Five Categories of Plant Protein Sources
Did you know you only need 0.8 g of protein per kg of body weight per day? For a person of 55 kg, this would mean 44 g of protein a day. This can be easily achieved fully from plant-based protein. Let’s see what are the main plant protein sources and how we can optimise their bioavailability and absorption.
The five main categories of plant protein are:
- Nuts and Seeds – almond, cashew, pumpkin seed, chia seed, flaxseed, sunflower seed, sesame
- Grains and Pseudograins – amaranth, buckwheat, millet, quinoa, oat, rice, spelt, wheat
- Beans – chickpeas, black beans, lentils, soy beans and soy products (tofu, tempeh), mung beans
- Vegetables – spinach, broccoli, artichokes, asparagus
- Supplements - spirulina, nutritional yeast, plant based protein powders
Plant Protein Quality and Complementation
To begin, proteins are made of chains of amino acids, some of which are made by the body, while others aren't. Those not produced by the body are called essential amino acids, of which there are nine: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
All plant foods contain at least some of every essential amino acid, but in general, legumes are limiting in methionine, and grains are limiting in lysine. Hence it is commonly advised to consume both legume and grain proteins to supply complementary proteins. Such time-tested food combinations are found in the traditional cuisines of Central and South America as rice and beans, in Asia as rice and tofu, and in America as peanut butter and wheat-based sandwiches.
Of course there are exceptions such as quinoa, which is recognised as a source of complete plant protein.
Plant Protein Digestibility
It is recognised that plant proteins in whole plant foods have lower digestibility compared with isolated proteins or animal proteins. For example, whole beans contain anti-nutritional factors (phytic acid, lectins) that inhibit protein digestion. Hence it is recommended to aim for a protein intake that is about 10% higher than the RDA (1-1.1 g/kg) to compensate for digestibility.
The processing and transformation of plant foods can also affect its digestibility and protein bioavailability. For example, in fermentation of soybeans to tempeh, the fungus helps to break down complex compounds into shorter molecules that is more readily absorbed by the gut.
We can also activate nuts and seeds, or sprout grains and beans such as buckwheat, quinoa, and mung beans, as the sprouting process activates enzymes that break down the anti-nutritional factors.
Top Five Plant Based Protein Sources
- Soybean Tempeh
- Dark Leafy Greens (e.g. spinach)
#1. Soybean Tempeh
Soybean Tempeh - 19 g protein per 100 g or 15 g per 3 oz (85 g) serving. Tempeh is fermented soybean cake. The protein content for soybean tempeh (19.5%) is comparable to chicken (21%), beef (20%), and eggs (13%).
Unfermented beans including soybeans, chickpeas and adzuki beans are known to be high in anti-nutritional factors such as protein-inhibiting trypsin inhibitor (TI) and tannins, mineral-binding phytic acid, flatulence-causing oligosaccharides, and lectins. Fermented beans such as tempeh have two-fold benefits: the reduction of anti-nutritive compounds and increased bioavailability of nutrients. Anti-nutritional compounds are shown to be reduced between 65 and 90 percent during tempeh fermentation. Hence it is a better choice to choose fermented over non-fermented beans for better bioavailability of protein. Of all beans, soybean tempeh has the highest protein content.
Quinoa - 14 g protein per 100 g uncooked or 8.1 g per 1 cup cooked serving. Quinoa is not a true cereal grain but a seed and naturally gluten-free. Apart from having a high protein content, it is also loaded with minerals including magnesium, manganese, iron, copper, phosphorous, and vitamin B2.
Quinoa is available in white, red, and black varieties. It is easy and quick to cook like rice - use 1 part quinoa to 2 parts water, simmer in stock or water for 15 minutes, then let stand for 5 minutes before fluffing. It can be used in sweet breakfast porridge and puddings or and savoury dishes and pilafs.
Flaxseeds - 18 g protein per 100 g or 2.1 g per 2 tablespoons serving. Flaxseeds are the most amazing seeds for protein as they contain about 22.4% protein by dry weight. It is also 40% oil, particularly high in a type called alpha linolenic acid (ALA), an essential omega-3 fatty acid that is the dietary precursor for the long-chain omega-3 PUFA eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), and docosahexaenoic acid (DHA). Flaxseeds are also high in a type of phytoestrogen called lignans, which have been shown to have antioxidant and anti-cancer effects.
Besides their high protein, ALA, and lignan content, they also contain a high amount of soluble dietary fibre that gives an mucilanginous, egg-like texture when soaked in water. In fact, you can easily use flaxseed meal to create a ‘vegan egg’ also known as flax egg to replace eggs in baking or as breading. For one egg, simply combine 1 tablespoon of flaxseed meal with 2 1/2 tablespoons of water. Then stir and set aside for 10 minutes to let it gel.
Because flax seeds are very hard, they should be always ground using a blender or spice grinder into fine flour before use, to maximise their digestion and absorption. Unground flaxseeds can pass through the body undigested, and not provide the nutritional benefits. Once ground, it must be used immediately or refrigerated as flax seed oil oxidises rapidly.
#4. Dark Leafy Greens
Spinach - 5 g protein per 1 cup cooked. Some vegetables can offer a good protein boost, in particular, dark leafy green vegetables such as spinach, kai lan, chye sim, and watercress. In addition to protein, they are high in many other vitamins and minerals including iron, calcium, and vitamin K.
Spirulina - 57 g protein per 100 g or 1.5 g per 1 teaspoon serving. Spirulina is a multicellular cyanobacteria that was one of the earliest photosynthesising organism evolved over 2 billion years ago.
Spirulina is a highly concentrated source of protein, between 55 and 70% by dry weight.
Unlike most plant-derived proteins, Spirulina is a complete protein as it contains all the essential amino acids, making it an ideal plant based dietary supplement. Moreover, spirulina cells do not have cellulose walls but relatively fragile envelope of murein which is one of its kinds in plant kingdom. This explains the very high digestibility of its proteins.
In addition to protein, spirulina is also an excellent source of many other vitamins and minerals, mainly B vitamins (thiamine, riboflavin, nicotinamide, pyridoxine, folic acid, cyanocobalamin), vitamin C, vitamin D, vitamin E, iron, and most important provitamin A (β-carotene). It contains the highest amount of beta-carotene much more than carrots, which is a precursor of vitamin A and is not dose dependent. Spirulina is the only vegetable source of vitamin B12, having two and half times more than meat. It is also nature's highest available source of a rare essential fatty acid called γ-linolenic acid (GLA), which has been shown to be an effective immuno- and cardio-protector.
There are several species of Spirulina, but the most widely used species as food supplement are S. platensis and S. maxima. It is now widely available in powder or tablets and can be eaten in smoothies, ice creams, or protein shake to boost your protein intake.