Lectins in Peanuts and Other Foods

Background: What are lectins?

Lectins are naturally occurring proteins that can bind to carbohydrates. They are found in animals, plants, bacteria, and viruses[4]. A wide variety of plants contain them including wheat, soy, peanuts, tomatoes, and potatoes [4]. Lectins defend plants from predators through antibacterial, antiviral, antifungal, and anti-insect activity [5]. Other compounds in plants are protective in similar ways, including antioxidants like anthocyanins, isoflavonoids, and other phenolics [6]. Lectins are classed as an anti-nutrient because they can potentially interfere with the absorption of other nutrients in humans. Other anti-nutrients include phytates (found in seeds and grains), calcium oxalate (found in spinach), and tannins (found in tea) [4]. While the majority of lectins are eliminated by cooking, others are resistant to heat and the digestion process [4].

In animals, lectins are found all throughout the organism and have various functions involving cell signaling and inflammation [7]. Evidence suggests that some lectins may increase inflammation and promote disease progression [7]. However, little is known about the physiological function of lectins found in plants. Some functions may include pathogen defense, mapping of neuronal pathways, assisting in tumor cell killing, and bloody typing.
A February 2020 study published in International Journal of Biological Macromolecules reviewed the current knowledge about the structure and function of various plant lectins, specifically the ones found in legumes. The study noted that there are a large number of lectins in legumes. They all have the unique ability to bind to specific carbohydrates, which allows them to have various applications in biological processes like infections (viral, bacterial, etc.), inflammation, and cancer [8]. Some lectins have been shown to induce inflammation (like PELa, a lectin found in the P. Elegans tropical tree), while others are anti-inflammatory (like LAL, a lectin found in the Lonchocarpus araripensis deciduous tree native to Brazil). Some lectins can activate pain receptors (eg., PELa), while others seem to reduce pain (eg., LCaL) [8].

Which plant foods contain lectins?

Lectins are widespread in foods. Here is a list of some plant foods that contain them [9]:

• Apples
• Watermelon
• Bananas
• Grapefruit
• Pumpkin
• Rice
• Barley
• Coffee
• Papaya
• Beets
• Cucumbers
• Sweet peppers

Are lectins harmful for human health?

As with many foods and even water, consuming large amounts of lectins could be toxic for humans [4]. However, there is no current recommended daily limit. Between 1976 and 1989, there were 50 reported cases of kidney bean food poisoning [10]. Symptoms of acute poisoning included abdominal pain, vomiting, and diarrhea. The source of the outbreak was identified as lectins in uncooked or partially cooked kidney beans. Cooking kidney beans significantly decreased the amount of lectins in them from between 80-100,000 haemagglutinin units (HAU) to <400 HAU [10]. Therefore, care should be taken to properly cook legumes like kidney beans to avoid adverse effects.

How much lectins do peanuts contain?

A 1986 study published in Peanut Science measured the quantity of lectins in raw, roasted, and boiled peanuts, as well as in raw, roasted, and boiled soybeans [11]. The following table adapted from the manuscript details the amount of lectins in peanuts over the course of 30 minutes of being cooked, either by roasting or boiling [11]:

Table 1. Active lectin content (µg/g defatted meal) of dry and moist heated Florunner peanuts¹

Data presented as mean + standard error of the mean (3 replicates)
¹This table was adapted from Ahmed, E.M., Lectin Quantitation in Peanut and Soybean Seeds. Peanut Science, 1986
²Air heated at 177 C
³Steam heated at 121 C

As noted in the table above, the uncooked peanuts contained 144.7 micrograms (µg) of lectins per gram of peanuts (there are 1,000,000 micrograms in one gram). Lectin content significantly decreased with cooking time in both roasted and boiled peanuts, with the content in boiled peanuts decreasing more rapidly than in roasted peanuts. However, after 30 minutes of cooking, the amount of active lectin was zero regardless of the method. The author concluded that “the amount of heat used in roasting or boiling peanuts in the present study was sufficient for 100% inactivation of peanut lectin.”

The study also looked at active lectin content of some commercially available peanuts and peanut butter randomly drawn from supermarket shelves. The following table from the manuscript highlights those amounts [11]:

Table 2. Active lectin content (µg/g defatted meal) of commercially available1 peanut seed products²

Data presented as mean + standard error of the mean (3 replicates)
¹Available from various large-chain supermarkets
²This table was adapted from Ahmed, E.M., Lectin Quantitation in Peanut and Soybean Seeds. Peanut Science, 1986

Raw peanuts contained the highest amount of lectins, but this was significantly reduced with cooking. Both dry and oil roasted peanuts contained very small amounts of lectins, as did peanut flour and peanut butter.

A 2011 study found similar results when measuring the lectin activity of peanuts on red blood cells (types A, B, and O) in hemagglutinin units (HAU) [12]:

Table 3. Effect of heat treatments on hemagglutinating activity (units) in peanut seeds¹

¹This table was adapted from Embaby, H.E.-S., Effect of heat treatments on certain antinutrients and in vitro protein digestibility of peanut and sesame seeds. Food science and technology research, 2011.

As shown in the table above, peanuts contained between 1024-2048 HAU when raw. However, after either boiling, microwaving, or roasting, between 98-100% of the lectins were destroyed. As shown in the table above, peanuts contained between 1024-2048 HAU when raw. However, after either boiling, microwaving, or roasting, between 98-100% of the lectins were destroyed.

What do we still not know?

The reality is that there is still much to learn about the role that lectins play in human health. Most of the available studies that have been conducted were done in ways that make it difficult to translate the findings in a public health setting, including:

• Studies conducted in animals
• Studies conducted on isolated human cells
• Studies where lectins were isolated from the foods that contain them

Could lectins be beneficial?

We do not fully understand the actions of lectins either by themselves or within foods. Available evidence suggests that in some cases, lectins may even be protective against cancer, some viruses, and infections. For example, PNA, the main lectin in peanuts, seems to be highly reactive to tumor cells. It has been associated with suppression of tumor development in mice [8]. When used in medical applications, it also helps drugs to be more effective because they can recognize the tumor cells better with PNA than without it [8]. PNA seems to be effective at suppressing the Herpes Simplex virus, while Machaerium lectins in the sunflower plant have been shown to effectively inhibit HIV [8]. Research surrounding the structure and biological functions of lectins is still ongoing, and more needs to be done to understand the unique characteristics of various plant lectins and their application in humans.

The problem of eliminating plants

It is also crucial to consider the synergistic actions of nutrients within a whole food and their physiological responses. Most of the lectin-containing plant foods listed above have been shown to provide multiple health benefits for humans. For example, regular peanut consumption has been associated with reduced risks of heart disease, cancer, diabetes, inflammation, obesity, Alzheimer’s, and gallstone disease [13]. Therefore, the recommendation to eliminate lectin-containing plant foods runs against the larger body of scientific evidence that promotes them for better health. To eliminate plant foods from one’s diet would be to forfeit that protection they provide for the body.

Conclusion

It is not necessary to eliminate lectin-containing foods from the diet. Proper cooking methods can significantly reduce or eliminate lectins in foods, making them safe for consumption. Further, the large body of scientific evidence shows that these foods (including peanuts) are overwhelmingly associated with positive health outcomes.