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We hear a lot about proteins and the importance of having them. Indeed, they are important, but what is a protein? Can protein from plant sources be enough? And what are the lowest calorie proteins in plants?
What is a Protein?
Proteins are large, complex molecules that play an important role in the body’s function. They do most of the work in the cells and are needed for the proper function of the body’s tissues and organs.
To understand how proteins work, we need to understand amino acids. These fundamental smaller molecules, called amino acids, are composed of basic elements: carbon, hydrogen, nitrogen, oxygen, and occasionally sulfur. There are two groups: an “amino group” and a “carboxyl group,” and they are connected by a carbon atom. The amino group is very base, or alkaline, like ammonia, and is composed of hydrogen and nitrogen. The carboxyl group is acidic (thus “acid” in amino acid), and is composed of carbon and oxygen. Animals and plants make amino acids when these atoms are combined in a certain way, resulting in amino acids. There are core 20 amino acids, of which 8 are essential for humans.
Proteins are complex molecules made up of a chain of amino acids. There could be a few dozen amino acids chained together to make a protein, or thousands, but they are a mix of the same 20 amino acids. When you eat a protein source, the protein is broken down in the body into amino acids and absorbed through the small intestines. The body needs a “pool” of essential amino acids to make new proteins for functions in the body, and draws from the amino acids taken through food. If the body doesn’t have enough essential proteins from the food we intake, it will be taken from other places in the body, like muscles. This is why it’s important to have essential protein sources in our diet.
Benefits of Plant-Based Proteins
Frequently when people hear the word protein, they almost exclusively think of animal products: meat, eggs, and milk. For instance, when we go to a quick service restaurant to get a sandwich or bowl, the question will be asked, “What will be your protein?” However, vegetables, nuts, legumes, and fruits all offer proteins.
Concerns about plant-based protein include not being a complete protein and not getting enough B12 and calcium. These can be addressed by eating a variety of plant-based proteins and other foods that have essential vitamins. For example, leafy greens are good for calcium. Nutritional yeast could be an excellent source of B12, and supplements could be taken for this vitamin.
The benefits of plant-based protein include protection against heart disease, stroke, type two diabetes, and cancer. In a large study, swapping just 3% of energy from animal protein was associated with a 10% decreased mortality. Plant-based protein has fewer calories and saturated fat than animal-based protein and is high in fiber. This has consistently been shown to be an effective strategy in controlling obesity.
Complete Plant-Based Protein Sources
One benefit of animal-based protein sources is that they are considered “complete” protein sources. This means they have the eight essential amino acids needed by the body that it can’t make by itself. Plant-based protein sources are frequently incomplete, with a few exceptions. This means that most plant-based protein sources do not have all of the eight essential amino acids or significantly lack the amount needed.
However, this is not a problem for those on a plant-based diet if a variety of food is eaten. Individuals on a plant-based diet can easily get all the essential amino acids by mixing their sources of proteins. One example is eating beans and rice provides all the essential proteins. Rice is low in the amino acid lysine but high in methionine. Beans are the opposite. They are high in lysine and low in methionine. When you intake a variety of protein sources from plants, getting all the amino acids necessary is not a problem. There are some plants that are considered complete, with all these essential amino acids needed, such as:
- Soy
- Quinoa
- Hemp seeds
- Amaranth
- Buckwheat
- Chia seeds
Vegetables Rich in Protein
There are some power vegetables that have a lot of protein for the calorie count. High on the list are alfalfa sprouts, fresh basil, raw spinach, bok choy, and asparagus. Although the percentage of protein in the food is lower because of higher water and carb count and you don’t get as much protein from these, these have very low calories so you can eat a lot of this and feel full faster.
Legumes and Beans: High Protein with Fewer Calories
Legumes, such as beans, are a great source of protein and are lower in calories. They have the benefit of being high in fiber and packed with nutrients. These are fairly high on the list of low-calorie protein sources because of their low calorie for the amount of protein you get.
Seeds and Nuts: Nutrient-Dense Protein Sources
Seeds and nuts are a great way to stay healthy. They are packed with nutrients, such as vitamins E, B6, niacin, and folate. They are sources of healthy fats, such as polyunsaturated fats, and low in saturated fats. Nuts and seeds are good sources of protein and high in dietary fiber. They are rich in minerals, such as iron, magnesium, zinc, calcium, and potassium. Also, they have good amounts of antioxidants. Some seeds, such as flax and hemp seeds, are great sources of omega-3.
On the chart, you will note that seeds and nuts are toward the end of the list. This is because the higher amount of oils in seeds and nuts make them higher in calories. This shouldn’t scare you off from eating them. They are a great source of plant-based protein and other nutrients.
Protein in Whole Grains
Whole grains also play a part in obtaining protein on a plant-based diet. Although the amount of calories per gram of protein is higher for these, they have many other benefits. The most benefit comes from whole grains. These are higher in iron, magnesium, manganese, phosphorus, B vitamins, and dietary fiber. Also, whole grains help with blood sugar regulation and body weight.
Where Do Calories Come From?
A calorie is the amount of energy in food. The body uses this energy to function, and if not used, it is stored in the body as fat. Calories come from only 4 sources: carbohydrates, proteins, fats, and alcohol. Of these, only 3 are nutrients: carbohydrates, proteins, and fats. Each provides slightly different calories per gram of mass, but as an approximation, carbohydrates and proteins have about 4 calories per gram, and fat has about 9 calories per gram.
- Carbohydrates are about 4 calories per gram
- Proteins are about 4 calories per gram
- Fats are about 9 calories per gram
See our page on how Food Calories are Calculated for more information.
Plant Protein and Calories
Some plant-based foods are high in protein but low in calories. This means most of the calories come from proteins, and few, if any, from oils and carbs. To put this another way, high-protein and low-calorie means few carbohydrates and fats. Eating plenty of these foods, especially leafy green vegetables and legumes, or beans, will help fill you up with plenty of protein and vitamins, but with few calories.
Most raw foods have a combination of all three nutrient calorie sources. A food with very few fats and carbs would be close to 4 calories per gram, the amount produced by the protein. The more calories per gram over 4, the more there is a combination of fats and carbohydrates in the food. Other nutrients, such as water, minerals, and vitamins, although very useful to the body, do not have calories, but they do add to the mass.
How to read this chart:
- Column one is the food or food group, as well as the form, if applicable (e.g. cooked, raw, roasted)
- Column two is the calories per gram of protein. For example, 8 means 8 calories per 1 gram of protein. The lower the calories, the less carbs and fats.
- Column three is the percent protein in the mass of the food. Most of the mass may be water and other nutrients. For example, 10% means 10 grams of protein for 100 grams of food. Cooked foods have added water, and although the calories for each protein gram are the same, the percent of protein drops because of the water being added to the total mass. (Percentages under 10% are listed to a tenth of a percent)
Lowest Calorie Plant-Based Protein Sources Chart
| No | Food | Calories per gram of protein | Percent protein |
|---|---|---|---|
| 1 | Seitan | 5 – 8 | 18 – 25% |
| 2 | Raw alfalfa sprouts | 6 | 4% |
| 5 | Mushrooms (white, portabella) | 7 – 9 | 3.1 – 3.3% |
| 4 | Fresh basil | 7 | 3.20% |
| 3 | Sprouted kidney beans | 7 | 4.20% |
| 7 | Raw spinach | 8 | 2.90% |
| 8 | Tofu | 8 – 10 | 6.6 – 17% |
| 6 | Nutritional yeast | 8 | 45 – 53% |
| 9 | Bok choy | 9 | 1.50% |
| 11 | Asparagus (cooked) | 9 | 2.40% |
| 10 | Tempeh | 9 – 13 | 13 – 20% |
| 13 | Raw arugula | 10 | 2.60% |
| 12 | Sprouted soy or mung beans | 10 | 3% |
| 16 | Raw chard | 11 | 1.80% |
| 15 | Raw cilantro, parsley | 11 – 12 | 2.1 – 3% |
| 14 | Raw soybeans | 11 | 13% |
| 18 | Raw kale or broccoli | 12 | 2.8 – 2.9% |
| 17 | Lentil sprouts | 12 | 9% |
| 19 | Lentils (sprouted or cooked) | 12 – 13 | 9% |
| 20 | Raw cauliflower | 13 | 1.90% |
| 21 | Brussels sprouts (cooked) | 14 | 2.60% |
| 24 | Green peas (cooked) | 15 | 5.20% |
| 22 | Black eyed peas (cooked) | 15 | 7.70% |
| 23 | Kidney, lima, black beans (cooked) | 15 | 7.8 – 8.9% |
| 25 | Pinto or navy beans (cooked) | 16 – 17 | 8.2 – 9% |
| 26 | Chickpea flour | 17 | 22% |
| 28 | Chickpeas (cooked) | 18 | 7.70% |
| 27 | Hemp seeds (raw and hulled) | 18 | 33% |
| 30 | Raw tomatoes | 20 | 0.90% |
| 29 | Raw shelled pumpkin seeds | 20 | 30% |
| 31 | Roasted peanuts | 21 | 28% |
| 33 | Wild rice | 25 | 4% |
| 32 | Wheat | 25 | 13.7% |
| 35 | Buckwheat | 26 | 13.2% |
| 34 | Quinoa | 26 | 14.1% |
| 37 | Raw guava | 27 | 3.40% |
| 36 | Amaranth | 27 | 13.6% |
| 38 | Almonds (unroasted) | 27 | 21% |
| 40 | Sweet corn (cooked) | 28 | 3.40% |
| 39 | Raw pistachio nuts | 28 | 20% |
| 43 | Raw oats | 29 | 13% |
| 42 | Chia seeds | 29 | 17% |
| 41 | Flax seeds | 29 | 18% |
| 44 | Raw cashew nuts | 30 | 18% |
| 45 | Sesame seeds | 33 | 17% |
| 46 | Potatoes (roasted, cooked) | 38 – 43 | 2.4 – 2.5% |
| 47 | Hazelnuts | 42 | 15% |
| 48 | Walnuts (roasted) | 43 | 15% |
| 49 | Pecans | 75 | 9.20% |
| 50 | Avocado | 80 | 2% |
Research in food protein and calorie chart is based on U.S. Department of Agriculture, Agricultural Research Service. FoodData Central, 2019. fdc.nal.usda.gov.