Many vegans follow their chosen diet, not (only) because they believe this diet improves their overall health, but because they believe it is morally wrong to kill animals or because they believe the way we raise and hunt the animals we eat places a heavy cost on the Earth’s ecosystem. Yet vegans also sometimes struggle to consume adequate amounts of protein. Here's how to get the best of both worlds. (For sustainable protein options for vegetarians, pescetarians and omnivores, check out The Complete Guide to Sustainable Protein.)

A 2013 study showed that European Union citizens can reduce their water footprint (the amount of water used to produce a particular product) by cutting meat consumption by half or by transitioning to a completely vegetarian diet. California residents can save 2.9 times more water, 2.5 times more energy, 13 times more fertilizer and 1.4 times more pesticides by switching from a non-vegetarian to a vegetarian diet. And a 2006 study demonstrated that an omnivorous diet produces 1500 kilograms (kg) more carbon dioxide (CO2) than a vegetarian diet with the same number of calories.

But these studies don’t necessarily maintain a constant macronutrient breakdown across different diets. Not all plants are equal when it comes to sustainability. Per calorie, fruits require more energy, use more water and produce more CO2 than vegetables. Both fruits and vegetables require more energy and use more water per calorie than poultry, although poultry produces more greenhouse gasses (see the chart below for indices of average energy use, blue water footprint and greenhouse gas (GHG) emissions per calorie of food for each food group). In fact, transitioning from the current American diet to the United States Department of Agriculture (USDA)-recommended diet would increase energy use, water consumption and greenhouse gas emissions.

Furthermore, many vegan foods claiming to be high in protein really aren't. The predominant macronutrient in legumes and nuts isn't protein, but carbohydrates and fat, respectively. Saying you eat black beans for protein is like saying you buy Playboy for the articles. Protein and prose exist in these entities, but they are not the main attractions. Bearing all these considerations in mind, here's a breakdown of the nine most protein-rich and sustainable foods for vegans, based on protein content, cost, greenhouse gas emissions, water footprint and energy use (based a 0-100 scaled index, where lower values indicate less energy was used to produce the food). (Not a vegan? Here's a complete guide to sustainable protein regardless of diet.)

9. Soy milk (unsweetened) Protein content: 3.0 grams (g) protein per liter (L) Cost: $1.01 per 10 g protein Greenhouse gas emissions: 0.85 kg CO2 per liter produced (2.51 g CO2 per calorie) Water footprint: 3,763 cubic meters of water per ton produced (11.13 L water per calorie) Energy use index: 1

8. Tofu Protein content: 9.9 g protein per 100 g Cost: $0.68 per 10 g protein Greenhouse gas emissions: 3.50 kg CO2 per kg produced (3.85 g CO2 per calorie) Water footprint: 2,523 cubic meters of water per ton produced (2.77 L water per calorie) Energy use index: 1

7. Soybeans Protein content: 13.0 g protein per 100 g Cost: $0.35 per 10 g protein Greenhouse gas emissions: 2.10 kg CO2 per kg produced (2.38 g CO2 per calorie) Water footprint: 2,145 cubic meters of water per ton produced (1.46 L water per calorie) Energy use index: 1

6. Tempeh (dried) Protein content: 18.5 g protein per 100 g Cost: $0.72 per 10 g protein Greenhouse gas emissions: 3.50 kg CO2 per kg produced (1.81 g CO2 per calorie) Water footprint: not available Energy use index: 1

Soy products (soybeans/edamame, soy milk, tempeh and tofu) have long been a staple in vegan diets. Americans are now eating more soy than ever before, and the Food and Drug Administration (FDA) promotes soy as reducing the risk of heart disease. However, animal studies show that genistein, an isoflavone in soy, alters reproduction and embryonic development, and high doses of this compound lead to abnormal growth of breast cells in male rats. A 2001 study concluded the soy isoflavones “genistein and daidzein may stimulate existing breast tumor growth” and “women with current or past breast cancer should be aware of the risks of potential tumor growth when taking soy products." Another study found that genistein stimulated the growth of estrogen-depended human breast cancer cells and that soy protein diets increased estrogen-dependent tumor growth as soy intake increased.

Osteopathic physician Dr. Joseph Mercola and the nutrition-focused Weston A. Price Foundation point to phytic acid as another potentially dangerous component of soy, and Mercola and dietician Emily DeLacey recommend fermented soy products like tempeh and fermented tofu over unfermented products like soy beans, soy milk and unfermented tofu since fermentation supposedly neutralizes the phytic acid in raw soy. At the very least, the jury is still out on soy, but it would seem wise to avoid relying on soy as a primary source of protein.

5. Seitan Protein content: 21.2 g protein per 100 g Cost: $0.69 per 10 g protein Greenhouse gas emissions (based on “grains” category): 0.49 kg CO2 per kg produced (0.46 g CO2 per calorie) Water footprint: 4,189 cubic meters of water per ton produced (3.96 L water per calorie) Energy use index: 7

The Japanese word seitan refers to wheat gluten, the main protein of wheat. As such, seitan is obviously off the table if you're gluten-free. It often has a chewy or stringy texture when cooked that more closely resembles the texture of meat in comparison to other substitutes.

4. Nori (dried) Protein content: 33.3 g protein per 100 g Cost: $5.66 per 10 g protein Greenhouse gas emissions (based on “other vegetable” category): 0.40 kg CO2 per kg produced (0.35 g CO2 per calorie) Water footprint: NA Energy use index: 72

Nori is the seaweed used to wrap sushi rolls, and while it’s protein-dense, it’s also very light, meaning you’ll need to eat a lot of it in order to get a significant amount of nutrients.

3. Nutritional yeast Protein content: 50.0 g protein per 100 g Cost: $1.25 per 10 g protein Greenhouse gas emissions: NA Water footprint: NA Energy use index: NA

Nutritional yeast, the third most protein-rich food on the list, is deactivated yeast that is sold as yellow flakes or powder. Many vegan recipes describe nutritional yeast as having a flavor similar to cheese or butter, but it also has an earthy undertone that makes it unique. Instead of using it as a flavoring agent or cheese replacement, try nutritional yeast to smooth out the rough edges of sharp flavors like cilantro, garlic and lime in a chimichurri sauce.

2. Spirulina (dried) Protein content: 57.5 g protein per 100 g Cost: $0.66 per 10 g protein Greenhouse gas emissions (based on “other vegetable” category): 0.40 kg CO2 per kg produced (0.39 g CO2 per calorie) Water footprint: NA Energy use index: 72

1. Chlorella Protein content: 58.0 g protein per 100 g Cost: $1.12 per 10 g protein Greenhouse gas emissions (based on “other vegetable” category): 0.40 kg CO2 per kg produced (0.16 g CO2 per calorie) Water footprint: NA Energy use index: 72

Two species of algae, chlorella and spirulina, have the most protein per mass of any food on this list. Chlorella can also assist with detoxifying the body of pesticides and heavy metals like lead and mercury. Spirulina is rich in vitamins B, C, D and E and contains all of the essential amino acids.

For sustainable protein options for vegetarians, pescetarians and omnivores, check out The Complete Guide to Sustainable Protein.