia seeds provide ALA, a precursor to EPA and DHA. Conversion rates are extremely low, especially for DHA, and are further reduced by high omega-6 intake. Fish and shellfish provide EPA and DHA directly, which the body absorbs and uses.
Vitamin B12: True B12 is absent from plants. Spirulina contains mostly pseudovitamin B12, which humans cannot use. Some seaweeds, like nori, contain trace amounts of active B12, but they are unreliable. Relying on these sources risks deficiency even if blood tests look normal.
Iron: Spinach contains non-heme iron, the less bioavailable form, and its oxalates block even more absorption. By contrast, the heme iron in red meat, poultry, and seafood is the form the human body is designed to use efficiently.
These examples show why nutrition labels overstate the benefits of plant foods. The nutrients listed on paper do not guarantee absorption or usability in the body.
If you choose to consume plants, here are ways to improve absorption
Certain preparation methods and pairings reduce antinutrient effects and improve nutrient uptake.
Soak, sprout, and ferment: These processes lower phytate levels in beans, grains, and seeds, making minerals like zinc and iron more available. Sprouted lentils or fermented sourdough bread provide more usable minerals than their unprocessed versions.
Cook properly: Pressure cooking beans reduces lectins. Traditional nixtamalization of corn (lime soaking) lowers phytates and oxalates while boosting calcium and iron availability.
Pair foods smartly: Adding vitamin C to plant-iron meals boosts absorption. Squeeze lemon on beans, include tomatoes in lentil soup, or add bell peppers to a quinoa dish. Avoid tea and coffee with iron-rich meals.
Diversify nutrient sources: Even with preparation, plants often provide limited or inactive forms of nutrients. Including animal foods ensures direct access to heme iron, retinol, DHA, EPA, and true vitamin B12.
These steps will not remove antinutrients completely, but they can lessen their effects.
Final thoughts
Plants are often promoted as nutrient-rich, but much of what they contain is locked away by antinutrients or delivered in forms the human body cannot use efficiently. Labels may list iron, calcium, or vitamin A, but absorption is far lower than expected. Many nutrients are either poorly absorbed, blocked, or dependent on genetic factors for conversion.
If you choose to include plants in your diet, be aware that some unexplained symptoms or nutrient deficiencies may be linked to these compounds. Awareness is key. Once you understand the limits of plant nutrition, you can make informed decisions and choose sources that provide your body with what it can truly absorb and use.
References
Gupta RK, Gangoliya SS, Singh NK. Reduction of phytic acid and enhancement of bioavailable iron and zinc in food grains. Crit Rev Food Sci Nutr. 2015.
Salgado N, Pinho O, Ferreira IMPLVO. Oxalates in foods: analysis, content, and bioavailability. Foods. 2020.
Lietz G, Oxley A, Boesch-Saadatmandi C, Kobayashi D. Importance of β-carotene 15,15’-monooxygenase 1 (BCMO1) polymorphisms in vitamin A nutrition. J Nutr. 2012.
Anderson BM, Ma DW. Are all n-3 polyunsaturated fatty acids created equal? Lipids Health Dis. 2009.
Takic M, Pokimica B, Demin MA, Zekovic M. Effects of alpha-linolenic acid on omega-3 status. Nutrients. 2020.
Dagnelie PC, van Staveren WA. B12 status of macrobiotic and vegan children. Am J Clin Nutr. 1989.
Watanabe F, Takenaka S, Katsura H, et al. Pseudovitamin B12 is the predominant cobamide of Spirulina. J Nutr Sci Vitaminol. 1999.
Disclaimer: The content shared here is for informational and educational purposes only and should never be taken as medical advice.
In writing this blog post, my goal is to distill research findings into a clear, approachable format that encourages critical thinking and empowers you to make informed decisions about your health.
