Is there value in combining “complementary” plant proteins?

Is there value in eating hummus with pita bread? Yes, absolutely… it’s delicious! Ditto rice and beans; bean burgers in a bun; beans on toast; lentil & nut roasts and so on. But in this blog, I’m specifically addressing the question of whether there’s a benefit to these food combinations in terms of anabolism (i.e., the building and maintenance of body tissues).
 
According to some, the answer is “yes”. Eating grains (or nuts) with legumes improves anabolism because – so the theory goes – these foods have “complementary” essential amino acid profiles. Certainly, it is true that many legumes are relatively low in methionine and high in lysine; and grains/nuts tend to be low in lysine and high in methionine. The protein in these foods can be considered “incomplete” to the extent that solely relying on them to meet the RDA for protein (0.8g/kg/d) will provide insufficient amounts of at least one essential amino acid (EAA). Whereas an equal amount of the two will provide a “complete” amino acid profile.

 

However, in recent times this idea has fallen out of favour (especially among vegans and vegetarians). It’s often argued that there is no need for plant-based dieters in developed nations to consciously combine “complementary” foods when planning meals. This is because – so the theory goes – meeting the RDA for all EAAs on plant-based diets is all but guaranteed provided they’re somewhat varied and meets (or exceeds) energy requirements^1,2,3,4.  This rule of thumb has some merit; but it’s dependent on some assumptions which may not hold true at the level of the individual.

I agree that eucaloric whole-food plant-based diets are likely to hit the target for each EAA. Vegans that regularly consume high protein foods (e.g., legumes, protein shakes, seitan etc.) are also highly likely meet the RDAs for all EAAs. But it’s less clear that a “junk food vegan” would get enough. Convenience foods can be low in both whole plant foods and protein. Further, not everyone eats a eucaloric (or hypercaloric) diet. Naturally, the risk of EAA inadequacy increases when less food is going into the system (due to a weight loss program or poor appetite for instance).

Importantly, based on the evidence we have, it seems that a non-negligible number of vegans fail to meet the RDA for protein (which in turn will increase their risk of falling short in terms of EAAs):

• A study based on the EPIC-Oxford cohort found that 16.5% of vegan males and 8.1% of vegan females consumed less than 0.6g/kg/d⁵.
• A Germany cross-sectional study found that more than 30% of vegan males and more than 40% of vegan females failed to reach 0.8g/kg/day⁶. 
• 27% of vegans failed to eat above 10% of their calories from protein in a French cross-sectional study⁷.
• A small Slovenian cross-sectional study found that 31% of vegans failed to reach 0.8g/kg/day⁸.

Admittedly, I don’t think we can discount the idea that these estimates may be exaggerated due to underreporting of energy/protein intake. Nonetheless, I think the above-mentioned studies to some extent challenges the idea that vegan diets rarely lead to EAA insufficiency in practice. Moreover, I think that vegans should probably aim above the RDA for protein: I have made my case for this in a previous blog (tl;dr: I think the benefits of higher intakes far outweigh any potential downsides).

I’m not quite so assured that vegans will “naturally” hit protein and EAA targets: I think many plant-based dieters would benefit from giving protein more thought. But (to get back to the topic at hand) might “protein combining” be a useful strategy in this regard? In my view, absolutely not! If our goal is to obtain a complete EAA profile, adding grains (or nuts) is if anything counterproductive when we have access to more legumes or other protein-rich foods. Indeed, per calorie legumes will reliably provide more of every single essential amino acid. To illustrate this, take a look at the graph below:

Now, there are plenty of good reasons for eating chapatis (a type of flatbread) with dahl. Here’s a few of them: they may enhance one’s enjoyment of the dish; whole grains are positively associated with good health outcomes; they’re a source of nutrients. But simply eating more dahl will certainly be far more helpful with respect to EAAs.

Grains and legumes do have complementary EAA profiles, the problem is that grains and nuts are too low in protein per calorie to make an equal contribution to the EAA content of a meal. To illustrate this, let’s return to the bar chart on lentils and rice.

56 grams of protein from a 50/50 mix of lentils and rice supplies over 100% of the RDA for lysine and methionine; but it comes with 1617 calories. On the other hand, while 56g of protein from lentils does not quite provide enough methionine, it contains less than half the calories.

To be clear, I’m not saying that you should avoid grains (or nuts). My point is that adding grains to legumes is a poor strategy with respect to upping the EAA content of animal-free meals. The solution to improving the EAA content of animal-free meals is simple: add more protein rich foods. Now replacing grains or potatoes with (more) legumes is one way to go about this. However, adding a more refined plant-based source of protein to meals is even better. Here are some examples of what I’m talking about:

⦾ Add gluten flour to bean burgers.
⦾ Include tofu in a split pea dahl (with or without chapatis)
⦾ Fortify your morning oats with a flavoured soy protein isolate.
⦾ Throw in some TVP to your 5-bean chilli.
⦾ Add some high protein vegan sausages to a stew.

In summary, combining grains/nuts with legumes is an inefficient means of ensuring one gets enough essential amino acids. By simply incorporating plant-based foods that are high in protein per calorie, one can be reasonably assured that all RDAs for essential amino acids are being met.

PS: As an aside, combining plant-based protein isolates seems like a legitimate way of exploiting complementary essential amino acid profiles. Currently, such products are not commercially available. But I would not be surprised if plant-based protein blends will prove to be very popular in the near future. A recent study found that a blend of wheat, corn and pea protein isolates stimulated muscle protein synthesis to a similar extent to an equal amount of milk protein in young active males⁹.

Incidentally, check out my recipe for “anabolic bangers” that combines protein from soy, pea and wheat:

References 
 
1. Mariotti, F. and Gardner, C.D., 2019. Dietary protein and amino acids in vegetarian diets—A review. Nutrients, 11(11), p.2661. 

2. https://www.nutrition.org.uk/healthy-sustainable-diets/protein/?level=Health%20professional 

3. Mariotti F. Plant protein, animal protein, and protein quality. In: Vegetarian and plant-based diets in health and disease prevention 2017 Jan 1 (pp. 621-642). Academic Press. 

4. McDougall, J., 2002. Plant foods have a complete amino acid composition. Circulation, 105(25), pp.e197-e197. 

5. Sobiecki, J.G., Appleby, P.N., Bradbury, K.E. and Key, T.J., 2016. High compliance with dietary recommendations in a cohort of meat eaters, fish eaters, vegetarians, and vegans: results from the European Prospective Investigation into Cancer and Nutrition–Oxford study. Nutrition Research, 36(5), pp.464-477. 

6. Waldmann, A., Koschizke, J.W., Leitzmann, C. and Hahn, A., 2003. Dietary intakes and lifestyle factors of a vegan population in Germany: results from the German Vegan Study. European journal of clinical nutrition, 57(8), pp.947-955.

7. Allès, B., Baudry, J., Méjean, C., Touvier, M., Péneau, S., Hercberg, S. and Kesse-Guyot, E., 2017. Comparison of sociodemographic and nutritional characteristics between self-reported vegetarians, vegans, and meat-eaters from the NutriNet-Santé study. Nutrients, 9(9), p.1023.

8. Bogataj Jontez, N., Kenig, S., Šik Novak, K., Petelin, A., Jenko Pražnikar, Z. and Mohorko, N., 2023. Habitual low carbohydrate high fat diet compared with omnivorous, vegan, and vegetarian diets. Frontiers in Nutrition, 10, p.572.

9. Pinckaers, P.J., Kouw, I.W., Gorissen, S.H., Houben, L.H., Senden, J.M., Wodzig, W.K., de Groot, L.C., Verdijk, L.B., Snijders, T. and van Loon, L.J., 2022. The Muscle Protein Synthetic Response to the Ingestion of a Plant-Derived Protein Blend Does Not Differ from an Equivalent Amount of Milk Protein in Healthy Young Males. The Journal of nutrition, 152(12), pp.2734-2743.