Vegan Diets and Protein: Careful Combining Not Required

Why vegans need not worry about this important nutrient more than non-vegans do

Dried beans and grains
Darren Muir/Stocksy United

There is a long-standing belief that in order to be vegan and not succumb to overt malnutrition, foods must be carefully combined to provide complete protein. This thinking is obsolete at best, and at worst was always more about bias than dietary balance.

The notion is obsolete because we now know some things about protein we didn’t know in the past. For starters, all plant foods contain all of the amino acids humans need, both those that are essential components of diet (i.e., those we cannot manufacture in our own metabolism) and those that are non-essential. My colleague, Prof. Christopher Gardner of Stanford University and I pointed this out in a correction to an otherwise sound column by Jane Brody on vegan diets in the The New York Times.

Plant foods vary markedly in the concentrations of amino acids. So, for example, grains are generally low in the amino acid lysine, but high in cysteine. Beans, in contrast, are low in cysteine, but rich in lysine. Combining them does, indeed, produce a full assembly of amino acids in something close to the ideal proportions.

This matters.

Piecing Together Proteins

Amino acids are best thought of as construction material for complex protein molecules, and those, in turn, are the construction material for almost everything our bodies need to build every day: enzymes, hormones, and cells.

The analogy is reasonably robust. If you were building a house and had an enormous supply of wood, but hardly any nails, construction would be impeded. So, too, if it were the other way around, or if you had a huge surplus of roofing tiles, but not windows; or excess sheetrock, but no wiring. You get the idea.

Construction of a body in all its complex parts, like the construction of a house in its less complex parts, requires all the right construction materials in the right proportions.

But, of course, you don’t need to deliver the building materials to the construction site simultaneously. House construction proceeds just fine if the lumber is all dropped off on Monday, the bricks and windows arrive Tuesday morning, and the wiring and tiles Tuesday afternoon. Contractors can generally get started with any reasonable array of supplies at the start, and then continue and finish with more supplies delivered over time.

The contribution of proteins to the daily construction within us is just the same. The body is able to retain amino acids from prior meals, and even the prior day, wait for any missing items to arrive, and then proceed with construction, known in the body as anabolism.

So, while on the one hand, the complementarity of amino acids in different plant foods is obviously important, an emphasis on combining foods in a vegan diet to get “complete” protein is not. As Prof. Gardner and I noted in our letter to the The New York Times, the differing concentrations of select, rate-limiting amino acids in grains and beans would only be of practical importance to people eating nothing but grains, or nothing but beans, all day long.

While such a misguided approach to vegan eating would, indeed, lead to trouble, so would such a misguided approach to any kind of diet.

Vegan Diets Lacking Protein Are Rare

That’s where bias seems to enter into the picture. Foods do need to be combined in a vegan diet to get all the needed amino acids in all of the necessary quantities. But unless a diet is both vegan and badly misguided, this is not a realistic concern. Protein deficiency, among vegans along with everyone else, is all but unheard of in the United States. Around the world, protein deficiency tends only to be seen in the context of overt and serious malnutrition and starvation.

Because our culture has made mixed or omnivorous diets containing meat, dairy, and eggs the norm, a decision was reached somewhere, some time that vegan diets made up exclusively of plant foods were not “normal” and required provisos. Those provisos have always come in the form of specific counsel to combine foods to avoid nutrient deficiencies.

But this is in no way different from the potential liabilities of any diet practiced in a silly, unbalanced manner. We do not warn those eating meat to carefully “combine” that meat with citrus fruit, or potatoes for that matter, to avoid the risk of scurvy. Why not? Meat does not provide vitamin C, so unless such diets combine foods judiciously, a deficiency could of course result. Even if potatoes and meat are combined, there is still the risk of deficiency of omega-3 essential fatty acids, for instance. So advice to combine meat and potatoes carefully with flaxseeds, salmon, or walnuts seems warranted—but I have never heard it issued.

Variety Is Key—For Everyone

Quite simply, any kind of diet imposes the risk of nutrient deficiencies if foods are not combined in some sensible, balanced array. There is nothing about that unique to vegan diets.

As for nutrient deficiencies left behind by even optimal diets, it is true that vitamin B12 supplementation (or fortification, meaning an addition to foods) is warranted on even the best of vegan diets. But vitamin D supplementation (or fortification) is warranted on even the best of mixed diets for people who spend their time dressed, indoors, and/or in northern climes. We can make vitamin D from sun exposure but need either sun exposure or the nutrient added to our diets.​

The need for certain amino acids from certain plant foods and other amino acids from other plant foods in a vegan diet is not meaningfully different from the need for certain nutrients from certain foods and other nutrients from other foods in an omnivorous diet. In neither case is there a need to combine foods carefully at all eating occasions to get the essential nutrients. In both cases, wholesome foods in a generally sensible, balanced assembly will reliably get the job done.

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