Another myth: vegans need to combine beans and grains to create “complete proteins”

July 18, 2015 at 8:20 pm (Uncategorized)


My mom recently asked me if wild rice combines with beans to form a “complete protein,” as brown rice does. I thought I’d post my answer to her here, as the myth about complete proteins is pretty widespread.

The “incomplete protein” myth was first popularized by Frances Moore Lappé in her 1971 book, Diet for a Small Planet. The main concern is that, as a general rule, legumes are lower in the essential amino acid methionine while most other plants foods (including grains) are lower in the essential amino acid lysine. So to get a “complete protein” you have to eat grains and beans together.

An aside: I’m not sure how “most other plant foods” became “grains.” Maybe it’s because most people think beans and grains are the only plant foods that have much protein? They seem to not realize that nuts and vegetables can also provide non-negligible amounts of protein.

But going back to the complete protein myth. Today, most vegetarians (at least) know that your body can store the essential amino acids and so you don’t have to actually eat grains and beans together at every meal. But many vegetarians still seem to believe that you have to eat both grains and beans in order to get all the essential amino acids. But that is also a myth.

To see why, consider the amount of methionine and lysine in various foods:

Food Serving Calories Protein (g) Lysine (mg) Methionine (mg)
Brown rice 100g 112 2.3 88 52
Wild rice 100g 101 4 170 119
Quinoa 100g 120 4.4 239 96
 Grains  125 4.9 170  90
 Lentils 100g  116  9  630  77
 Black beans 100g  132  9  608  133
 Mung beans 125g  131  8.8  613  105
 Beans  127  8.2  570  100
 Nuts  3/4 oz  129  4.0  140  60
 Seeds  3/4 oz  122  5.1  120  70
 Green leafies 1 cup  38  3.7 200 100
 Mushroom  39  4.2  260 60
 Crucifers  39  3.0  160  30
 Other veg  40  2.7  120  30
 Cheese  1 – 1.5 oz  112  7.7  640  210
 Yogurt  200g  122  6.9  620  200
 Eggs  1.5 med  102  8.3  600  260

About this table: Note that the weight when stated is of the cooked food, except for the dairy, nuts, and seeds entries, which are assumed to be raw. When no serving size is stated the protein numbers are based on the specified number of calories. The general category entries (e.g., grains, beans, nuts, cheese…) are based on a manual averaging of 5 to 10 common varieties from each category. The serving size for cheese was taken to be 1 ounce for hard cheeses and about 1.5 ounces for softer, fresh cheeses.

First let’s take a look at lysine. The first thing to notice is that a serving of beans has about the same amount of lysine as the “complete proteins” of dairy and eggs. But most other plant foods have very little lysine. And the RDA for lysine for a 125 pound person is 2159 mg. There are a few non-legume foods with reasonable amounts of lysine (most notably pepitas, which have 575 mg in 3/4 oz). But even if you were to eat a lot of pumpkin seeds, it would be hard to get close to 2159 mg without eating beans. And if you weigh more than 125 pounds then you’ll need even more lysine. So if you’re a vegan you will either need to eat a huge amount of calories or you will need to eat beans! You can read more about lysine requirements for vegans on the excellent www.veganhealth.org website.

Now methionine. None of the plant foods have as much methionine per serving as a serving of eggs or dairy. But the RDA is also lower, at about 540 for a 25 pound person. (The RDA is actually 1080 for methionine + cysteine, but the amounts are usually similar in most plant foods, so I divided it in half to get an estimated RDA for methionine alone.) While it’s true that grains have a higher percentage of methionine than beans and other plant foods (except for green leafies apparently), the total amount of methionine from a serving of grains is actually slightly less than the total amount from a serving of beans, since beans have quite a bit more protein than grains on average.  That said, it would be almost impossible to meet the RDA for methionine from beans alone. Our prototypical 125-pound woman would need to eat about 5 cups of beans! Not impossible, but probably a bad idea. To get enough methionine, you need to eat a range of protein-containing foods (i.e., not just fruit and starchy vegetables), but you don’t need to eat either beans or grains in particular. If you’re eating enough protein then it it’s be hard to not get enough methionine.

Here’s a sample diet that has no grains and meets the RDA for both lysine and methionine:  2 cups of beans, 2 ounces nuts/seeds, 1 cup green leafies, a serving of mushrooms, a serving of cruciferous vegetables, and 2 servings of other veggies. Yes, that is 6 servings of vegetables, but you’re supposed to get 9 servings a day of fruits and vegetables, so that seems reasonable.

All of that said, in practice many people (even supposedly health-conscious vegans) do not eat 2 cups of beans and 6 servings of vegetables on a daily basis. But they probably eat quite a bit of grains, so they can get the rest of their lysine and methionine that way.

To recap: If you’re eating beans and a variety of vegetables along with some nuts and seeds, you don’t have to eat grains, at least in terms of protein requirements. But a vegan most likely can’t hit the RDA for lysine with just grains, nuts, and vegetables — most days you’ll need to eat beans as well.

Of course, some might argue that the RDA for lysine is unnecessarily high, but that’s a topic for another day….

3 Comments

  1. joyinthearts said,

    My doc used to tell me that corn was high in lysine when I complained of craving it. He said it help prevent cold sores, etc. But I do not see it mentioned on your list.

  2. joyinthearts said,

    Thank you . That is very helpful. I do routinely eat a lot of beans. Your site is beautiful.

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