Back in November of 2013, Dr. Stephan Guyenet posted an article outlining the evolutionary history of legume consumption. He demonstrates that, contrary to popular belief, legumes were part of our ancestral diet. Recent analysis of Neanderthal tooth plaque revealed that they consumed wild varieties of peas and fava beans. Since early humans are thought to have eaten a more diverse diet than Neanderthals, it is safe to assume that our human ancestors also ate legumes.
Dr. Guyenet also points to several contemporary hunter-gatherer groups that consumed significant amounts of legumes, including the !Kung San of the Kalahari desert (who relied heavily on a legume called the tsin bean) and the Australian Aborigines (who extensively harvested the seeds and gum of Acaciatrees, another legume).
This research suggests that legumes are, in fact, “Paleo”. But even if Paleolithic people didn’t eat legumes, is that reason enough to avoid them? If it is, then shouldn’t we also strictly avoid dark chocolate, coffee, green tea, and alcohol? What about the glut of breads, muffins, packaged snacks, desserts, and even candy (no, I’m not kidding) claiming to be “Paleo” that have recently become so popular? It should be obvious that our ancestors were not baking with nut flour, chowing down on truffles or drinking “Paleo” cocktails. Yet even the most die-hard, self-identified Paleo purists typically consume at least some of these foods and beverages, and don’t seem to see a contradiction in that. Why should legumes be any different?
A more important question to ask than whether a food is “Paleo” is how it impacts human health. Fortunately, in the case of legumes, we have a lot of modern research that can help us to answer that question.
Should we avoid legumes because of the anti-nutrients they contain?
Paleo dogma on legumes holds that we should avoid them because they contain toxic anti-nutrients called lectins and phytic acid (aka phytate). Let’s take a look at each of these compounds in legumes and see if this argument holds up.
Lectins are a type of protein that can bind to cell membranes. Studies have shown that lectins can impair growth, damage the lining of the small intestine, destroy skeletal muscle, and interfere with the function of the pancreas. Sounds serious, right?
Not so fast. There are several reasons that these results cannot be extrapolated to humans. First, the animals consumed very large amounts of lectins—much larger than a human would get from a varied diet which includes legumes. Second, the lectins were from raw legumes. Why is this significant? Because humans eat primarily cooked legumes, and cooking neutralizes the lectins found in most legumes.
In fact, cooking legumes for as little as 15 minutes or pressure-cooking them for 7.5 minutes almost completely inactivates the lectins they contain, leaving no residual lectin activity in properly cooked legumes.
What’s more, other components in food (e.g. simple sugars) can bind to lectins and diminish their toxic effect. So even if there is a small amount of lectin left after cooking, it’s unlikely that it will have a detrimental effect given the presence of simple carbohydrates in legumes that can bind to the proteins.
Finally, if lectins really are a problem then we’ll have to cut out a lot more than legumes from our diet in order to avoid them. It turns out that lectins are present in at least 53 fruits, vegetables, spices and other commonly eaten plants, including carrots, zucchini, melon, grapes, cherries, raspberries, blackberries, garlic and mushrooms—to name a few.
This is not an invitation to stop eating these foods! It’s simply a reminder that almost every plant we eat contains small amounts of toxins, since this is how plants defend themselves. In the majority of cases these low levels of toxins don’t harm us, and in fact, they may even provide health benefits. For example, many of the compounds we call “antioxidants”—like polyphenols found in blueberries, dark chocolate, etc.—are actually “pro-oxidants” that cause mild oxidative stress and thus upregulate our body’s natural defense systems.
To my knowledge there’s only one study demonstrating humans being harmed by consuming legumes. This is the study often used by Paleo advocates to “prove” that legumes are dangerous. However, what is often neglected is that this study described a case of food poisoning that occurred in hospital patients who ate legumes that hadn’t been cooked properly. Suggesting that we shouldn’t eat cooked legumes because raw legumes cause disease is like saying that we shouldn’t eat cooked chicken because we can get Salmonella from eating raw chicken.
The one lectin we may want to exercise caution with is peanut lectin, since both raw peanuts and peanut oil have relatively high lectin content. Some data in animals suggest that peanut lectin may contribute to atherosclerosis by stimulating the growth of smooth muscle and pulmonary arterial cells. However, other research (including clinical trials) in both animals and humans have found that peanuts and even peanut oil reduce cardiovascular risk factors and thus may protect against heart disease. In light of this conflicting data, and because of other risks associated with peanut consumption such as exposure to aflatoxin, I recommend either minimizing your intake of peanuts or avoiding them entirely.
Phytic acid (aka phytate)
Phytic acid is the storage form of phosphorus found in many plants, especially in the bran or hull of grains and in nuts and seeds. Although herbivores like cows and sheep can digest phytic acid, humans can’t. This is bad news because phytic acid binds to minerals (especially iron and zinc) in food and prevents us from absorbing them. (It’s important to note that phytic acid does not leach minerals that are already stored in the body; it only inhibits the absorption of minerals from food in which phytic acid is present.)
Phytic acid interferes with enzymes we need to digest our food, including pepsin, which is needed for the breakdown of proteins in the stomach, and amylase, which is required for the breakdown of starch. Phytic acid also inhibits the enzyme trypsin, which is needed for protein digestion in the small intestine.
Sounds pretty bad, right? While it is true that diets high in phytic acid contribute to mineral deficiencies, it’s also true that humans can tolerate moderate amounts of it without harm (perhaps because our gut bacteria produce enzymes that break down phytate and extract the nutrients the body needs). In fact, there’s even evidence that phytic acid may have some beneficial effects. It prevents the formation of free radicals (making it an antioxidant), prevents the accumulation of heavy metals in the body, and plays a role in cellular communication.
The problem with telling people to avoid legumes because they contain phytic acid is that many other foods in the diet—including “Paleo-friendly” foods—contain substantially higher amounts of phytic acid than legumes. For example, a serving of trail mix, that beloved Paleo favorite, is likely to be much higher in phytic acid than a serving of lentils. Cacao beans (chocolate) have about the same amount of phytic acid as most beans. And spinach and swiss chard are higher in phytic acid than almost any legume, nut or seed!
Are there any reasons we might want to limit legumes in our diet? Read more here