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Lesson 3 of 7 7–10 min read
Protein: What It Is and Where It Comes From
Protein is one of the three macronutrients. This lesson explains what protein is made of, what it does in the body, which foods provide it, and how to read protein on a food label.
What is protein?
Protein is a macronutrient — a nutrient the body needs in relatively large amounts, alongside carbohydrate and fat. As you saw in Lesson 2: Calories & Energy, protein provides about 4 kcal per gram, the same as carbohydrate.
Protein is found in many foods, not only meat. Beans, lentils, dairy, eggs, nuts and grains all contribute.
Amino acids: the building blocks
Proteins are made of smaller units called amino acids, joined together in chains. About 20 different amino acids are used to build the proteins in the human body. When you eat protein, digestion breaks it down into amino acids, which the body then uses to build the proteins it needs.
Essential amino acids
For adults, 9 amino acids are essential (also called indispensable): the body cannot make them in sufficient amounts, so they must come from food. They are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
The other amino acids can be made by the body.
What protein does in the body
Protein is needed for growth, repair and maintenance of the body. It is a structural part of every cell, including muscle, skin, hair and bone. Proteins also act as enzymes (which help chemical reactions happen), some hormones, and parts of the immune system. Protein can also be used as a source of energy.
Where protein comes from
Protein is found in both animal and plant foods:
| Animal sources | Plant sources |
|---|---|
| Meat and poultry | Beans, lentils and peas |
| Fish and seafood | Tofu, tempeh and other soya foods |
| Eggs | Nuts and seeds |
| Milk, yoghurt and cheese | Grains such as wheat, oats and rice |
Protein foods also bring other nutrients — and sometimes things to watch, such as salt or saturated fat in some processed meats. That is one reason a variety of sources is useful. To compare real foods, the Food Calorie Calculator shows protein values from USDA FoodData Central. You can also compare the highest protein foods, estimate an adult reference amount with the Protein Calculator, or browse high-protein meals.
Plant proteins and variety
Individual plant foods often contain smaller amounts of one or more essential amino acids than animal foods do. Eating a variety of plant proteins across the day — for example, beans, grains, nuts and seeds — helps provide all the essential amino acids.
Common myth
You may hear that vegetarians and vegans must “combine” proteins (such as beans with rice) at the same meal. This isn't necessary: what matters is variety over the course of the day.
Reading protein on a food label
Most nutrition labels list protein in grams, often per 100g and per portion. Just like energy in Lesson 2, you can scale the per-100g figure to the amount you actually eat:
(protein per 100g ÷ 100) × grams eaten
Hypothetical example
A fictional food label shows 8g protein per 100g. You eat 150g.
8 ÷ 100 × 150 = 12g protein.
This food is invented for learning; it is not a real product value.
Try it yourself (hypothetical food)
A fictional food label shows 8g protein per 100g. How much protein is in a 150g portion?
Use (protein per 100g ÷ 100) × portion in grams.
How much protein do people need?
Protein needs vary between people. They depend on factors such as age, body size, life stage (for example pregnancy, breastfeeding or older age), physical activity and health. This course does not give personal protein targets.
European reference (EFSA)
The European Food Safety Authority (EFSA) set a population reference intake for adults of 0.83g of protein per kilogram of body weight per day.
This is a European population reference value used for planning and assessing diets in groups of people. It is not a personal target, and other countries publish their own reference values. For individual advice, speak to a doctor or registered dietitian.
Food first
Most people can get enough protein from a varied diet of everyday foods, and protein supplements are not necessary for most people. Whole foods also provide other nutrients, such as fibre, vitamins and minerals, that a supplement may not. If you are considering supplements or a major change to your diet, check with a qualified healthcare professional.
Look up protein in real foods
See protein, energy and other nutrients for everyday foods using USDA FoodData Central data.
Check your understanding
4 questions. Choose an answer for each, then check.
Answer all 4 questions to check your work.
Lesson summary
- Protein is a macronutrient providing about 4 kcal per gram.
- Proteins are built from amino acids; 9 are essential for adults and must come from food.
- Protein supports growth, repair and maintenance, and has many other roles in the body.
- Both animal and plant foods provide protein; variety across the day matters for plant proteins.
- Scale label values with (protein per 100g ÷ 100) × grams eaten.
- Needs vary between people; EFSA's 0.83g/kg/day is a European population reference, not a personal target.
- Most people can meet their needs from food without supplements.