Have you been hearing all the buzz about anti-nutrients in plant foods and how harmful they are? Is it true? We know how confusing it is to be bombarded with conflicting information online, so if you are interested in the potential for harm related to anti-nutrients then keep reading!
There is ample research regarding the benefits of healthful fruits and vegetables and other plant-derived foods that are protective against chronic disease like cardiovascular disease and cancers. So much so that the Dietary Guidelines for Americans recommends eating a variety of plants like leafy greens, red and orange vegetables, and legumes. Recently there has been more and more information circulating about the potential for anti-nutrients that may be negatively impacting absorption of some nutrients. But, should you worry about anti-nutrients?
Let’s dive in and see what the evidence says!
What are anti-nutrients?
First of all we need to identify what anti-nutrients are. Anti-nutrients are plant compounds that are thought to restrict or inhibit the bioavailability of certain key nutrients. Anti-nutrients include:
- Lectins
- Tannins
- Goitrogens
- Oxalates
- Phytates
- Phytoestrogens
Each compound has a different mechanism of action, mostly actions that help them survive in the wild, and have a purported effect on key nutrients. For example, lectins which are carbohydrate-binding proteins and found in legumes, seeds, and nuts are thought to alter gut function and potentially increase inflammation. In our recent blog post all about lectins, we talk all about this!
| ‘Anti-nutrient’ | Food sources | Proposed Clinical Implications |
| Lectins | Legumes, cereal grains, seeds, nuts, fruits, vegetables | Altered gut function; inflammation |
| Oxalates | Spinach, Swiss chard, sorrel, beet greens, beet root, rhubarb, nuts, legumes, cereal grains, sweet potatoes, potatoes | May inhibit calcium absorption; May increase calcium kidney stone formation |
| Goitrogens | Brassica vegetables (kale, Brussels sprouts, cabbage, turnip greens, Chinese cabbage, broccoli), millet, cassava | Hypothyroidism and/or goiter; Inhibit iodine uptake |
| Phytates | Legumes, cereal grains, pseudocereals (amaranth, quinoa, millet), nuts, seeds | May inhibit absorption of iron, zinc and calcium; Acts as an antioxidant; Antineoplastic effects |
| Phytoestrogens | Soy and soy products, flaxseeds, nuts (negligible amounts), fruits and vegetables (negligible amounts) | Endocrine disruption; Increased risk of estrogen-sensitive cancers |
| Tannins | Tea, cocoa, grapes, berries, apples, stone fruits, nuts, beans, whole grains | Inhibit iron absorption; Negatively impact iron stores |
Table adapted from Petroski W, Minich DM. Is There Such a Thing as “Anti-Nutrients”? A Narrative Review of Perceived Problematic Plant Compounds. Nutrients. 2020; 12(10):2929. https://doi.org/10.3390/nu12102929
Let’s explore if these claims are true.
Are anti-nutrients harmful?
It seems that each family of anti-nutrient has been demonized in a different way based on their mechanism of action, so let’s take a closer look at each one to see if they are in fact harmful to human health.
Lectins
Again, lectins are carbohydrate-binding proteins that are found in almost all organisms. In fact, over 500 different lectins have been identified in plants and animals! Recently, bean lectins and wheat germ agglutinin have also gotten a lot of attention from scientists due to their proposed link to gut permeability and toxicity.
Because they bind to specific carbohydrates, lectins help protect the plant by functioning in cell recognition, tissue development, and host defense. And also seem to be highest when the plant is growing and diminishes with plant maturity.
Lectins, particularly in legumes and beans like kidney and white beans, contain Phytohaemagglutinin (PHA) which is a protein that causes red blood cells to agglutinate or clump together. However, in high amounts, it is toxic to humans and mammals.
It’s also true that lectins resist enzymatic digestion in the gastrointestinal tract of humans, However, traditional processing methods like boiling or soaking causes irreversible lectin denaturation. Making both the potential for toxicity and gut permeability obsolete. In fact, human clinical studies show that using whole (cooked) beans “do not exhibit the same effects as in vitro or in vivo animal models that use isolated lectins and raw bean flours.”
Key Takeaway: Research does show that food poisoning can occur if lectin-rich foods are not prepared properly, however traditional processes of preparing these foods are very safe. In fact, studies show lectin-rich foods that have been prepared properly have powerful disease fighting compounds, prebiotic fiber, vitamins, minerals, and essential amino acids that are beneficial to our health!
Oxalates
Oxalates are another plant compound that can form insoluble salts with minerals like sodium and potassium. In fact, all photosynthetic organisms produce some level of oxalates. It’s thought that plants produce oxalates to protect the plant, detoxify heavy metals, and also regulate minerals like calcium.
Oxalates can be water-soluble or insoluble. Unbound oxalates that are water-soluble can bind to minerals and decrease their absorption. They can also be absorbed in the intestines of humans, which is thought to contribute to calcium oxalate kidney stone formation. Insoluble oxalates are excreted in the stool.
A balanced human diet only contains a little bit of oxalates. Plant foods with the highest amounts of oxalates include:
- Spinach
- Swiss chard
- Sweet potatoes
- Beets
- Amaranth
- Rhubarb
Different parts of the plant can vary greatly in its oxalate content.
For example, the leaves of plants contain a lot more than the stalks or roots. It is also important to note that the amount of total oxalate versus how much is soluble or insoluble would be significant in how much of a role it would play in the bioavailability of nutrients and/or the formation of kidney stones.
Like lectins, the cooking and preparation of oxalate-rich foods significantly impacts the oxalate content and mineral availability. Since oxalates can be water soluble, wet processing methods like soaking or boiling are very efficient at reducing oxalate content.
Even though oxalates have been demonized as the cause of calcium oxalate kidney stone formation, recent studies have also proven that minerals like potassium and magnesium (that are rich in oxalate-rich foods) also play a protective role in kidney stone formation. This is demonstrated in the DASH dietary pattern, which is rich in nuts, legumes, whole grains, and vegetables with a fairly high oxalate content. In fact, this dietary pattern has a 40-50% reduction in risk of kidney stones.
Intestinal health and barrier function play a key role in the potential for kidney stone formation, making those with Inflammatory Bowel Disease (IBD) potentially at a greater risk. However, the pathway between digestive disorders and hyper oxalate absorption has yet to be proven. Some other genetic factors are also at play, making some simply more susceptible.
Key Takeaway: Some people are at higher risk for kidney stone function and there is potential for oxalates playing a role in that pathway. However, cooking oxalate-rich foods and eating enough calcium and potassium significantly reduce how much oxalate is available. Plus, oxalate containing foods have a variety of helpful, disease-fighting compounds!
Goitrogens
A goitrogen is a term that is used to broadly describe agents that interfere with thyroid function. In fact, they can increase risk of goiter and other thyroid diseases. The most common sources of goitrogens are medications, environmental toxins, and foods in the Brassica family like broccoli and cabbage.
Although the majority of research related to goitrogens have been around their ability to reduce cancer risk and regulate redox reactions, some animal and cell model studies have shown a potential for goitrogens interfering with the thyroid and uptaking iodine properly.
The most well-known goitrogen-containing foods, those in the Brassica family, vary greatly in their actual amounts of the goitrogenic compounds thought to interfere with thyroid function.
Additionally, species, plant growing conditions, soil, growing location, plant stressors, pesticides, and other factors all affect goitrogenic compound amounts.
Cooking goitrogen-containing foods does reduce goitrogenic compounds, however it also reduces beneficial compounds. For example, one study found that boiling broccoli for just 5 minutes reduced the total glucosinolate content by 51% and reduced glucoraphanin (helpful compound) by 57%. It’s a bit of a give and take that requires staying up to date on current research if you are worried about goitrogen-containing foods on thyroid function.
The available evidence on goitrogens related to healthy individuals are very mixed and quite complex to extrapolate.
Many of the studies are done in rats as well as in a diet that is not representative of a balanced human diet. However, due to the potential inhibitory effects on thyroid function, those that are at increased risk or have underlying iodine deficiency, it is wise to pay attention to goitrogen-containing foods and not consume too many. So always discuss this with your healthcare provider if you feel like this should be attended to.
Key Takeaway: Goitrogen-containing foods vary greatly in the actual amounts of compounds that are thought to interfere with thyroid function. Overall, plant foods that contain these compounds are safe as part of a varied, balanced diet full of plants. In fact, doing so actually comes with a lot of beneficial benefits like phytochemicals, fiber, and essential nutrients. For those with thyroid disease, iodine deficiency, or at high risk for thyroid disease, careful consideration of goitrogen-containing foods like kale and broccoli should be cooked with iodized salt to avoid any inhibition of iodine uptake.
Phytoestrogens
Phytoestrogens are plant-compounds that are structurally similar to estradiol, the primary circulating sex hormone in females. Because they are structurally similar, phytoestrogens can bind to estrogen receptors and modulate estrogen activity in the body.
A whole umbrella of phenolic compounds are part of the phytoestrogen family, such as lignans and isoflavones which are the most relevant to the human diet. Food sources of lignans are found in flaxseeds and sesame seeds whereas isoflavones are found primarily in soy.
Fun fact: Your microbiome is responsible for converting these phenolic compounds to the estrogen-like substance!
Traditional cooking methods like boiling, steaming, and fermenting increase a specific isoflavone content. As far as safety is concerned, there have not been any definitive studies that have shown adverse effects of phytoestrogens. For more information, check out our latest post here.
Key Takeaway: There are lots of different factors like the makeup of the gut microbiome and the phytoestrogen source that impact actual phytoestrogen delivery. The current landscape of the literature is quite mixed, with much of the observational and epidemiological studies showing that the inclusion of phytoestrogens in the diet is not of concern, in fact it’s showing to be beneficial. Consuming a varied, colorful, plant-based diet containing phytoestrogens provides health-promoting compounds, fiber, and essential nutrients.
Phytates
Phytates, or phytic acid, primarily serves as storage for plant phosphate which acts as an energy source and an antioxidant for the growing seed of a plant. In fact, phytates are produced as the plant is growing and can account for up to 60-90% of total phosphorus in cereals, legumes, nuts, and seeds.
The phosphate groups on phytates act as strong chelators – a chemical ion that binds tightly to metal ions. Since phytates are insoluble and indigestible by human enzymes, high phytate foods could potentially cause nutritional deficiencies, such as iron, calcium, and zinc.
The same chelating properties of phytates also allow it to act as an antioxidant, which is beneficial for humans to consume. When a balanced diet with adequate variety is consumed to ensure appropriate phytate to mineral amounts, the concern for nutritional deficiencies becomes insignificant. In developing countries where variety is less likely, nutrient status is of higher concern.
Growing methods and seasons not only impact phytate amounts in plants. But so do cooking methods. Soaking, boiling, germination, and fermentation significantly reduce the amount of phytates in plant sources. These methods utilize naturally occurring enzymes within the plant to break down and reduce phytate amounts. Another important way to improve iron bioavailability is to consume sources of vitamin C with plant-foods to increase absorption.
Key Takeaway: Phytate rich foods both have the potential to inhibit key nutrients, iron, calcium, and zinc, while also serving as potent antioxidants in the diet. When phytates are eaten as part of a varied, balanced, colorful diet, mineral status should not be compromised. By pairing phytate rich food with complementary foods, like vitamin C rich fruits and vegetables, as well as preparing them via traditional cooking methods, mineral status can be optimized.
Tannins
Have you ever tasted a really acidic or bitter plant-food? Think tea, cocoa beans, or wine! That astringent taste is because of the tannins present!
Tannins are polyphenolic compounds that have a high molecular weight and are chemically reactive which allows them to bond with proteins and carbohydrates. This action allows them to play a role in defending the plant as well as how they act as antioxidants, anticancer, immunomodulatory, and cardioprotective for us!
Similar to phytates, they can also be chelators, making it possible for tannin rich foods to inhibit the absorption of minerals like iron, copper, and zinc. Studies have demonstrated that tannins may interfere with iron absorption in isolation, however not so when a varied diet is consumed.
Tannins are particularly demonized related to iron absorption and iron deficiency anemia and in developing countries. Tannin content can be reduced by cooking tannin rich foods, peeling skins of nuts, and brewing tea between 2-3 minutes (rather than 4-5, which is when tannin content peaks).
Many studies regarding iron absorption and tannins are highly debatable and varied. At any rate, potential inhibitory effects of tannins may be offset by the addition of vitamin C rich foods with non-heme iron food sources, such as beans or spinach. Those that are at risk for iron deficiency may choose to consume tannin-rich foods and beverages between meals. As well to help offset any potential inhibition of iron absorption.
Key Takeaway: Tannins are not consumed in isolation, but rather alongside a plethora of other helpful bioactive compounds. The potential inhibitory effects of tannins are highly debated but the health benefits of tannin rich foods are demonstrated time and again. Positive effects can be seen in cognition, heart health, cancer risk, etc. All in all, a varied, balanced plant-rich diet is far more beneficial than it is harmful.
How to reduce anti-nutrients in foods
As demonstrated above, traditional cooking methods and processing can have a profound impact on the amount of available anti-nutrient quantity in foods. Utilizing methods such as boiling, soaking, and sprouting can efficiently reduce the worrisome anti-nutrient content and therefore pose no health risk. Additionally, the vast array of nutrients these foods provide that are disease-fighting and body protecting far outweigh any negative effects!
| ‘Antinutrient’ | Food preparations that reduce | Food preparations that increase |
| Lectins | Soaking, boiling, autoclaving, germination, fermentation | Roasting, baking |
| Oxalates | Soaking, boiling, steaming, pairing with high calcium foods | Roasting, grilling, baking, low-calcium diet |
| Goitrogens | Steaming, boiling | n/a |
| Phytates | Soaking, boiling, germination, fermentation | n/a |
| Phytoestrogens | n/a | Boiling, steaming, fermenting (increases aglycone content) |
| Tannins | Cooking, peeling skins of fruits and nuts | n/a |
Table adapted from Petroski W, Minich DM. Is There Such a Thing as “Anti-Nutrients”? A Narrative Review of Perceived Problematic Plant Compounds. Nutrients. 2020; 12(10):2929. https://doi.org/10.3390/nu12102929
Should you worry about anti-nutrients in foods?
Although the term anti-nutrients has a negative connotation and has led many to believe that plants are more harmful than good, this couldn’t be further from the truth! With the available evidence we have at this point, it’s clear that plants have powerful compounds that protect both them AND us. By eating these powerful plant compounds, we can reap the benefits and help our bodies thrive!
If you have more specific questions about nutrition, continue to explore our blog and other resources we have available. Additionally, if you are ready to have a specialized dietitian on your healthcare team, schedule a free consultation call today to see how we can support you!
References
Petroski W, Minich DM. Is There Such a Thing as “Anti-Nutrients”? A Narrative Review of Perceived Problematic Plant Compounds. Nutrients. 2020; 12(10):2929. https://doi.org/10.3390/nu12102929
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